COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC

2

8

14

16

21

TABLE OF CONTENTS

ScientificAmerican.comspecial online issue no. 4

THE SEAR CH FOR ALIEN LIFE

Are we alone in the universe? It’s a question that every school kid has probably asked at some time—and scientists in particular want an answer.In their quest after alien beings, astronomers have scanned the heavens for radio signals from another technologically advanced civilization;they’ve sent probes to all but one of the planets around our Sun; they’ve studied extreme life forms on Earth to better understand the conditionsunder which life can take root; and they’ve scrutinized the neighborhoods around distant stars.

We may never discover whether or not extraterrestrials exist—at least not until they contact us. But researchers continue to refine their search.Discoveries that water likely flowed on Mars at one time and that Jupiter’s moon Europa may house a subterranean sea have intensified the huntfor alien organisms in our own solar system. And the identification of approximately 100 extrasolar planets in recent years has raised hopes offinding inhabited worlds similar to Earth elsewhere in our galaxy.

In this special online issue, Scientific American authors review the evidence for and against the existence of ETs. In Where Are They?, Ian Crawfordponders what it means that all of our surveys so far have come up empty handed. In Is There Life Elsewhere in the Universe?, Jill C. Tarter, direc-tor of research for the Search for Extraterrestrial Intelligence (SETI) Institute, and her colleague Christopher F. Chyba assert that the search hasonly just begun. Other articles examine the cases to be made for relic life on Mars and other bodies in our solar system, as well as the plans tolaunch a new space telescope for spying on distant worlds. Buy the issue, read the articles and, the next time you gaze up at the night sky, makeup your own mind.—the Editors

Where Are They?BY IAN CRAWFORD, SIDEBAR BY ANDREW J. LEPAGE; SCIENTIFIC AMERICAN, JULY 2000Maybe we are alone in the galaxy after all

Is There Life Elsewhere in the Universe?BY JILL C. TARTER AND CHRISTOPHER F. CHYBA; SCIENTIFIC AMERICAN, DECEMBER 1999The answer is: nobody knows. Scientists' search for life beyond Earth has been less thorough than commonly thought.But that is about to change

An Ear to the StarsBY NAOMI LUBICK; SCIENTIFIC AMERICAN, NOVEMBER 2002Despite long odds, astronomer Jill C. Tarter forges ahead to improve the chances of picking up signs of extraterrestrial intelligence

Searching for Life in Our Solar SystemBY BRUCE M. JAKOSKY; SCIENTIFIC AMERICAN, MAGNIFICENT COSMOS-SPRING 1998

If life evolved independently on our neighboring planets or moons, then where are the most likely places to look for evidence of extraterrestrial organisms?

Searching for Life on Other PlanetsBY J. ROGER P. ANGEL AND NEVILLE J. WOOLF; SCIENTIFIC AMERICAN, APRIL 1996Life remains a phenomenon we know only on Earth. But an innovative telescope in space could change that by detecting signs of life on distant planets

The Case for Relic Life on MarsBY EVERETT K. GIBSON JR., DAVID S. MCKAY, KATHIE THOMAS-KEPRTA AND CHRISTOPHER S. ROMANEK; SCIENTIFIC AMERICAN, DECEMBER 1997A meteorite found in Antarctica offers strong evidence that Mars has had—and may still have—microbial life

1 SCIENTIFIC AMERICAN SPECIAL ONLINE ISSUE NOVEMBER 2002COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.

28

H ow common are other civilizations in the uni-verse? This question has fascinated humanityfor centuries, and although we still have no de-finitive answer, a number of recent develop-ments have brought it once again to the fore.Chief among these is the confirmation, after a

long wait and several false starts, that planets exist outsideour solar system.

Over the past five years more than three dozen stars like thesun have been found to have Jupiter-mass planets. And eventhough astronomers have found no Earth-like planets so far,we can now be fairly confident that they also will be plentiful.To the extent that planets are necessary for the origin and evo-lution of life, these exciting discoveries certainly augur well forthe widely held view that life pervades the universe. This viewis supported by advances in our understanding of the historyof life on Earth, which have highlighted the speed with whichlife became established on this planet. The oldest direct evi-dence we have for life on Earth consists of fossilized bacteria in3.5- billion-year-old rocks from Western Australia, announced

in 1993 by J. William Schopf of the University of California atLos Angeles. These organisms were already quite advancedand must themselves have had a long evolutionary history.Thus, the actual origin of life, assuming it to be indigenous toEarth, must have occurred closer to four billion years ago.

Earth itself is only 4.6 billion years old, and the fact that lifeappeared so quickly in geologic time—probably as soon asconditions had stabilized sufficiently to make it possible—sug-gests that this step was relatively easy for nature to achieve.Nobel prize–winning biochemist Christian de Duve has goneso far as to conclude, “Life is almost bound to arise . . . wher-ever physical conditions are similar to those that prevailed onour planet some four billion years ago.” So there is every rea-son to believe that the galaxy is teeming with living things.

Does it follow that technological civilizations are abundantas well? Many people have argued that once primitive life hasevolved, natural selection will inevitably cause it to advancetoward intelligence and technology. But is this necessarily so?That there might be something wrong with this argumentwas famously articulated by nuclear physicist Enrico Fermi in

S E A R C HING FOR E X T R AT E R R E S T RI A LS


originally published July 2000

Where Are

They?

1950. If extraterrestrials are commonplace, he asked, whereare they? Should their presence not be obvious? This ques-tion has become known as the Fermi Paradox.

This problem really has two aspects: the failure of searchfor extraterrestrial intelligence (SETI) programs to detect ra-dio transmissions from other civilizations, and the lack of evi-dence that extraterrestrials have ever visited Earth. The possi-bility of searching for ETs by radio astronomy was first seri-ously discussed by physicists Giuseppe Cocconi and PhilipMorrison in a famous paper published in the journal Naturein 1959. This was followed the next year by the first actualsearch, Project Ozma, in which Frank D. Drake and his col-leagues at the National Radio Astronomy Observatory inGreen Bank, W.Va., listened for signals from two nearby stars.Since then, many other SETI experiments have been per-formed, and a number of sophisticated searches, both all-skysurveys and targeted searches of hundreds of individual stars,are currently in progress [see “The Search for ExtraterrestrialIntelligence,” by Carl Sagan and Frank Drake; ScientificAmerican, May 1975; “Is There Intelligent Life Out There?”

by Guillermo A. Lemarchand; Scientific American Pre-sents: Exploring Intelligence, Winter 1998]. In spite of allthis activity, however, researchers have made no positive de-tections of extraterrestrial signals.

Of course, we are still in the early days of SETI, and the lackof success to date cannot be used to infer that ET civilizationsdo not exist. The searches have so far covered only a small frac-tion of the total “parameter space”—that is, the combinationof target stars, radio frequencies, power levels and temporalcoverage that observers must scan before drawing a definitiveconclusion. Nevertheless, initial results are already beginningto place some interesting limits on the prevalence of radio-transmitting civilizations in the galaxy [see box on next page].

Maybe we are alone in the galaxy after all

by Ian Crawford

ZIP, ZILCH, NADA has come out of any aliens with whom weshare the galaxy. Searches for extraterrestrial intelligence have atleast partially scanned for Earth-level radio transmitters out to4,000 light-years away from our planet (yellow circle) and for so-called type I advanced civilizations out to 40,000 light-years (redcircle). The lack of signals is starting to worry many scientists.

DON

DIX

ON; C

ALCU

LATI

ONS

BY A

NDR

EW J.

LE P

AGE

SCIENTIFIC AMERICAN SPECIAL ONLINE ISSUE 3The Search for Alien LifeCOPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.

The Fermi Paradox becomes evidentwhen one examines some of the as-sumptions underlying SETI, especiallythe total number of galactic civiliza-tions, both extant and extinct, that itimplicitly assumes. One of the currentleaders of the field, Paul Horowitz ofHarvard University, has stated that heexpects at least one radio-transmittingcivilization to reside within 1,000 light-years of the sun, a volume of space that

contains roughly a million solar-typestars. If so, something like 1,000 civi-lizations should inhabit the galaxy as awhole.

This is rather a large number, and un-less these civilizations are very long-lived, it implies that a truly enormousnumber must have risen and fallen overthe course of galactic history. (If theyare indeed long-lived—if they manageto avoid natural or self-induced catas-

trophes and to remain detectable withour instruments—that raises other prob-lems, as discussed below.) Statistically,the number of civilizations present atany one time is equal to their rate offormation multiplied by their mean life-time. One can approximate the forma-tion rate as the total number that haveever appeared divided by the age of thegalaxy, roughly 12 billion years. If civi-lizations form at a constant rate and

No SETI program has ever found averifiable alien radio signal. Whatdoes that null result mean? Anyanswer must be highly qualified,

because the searches have been so incom-plete. Nevertheless, researchers can drawsome preliminary conclusions about thenumber and technological sophistication ofother civilizations.

The most thoroughly examined frequencychannel to date, around 1.42 gigahertz, cor-responds to the emission line of the mostcommon element in the universe, hydro-gen—on the premise that if extraterrestrialshad to pick some frequency to attract our at-tention, this would be a natural choice. The di-agram on the opposite page, the first of itskind, shows exactly how thoroughly the uni-verse has been searched for signals at ornear this frequency. No signal has ever beendetected, which means that any civilizationseither are out of range or do not transmitwith enough power to register on our instru-ments. The null results therefore rule outcertain types of civilizations, including prim-itive ones close to Earth and advanced onesfarther away.

The chart quantifies this conclusion. Thehorizontal axis shows the distance fromEarth. The vertical axis gives the effectiveisotropic radiated power (EIRP) of the trans-mitters. The EIRP is essentially the transmit-ter power divided by the fraction of the skythe antenna covers. In the case of an omni-directional transmitter, the EIRP is equal tothe transmitter power itself. The most pow-erful on this planet is currently the Arecibo

radio telescope in Puerto Rico, which couldbe used as a narrowly beamed radar systemwith an EIRP of nearly 1014 watts.

The EIRP can serve as a crude proxy forthe technological level of an advanced civi-lization, according to a scheme devised byRussian SETI pioneer Nikolai S. Kardashev inthe early 1960s and later extended by CarlSagan. Type I civilizations could transmit sig-nals with a power equivalent to all the sun-light striking an Earth-like planet, about 1016

watts. Type II civilizations could harness theentire power output of a sunlike star, about1027 watts. Still mightier type III civilizationscommand an entire galaxy, about 1038

watts. If the capability of a civilization falls inbetween these values, its type is interpolat-ed logarithmically. For example, based onthe Arecibo output, humanity rates as a type0.7 civilization.

For any combination of distance andtransmitter power, the diagram indicateswhat fraction of stars has been scanned sofar without success. The white and coloredareas represent the civilizations whose exis-tence we therefore can rule out with varyingdegrees of confidence. The black area repre-sents civilizations that could have evadedthe searches. The size of the black area in-creases toward the right—that is, going far-ther away from Earth.

SETI programs completely exclude Areci-bo-level radio transmissions out to 50 or solight-years. Farther away, they can rule outthe most powerful transmitters. Far beyondthe Milky Way, SETI fails altogether, becausethe relative motions of galaxies would shift

any signals out of the detection band.These are not trivial results. Before scien-

tists began to look, they thought that type IIor III civilizations might actually be quitecommon. That does not appear to be thecase. This conclusion agrees with other as-tronomical data. Unless supercivilizationshave miraculously repealed the second lawof thermodynamics, they would need todump their waste heat, which would showup at infrared wavelengths. Yet searchesperformed by Jun Jugaku of the ResearchInstitute of Civilization in Japan and his col-leagues have seen no such offal out to a dis-tance of about 80 light-years. Assuming thatcivilizations are scattered randomly, thesefindings also put limits on the average spac-ing of civilizations and thus on their inferredprevalence in unprobed areas of the galaxy.

On the other hand, millions of undetectedcivilizations only slightly more advancedthan our own could fill the Milky Way. A hun-dred or more type I civilizations could alsoshare the galaxy with us. To complicate mat-ters further, extraterrestrials might be usinganother frequency or transmitting sporadi-cally. Indeed, SETI programs have logged nu-merous “extrastatistical events,” signals toostrong to be noise but never reobserved.Such transmissions might have been way-ward radio waves from nearby cell phones—or they might have been intermittent extrater-restrial broadcasts.

No one yet knows. Although the cuttingedge of technology has made SETI evermore powerful, we have explored only amere fraction of the possibilities.

Where They Could HideThe galaxy appears to be devoid of supercivilizations, but lesser cultures could have eluded the ongoing searches

by Andrew J. LePage


live an average of 1,000 years each, atotal of 12 billion or so technologicalcivilizations must have existed over thehistory of the galaxy for 1,000 to be ex-tant today. Different assumptions forthe formation rate and average lifetimeyield different estimates of the numberof civilizations, but all are very largenumbers. This is what makes the FermiParadox so poignant. Would none ofthese billions of civilizations, not even a

single one, have left any evidence oftheir existence?

Extraterrestrial Migration

This problem was first discussed indetail by astronomer Michael H.

Hart and engineer David Viewing in independent papers, both published in1975. It was later extended by various re-searchers, most notably physicist Frank

J. Tipler and radio astronomer RonaldN. Bracewell. All have taken as theirstarting point the lack of clear evidencefor extraterrestrial visits to Earth. What-ever one thinks about UFOs, we can besure that Earth has not been taken overby an extraterrestrial civilization, as thiswould have put an end to our own evo-lution and we would not be here today.

There are only four conceivable waysof reconciling the absence of ETs withthe widely held view that advanced civ-ilizations are common. Perhaps inter-stellar spaceflight is infeasible, in whichcase ETs could never have come hereeven if they had wanted to. Perhaps ETcivilizations are indeed actively explor-ing the galaxy but have not reached usyet. Perhaps interstellar travel is feasi-ble, but ETs choose not to undertake it.Or perhaps ETs have been, or still are,active in Earth’s vicinity but have decid-ed not to interfere with us. If we caneliminate each of these explanations ofthe Fermi Paradox, we will have to facethe possibility that we are the most ad-vanced life-forms in the galaxy.

The first explanation clearly fails. Noknown principle of physics or engineer-ing rules out interstellar spaceflight.Even in these early days of the space age,engineers have envisaged propulsionstrategies that might reach 10 to 20 per-cent of the speed of light, thereby per-mitting travel to nearby stars in a mat-ter of decades [see “Reaching for theStars,” by Stephanie D. Leifer; Scien-tific American, February 1999].

For the same reason, the second expla-nation is problematic as well. Any civi-lization with advanced rocket technolo-gy would be able to colonize the entiregalaxy on a cosmically short timescale.For example, consider a civilization thatsends colonists to a few of the planetarysystems closest to it. After those colonieshave established themselves, they sendout secondary colonies of their own, andso on. The number of colonies grows ex-ponentially. A colonization wave frontwill move outward with a speed deter-mined by the speed of the starships andby the time required by each colony toestablish itself. New settlements willquickly fill in the volume of space be-hind this wave front [see illustration onnext page].

Assuming a typical colony spacing of10 light-years, a ship speed of 10 percentthat of light, and a period of 400 yearsbetween the foundation of a colony andits sending out colonies of its own, thecolonization wave front will expand at

1010

101 102 103 104 105 106 107 108 109

1015

1020

1025

1030

Effe

ctiv

e Is

otr

op

ic R

adia

ted

Po

wer

Distance from Earth (light-years)

Percentage of Star Systems Searched

THOROUGHLYSEARCHED

Earth-levelcivilization(radio leakage)

Earth-levelcivilization(Arecibo)

Type Icivilization

Type IIcivilization

Extent ofMilky Way galaxy

Extent of local group of galaxies

NOT YET SEARCHED

0 10 20 30 40 50 60 70 80 90 100

RESULTS OF SETI PROGRAMS are summarized in this diagram. The blackarea shows which civilizations could have eluded our radio searches, either be-cause they are too far away or because their transmitters are too weak. To makesense of this diagram, choose a transmitter strength (vertical axis), read acrossto the edge of the black area and go down to find the distance from Earth (hor-izontal axis). For example, an Arecibo-class transmitter of 1014 watts must befarther away than about 4,000 light-years to have eluded the searches altogether.The color code provides more detailed information—namely, the estimated per-centage of all star systems that have been examined for transmitters of a givenpower or greater.

ANDR

EW J.

LE P

AGE

ANDREW J. LEPAGE is a physicist at Visidyne, Inc., in Burlington, Mass., where he ana-lyzes satellite remote-sensing data. He has written some three dozen articles on SETI andexobiology.

SCIENTIFIC AMERICAN SPECIAL ONLINE ISSUE 5The Search for Alien Life COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.

an average speed of 0.02 light-year ayear. As the galaxy is 100,000 light-yearsacross, it takes no more than about fivemillion years to colonize it completely.Though a long time in human terms, thisis only 0.05 percent of the age of thegalaxy. Compared with the other rele-vant astronomical and biological time-scales, it is essentially instantaneous.The greatest uncertainty is the time re-quired for a colony to establish itself and

spawn new settlements. A reasonableupper limit might be 5,000 years, thetime it has taken human civilization todevelop from the earliest cities to space-flight. In that case, full galactic coloniza-tion would take about 50 million years.

The implication is clear: the first tech-nological civilization with the ability andthe inclination to colonize the galaxycould have done so before any competi-tors even had a chance to evolve. In prin-

ciple, this could have happened billionsof years ago, when Earth was inhabitedsolely by microorganisms and was wideopen to interference from outside. Yetno physical artifact, no chemical traces,no obvious biological influence indicatesthat it has ever been intruded upon.Even if Earth was deliberately seededwith life, as some scientists have specu-lated, it has been left alone since then.

It follows that any attempt to resolvethe Fermi Paradox must rely on as-sumptions about the behavior of othercivilizations. For example, they might de-stroy themselves first, they might have nointerest in colonizing the galaxy, or theymight have strong ethical codes againstinterfering with primitive life-forms.Many SETI researchers, as well as oth-ers who are convinced that ET civiliza-tions must be common, tend to dismissthe implications of the Fermi Paradoxby an uncritical appeal to one or moreof these sociological considerations.

But they face a fundamental problem.These attempted explanations are plau-sible only if the number of extraterres-trial civilizations is small. If the galaxyhas contained millions or billions oftechnological civilizations, it seems veryunlikely that they would all destroythemselves, be content with a sedentaryexistence, or agree on the same set ofethical rules for the treatment of less de-veloped forms of life. It would take onlyone technological civilization to em-bark, for whatever reason, on a pro-gram of galactic colonization. Indeed,the only technological civilization weactually know anything about—namely,our own—has yet to self-destruct,shows every sign of being expansionist,and is not especially reticent about in-terfering with other living things.

Despite the vastness of the endeavor, Ithink we can identify a number of rea-sons why a program of interstellar colo-nization is actually quite likely. For one,

HOME PLANET

0 1

Colonization Timeline (millions of years)

2 3 3.75

0 2,500 5,000

EVOLUTION OFHUMANS

TODAY

OLDEST KNOWNFOSSILS

FORMATIONOF EARTH

OLDEST STARIN THE GALAXY

7,500 10,000 12,500

Cosmic Timeline (millions of years)

HOME PLANET

STEP 1: 500 Years

STEP 7,500: 3.75 Million Years (Galaxy Completely Colonized)

STEP 4: 2,000 Years

STEP 7: 3,500 Years STEP 10: 5,000 Years

COLONIZATION OF THE GALAXYis not as time-consuming as one mightthink. Humans could begin the processby sending colonists to two nearby stars,a trip that might take 100 years withforeseeable technology. After 400 years todig in, each colony sends out two of itsown, and so on. Within 10,000 years ourdescendants could inhabit every star sys-tem within 200 light-years. Settling theentire galaxy would take 3.75 millionyears—a split second in cosmic terms. Ifeven one alien civilization has ever under-taken such a program, its colonies shouldbe everywhere we look.

BRYA

N C

HRI

STIE

; CAL

CULA

TION

S BY

GEO

RGE

MUS

SER


a species with a propensity to colonizewould enjoy evolutionary advantageson its home planet, and it is not difficultto imagine this biological inheritancebeing carried over into a space-age cul-ture. Moreover, colonization might beundertaken for political, religious or sci-entific reasons. The last seems especiallyprobable if we consider that the first civ-ilization to evolve would, by definition,be alone in the galaxy. All its SETIsearches would prove negative, and itmight initiate a program of systematicinterstellar exploration to find out why.

Resolving the Paradox?

Furthermore, no matter how peace-able, sedentary or uninquisitive most

ET civilizations may be, ultimately theywill all have a motive for interstellarmigration, because no star lasts forever.Over the history of the galaxy, hun-dreds of millions of solar-type starshave run out of hydrogen fuel and end-ed their days as red giants and whitedwarfs. If civilizations were commonaround such stars, where have theygone? Did they all just allow themselvesto become extinct?

The apparent rarity of technologicalcivilizations begs for an explanation. Onepossibility arises from considering thechemical enrichment of the galaxy. Alllife on Earth, and indeed any conceiv-able extraterrestrial biochemistry, de-pends on elements heavier than hydro-gen and helium—principally, carbon, ni-trogen and oxygen. These elements,produced by nuclear reactions in stars,have gradually accumulated in the inter-stellar medium from which new starsand planets form. In the past the concen-trations of these elements were lower—possibly too low to permit life to arise.Among stars in our part of the galaxy,the sun has a relatively high abundanceof these elements for its age. Perhaps oursolar system had a fortuitous head startin the origins and evolution of life.

But this argument is not as compellingas it may at first appear. For one, re-searchers do not know the critical thresh-old of heavy-element abundances thatlife requires. If abundances as low as atenth of the solar value suffice, as seemsplausible, then life could have arisenaround much older stars. And althoughthe sun does have a relatively highabundance of heavy elements for its age,it is certainly not unique [see “HereCome the Suns,” by George Musser;Scientific American, May 1999].

Consider the nearby sunlike star 47 Ur-sae Majoris, one of the stars aroundwhich a Jupiter-mass planet has recentlybeen discovered. This star has the sameelement abundances as the sun, but itsestimated age is seven billion years. Anylife that may have arisen in its planetarysystem should have had a 2.5-billion-year head start on us. Many millions ofsimilarly old and chemically rich starspopulate the galaxy, especially towardthe center. Thus, the chemical evolutionof the galaxy is almost certainly not ableto fully account for the Fermi Paradox.

To my mind, the history of life onEarth suggests a more convincing expla-nation. Living things have existed herealmost from the beginning, but multicel-lular animal life did not appear untilabout 700 million years ago. For morethan three billion years, Earth was in-habited solely by single-celled microor-ganisms. This time lag seems to implythat the evolution of anything more com-plicated than a single cell is unlikely.Thus, the transition to multicelled ani-mals might occur on only a tiny fractionof the millions of planets that are inhab-ited by single-celled organisms.

It could be argued that the long soli-tude of the bacteria was simply a neces-sary precursor to the eventual appear-ance of animal life on Earth. Perhaps ittook this long—and will take a compa-rable length of time on other inhabitedplanets—for bacterial photosynthesis toproduce the quantities of atmosphericoxygen required by more complex formsof life. But even if multicelled life-formsdo eventually arise on all life-bearingplanets, it still does not follow that thesewill inevitably lead to intelligent crea-tures, still less to technological civiliza-tions. As pointed out by Stephen JayGould in his book Wonderful Life, theevolution of intelligent life depends on ahost of essentially random environmen-tal influences.

This contingency is illustrated mostclearly by the fate of the dinosaurs. Theydominated this planet for 140 millionyears yet never developed a technologi-cal civilization. Without their extinction,the result of a chance event, evolutionaryhistory would have been very different.The evolution of intelligent life on Earthhas rested on a large number of chanceevents, at least some of which had a verylow probability. In 1983 physicist Bran-don Carter concluded that “civilizationscomparable with our own are likely tobe exceedingly rare, even if locations asfavorable as our own are of common oc-

currence in the galaxy.” Of course, allthese arguments, though in my view per-suasive, may turn out to be wide of themark. In 1853 William Whewell, aprominent protagonist in the extrater-restrial-life debate, observed, “The dis-cussions in which we are engaged be-long to the very boundary regions of sci-ence, to the frontier where knowledge. . . ends and ignorance begins.” In spiteof all the advances since Whewell’s day,we are in basically the same position to-day. And the only way to lessen our ig-norance is to explore our cosmic sur-roundings in greater detail.

That means we should continue theSETI programs until either we detectsignals or, more likely in my view, we canplace tight limits on the number of radio-transmitting civilizations that may haveescaped our attention. We should pur-sue a rigorous program of Mars explo-ration with the aim of determiningwhether or not life ever evolved on thatplanet and, if not, why not. We shouldpress ahead with the development oflarge space-based instruments capableof detecting Earth-size planets aroundnearby stars and making spectroscopicsearches for signs of life in their atmo-spheres. And eventually we should de-velop technologies for interstellar spaceprobes to study the planets around near-by stars.

Only by undertaking such an ener-getic program of exploration will wereach a fuller understanding of ourplace in the cosmic scheme of things. Ifwe find no evidence for other technolog-ical civilizations, it may become our des-tiny to embark on the exploration andcolonization of the galaxy. SA

The Author

IAN CRAWFORD is an astronomer inthe department of physics and astronomyat University College London. His researchinterests mostly concern the study of in-terstellar and circumstellar environments,including circumstellar disks thought tobe forming planets. He believes that thecosmic perspective provided by the ex-ploration of the universe argues for thepolitical unification of our world. He ex-plains: “This perspective is already ap-parent in images of Earth taken fromspace, which emphasize the cosmic in-significance of our entire planet, nevermind the national boundaries we havedrawn upon its surface. And if we doever meet other intelligent species outthere among the stars, would it not bebest for humanity to speak with a unitedvoice?”


One of the ongoing searches foralien radio signals, SETI@home,

scans a stripe across the sky. Be-cause the Arecibo Observatory in

Puerto Rico has only a limited abilityto steer, the stripe extends from thecelestial equator up to a declination (celestial latitude) of 35 degrees—

which fortuitously includes many ofthe recently discovered planetary

systems. To observe year-round and avoid interfering with other

astronomical observations,SETI@home simply tags along

wherever the telescope happens tobe pointing. Over time, it sweeps

across the band.

Is There Life Elsewherein the Universe?

by Jill C. Tarter and Christopher F. Chyba

The answer is: nobody knows.

Scientists’ search for life beyond Earth has been less

thorough than commonly thought.

But that is about to changeCE

LEST

IAL

LATI

TUDE

originally published December 1999


For 40 years, scientists have conducted searches for radio signals from an extraterrestrial technology, sentspacecraft to all but one of the planets in our solar system, and greatly expanded our knowledge of theconditions in which living things can survive. The public perception is that we have looked extensively forsigns of life elsewhere. But in reality, we have hardly begun our search.

Assuming our current, comparatively robust space program continues, by 2050 we may finally knowwhether there is, or ever was, life elsewhere in our solar system. At a minimum we will have thoroughlyexplored the most likely candidates, something we cannot claim today. We will have discovered whether lifedwells on Jupiter’s moon Europa or on Mars. And we will have undertaken the systematic exobiologicalexploration of planetary systems around other stars, looking for traces of life in the spectra of planetaryatmospheres. These surveys will be complimented by expanded searches for intelligent signals.

We may find that life is common but technical intelligence is extremely rare or that both arecommon or rare.

CELESTIAL LONGITUDE

SETI@HOME, UNIVERSITY OF CALIFORNIA, BERKELEY


For now, we just don’t know. The Milky Waygalaxy is vast, and we have barely stirred its depths.Indeed, we have so poorly explored our own solarsystem that we cannot even rule out exotic possibili-ties such as the existence of a small robotic craft senthere long ago to await our emergence as a techno-

logical species. Over the next 50years, our searches for extrater-restrial intelligence will perhapsmeet with success. Or the situa-tion may remain the same as itwas in 1959, when astrophysi-cists Giuseppe Cocconi andPhilip Morrison concluded,“The probability of success isdifficult to estimate, but if wenever search, the chance ofsuccess is zero.”

A search for life elsewheremust by guided by a practical definition of life.Many researchers studying the origins of life haveadopted a “Darwinian” definition, which holdsthat life is a self-sustained chemical system capableof undergoing Darwinian evolution by natural se-lection. By this definition, we will have made livingsystems of molecules in the laboratory well before2050. The extent to which these systems will in-form us about the early history of life here or else-where is unclear, but at least they will give us someexamples of the diversity of plausible biologicalstyles.

Unfortunately, the Darwinian definition is notterribly useful from the point of view of spacecraftexploration. How long should one wait to seewhether a chemical system is capable of undergoingevolution? As a practical matter, the Darwinian ap-

proach must give way to less precise but opera-tionally more useful definitions. Consider the biolo-gy experiments that the twin Viking spacecraft car-ried to Mars in 1976. Researchers implicitly adopt-ed a metabolic definition: they hoped to recognizeMartian life through its consumption of chemicals.One of the tests they conducted, the labeled-releaseexperiment (which checked whether a soil samplefed with nutrients gave off gaseous carbon), did infact suggest the presence of organisms. In the wordsof Viking biology team leader Chuck Klein, its find-ings “would almost certainly have been interpretedas presumptive evidence for biology” were it not forcontradictory data from other experiments.

Lessons from Viking

Foremost among these other experiments wasthe Viking gas chromatograph and mass spec-

trometer, which searched for organic molecules.None were found; consequently, scientists ex-plained the labeled-release results as unanticipatedchemistry rather than biology [see “The Search forLife on Mars,” by Norman H. Horowitz; Scien-tific American, November 1977]. In effect, theyadopted a biochemical definition for life: Martianlife, like that on Earth, would be based on organiccarbon.

The Viking experience holds important lessons.First, although we should search for life from theperspective of multiple definitions, the biochemicaldefinition seems likely to trump others wheneverthe sensing is done remotely; in the absence of or-ganic molecules, biologically suggestive results willprobably be distrusted. Second, researchers mustestablish the chemical and geological context in

Large arrays of small dishes will conduct the next

generation of searches forextraterrestrial intelligence.

The plans call for hundredsor even thousands of

satellite-television antennas,which collectively offer

higher sensitivity, broaderfrequency coverage and

better resilience to interference. The first such instrument, the 1hT (which

will have a total collectingarea of one hectare, or 2.5acres), is projected to cost

$25 million.

Radio-frequency interference mightforce us to take oursearch to the far side ofthe moon, maybe to theSaha crater.

SETH

SHO

STAK

SET

IIns

titut

e

SETH SHOSTAK SETI Institute (Arecibo Observatory)


order to interpret putative biological findings. Fi-nally, life detection experiments should be de-signed to provide valuable information even in thecase of a negative result. All these conclusions arebeing incorporated into thinking about future mis-sions, such as the experiments to be flown on thefirst Europa lander.

In addition to a biochemical instrument, a valu-able life detection experiment might involve a mi-croscope. The advantage of a microscope is that itmakes so few assumptions about what might befound. But the recent controversy over Allan Hills84001, the Martian meteorite in which some re-searchers have claimed to see microfossils, remindsus that the shape of microscopic features is unlikelyto provide unambiguous evidence for life. There arejust too many nonbiological ways of producingstructures that appear biological in origin.

Europa may be the most promising site for lifeelsewhere in the solar system. Growing evidenceindicates that it harbors the solar system’s secondextant ocean—a body of water that has probablylasted for four billion years underneath a surfacelayer of ice. The exploration of Europa will beginwith a mission, scheduled for launch in 2003, de-signed to prove whether or not the ocean is reallythere [see “The Hidden Ocean of Europa,” byRobert T. Pappalardo, James W. Head and RonaldGreeley; Scientific American, October]. A posi-tive answer will inspire a program of detailed ex-ploration—including landers and perhaps, ulti-mately, ice-penetrating submarines—that willcheck whether the ocean is home to life. Whateverthe outcome, we will certainly learn a great dealmore about the limits of life’s adaptability and theconditions under which it can arise. On Earth,wherever there is liquid water, there is life, even inunexpected places, such as deep within the crust.

Another Jovian satellite, Callisto, also showssigns of a sea. In fact, subsurface oceans might bestandard features of large icy satellites in the outersolar system. Saturn’s moon Titan could be anoth-er example. Because Titan is covered with a kind ofatmospheric organic smog layer, we have not yetseen its surface in any detail [see “Titan,” by To-bias Owen; Scientific American, February1982]. In 2004 the Huygens probe will drop intoits atmosphere, floating down for two hours andsending back images. Some models suggest thatthere may be liquid hydrocarbons flowing on Ti-tan’s surface. If these organics mix with subsurfaceliquid water, what might be possible?

Inter(pla)net

By 2050 we will have scoured the surface andsome of the subsurface of Mars. Already the

National Aeronautics and Space Administration islaunching two spacecraft to Mars each time it and Earth are suitably aligned, every 26 months.In addition, researchers now plan a series of Marsmicromissions: infrastructure and technologydemonstrations that take advantage of surplus pay-

load available on launches of the European SpaceAgency’s Ariane 5 rocket. By 2010 we expect tohave established a Mars global positioning systemand computer network. Computer users on Earthwill be able to enjoy continuous live video returnedfrom robot rovers exploring Mars on the groundand in the air. In a virtual sense, hundreds of mil-lions of people will visit Mars regularly, and it willcome to seem a familiar place. As the Internet be-comes interplanetary, we will inevitably come tothink of ourselves as a civilization that spans the so-lar system.

Within a decade, we will begin returning sam-ples from Mars to Earth. But the best places tolook for extant life—Martian hot springs (if theyexist) and deep niches containing liquid water—may well be the most demanding for robot explor-ers. In the end, we will probably need to send hu-man explorers. Despite the difficulties, we foreseethe first permanent human outposts on Mars, withregularly rotating crews, by 2050. Humans willwork closely with robots to explore in detail thosesites identified as the most likely venues for life orits fossil remains.

If researchers discover life on Mars, one of thefirst questions they will ask is: Is it related to us?An important realization of the past 10 years isthat the planets of the inner solar system may nothave been biologically isolated. Viable organismscould have moved among Mars, Earth and Venus

An extraterrestrial signal shows up as aslightly tilted streak on aplot such as this one. Eachdot denotes the detectionof radio energy at a given frequency (horizontalaxis) and time (verticalaxis). Scattered dots arenoise; a line represents aregular signal. For an extraterrestrial signal, theline is slanted, becauseEarth’s rotation shifts the frequency. In this case, thetransmission (arrow)comes from a spaceship—one of ours, Pioneer 10,which is now 73 times asfar from the sun as Earth is.

SETI INSTITUTE


enclosed in rocks ejected by large impacts. Thus,whichever world first developed life may have theninoculated the others. If life exists on Mars, wemay share a common ancestor with it. If so, DNAcomparison could help us determine the world oforigin. Of course, should Martian life be of inde-pendent origin from life on Earth, it may lackDNA altogether. The discovery of a second genesiswithin our solar system would suggest that life de-velops wherever it can; such a finding would but-tress arguments for the ubiquity of life throughoutthe universe [see “The Search for ExtraterrestrialLife,” by Carl Sagan; Scientific American, Octo-ber 1994].

An essential part of our exploration of Marsand other worlds will be planetary protection.NASA now has guidelines to protect the worldsthat it visits against contamination with micro-organisms carried from Earth. We have much tolearn about reducing the bioload of spacecraft we

launch elsewhere. Progress is demanded—scien-tifically by the requirement of not introducingfalse positives, legally by international treaty and,we believe, ethically by the imperative to protectany alien biospheres.

And what about other planetary systems? Al-ready we know of more planets outside our solarsystem than within it. Well before 2050 the firsttruly interstellar missions will be flying out of oursolar system, perhaps sent on the wings of giant so-lar sails. They will directly sample the prolific or-ganic chemistry (already revealed by radio tele-scopes) present between the stars. They will notreach the nearest systems by 2050—with presenttechnology, the trip would take tens of thousandsof years—so we will have to study those systemsremotely.

By 2050 we will have catalogues of extrasolarplanetary systems analogous to our current cata-logues of stars. We will know whether our particu-lar planetary system is typical or unusual (we sus-pect it will prove to be neither). Currently the onlyworlds our technology routinely detects are giantplanets more massive than Jupiter. But advancedspace-based telescopes will regularly detect Earth-size worlds around other stars, if they exist, andanalyze their atmospheres for hints of biologicalprocesses. Such worlds would then become com-pelling targets for additional observations, includ-ing searches for intelligent signals.

Window on the Worlds

Although we talk of searching for extraterrestri-al intelligence (SETI), what we are seeking is

evidence of extraterrestrial technologies. It mightbe better to use the acronym SET-T (pronounced

the same) to acknowledge this. To date, we haveconcentrated on a very specific technology—radiotransmissions at wavelengths with weak naturalbackgrounds and little absorption [see “The Searchfor Extraterrestrial Intelligence,” by Carl Sagan andFrank Drake; Scientific American, May 1975].No one has yet found any verified signs of a distanttechnology. But the null result may have more to dowith limitations in range and sensitivity than withactual lack of civilizations. The most distant starprobed directly is still less than 1 percent of the dis-tance across our galaxy.

SETI, like all of radio astronomy, now faces acrisis. Humanity’s voracious appetite for technolo-gies that utilize the radio spectrum is rapidly ob-scuring the natural window with curtains of radio-frequency interference. This trend might eventual-

Lurking in the depths of Europa could be our fellow

inhabitants of the solar system. The surface of

Europa, mishmashed by icebergs, hints at a

subterranean ocean. Life survives deep in Earth’s crust

and oceans. Could it survive on this world, too?

ILLU

STRA

TION

BY

RON

MIL

LER


ly force us to take our search to the far side of themoon, the one place in the solar system that neverhas Earth in its sky. International agreements havealready established a “shielded zone” on themoon, and some astronomers have discussed re-serving the Saha crater for radio telescopes. If thepath for human exploration of Mars proceeds viathe moon, then by 2050 the necessary infrastruc-ture may be in place.

Plans for the next few decades of SETI also envi-sion the construction of a variety of ground-basedinstruments that offer greater sensitivity, frequencycoverage and observing time. Currently all theseplans rely on private philanthropic funding. Forsearches at radio frequencies, work has com-menced on the One Hectare Telescope (1hT),which will permit simultaneous access to the entiremicrowave window. A large field of view—and alarge amount of computational power—will en-able dozens of objects to be observed at the sametime, a mix of SETI targets and natural astronomi-cal bodies. Radio astronomy and SETI will thus beable to share telescope resources, rather than com-pete for them, as is frequently the case now. The1hT will also demonstrate one affordable way tobuild a still larger Square Kilometer Array (SKA)that could improve sensitivity by a factor of 100over anything available today. For SETI, this factorof 100 translates into a factor of 10 in distance and1,000 in the number of stars explored.

These arrays will be affordable because theirhardware will derive from recent consumer prod-ucts. To the extent possible, complexity will betransferred from concrete and steel to silicon andsoftware. We will be betting on Moore’s Law—theexponential increase in computing power overtime. The SETI@home screensaver, which morethan a million people around the globe have down-loaded (from www.setiathome.ssl.berkeley. edu), il-lustrates the kind of parallel computation availableeven today. By 2050 we may have built manySKAs and used them to excise actively the growingamount of interference. If successful, such instru-ments will certainly be more affordable than an ob-servatory on the lunar far side.

Recently other wavelength bands besides the ra-dio have been receiving attention. Generations ofstargazers have scanned the heavens with nakedeyes and telescopes without ever seeing an artifact

of astroengineering. But what if it flashed for onlya billionth of a second? Limited searches for opti-cal pulses have just begun. In the coming decades,optical SETI searches may move on to larger tele-scopes. If these initial searches do not succeed infinding other civilizations, they will at least probeastrophysical backgrounds at high time resolution.

The increased pace of solar system explorationwill provide additional opportunities for SETI. Weshould keep our robotic eyes open for probes orother artifacts of an extraterrestrial technology. De-spite tabloid reports of aliens and artifacts every-where, scientific exploration so far has revealed nogood evidence for any such things.

Sharing the Universe

Although we cannot state with confidence whatwe will know about other intelligent occupants

of the universe in 2050, we can predict that whatev-er we know, everyone will know. Everyone willhave access to the process of discovery. Anyonewho is curious will be able to keep score of whatsearches have been done and which groups arelooking at what, from where, at any given moment.The data generated by the searches will flow tooquickly for humans to absorb, but the interestingsignals, selected by silicon sieves, will be availablefor our perusal. In this way, we hope to supplantthe purveyors of pseudoscience who attract the cu-rious and invite them into a fantastic (and lucrative)realm of nonsense. Today the real data are too ofteninaccessible, whereas the manufactured data arewidely available. The real thing is better, and it willbe much easier to access in the future.

If by 2050 we have found no evidence of an ex-traterrestrial technology, it may be because technicalintelligence almost never evolves, or because techni-cal civilizations rapidly bring about their own de-struction, or because we have not yet conducted anadequate search using the right strategy. If hu-mankind is still here in 2050 and still capable of do-ing SETI searches, it will mean that our technologyhas not yet been our own undoing—a hopeful signfor life generally. By then we may begin consideringthe active transmission of a signal for someone elseto find, at which point we will have to tackle thedifficult questions of who will speak for Earth andwhat they will say.

SETI

INST

ITUT

E

JILL C. TARTER participated inher first search for extraterres-trial intelligence in 1976 whilean astrophysics graduate student at the University of California, Berkeley. Her currenteffort is over 1,000 times assensitive. Tarter’s career bears a striking resemblance to thatof Ellie Arroway, the heroine ofCarl Sagan’s novel Contact. Today she is the director of research for the SETI Institute inMountain View, Calif. When not observing, lecturing or fund-raising, she enjoys flying a private plane and dancing thesamba.

CHRISTOPHER F. CHYBA is a planetary scientist whose research focuses on the origins oflife and exobiology. He recentlyled the Science Definition Teamfor NASA’s 2003 Orbiter mission toEuropa. He now chairs the spaceagency’s Solar System Exploration Subcommittee, which recommends priorities for solar system exploration. Chyba is a former director for international environmental affairs on the National SecurityCouncil staff at the White House.At the SETI Institute, he holds the endowed chair named for his graduate school adviser, Carl Sagan.

The Authors

Further Information

INTELLIGENT LIFE IN THE UNIVERSE. I. S. Shklovskii and Carl Sagan. Holden-Day, 1966.EXTRATERRESTRIALS: SCIENCE AND ALIEN INTELLIGENCE. Edited by Edward Regis, Jr. Cambridge UniversityPress, 1985.

THE SEARCH FOR LIFE IN THE UNIVERSE. Donald Goldsmith and Tobias Owen. Addison-Wesley, 1992.IS ANYONE OUT THERE? THE SCIENTIFIC SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE. Frank Drake andDava Sobel. Delacorte Press, 1992.

EXTRATERRESTRIALS—WHERE ARE THEY? Edited by Ben Zuckerman and Michael H. Hart. Cambridge Uni-versity Press, 1995.

THE ORIGIN OF LIFE IN THE SOLAR SYSTEM: CURRENT ISSUES. Christopher F. Chyba and Gene D. McDonald inAnnual Review of Earth and Planetary Sciences, Vol. 23, pages 215–249; 1995.

SHARING THE UNIVERSE: PERSPECTIVES ON EXTRATERRESTRIAL LIFE. Seth Shostak. Berkeley Hills Books, 1998.

SETI

INST

ITUT

E


In a photograph hanging outside her office, Jill C.Tarter stands a head taller than Jodie Foster, the actresswho played an idealistic young radio astronomernamed Ellie Arroway in the film Contact. Tarter wasnot the model for the driven researcher at the center ofCarl Sagan’s book of the same name, although she un-derstands why people often make that assumption. Infact, she herself did so after reading the page proofs thatSagan had sent her in 1985. After all, both she and Ar-roway were only children whose fathers encouragedtheir interest in science and who died when they werestill young girls. And both staked their lives and careerson the search for extraterrestrial intelligence (SETI), nomatter how long the odds of detecting an otherworldlysign. But no, Tarter says, the character is actually Saganhimself—they all just share the same passion.

In her position as director of the Center for SETI Re-search at the SETI Institute in Mountain View, Calif.,Tarter has recently focused on developing new tech-nology for observing radio signals from the universe.The concept, first presented in the 1950s, is that a tech-nologically advanced civilization will leak radio signals.Some may even be transmitting purposefully.

So far there haven’t been any confirmed detections.Amid the radio chatter from natural and humansources, there have been some hiccups and a few heart-stoppingly close calls. On her first observing run atGreen Bank Observatory in West Virginia, Tarter de-tected a signal that was clearly not natural. But it turned

out to come from a telescope operator’s CB radio.Tarter’s current project is the Allen Telescope Ar-

ray, consisting of a set of about 350 small satellite dish-es in Hat Creek, Calif. The system, which will spanabout 10,000 square meters and will be the first radio-telescope array built specifically for SETI projects, isfunded by private investors. Its observing speed will be100 times as fast as that of today’s equipment, and itwill expand observable frequency ranges.

Tarter has often been a lone and nontraditional en-tity in her environment. Her interest in science, whichbegan with engineering physics, was nurtured by her fa-ther, who died when she was 12. As with most other fe-male scientists of her generation, Tarter says, a father’sencouragement was “just enough to make the differenceabout whether you blew off the negative counseling”that girls interested in science often got. Her motherworried about her when she departed in the 1960s fromtheir suburban New York home for Cornell University,when women there were still locked in their dormsovernight. She was the only female student in the engi-neering school that year. (Tarter is a descendant of EzraCornell, the university’s founder, although at the timeher gender meant that she would not receive the familyscholarship.)

“There’s an enormous amount of problem solving,of homework sets to be done as an engineering stu-dent,” Tarter recalls. Whereas male students formedteams, sharing the workload, “I sat in my dorm and didthem all by myself.” Puzzling out the problems alonegave her a better education in some ways, she says, but“it was socially very isolating, and I lost the ability tobuild teaming skills.”

Her independence and eventual distaste for engi-neering led her to do her graduate work in physics atCornell, but Tarter soon left for the University of Cal-ifornia at Berkeley to pursue a doctorate in astronomy.While working on her Ph.D., which she completed in1975, Tarter was also busy raising a daughter from her

Profile

An Ear to the StarsDespite long odds, astronomer Jill C. Tarter forges ahead to improve the chances of picking up signs of extraterrestrial intelligence By NAOMI LUBICK

■ Grew up in Scarsdale, N.Y., and is a descendant of Cornell University’s founder.■ Most influential cartoon: Flash Gordon.■ In the July Astronomical Journal, she and two colleagues conclude that

there are no more than 10,000 civilizations in the Milky Way at about ourlevel of technological advancement.

■ “I just can’t ever remember a time when I didn’t assume that the stars weresomebody else’s suns.”

JILL C. TARTER: SETI SEARCHER

14 SCIENTIFIC AMERICAN SPECIAL ONLINE ISSUE NOVEMBER 2002

originally published November 2002


LY L

Y SE

TIIn

stit

ute

first marriage, to C. Bruce Tarter, who has directed LawrenceLivermore National Laboratory for the past eight years. Thetwo had married in Tarter’s junior year of college and movedto California together. Tarter’s postdoctoral work there was onbrown dwarfs, a term she coined in the 1970s for what wasthen a hypothetical planetlike body (only recently have theybeen observed directly).

By chance, an ancient computer led Tarter to SETI. She hadprogrammed a signal-processing machine as a first-year gradu-ate student. When astronomer Stuart Bowyer acquired the com-puter from a colleague several years later for a SETI project—lackof funds forced Bowyer into looking for handouts—he ap-proached Tarter, because someone remembered that she hadused it.

To persuade her to join the project, Bowyer placed a copy ofa report on her desk called Project Cyclops, a NASA study con-ducted by Bernard M. Oliver of Hewlett Packard Corp. on pos-sible system designs for detecting extraterrestrial life. Tarter readthe hefty volume cover to cover in one night. Hooked on the ideaof SETI, she would work with Frank Drake, who in 1960 con-ducted Ozma, the first American SETI project, and with William“Jack” Welch, who taught her radio astronomy and would be-come her second husband in 1980. Astronomer John Billinghamhired her to join the small group of SETI researchers at NASA, agroup that Tarter helped to turn into the SETI Institute in 1984.She became director of SETI’s Project Phoenix in 1993, so namedbecause it was resurrected after Congress removed its funding.

The SETI project has always seemed to be NASA’s astro-nomical stepchild, Tarter explains, partly because of the “littlegreen men” associations. But the congressional rejection of thesearch for intelligent life paradoxically gave new life to its pursuit.

Operating outside the confines of NASA’s bureaucracy,Tarter says, the SETI Institute runs like a nonprofit business. Thecurrent funding for projects has come from venture capitalists—

wealthy scientific philanthropists such as Paul G. Allen andNathan P. Myhrvold, both formerly at Microsoft. Some con-tributors also serve with scientists on a board that supervisesSETI’s business plan, procedures and results.

Tarter’s efforts to push SETI forward with private financingimpress even skeptics of the enterprise. Benjamin M. Zucker-man, a radio astronomer who began his career with SETI, isblunt in his disbelief in both the search for and the existence ofextraterrestrial intelligence. Still, he finds Tarter’s work excep-tional and notes that by keeping the public interested in SETI,Tarter has enabled astronomers to continue esoteric work.

Tarter, too, has been able to overcome her solo work ten-dencies. Her SETI collaborators say she has been an indomitableand tireless team leader. Yet a bout with breast cancer in 1995may have been a defining moment of her ability to delegate au-thority. Radiation and chemotherapy treatment required thatshe step down temporarily as Phoenix project manager and cutback on her travel, thereby forcing her to assign tasks to oth-

ers. She picked up her grueling pace of going to observatoriesand attending meetings—not to mention consulting for themovie version of Contact—as soon as her therapy ended.

The SETI Institute’s Allen Telescope Array, to start up in2005, will be Tarter’s largest contribution to instrumentationyet. Thanks to advances in computers and telecommunications,the cost of the array is much lower than that of past setups. Forinstance, each 27-meter-wide dish of the Very Large Array inSocorro, N.M., cost several million dollars in the late 1970s,whereas the SETI Institute paid only $50,000 per dish for theAllen array. Each dish measures 6.1 meters wide and will be setup in a carefully selected, random pattern. The U.C. BerkeleyRadio Astronomy Lab and the SETI Institute will co-manage it.

The small dishes will be more mobile than the 305-meter-widestationary dish at Arecibo, Puerto Rico, where Tarter currentlydoes most of her observing. The Allen array will hear frequenciesfrom 0.5 to 11.2 gigahertz, a span 20 times as wide as what ra-dio telescopes can detect, and results will be high-resolution im-ages of the sky, with a dozen target stars observed at once. Plus,the institute will be able to give time to other observers—insteadof competing for it elsewhere.

Tarter strongly believes in the search for extraterrestrial in-telligence, although unlike Ellie Arroway, she seems to acceptthat a momentous signal may not come in her lifetime. Mean-while she is happy to push the technological boundaries of theearth’s listening posts and is already planning even larger tele-scopes for future Arroways to use.

Naomi Lubick is based in Palo Alto, Calif.

ALLEN TELESCOPE ARRAY (based on artist’s conception) will begin workingin 2005. Each antenna has a shroud to block ground reflections.


Searching forLifeinOur Solar

SystemIf life evolved

independently on our neighboring

planets or moons, then where are the

most likely places to look for evidence of

extraterrestrial organisms?

by Bruce M. Jakosky

Since antiquity, human beings have imagined lifespread far and wide in the universe. Only recentlyhas science caught up, as we have come to under-stand the nature of life on Earth and the possibility

that life exists elsewhere. Recent discoveries of planets orbit-ing other stars and of possible fossil evidence in Martian me-teorites have gained considerable public acclaim. And the sci-entific case for life elsewhere has grown stronger during thepast decade. There is now a sense that we are verging on thediscovery of life on other planets.

To search for life in our solar system, we need to start athome. Because Earth is our only example of a planet endowed

with life, we can use it to understand the conditions neededto spawn life elsewhere. As we define these conditions, though,we need to consider whether they are specific to life on Earthor general enough to apply anywhere.

Our geologic record tells us that life on Earth started shortlyafter life’s existence became possible—only after protoplanets(small, planetlike objects) stopped bombarding our planet nearthe end of its formation. The last “Earth-sterilizing” giant im-pact probably occurred between 4.4 and 4.0 billion years ago.Fossil microscopic cells and carbon isotopic evidence suggestthat life had grown widespread some 3.5 billion years ago andmay have existed before 3.85 billion years ago.


originally published in Magnificent Cosmos-Spring 1998


Once it became safe for life to exist, no more than half abillion years—and perhaps as little as 100 million to 200 mil-lion years—passed before life rooted itself firmly on Earth. Thisshort time span indicates that life’s origin followed a relativelystraightforward process, the natural consequence of chemicalreactions in a geologically active environment. Equally impor-tant, this observation tells us that life may originate along sim-ilar lines in any place with chemical and environmental condi-tions akin to those of Earth.

The standard wisdom of the past 40 years holds that prebio-logical organic molecules formed in a so-called reducing atmo-sphere, with energy sources such as lightning triggering chem-

ical reactions to combine gaseous molecules. A more recenttheory offers a tantalizing alternative. As water circulatesthrough ocean-floor volcanic systems, it heats to temperaturesabove 400 degrees Celsius (720 degrees Fahrenheit). Whenthat superhot water returns to the ocean, it can chemically re-duce agents, facilitating the formation of organic molecules.This reducing environment also provides an energy source tohelp organic molecules combine into larger structures and tofoster primitive metabolic reactions.

Where Did Life Originate?

The significance of hydrothermal systems in life’s his-tory appears in the “tree of life,” constructed re-cently from genetic sequences in RNA molecules,

which carry forward genetic information. This tree arisesfrom differences in RNA sequences common to all of Earth’sliving organisms. Organisms evolving little since their sepa-ration from their last common ancestor have similar RNAbase sequences. Those organisms closest to the “root”—orlast common ancestor of all living organisms—are hyper-thermophiles, which live in hot water, possibly as high as115 degrees C. This relationship indicates either that terres-trial life “passed through” hydrothermal systems at someearly time or that life’s origin took place within such systems.Either way, the earliest history of life reveals an intimate con-nection to hydrothermal systems.

As we consider possible occurrences of life elsewhere in thesolar system, we can generalize environmental conditions re-quired for life to emerge and flourish. We assume that liquidwater is necessary—a medium through which primitive or-ganisms can gain nutrients and disperse waste. Althoughother liquids, such as methane or ammonia, could serve thesame function, water is likely to have been much more abun-dant, as well as chemically better for precipitating reactionsnecessary to spark biological activity.

To create the building blocks from which life can assembleitself, one needs access to biogenic elements. On Earth, theseelements include carbon, hydrogen, oxygen, nitrogen, sulfurand phosphorus, among the two dozen or so others playinga pivotal role in life. Although life elsewhere might not useexactly the same elements, we would expect it to use manyof them. Life on Earth utilizes carbon (over silicon, for ex-ample) because of its versatility in forming chemical bonds,rather than strictly its abundance. Carbon also exists readily ascarbon dioxide, available as a gas or dissolved in water. Sili-con dioxide, on the other hand, exists plentifully in neitherform and would be much less accessible. Given the ubiquityof carbon-containing organic molecules throughout the uni-verse, we would expect carbon to play a role in life anywhere.

Of course, an energy source must drive chemical disequi-

DENDRITIC VALLEYS ON MARS resemble river drainage systems on Earth,spanning roughly one kilometer across and several hundred meters deep.Occurring primarily on ancient, cratered terrain, the valleys may haveformed from atmospheric precipitation or from underground water thatflowed onto the surface. Compared with Earth’s drainage systems, theMartian valleys show a lower channel density (number of channels persquare kilometer), suggesting that on early Mars water was less abundantthan it is on Earth.

COURTESY OF BRUCE M. JAKOSKY


librium, which fosters the reactions necessary to spawn livingsystems. On Earth today, nearly all of life’s energy comes fromthe sun, through photosynthesis. Yet chemical energy sourcessuffice—and would be more readily available for early life.These sources would include geochemical energy from hy-drothermal systems near volcanoes or chemical energy fromthe weathering of minerals at or near a planet’s surface.

Possibilities for Life on Mars

Looking beyond Earth, two planets show strong evi-dence for having had environmental conditions suit-able to originate life at some time in their history—

Mars and Europa. (For this purpose, we will consider Europa,a moon of Jupiter, to be a planetary body.)

Mars today is not very hospitable. Daily average tempera-tures rarely rise much above 220 kelvins, some 53 kelvins be-low water’s freezing point. Despite this drawback, abundantevidence suggests that liquid water has existed on Mars’s sur-face in the past and probably is present within its crust today.

Networks of dendritic valleys on the oldest Martian sur-faces look like those on Earth formed by running water. Thewater may have come from atmospheric precipitation or “sap-ping,” released from a crustal aquifer. Regardless of where itcame from, liquid water undoubtedly played a role. The val-leys’ dendritic structure indicates that they formed gradually,meaning that water once may have flowed on Mars’s surface,although we do not observe such signs today.

In addition, ancient impact craters larger than about 15kilometers (nine miles) in diameter have degraded heavily,showing no signs of ejecta blankets, the raised rims or centralpeaks typically present on fresh craters. Some partly erodedcraters display gullies on their walls, which look water-carved.Craters smaller than about 15 kilometers have eroded awayentirely. The simplest explanation holds that surface watereroded the craters.

Although the history of Mars’s atmosphere is obscure, theatmosphere may have been denser during the earliest epochs,3.5 to 4.0 billion years ago. Correspondingly, a denser atmo-sphere could have yielded a strong greenhouse effect, which

would have warmed the planet enough to permit liquid waterto remain stable. Subsequent to 3.5 billion years ago, evidencetells us that the planet’s crust did contain much water. Evi-dently, catastrophic floods, bursting from below the planet’ssurface, carved out great flood channels. These floods oc-curred periodically over geologic time. Based on this evidence,liquid water should exist several kilometers underground,where geothermal heating would raise temperatures to themelting point of ice.

Mars also has had rich energy sources throughout time. Vol-canism has supplied heat from the earliest epochs to the re-cent past, as have impact events. Additional energy to sustainlife can come from the weathering of volcanic rocks. Oxida-tion of iron within basalt, for example, releases energy thatorganisms can use.

The plentiful availability of biogenic elements on Mars’s sur-face completes life’s requirements. Given the presence of waterand energy, Mars may well have independently originated life.Moreover, even if life did not originate on Mars, life stillcould be present there. Just as high-velocity impacts have jet-tisoned Martian surface rocks into space—only to fall onEarth as Martian meteorites—rocks from Earth could similarlyhave landed on the red planet. Should they contain organ-isms that survive the journey and should they land in suitableMartian habitats, the bacteria could survive. Or, for all weknow, life could have originated on Mars and been trans-planted subsequently to Earth.

An inventory of energy available on Mars suggests thatenough is present to support life. Whether photosynthesisevolved, and thereby allowed life to move into other ecologicalniches, remains uncertain. Certainly, data returned from theViking spacecraft during the 1970s presented no evidence thatlife is widespread on Mars. Yet it is possible that some Mar-tian life currently exists, cloistered in isolated, energy-richand water-laden niches—perhaps in volcanically heated, subsur-face hydrothermal systems or merely underground, drawingenergy from chemical interactions of liquid water and rock.

Recent analysis of Martian meteorites found on Earth has ledmany scientists to conclude that life may have once thrived onMars—based on fossil remnants seen within the rock [see boxbelow]. Yet this evidence does not definitively indicate bio-logical activity; indeed, it may result from natural geochemicalprocesses. Even if scientists determine that these rocks con-tain no evidence of Martian life, life on the red planet mightstill be possible—but in locations not yet searched. To draw adefinitive conclusion, we must study those places where life(or evidence of past life) will most likely appear.

Europa

Europa, on the other hand, presents a different possi-ble scenario for life’s origin. At first glance, Europaseems an unlikely place for life. The largest of

Jupiter’s satellites, Europa is a little bit smaller than ourmoon, and its surface is covered with nearly pure ice. Yet Eu-ropa’s interior may be less frigid, warmed by a combination of

CATASTROPHIC OUTFLOW CHANNEL on Mars—Dao Vallis—is on the flanks ofthe volcano Hadriaca Patera. Scientists believe the volcano’s heat may havecaused groundwater to well up, or erupt, onto Mars’s surface at this location.The possible combination of volcanic energy and water makes this anintriguing place to search for life.

COUR

TESY

OF

BRUC

E M

. JAK

OSKY


radioactive decay and tidal heating, which could raise thetemperature above the melting point of ice at relatively shal-low depths. Because the layer of surface ice stands 150 to300 kilometers thick, a global, ice-covered ocean of liquidwater may exist underneath.

Recent images of Europa’s surface from the Galileo space-craft reveal the possible presence of at least transient pocketsof liquid water. Globally, the surface appears covered withlong grooves or cracks. On a smaller scale, these quasilinearfeatures show detailed structures indicating local ice-relatedtectonic activity and infilling from below. On the smallestscale, blocks of ice are present. By tracing the crisscrossinggrooves, the blocks clearly have moved with respect to thelarger mass. They appear similar to sea ice on Earth—as iflarge ice blocks had broken off the main mass, floated asmall distance away and then frozen in place. Unfortunately,we cannot yet determine if the ice blocks floated through liq-uid water or slid on relatively warm, soft ice. The dearth of im-pact craters on the ice indicates that fresh ice continuallyresurfaces Europa. It is also likely that liquid water is presentat least on an intermittent basis.

If Europa has liquid water at all, then that water probablyexists at the interface between the ice and underlying rocky in-terior. Europa’s rocky center probably has had volcanic activ-ity—perhaps at a level similar to that of Earth’s moon, whichrumbled with volcanism until about 3.0 billion years ago.The volcanism within its core would create an energy source

for possible life, as would the weathering of minerals reactingwith water. Thus, Europa has all the ingredients from whichto spark life. Of course, less chemical energy is likely to existon Europa than Mars, so we should not expect to see anabundance of life, if any. Although the Galileo space probehas detected organic molecules and frozen water on Callistoand Ganymede, two of Jupiter’s four Galilean satellites, thesemoons lack the energy sources that life would require to takehold. Only Io, also a Galilean satellite, has volcanic heat—yetit has no liquid water, necessary to sustain life as we know it.

Mars and Europa stand today as the only places in our solarsystem that we can identify as having (or having had) all ingre-dients necessary to spawn life. Yet they are not the only plane-tary bodies in our solar system relevant to exobiology. In partic-ular, we can look at Venus and at Titan, Saturn’s largest moon.Venus currently remains too hot to sustain life, with scorchingsurface temperatures around 750 kelvins, sustained by green-house warming from carbon dioxide and sulfur dioxide gases.Any liquid water has long since disappeared into space.

Venus and Titan

Why are Venus and Earth so different? If Earthorbited the sun at the same distance thatVenus does, en Earth, too, would blister with

heat—causing more water vapor to fill the atmosphere andaugmenting the greenhouse effect. Positive feedback would

In 1984, sur veying the Far WesternIcefield of the Allan Hills Region of

Antarctica, geologist Roberta Score pluckedfrom a plain of wind-blasted, bluish,10,000-year-old ice an unusual greenish-gray rock. Back at the National Aeronauticsand Space Administration JohnsonSpace Center and at Stanford University,researchers confirmed that the 1.9-kilogram (four-pound), potato-size rock—designated ALH84001—was a meteoritefrom Mars, one with a remarkable history.

Crystallizing 4.5 billion years ago, shortlyafter Mars’s formation, the rock was ejectedfrom the red planet by a powerful impact,which sent it hurtling through space for 16million years until it landed in Antarctica13,000 years ago. Geochemists concludedthat the rock’s distribution of oxygen iso-topes, minerals and structural features wasconsistent with those of five other meteoritesidentified as coming from Mars. Lining thewalls of fractures within the meteorite arecarbonate globules, each a flattened spheremeasuring 20 to 250 microns (millionths ofmeters). The globules appear to have formedin a carbon-dioxide-saturated fluid, possiblywater, between 1.3 and 3.6 billion years ago.Within those globules, provocative featuresvaguely resemble fossilized remnants of an-cient Martian microbes.

Tiny iron oxide and iron sulfide grains, re-sembling ones produced by bacteria onEarth, appear in the globules, as do particularpolycyclic aromatic hydrocarbons, oftenfound alongside decaying microbes. Otherovoid and tubular structures resemble fos-silized terrestrial bacteria themselves. Al-though the structures range from 30 to 700nanometers (billionths of meters) in length,some of the most intriguing tubes measureroughly 380 nanometers long—a size near-ing the low end of that for terrestrial bacteria,which are typically one to 10 microns long.

The tubes’ size and shape indicate they maybe fossilized pieces of bacteria, or tinier“nanobacteria,” which on Earth measure 20 to400 nanometers long.

These findings collectively led NASA scien-tists Everett K. Gibson, David S. McKay andtheir colleagues to announce in August 1996that microbes might once have flourished onthe red planet. Recent chemical analyses re-veal, however, that ALH84001 is heavily con-taminated with amino acids from Antarcticice, a result that weakens the case for micro-fossils from Mars. —Richard Lipkin

Microbial Remnants from Mars?

SEEMINGLY FORMED in the Martian meteorite ALH84001, a segmented object (above), some 380nanometers long, vaguely resembles fossilized bacteria from Earth.

NASA

JOH

NSON

SPA

CE C

ENTE

R


spur this cycle, with more water, greater greenhouse warm-ing and so on saturating the atmosphere and sending temper-atures soaring. Because temperature plays such a strong rolein determining the atmosphere’s water content, both Earthand Venus have a temperature threshold, above which thepositive feedback of an increasing greenhouse effect takes off.This feedback loop would load Venus’s atmosphere with wa-ter, which in turn would catapult its temperatures to very highvalues. Below this threshold, its climate would have been morelike that of Earth.

Venus, though, may not always have been so inhospitable.Four billion years ago the sun emitted about 30 percent lessenergy than it does today. With less sunlight, the boundary be-tween clement and runaway climates may have been insideVenus’s orbit, and Venus may have had surface temperaturesonly 100 degrees C above Earth’s current temperature. Lifecould survive quite readily at those temperatures—as we ob-serve with certain bacteria and bioorganisms living near hotsprings and undersea vents. As the sun became hotter, Venuswould have warmed gradually until it would have undergonea catastrophic transition to a thick, hot atmosphere. It is pos-sible that Venus originated life several billion years ago butthat high temperatures and geologic activity have since oblit-erated all evidence of a biosphere. As the sun continues toheat up, Earth may undergo a similar catastrophic transitiononly a couple of billion years from now.

Titan intrigues us because of abundant evidence of organicchemical activity in its atmosphere, similar to what mighthave occurred on the early Earth if its atmosphere had potentabilities to reduce chemical agents. Titan is about as big asMercury, with an atmosphere thicker than Earth’s, consistingpredominantly of nitrogen, methane and ethane. Methanemust be continually resupplied from the surface or subsur-face, because photochemical reactions in the atmospheredrive off hydrogen (which is lost to space) and convert themethane to longer chains of organic molecules. These longer-chain hydrocarbons are thought to provide the dense hazethat obscures Titan’s surface at visible wavelengths.

Surface temperatures on Titan stand around 94 kelvins,too cold to sustain either liquid water or nonphotochemicalreactions that could produce biological activity—although

Titan apparently had some liquid water during its early histo-ry. Impacts during its formation would have depositedenough heat (from the kinetic energy of the object) to meltfrozen water locally. Deposits of liquid water might have per-sisted for thousands of years before freezing. Every part of Ti-tan’s surface probably has melted at least once. The degree towhich biochemical reactions may have proceeded during sucha short time interval is uncertain, however.

Exploratory Missions

Clearly, the key ingredients needed for life have beenpresent in our solar system for a long time and maybe present today outside of Earth. At one time or

another, four planetary bodies may have contained the neces-sary conditions to generate life.

We can determine life’s actual existence elsewhere only em-pirically, and the search for life has taken center stage in theNational Aeronautics and Space Administration’s ongoingscience missions. The Mars Surveyor series of missions,scheduled to take place during the coming decade, aims todetermine if Mars ever had life. This series will culminate in amission currently scheduled for launch in 2005, to collectMartian rocks from regions of possible biological relevanceand return them to Earth for detailed analysis. The Cassinispacecraft currently is en route to Saturn. There the Huygensprobe will enter Titan’s atmosphere, its goal to decipher Ti-tan’s composition and chemistry. A radar instrument, too,will map Titan’s surface, looking both for geologic clues to itshistory and evidence of exposed lakes or oceans of methaneand ethane.

Moreover, the Galileo orbiter of Jupiter is focusing its ex-tended mission on studying the surface and interior of Eu-ropa. Plans are under way to launch a spacecraft missiondedicated to Europa, to discern its geologic and geochemicalhistory and to determine if a global ocean lies underneath itsicy shell.

Of course, it is possible that, as we plumb the depths of ourown solar system, no evidence of life will turn up. If life as-sembles itself from basic building blocks as easily as we be-lieve it does, then life should turn up elsewhere. Indeed, life’sabsence would lead us to question our understanding of life’sorigin here on Earth. Whether or not we find life, we will gaina tremendous insight into our own history and whether life israre or widespread in our galaxy.

The Author

BRUCE M. JAKOSKY is professor of geology and a memberof the Laboratory for Atmospheric and Space Physics at the Uni-versity of Colorado at Boulder. He is also an investigator on theMars Global Surveyor mission currently in orbit around Mars.His book The Search for Life on Other Planets will be publishedin the summer of 1998 by Cambridge University Press.

JPL/

NASA

EUROPA’S SURFACE is lined with features that suggest “ice tectonics.” Blocks of ice appear to have broken up and shifted, perhaps sliding on slush or possibly even floating on liquid water. Either way, spectral analysis of reflected light indicates nearly pure water ice on Europa’s surface. The horizontal black bars through the image designate data lost during interplanetary transmission.


The possibility that we are notalone in the universe has fasci-nated people for centuries. In

the 1600s Galileo Galilei peered into thenight sky with his newly invented tele-scope, recognized mountains on themoon, and noted that other planets werespheres like Earth. About 60 years laterother stargazers observed polar ice capson Mars, as well as color variations onthe planet’s surface, which they believedto be vegetation changing with the sea-sons (the colors are now known to bethe result of dust storms). During thelatter part of this century, cameras onboard unmanned spacecraft capturedimages from Mars of channels carvedby long gone rivers, offering hope thatlife once may have existed there. Butsamples of Martian soil obtained in the1970s by the Viking lander spacecraftlacked material evidence of any life. In-deed, the present conditions in the restof our solar system seem to be generallyincompatible with life like that foundon Earth.

But our search for extraterrestrial lifehas recently been extended—we cannow turn our attention to planets out-side our own solar system. After yearsof looking, astronomers have turned upevidence of planets orbiting three dis-tant stars similar to our sun [see box onpages 23 and 24]. Planets around these

and other stars may have evolved livingorganisms. Finding extraterrestrial lifemay seem a Herculean task, but withinthe next decade, we could build theequipment needed to locate planets withlife-forms like the primitive ones onEarth.

The largest and most powerful tele-scope now in space, the Hubble SpaceTelescope, can just make out mountainson Mars. Pictures sharp enough to dis-play geologic features of planets aroundother stars would require an array ofspace telescopes the size of the U.S. Fur-thermore, as Carl Sagan of Cornell Uni-versity has pointed out, pictures of Earthdo not reveal the presence of life unlessthey are taken at very high resolution.Detailed images could be obtained withunmanned spacecraft sent to other solarsystems, but the huge distance betweenEarth and any other planet is a distinctdrawback to this approach—it wouldtake millennia to travel to another solarsystem and send back useful images.

Taking photographs, however, is notthe best way to start studying distantplanets. Astronomers instead rely on thetechnique of spectroscopy to obtain mostof their information. In spectroscopy,light originating from an object in spacecan be analyzed for unique markers thathelp researchers piece together charac-teristics such as the celestial body’s tem-

Searching for Lifeon Other Planets

Life remains a phenomenon we know only on Earth. But an innovative telescope

in space could change that by detecting signs of life on distant planets

by J. Roger P. Angel and Neville J. Woolf

SPACE-BASED TELESCOPE SYSTEM that can search for life-bearing planets hasbeen proposed by the authors. The instrument, a type of interferometer, could be as-sembled at the proposed international space station (lower left). Subsequently, electricpropulsion would send the 50- to 75-meter-long device into an orbit around the sunroughly the same as Jupiter’s. Such a mission is at the focus of the National Aeronau-tics and Space Administration’s plans to study neighboring planetary systems.

ALFR

ED T.

KAM

AJIA

N


originally published April 1996


perature, atmospheric pressure andchemical composition.

The vital signs easiest to spot withspectroscopy are radio signals designedby extraterrestrials for interstellar com-munication. Such transmissions wouldbe totally unlike natural phenomena;such unexpected features are examplesof the kind of beacons that we mustlook for to locate intelligent life else-where. Yet sensitive scans of farawaystar systems have not come across anysignals, indicating only that extraterres-trials bent on interstellar radio commu-nication are uncommon.

But planets may be home to noncom-municating life-forms, so we need to beable to find evidence of even the sim-plest organisms. To expand our capacityto locate distant planets and determinewhether these worlds are inhabited, wehave proposed a powerful and novelsuccessor to Galileo’s telescope that will,we believe, enable us to detect life onother planets.

The simplest forms of life on ourplanet altered the conditions on Earthin ways that a distant observer couldperceive. The fossil records indicate that

within a billion years of Earth’s forma-tion, as soon as the heavy bombard-ment by asteroids ceased, primitive or-ganisms such as bacteria and algae hadspread around most of the globe. Theseorganisms represented the totality oflife here for the next two billion years;consequently, if life exists on otherplanets, it might well be in this highlyuncommunicative form.

Algae and the Atmosphere

Earth’s humble blue-green algae donot operate radio transmitters, but

they are chemical engineers par excel-lence. As algae became more wide-spread, they began adding large quanti-ties of oxygen to the atmosphere. Theproduction of oxygen is fundamental tocarbon-based life: the simplest organ-isms take in water, nitrogen and carbondioxide as nutrients and then releaseoxygen into the atmosphere as waste.Oxygen is a chemically reactive gas;without continued replenishment by al-gae and, later in Earth’s evolution, byplants, its concentration would fall.Thus, the presence of large amounts of

oxygen in a planet’s atmosphere is thefirst indicator that some form of car-bon-based life may exist there.

Oxygen leaves an unmistakable markon the radiation emitted by a planet.For example, some of the sunlight thatreaches Earth’s surface is reflectedthrough the atmosphere back towardspace. Oxygen in the atmosphere ab-sorbs some of this radiation, and thusan observer of Earth using spectroscopyto study the reflected sunlight could pickout the distinctive signature associatedwith oxygen.

In 1980 Toby C. Owen, then at theState University of New York at StonyBrook, suggested looking for oxygen’ssignal in the visible red light reflectedby planets, as a sign of life there. Closerto home, Sagan reported in 1993 thatthe Galileo space probe recorded thedistinctive spectrum of oxygen in thered region of visible light coming fromEarth. Indeed, this indication of life’sexistence has been radiating a recogniz-able signal into space for at least thepast 500 million years.

Of course, there could be some non-biological source of oxygen on a planet

Until recently, astronomers had no direct evidence that planets of anykind orbited other stars resembling the sun. Then, last October, Michel

Mayor and Didier Queloz of the Geneva Observatory announced the detectionof a massive planet circling the sunlike star 51 Pegasi [see “Strange Places,”by Corey S. Powell, “Science and the Citizen,” SCIENTIFIC AMERICAN, January1996]. Geoffrey W. Marcy and R. Paul Butler of San Francisco State Universi-ty and the University of California at Berkeley swiftly confirmed the findingand, just three months later, turned up two more bodies orbiting other, sim-ilar stars, proving the first discovery was no fluke.

Nobody has actually seen these alien worlds; all three were identified in-directly, by measuring the way they influenced the movement of their par-ent stars. As an object orbits a star, its gravitational pull causes the star towobble back and forth. That motion creates a periodic displacement, knownas a Doppler shift, in the spectrum of the star as seen from Earth. The pat-tern of the shift reveals the size and shape of the companion’s orbit; theshift’s magnitude indicates the companion’s minimum possible mass. Noother details (temperature or composition, for instance) can be discernedthrough the Doppler technique.

Even from that limited information, it is clear that the new planets are un-like anything seen before. The one around 51 Pegasi is the oddest of thebunch. Its mass is at least half that of Jupiter, and yet it orbits just sevenmillion kilometers from the parent star—less than one eighth Mercury’s dis-tance from the sun. At such proximity, the planet’s surface would be bakedto a theoretical temperature of 1,300 degrees Celsius. The planet’s orbitalperiod, or year, is just 4.2 days.

One of the planets found by Marcy and Butler orbits the star 47 Ursae Ma-joris; this body has somewhat less extreme attributes. Its three-year orbittakes it on a circular course about 300 million kilometers from its star (cor-responding to an orbit between Mars and Jupiter), and its mass is at mini-

mum 2.3 times that of Jupiter; it would not seem terribly out of place in ourown solar system.

The third new body, also identified by Marcy and Butler, circles the star 70Virginis. This “planet” is rather different from the other two. It is the heftiestof the group, having at least 6.5 times the mass of Jupiter, and its 117-dayorbit has a highly elliptical shape. Marcy has asserted that it lies in the“Goldilocks zone,” the range of distances where a planet’s temperaturecould be “just right” for water to exist in liquid form.

Despite such optimistic talk, this giant planet probably has a deep,suffocating atmosphere that offers poor prospects for life. In fact,based on its great mass and elliptical orbit, many scientists argue thatthe 70 Virginis companion should be classified not as a planet at allbut as a brown dwarf, a gaseous object that forms somewhat like a star butlacks enough mass to shine.

There is a reason why astronomers are finding only massive bodies infairly short-period orbits: these are the kind that are easiest to discern

using the Doppler technique. Uncovering a planet in a slow orbit akin toJupiter’s would require at least a decade of high-precision Doppler observa-tions. One possible way to broaden the search is to look at gravitationallensing, a process whereby the gravity of an intervening star temporarilymagnifies the light from a more distant one. If the lensing star has planets,they could produce additional, short-lived brightenings. Many stars can bemonitored at once, so this approach could yield statistics on the abundanceof planets. Unfortunately, it cannot be used to detect planets around near-by stars.

Another possibility involves searching directly for the radiation reflectedby large planets around other stars. Normally, Earth’s atmosphere wouldhopelessly blur together star and planet. Adaptive optics—a means for can-

New Planets around Sunlike Stars


without life, so this possibility must al-ways be explored. In addition, life couldbe based on some other brand of chem-istry that does not produce oxygen ascarbon-based life does. But compellingreasons lead us to expect that life onother planets would have a chemistrysimilar to our own. Carbon is particu-larly suitable as a building block of life:it is abundant in the universe, and noother known element can form themyriad of complex but stable moleculesnecessary for life as we know it.

Searching for Another Earth

Our water-rich planet is obviouslyfavorable to life. Water provides a

solvent for the biochemical reactions oflife to take place and serves as a sourceof needed hydrogen for living matter.Planets similar to Earth in size and dis-tance from their sun represent the mostplausible homes for carbon-based life inother solar systems, primarily becauseliquid water could exist on these worlds.A planet’s distance from its star deter-mines its temperature—whether it willbe too hot or too cold for liquid water.

We can easily estimate the “Goldi-locks orbit”—the distance at whichconditions are “just right” to generateand sustain life as it exists on Earth. Fora large, hot star, 25 times as bright asour sun, a hypothetical Earth-like plan-et would lie at about the distance thatJupiter circles the sun. For a small, coolstar, one tenth as bright as the sun, theplanet’s orbit would resemble Mercury’scourse.

But proper location means little if aplanet’s pull of gravity cannot hold onto oceans and an atmosphere. If distancefrom a star were the only factor to con-sider, Earth’s moon would have liquid

MIC

HAEL

GOO

DMAN

INFRARED SIGNATURE OF LIFE canbe seen only on Earth: although Venus,Earth and Mars all have atmospheres richin carbon dioxide (CO2), only Earth car-ries abundant water (H2O) and ozone(O3), a form of oxygen usually found highin the atmosphere. Water is a vital ingre-dient needed to sustain carbon-based life;oxygen is a sign of its presence. Infraredradiation emitted from planets in distantsolar systems might reveal other worldssimilar to our own.

MIC

HAEL

GOO

DMAN

WAVELENGTH (MICRONS)

SOURCE: R. Hanel, Goddard Space Flight Center

8 10 14 20

VENUS

EARTH

MARS

CO2

H2O

O3

CO2

CO2

MILLIONS OF KILOMETERS 150 300

AT LEAST 0.5 TIMES THE MASS OF JUPITER



51 PEGASI SYSTEM

70 VIRGINIS SYSTEM

47 URSAE MAJORIS SYSTEM

EARTH MARSVENUSMERCURY

INNER SOLAR SYSTEM

celing out atmospheric distortion—may offer a way to overcome thisproblem. In theory, an adaptive optics system conceived by J. Roger P.Angel and refined by David Sandler and Steve Stahl of Thermotrex Corpo-ration in San Diego could capture an image of a large planet at Jupiter’sorbital distance in a single night of observation.

The newfound planets represent only the tip of the iceberg. Continuedobservations, careful data analysis—and innovative technologies, suchas a space-based interferometer—will soon yield many more such dis-coveries, giving us a better sense of the true variety of worlds out there.

—Corey S. Powell, staff writer


water. But gravity depends on the sizeand density of the body: because themoon is smaller and less dense thanEarth, its pull of gravity is much weak-er. Any water or layers of atmospherethat might develop on or around such abody would quickly be lost to space.

Conversely, a very large planet, whichhas a strong pull of gravity, will attractgases from space. Scientists believe thatJupiter developed this way, graduallyaccumulating a huge outer shell of hy-drogen and helium. Life as we know itseems unlikely to exist on massive gase-ous planets like Jupiter.

Although we have a fairly specific de-scription of the kind of planet thatmight be hospitable to life, finding anyobject orbiting distant stars has proveddaunting. Currently the best methodsfor detecting such bodies actually in-volve looking not at the planets them-selves but at their stars. Astronomerswatch for slight variations in a star’s or-bit or light emission that can be ex-plained only by the presence of planets.Unfortunately, indirect observation ofplanets tells us little about their charac-teristics. Indeed, all indirect techniques

reveal only a body’s mass and position;ascertaining whether it carries inhabi-tants remains impossible.

Seeing Infrared

Clearly, we need a different techniqueto reveal characteristics as specific

as what chemicals can be found on aplanet. Previously we mentioned thatthe visible radiation coming from a plan-et can confirm the presence of certainmolecules, in particular oxygen, that weknow support life. But distinguishingfaint oxygen signals in light reflected bya small planet around even a star in ourown sun’s neighborhood would be ex-traordinarily difficult.

For example, the glow from a distantplanet’s sun would outshine the planetby a factor of 10 billion. So hunting forplanets can be as challenging as tryingto pick out a glowworm sitting next toa searchlight, both of which are thou-sands of kilometers away. Even if wecould pick out the light reflected by aplanet, any oxygen features in its visiblespectrum would be weak and remark-ably hard to spot.

Faced with this quandary, in 1986 weproposed, along with Andrew Y. S.Cheng, now at the University of HongKong, that monitoring the mid-infraredwavelengths (longer than visible redwavelengths) emitted by a planet wouldbe a better method for finding planetsand looking for extraterrestrial life. Thistype of radiation—really the planet’s ra-diated heat—has a wavelength 10 to 20times longer than that of visible light. Atthese wavelengths, a planet emits about40 times as many photons—particles oflight—as it does at shorter wavelengths,and the nearby star would outshine theplanet “only” 10 million times, a ratio1,000 times more favorable than thatwhich red light offers.

Moreover, three compounds thatshould appear together on inhabitedplanets—ozone (a form of oxygen usu-ally located high in the atmosphere),carbon dioxide and water—are easilyrecognizable by examining the infraredspectrum. Once again, our solar systemprovides promising support for thistechnique: a survey of the infrared emis-sions of local planets reveals that onlyEarth displays the infrared signature oflife [see lower illustration on precedingpage]. Although Earth, Mars and Ve-nus all have atmospheres with carbondioxide, only Earth shows the signatureof plentiful water and ozone.

What kind of telescope do we needto locate Earth-like planets and pick uptheir infrared emissions? Some of to-day’s ground-based telescopes can de-tect the strong infrared radiation ema-nating from stars. But the heat emittedby our atmosphere and by the telescopeitself would completely swamp any signof a planet. Even Antarctica is not near-ly cold enough to enable us to pick outsuch a faint image: the telescope mustbe cooled to at least minus 225 degreesCelsius (about 50 kelvins). More trou-blesome, radiation passing throughEarth’s atmosphere is imprinted withexactly the features of ozone, carbondioxide and water we hope to find onanother planet. Obviously, we reasoned,we must move the telescope into space.

Even then, to distinguish a planet’sradiation from that of its star, a tradi-tional telescope would have to be muchlarger than any ground-based or orbit-ing telescope built to date. Because lightcannot be focused to a spot smaller thanits wavelength, light from a distant pointin the sky can at best be focused to afuzzy core surrounded by a faint halo;even a perfect telescope mirror cannot

CANCELING STARLIGHT enables astronomers to see dim planets typically ob-scured by stellar radiance. Two telescopes focused on the same star (top) can cancelout much of its light: one telescope inverts the light—making peaks into troughs andvice versa (right ). When the inverted light is combined with the noninverted starlightfrom the second telescope (left), the light waves interfere with one another, and the im-age of the star vanishes (center).

MIC

HAEL

GOO

DMAN

MIRROR

WAVEFORM INVERTED

MIRROR

STAR OBSCURED

PLANET


form perfect images. If the halo sur-rounding the star extends beyond theplanet’s orbit, then we cannot discernthe much dimmer body of the planet in-side it. By making a telescope mirror andthe resulting image very large, we can,in principle, make the image of a star assharp as desired, but the size of theequipment needed to achieve such reso-lution renders the project infeasible.

We can predict the performance oftelescopes and thus know in advancewhat kind of image quality we can ex-pect. For example, to monitor the in-frared spectrum of an Earth-like planetcircling, say, a star 30 light-years away,we would need an enormous space tele-scope, close to 60 meters in diameter.With current technology, the cost ofsuch an instrument would rival the na-tional debt. And even telescope enthusi-asts such as ourselves regard the size ofthis device as daunting.

Rethinking the Telescope

To develop a more reasonably sizedtelescope that would allow us to lo-

cate small, perhaps habitable, planets,we knew we would have to play sometricks with our instruments. One usefulstratagem had been suggested 23 yearsago by Ronald N. Bracewell of StanfordUniversity. He showed how two smalltelescopes could be adapted to searchfor large, cool planets similar to Jupiter.The instrument he proposed consisted oftwo one-meter telescopes separated by20 meters. Each telescope alone wouldhave yielded blurred pictures that wouldnever have enabled Bracewell to resolvethe faint images of planets. But togetherthe two devices could be arranged toobserve distant worlds.

If he focused both telescopes on thesame star, Bracewell envisaged that hewould be able to invert the light wavesfrom one telescope, flipping peaks intotroughs and vice versa. Then he wouldcombine the inverted light with lightfrom the second telescope. Because thefirst image would be the reverse of thesecond, when Bracewell combined thetwo so that they overlapped precisely,the light from the star—both the coreand the surrounding halo—would becanceled out. (The light would not dis-appear, of course; energy must be con-served. Instead the light from the starwould be diverted to a separate part ofthe telescope.) Scientists refer to this typeof device as an interferometer because it

reveals details about the source of lightby employing the interference of lightwaves.

The interferometer designed by Brace-well can obscure a star only if the star isperpendicular to the line joining the cen-ters of each telescope. With such an ar-rangement, both telescopes receive ex-actly the same pattern of light wavesfrom the star. If we sweep the instru-ment through the sky, stars will appearto blink as they move in and out ofalignment.

A planet separated from its star byeven a fairly small distance, however,will not be aligned with the device whenits star is brought into alignment. Thetwo telescopes will register the planet’ssignal at slightly different times, so thelight waves from the planet will notcancel one another out. If light shinesthrough the interferometer after we havecanceled out the star’s image, we knowthat some additional source of infraredradiation—perhaps a planet—existsnear the star. We can analyze this signalby rotating the interferometer about theline joining the instrument and the star.The image will change intensity as thedevice rotates; planets should display arecognizable pattern of variation [see il-lustration on this page].

After working out the design for thisinterferometer, Bracewell realized thatthe main obstacle to locating a Jupiter-like planet would not be the overpow-

ering light from a nearby star; it wouldinstead be the heat radiated by dustparticles in our solar system, referred toas zodiacal glow. The faint signal froma distant planet would be almost imper-ceptible against the background glare.Any hope of discovering a planet would

ROTATING INTERFEROMETER couldreveal the existence of a planet around adistant star. The four telescopes arrangedas shown above in the authors’ proposedinstrument would produce a compositeview of the sky partially darkened by nu-merous bands; the star to be obscuredwould be hidden by one strip. As the in-strument rotates about the line connect-ing the center of the device with the star,the dark bands will also rotate. A nearbyplanet would pass in and out of the bands(panels a–c). Scientists could then ana-lyze the pattern of blinking to determinehow far the planet is from its star.

MIC

HAEL

GOO

DMAN

TARGET STAR

AXIS OF ROTATION

TELESCOPES TELESCOPES

a

b

c

STAR IN DARK BAND

PLANET IN DARK BAND

STAR

PLANET VISIBLE

STARPLANET

MIC

HAEL

GOO

DMAN


require averaging data for at least onemonth to see through this glowingbackground.

In addition, we found that when wetried to adapt Bracewell’s design to huntfor planets smaller than Jupiter that or-bit closer to a sun, a problem arose. Nointerferometer can perfectly cancel outstarlight—the area darkened is rathersmall, light from the star always leaksaround the edges, and any excesslight presents a significant obstaclewhen we try to see extremely dim,small planets such as Earth.

To tackle these restrictions, a num-ber of researchers, including the twoof us, have been working on alterna-tive strategies. In 1990 one of us (An-gel) suggested that arranging fourmirrors in a diamond pattern allowsbetter cancellation of starlight. But tosuppress the background glare of zo-diacal light, each telescope wouldhave to be eight meters in diameter.Alain Léger and his collaborators atthe University of Paris then suggestedthe first practical solution to thiscomplication. They proposed placingthe device in orbit around the sun, atabout the distance of Jupiter’s orbit,which would naturally cool the tele-scopes to an appropriate temperatureand would minimize backgroundglare from zodiacal light. Because ofthe decrease in background glare, theorbiting interferometer could be rela-tively small: a sensitive instrumentcould be built with four individualtelescopes as small as one meter in di-ameter. The instrument has one signifi-cant drawback, however. Because it isso effective at canceling out a star’s light,the device can sometimes conceal anearby planet as well.

Here the matter rested until 1995,when the National Aeronautics andSpace Administration solicited from re-searchers a road map for the explorationof other solar systems. nasa selectedthree teams to investigate various meth-

ods for discovering planets around oth-er stars. We assembled a team that in-cluded Bracewell, Léger and his col-league Jean-Marie Mariotti of the ParisObservatory, as well as some 20 otherscientists and engineers. In particular,the two of us at the University of Ari-zona have been studying the potentialof a new approach. We have designed

an interferometer with two pairs ofmirrors all arranged in a straight line.Each pair of mirrors will darken thestar’s main image, but significantly, eachpair will also cancel the starlight leak ofthe other pair.

It turns out that because this interfer-ometer cancels starlight very effectively,

it can be made rather long, roughly 50to 75 meters in length. The size of theinstrument offers an important advan-tage: with this arrangement, the signalsfrom planets are complex and unique.With the proper analysis, we can use thedata from the interferometer to recon-struct an image of a distant solar system[see illustration on this page]. As weenvision the orbiting interferometer, itwould point to a different star eachday but could return to interesting sys-tems for more extensive observations.

If pointed at our own solar systemfrom a nearby star, the interferometercould pick out Venus, Earth, Mars, Ju-piter and Saturn. And the data couldbe analyzed to determine the chemicalcomposition of each planet’s atmo-sphere. From our solar system, the de-vice could easily study the newly dis-covered planet around 47 Ursae Ma-joris. More important, thisinterferometer could identify Earth-like planets elsewhere that would oth-erwise elude us, and the device cancheck all these planets for the presenceof carbon dioxide, water and ozone.

Building such an instrument wouldbe a substantial undertaking, perhapsan international project, and many ofthe details have yet to be completelyworked out. We estimate that theproposed interferometer will cost lessthan $2 billion—about 10 percent ofnasa’s budget for space science re-search over the next decade. The dis-covery of life on another planet mayarguably be the crowning achieve-

ment in the exploration of space. Find-ing life elsewhere, nasa administratorDaniel S. Goldin has said, “wouldchange everything—no human endeav-or or thought would be unchanged bythat discovery.”

Remarkably, the technology to assistin this discovery is at our fingertips.Soon we should be able to answer thecenturies-old question, “Is life on Earthalone in the universe?”

The Authors

J. ROGER P. ANGEL and NEVILLE J. WOOLF have collaboratedfor the past 15 years on methods for making better telescopes. They arebased at Steward Observatory at the University of Arizona. Angel is afellow of the Royal Society and directs the Steward Observatory Mir-ror Laboratory. Woolf has pioneered techniques to minimize the distor-tion of images caused by the atmosphere. Angel and Woolf consider thequest for distant planets to be the ultimate test for telescope builders;they are meeting this challenge by pushing the limits of outer-space ob-servation technology, such as adaptive optics and space telescopes.

Further Reading

A Space Telescope for Infrared Spectroscopy of Earth-like Planets. J.R.P. Angel, A.Y.S. Cheng and N. J. Woolf inNature, Vol. 322, pages 341–343; July 24, 1986.

Use of a 16 Meter Telescope to Detect Earthlike Plan-ets. J. Roger P. Angel in The Next Generation Space Telescope.Edited by P. Bely and C. J. Burrows. Space Telescope Science In-stitute, Baltimore, 1990.

Life in the Universe. Special issue of Scientific American, Vol.271, No. 4; October 1994.

IMAGE OF DISTANT PLANETS, creat-ed from simulated interferometer signals,indicates what astronomers might reason-ably expect to see with a space-based tele-scope. This study displays a system about30 light-years away, with four planetsroughly equivalent in luminosity to Earth.(Each planet appears twice, mirroredacross the star.) With this sensitivity, theauthors speculate that the instrumentcould easily examine the planet recentlyfound orbiting 47 Ursae Majoris.

UNIVERSITY OF ARIZONA OASES PROJECT


The Case for Relic Life on MarsA meteorite found in Antarctica offers strong evidence that Mars has had—

and may still have—microbial life

by Everett K. Gibson, Jr., David S. McKay,Kathie Thomas-Keprta and Christopher S. Romanek

METEORITE TIMELINE beginswith the crystallization of the rock on

the surface of Mars, during the first 1 per-cent of the planet’s history. Less than a bil-

lion years later the rock was shocked and frac-tured by meteoritic collisions. Some time after

these impacts, a water-rich fluid flowed throughthe fractures, and tiny globules of carbonate min-erals formed in them. At the same time, molecularby-products, such as hydrocarbons, of the decayof living organisms were deposited in or near theglobules by that fluid. Impacts on the surface ofMars continued to shock the rock, fracturing theglobules, before a powerful collision ejected therock into space. After falling to Earth, the mete-orite lay in the Antarctic for millennia before itwas found and its momentous history revealed.


originally published December 1997

Of all the scientific subjects that have seizedthe public psyche, few have held on astightly as the idea of life on Mars. Starting

not long after the invention of the telescope and continuing fora good part of the past three centuries, the subject has inspired innu-

merable studies, ranging from the scientific to the speculative. But com-mon to them all was recognition of the fact that in our solar system, if a planet

other than Earth harbors life, it is almost certainly Mars.Interest in Martian life has tended to coincide with new discoveries about the myste-

rious red world. Historically, these discoveries have often occurred after one of the pe-riodic close approaches between the two planets. Every 15 years, Mars comes withinabout 56 million kilometers of Earth (the next approach will occur in the summer of2003). Typically, life on Mars was assumed to be as intelligent and sophisticated as thatof Homo sapiens, if not more so. (Even less explicably, Martian beings have been pop-ularly portrayed as green and diminutive.)

It was after one of the close approaches in the late 19th century that Italian as-tronomer Giovanni V. Schiaparelli announced that he had seen great lines stretchingacross the planet’s surface, which he called canali. At the turn of the century, U.S. as-tronomer Percival Lowell insisted that the features were canals constructed by an ad-vanced civilization. In the 1960s and 1970s, however, any lingering theories about thelines and elaborate civilizations were put to rest after the U.S. and the Soviet Union sentthe first space probes to the planet. The orbiters showed that there were in fact nocanals, although there were long, huge canyons. Within a decade, landers found no ev-idence of life, let alone intelligent life and civilization.

Although the debate about intelligent life was essentially over, the discussions aboutmicrobial life on the planet—particularly life that may have existed on the warmer, wet-ter Mars of billions of years ago—were just beginning. In August 1996 this subject wasthrust into the spotlight when we and a number of our colleagues at the National Aero-nautics and Space Administration Johnson Space Center and at Stanford University an-nounced that unusual characteristics in a meteorite known to have come from Marscould most reasonably be interpreted as the vestiges of ancient Martian bacterial life.The 1.9-kilogram, potato-sized meteorite, designated ALH84001, had been found inAntarctica in 1984.

Our theory was by no means universally embraced. Some researchers insisted thatthere were nonbiological explanations for the meteorite’s peculiarities and that these ra-tionales were more plausible than our biological explanation. We remain convincedthat the facts and analyses that we will outline in this article point to the existence of a

METEORITE COLLECTEDIN ANTARCTICA

SAMPLE ALH84001IDENTIFIED AS

METEORITE FROM MARS

PHOT

OGRA

PHS

BY N

ASA

JOH

NSON

SPA

CE C

ENTE

R; D

IGIT

AL C

OMPO

SITI

ON B

Y JE

NNI

FER

C. C

HRI

STIA

NSEN

1984

1993

1996

The Search for Alien Life COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.

primitive form of life. Moreover, suchlife-forms may still exist on Mars if, assome researchers have theorized, porespaces and cracks in rocks below thesurface of the planet contain liquid wa-ter. Why should researchers even careabout the possible existence of such asimple form of life billions of years agoon the red planet? Certainly, the preva-lence of life in the universe is among themost profound scientific questions. Yetalmost no hard data exist that can beused to theorize on that issue.

Confirmation that primitive life onceflourished on Mars would be extremelyuseful to those studying the range ofconditions under which a planet cangenerate the complex chemistry fromwhich life evolves. Then, too, the infor-mation could help us understand theorigin of life on Earth. Ultimately, thesekinds of insights could elucidate vari-ous hypotheses—which are currentlylittle more than guesses—about howcommon life is in the universe.

Inhospitable Planet

Conditions on Mars today are nothospitable to life as we know it.

The planet’s atmosphere consists of 95percent carbon dioxide, 2.7 percent ni-trogen, 1.6 percent argon and only traceamounts of oxygen and water vapor.Surface pressure is less than 1 percentof Earth’s, and daily temperatures rarelyexceed zero degrees Celsius, even in theplanet’s warmest regions in the summer.Most important, one of life’s most fun-damental necessities, liquid water, seemsnot to exist on the planet’s surface.

Given these realities, it is perhaps notsurprising that the two Viking spaceprobes that settled on the planet’s sur-face, in July and September of 1976,failed to find any evidence of life. Theresults cast doubt on—but did not com-pletely rule out—the possibility that thereis life on Mars. The landers, which wereequipped to detect organic compoundsat a sensitivity level of one part per bil-lion, found none, either at the surfaceor in the soil several centimeters down.Similarly, three other experiments foundno evidence of microbial organisms. Ul-timately, researchers concluded that thepossibility of life on Mars was quitelow and that a more definite statementon the issue would have to await theanalysis of more samples by future lan-ders—and, it was hoped, the return ofsome samples from the red planet fordetailed study on Earth.

Although the landers found no evi-dence of life on present-day Mars, pho-tographs of the planet taken from orbitby the Viking craft, as well as earlierimages made by the Mariner 9 probe,strongly suggest that great volumes ofwater had sculpted the planet’s surfacea few billion years ago and perhaps asrecently as several hundred million yearsago [see “Global Climatic Change onMars,” by Jeffrey S. Kargel and RobertG. Strom; Scientific American, No-vember 1996].

In addition, various meteorites foundon Earth and known to be of Martianorigin—including ALH84001 itself—of-fer tangible proof of Mars’s watery pastbecause they show unambiguous signsof having been altered by water. Specif-ically, some of these meteorites havebeen found to contain carbonates, sul-fates, hydrates and clays, which can beformed, so far as planetary scientistsknow, only when water comes into con-tact with other minerals in the rock.

Of course, the entire argument hingeson ALH84001’s having come from thered planet. Of this, at least, we can becertain. It is one of several meteoritesfound since the mid-1970s in meteor-ite-rich regions in Antarctica [see boxon next two pages]. In the early 1980sDonald D. Bogard and Pratt Johnson ofthe NASA Johnson Space Center beganstudying a group of meteorites found tocontain minute bubbles of gas trappedwithin glass inside the rock.

The glass is believed to have formedduring impacts with meteoroids orcomets while the rock was on the sur-face of Mars. Some of these glass-pro-ducing impacts apparently impartedenough energy to eject fragments outinto space; from there, some of theserocks were captured by Earth’s gravita-tional field. This impact scenario is theonly one that planetary scientists be-lieve can account for the existence onour world of bits of Mars.

Bogard and Johnson found that thetiny samples of gas trapped in the glassof some of the meteorites had the exactchemical and isotopic compositions asgases in the atmosphere of Mars, whichhad been measured by the Viking lan-ders in 1976. The one-to-one correlationbetween the two gas samples—over arange of nine orders of magnitude—strongly suggests that these meteoritesare from Mars. In all, five meteoriteshave been shown to contain samples oftrapped Martian atmosphere. ALH-84001 was not among the five so ana-

lyzed; however, its distribution of oxygenisotopes, minerology and other charac-teristics place it in the same group withthe other five Martian rocks.

The distribution of oxygen isotopeswithin a group of meteorites has beenthe most convincing piece of evidenceestablishing that the rocks—includingALH84001—come from Mars. In theearly 1970s Robert N. Clayton and hisco-workers at the University of Chicagoshowed that the isotopes oxygen 16,oxygen 17 and oxygen 18 in the silicatematerials within various types of mete-orites have unique relative abundances.The finding was significant because itdemonstrates that the bodies of our so-lar system formed from distinct regionsof the solar nebula and thus have uniqueoxygen isotopic compositions. Using thisisotopic “fingerprint,” Clayton helpedto show that a group of 12 meteorites,including ALH84001, are indeed close-ly related. The combination of trappedMartian atmospheric gases and thespecific distribution of oxygen isotopeshas led researchers to conclude that themeteorites must have come from Mars.

Invader from Mars

Other analyses, mainly of radioiso-topes, have enabled researchers to

outline ALH84001’s history from itsorigins on the red planet to the presentday. The three key time periods of inter-est are the age of the rock (the length oftime since it crystallized on Mars), howlong the meteorite traveled in space andhow long it has been on Earth. Analysisof three different sets of radioactive iso-topes in the meteorite have establishedeach of these time periods.

The length of time since the rock soli-dified from molten materials—the so-called crystallization age of the materi-al—has been determined through theuse of three different dating techniques.One uses isotopes of rubidium andstrontium, another, neodymium and sa-marium, and the third, argon. All threemethods indicated that the rock is 4.5billion years old. By geologic standardsthe rock is extremely old; the 4.5-bil-lion-year figure means that it crystallizedwithin the first 1 percent of Mars’s his-tory. In comparison, the other 11 Mar-tian meteorites that have been analyzedare all between 1.3 billion years old and165 million years old.

It is remarkable that a rock so old,and so little altered on Mars or duringits residence in the Antarctic ice, be-


came available for scientists to study.The duration of the meteorite’s space

odyssey was determined through theanalysis of still other isotopes, namelyhelium 3, neon 21 and argon 38. Whilea meteorite is in space, it is bombardedby cosmic rays and other high-energyparticles. The particles interact with thenuclei of certain atoms in the meteorite,producing the three isotopes listedabove. By studying the abundances andproduction rates of these cosmogenical-ly produced isotopes, scientists can de-termine how long the meteorite was ex-posed to the high-energy flux and,therefore, how long the specimen wasin space. Using this approach, research-ers concluded that after being torn freefrom the planet, ALH84001 spent 16million years in space before falling inthe Antarctic.

To determine how long the meteoritelay in the Antarctic ice, A. J. TimothyJull of the University of Arizona usedcarbon 14 dating. When silicates areexposed to cosmic rays in space, carbon14 is produced. In time, the rates of pro-duction and decay of carbon 14 balance,and the meteorite becomes saturatedwith the isotope. The balance is upsetwhen the meteorite falls from space andproduction of carbon 14 ceases. Thedecay goes on, however, reducing theamount in the rock by one half every5,700 years. By determining the differ-ence between the saturation level andthe amount measured in the silicates,researchers can determine how long themeteorite has been on Earth. Jull’s find-

ing was that ALH84001 fell from space13,000 years ago.

From the very moment it was discov-ered, the meteorite now known as ALH-84001 proved unusual and intriguing.In 1984 U.S. geologist Roberta Scorefound the meteorite in the Far WesternIcefield of the Allan Hills Region. Scorerecognized that the rock was unique be-cause of its pale greenish-gray color.The sample turned out to consist of 98percent coarse-grained orthopyroxene[(Mg,Fe)SiO3], a silicate mineral. Thereare also relatively minor amounts offeldspathic glass, which is also knownas maskelynite (NaAlSi3O8), olivine[(Mg,Fe)2SiO4], chromite (FeCr2O4) andpyrite (FeS2) as well as carbonate phas-es and phyllosilicates.

Carbonates Are Key

The most interesting aspect of ALH-84001 are the carbonates, which

exist as tiny discoids, like flattenedspheres, 20 to 250 microns in diameter.They cover the walls of cracks in themeteorite and are oriented in such away that they are flattened against theinside walls of the fractures [see illustra-tion on page 60]. The globules were ap-parently deposited from a fluid saturat-ed with carbon dioxide that percolatedthrough the fracture after the silicateswere formed. None of the other 11 me-teorites known to have come from Marshave such globules.

It was within the carbonate globulesthat our research team found the assort-

ment of unique features that led us tohypothesize that microbial organismscame into contact with the rock in thedistant past. Basically, the case for an-cient microbial life on Mars is built al-most entirely around the globules.

Individually, none of the features wefound are strongly indicative of life.Collectively, however—and especiallywithin the confines of the tiny discoids—the globules can be plausibly explainedas the ancient vestiges of microbial life.The features fall into several categoriesof evidence. One category centers on thepresence of tiny iron oxides and ironsulfide grains, which resemble thoseformed by terrestrial bacteria. The sec-ond group revolves around the presenceof organic carbon molecules in and onthe globules. Finally, unusual structuresfound within the globules bear a strikingresemblance to bacteria fossils found onEarth. Another relevant piece of evi-dence suggests the globules formed froma water-rich fluid below 100 degrees C.

NASA Johnson Space Center research-ers, along with Monica Grady of theBritish Museum of Natural History andworkers at the Open University in theU.K., performed the first isotopic analy-sis of carbon and oxygen in the carbon-ate globules. The carbon analysis indi-cates that the globules have more car-bon 13 than any carbonates found onEarth but just the right amount to havecome from Mars.

Most carbon on Earth is made up of98.9 percent carbon 12 and 1.1 percentcarbon 13. Various reactions, however,

BEDROCK

MOUNTAIN

BLUE ICE

METEORITE

SNOWFALL

ICE FLOW

WIND FLOW

EXPOSEDMETEORITE

EROSIONSURFACE

BURIEDMETEORITE

LAUR

IE G

RACE

A combination of geologic and meteorological phenomena gather mete-orites at the bases of Antarctica’s mountains. After landing, the meteoritesbecome buried in compressed snow, which eventually becomes ice.Sheets of ice move toward the edges of the continent, carrying the mete-orites with them. If a mountain blocks horizontal movement of the mete-

orites, they will in time become exposed near the mountain. The reason isthat the winds slowly but continuously “ablate” the ice above the mete-orites, turning it into a gas. Ablation exposes areas of ice that had beenburied deep under the surface, so meteorites are found on ice that is gen-erally more than 10,000 years old and is bluish in color.

The Budget Space Probe


can alter this ratio. For example, in gen-eral a sample of carbon that has been apart of an organic chemical system—say, in plant matter—is somewhat moreenriched in carbon 12, whereas carbonin limestone is relatively enriched incarbon 13. The carbon in the globulesof ALH84001 is more enriched in car-bon 13 than any natural materials onEarth. Moreover, the enrichment is dif-ferent from that of the other 11 Mar-tian meteorites. This fact suggests thatthe carbon in the globules—unlike thetrace amounts of carbon seen in theother Martian meteorites—may havebeen derived from Mars’s atmosphere.

Analysis of the distribution of oxygenisotopes in the carbonates can provideinformation about the temperature atwhich those minerals formed. The sub-ject bears directly on the question of

whether the carbonates were formed attemperatures that could support micro-bial life, because terrestrial organismsdo not survive at temperatures aboveabout 115 degrees C. The NASA-U.K.team analyzed the oxygen isotopes inthe carbonate globules. Those findingsstrongly suggest that the globules formedat temperatures no higher than 100 de-grees C. Earlier this year John W. Valleyof the University of Wisconsin–Madi-son used an ion microprobe techniqueto confirm our finding.

It should be noted that another re-search group, led by Ralph P. Harvey ofCase Western Reserve University, hasanalyzed the chemical composition ofthe minerals in the carbonates with anelectron microprobe and concluded thatthe carbonates formed at 700 degreesC. In our view, Harvey’s findings are at

odds with a growing body of evidencethat the globules formed at relativelylow temperatures.

We are extremely interested in theage of the carbonates, because it wouldallow us to estimate when microbial lifeleft its mark on the rock that becameALH84001. Yet all we can say for sureis that the carbonates crystallized in thefractures in the meteorite some time af-ter the rock itself crystallized. Variousresearch groups have come up with agesranging from 1.3 to 3.6 billion years;the data gathered so far, however, areinsufficient to date the carbonate glob-ules conclusively.

Biomineral Clues

The first category of evidence in-volves certain minerals found in-

In 1969 a team of Japanese glaciologists working near the YamatoMountains in Antarctica discovered eight meteorites in an ice field

known to be more than 10,000 years old. The discovery was remarkablebecause the meteorites were of four different types, indicating that theycould not have all fallen at the same time, as fragments of the same mete-orite. It did not take glaciologists long to figure out how ice flows consoli-date the rocks. Meteorites landing on the Antarctic ice become buried incompressed snow, called firn, which in time becomes ice. This ice eventu-ally becomes “old” ice, which is bluish in color.

Propelled by gravity, masses of ice move at a rate of about two to threemeters a year from the relatively lofty interior of the Antarctic continent tothe edges (left). If an obstacle such as a mountain range impedes the flowof ice, the ice mass pushes upward against the barrier. The ice—and themeteorites it contains—can make no more horizontal progress. In themeantime, as the winds blow over it, the surface layer of ice is slowly re-moved by a process known as ablation. Ablation, in which the solid iceconverts directly to a gas, removes typically two to three centimeters of

ice per year. As ice is removed, the meteorites within are exposed on thesurface of the ice sheet. The end result is that meteorites are continuous-ly accumulating and being exhumed at the base of Antarctica’s moun-tains. Because ablation exposes areas of ice that had been buried deepunder the firn, the meteorites are always found on regions of old, bluishice. Nowhere else in the world does this marvelous concentration mecha-nism occur. Only the Antarctic has the necessary combination of movingglaciers and mountainous barriers.

Over the past 28 years, scientific teams have recovered more than17,000 meteorites. The vast majority of samples came from the asteroidbelt, but the Antarctic harvest has also yielded 14 samples from the moonand six from Mars.

Delivered as they are free of charge, meteorites have been called thepoor man’s space probe. Before the discovery of the Antarctic meteoritecache, the world’s meteorite collections had only between 2,000 and2,500 different specimens.

—E.K.G., D.S.M., K.T.-K. and C.S.R.

NAS

A JO

HNS

ON S

PACE

CEN

TER

SEGMENTED OBJECT (above, left) is 380 nanometers in length and was found in a carbonate globule in meteorite ALH84001.The minute structure resembles fossilized bacteria, or microfossils, found on Earth. For example, the vertically oriented object to theright in the right-hand photograph is believed to be a microfossil. The object, which is also 380 nanometers long, was found 400meters below Earth’s surface (in Washington State) in a type of geologic formation known as Columbia River Basalt.


side the carbonate globules; the typeand arrangement of the minerals aresimilar, if not identical, to certain bio-minerals found on Earth. Inside, theglobules are rich in magnesite (MgCO3)and siderite (FeCO3) and have smallamounts of calcium and manganese car-bonates. Fine-grained particles of mag-netite (Fe3O4) and sulfides ranging insize from 10 to 100 nanometers on aside are present within the carbonatehost. The magnetite crystals are cuboid,teardrop or irregular in shape. Individ-ual crystals have well-preserved struc-tures with little evidence of defects ortrace impurities.

An analysis of the samples conductedwith high-resolution transmission elec-tron microscopy coupled with energy-dispersive spectroscopy indicates thatthe size, purity, morphology and crystalstructures of all these magnetites aretypical of magnetites produced by bac-teria on Earth.

Terrestrial magnetite particles associ-ated with fossilized bacteria are knownas magnetofossils. These particles arefound in a variety of sediments and soilsand are classified, according to size, assuperparamagnetic (less than 20 nano-meters on an edge) or single-domain (20to 100 nanometers). The magnetiteswithin ALH84001 are typically 40 to60 nanometers on an edge.

Single-domain magnetite has been re-ported in ancient terrestrial limestonesand is generally regarded as having beenproduced by bacteria. Most intriguing,some of the magnetites in ALH84001are arranged in chains, not unlike pearlsin a necklace. Terrestrial bacteria oftenproduce magnetite in precisely this pat-tern, because as they biologically pro-cess iron and oxygen from the water,they produce crystals that naturallyalign themselves with the Earth’s mag-netic field.

Organic Carbon Molecules

The presence of organic carbon mol-ecules in ALH84001 constitutes the

second group of clues. In recent years,researchers have found organic mole-cules not only in Martian meteoritesbut also in ones known to have comefrom the asteroid belt in interplanetaryspace, which could hardly support life.Nevertheless, the type and relative abun-dance of the specific organic moleculesidentified in ALH84001 are suggestiveof life processes. The presence of indig-enous organic molecules within ALH-

84001 is the first proof that such mole-cules have existed on Mars.

On Earth, when living organisms dieand decay, they create hydrocarbons as-sociated with coal, peat and petroleum.Many of these hydrocarbons belong toa class of organic molecules known aspolycyclic aromatic hydrocarbons(PAHs). There are thousands of differ-ent PAHs. Their presence in a sampledoes not in itself demonstrate that bio-logical processes occurred. It is the lo-cation and association of the PAHs inthe carbonate globules that make theirdiscovery so interesting.

In ALH84001 the PAHs are alwaysfound in carbonate-rich regions, includ-ing the globules. In our view, the rela-tively simple PAHs are the decay prod-ucts of living organisms that were car-ried by a fluid and trapped when theglobules were formed. In 1996 a teamat the Open University showed that thecarbon in the globules in ALH84001has an isotopic composition suggestiveof microbes that used methane as a foodsource. If confirmed, this finding will beone of the strongest pieces of evidenceto date that the rock bears the imprintof biological activity.

In our 1996 announcement, RichardN. Zare and Simon J. Clemett of Stan-ford used an extremely sensitive analyt-ical technique to show that ALH84001contains a relatively small number ofdifferent PAHs, all of which have beenidentified in the decay products of mi-crobes. Most important, the PAHs werefound to be located inside the meteorite,where contamination is very unlikely tohave occurred. This crucial finding sup-ports the idea that the carbonates areMartian and contain the vestiges of an-cient living organisms.

PAHs are a component of automobileexhaust, and they have also been foundin meteorites, planetary dust particlesand even in interstellar space. Signifi-cantly, ultrasensitive analysis of the dis-tribution of the PAHs in ALH84001 in-dicated that the PAHs could not havecome from Earth or from an extrater-restrial source—other than Mars.

Perhaps the most visually compellingpiece of evidence that at least vestiges ofmicrobes came into contact with the rockare objects that appear to be the fos-silized remains of microbes themselves.Detailed examination of the ALH-84001 carbonates using high-resolutionscanning electron microscopy (SEM)revealed unusual features that are simi-lar to those seen in terrestrial samples

associated with biogenic activity. Close-up SEM views show that the carbonateglobules contain ovoid and tube-shapedbodies [see photomicrographs on pre-ceding page]. The objects are around380 nanometers long, which means theycould very well be the fossilized remainsof bacteria. To pack in all the compo-nents that are normally required for atypical terrestrial bacterium to function,sizes larger than 250 nanometers seemto be required. Additional tubelikecurved structures found in the globulesare 500 to 700 nanometers in length.

Nanobacteria or Appendages?

Other objects found within ALH-84001 are close to the lower size

limit for bacteria. These ovoids are only40 to 80 nanometers long; other, tube-shaped bodies range from 30 to 170nanometers in length and 20 to 40nanometers in diameter. These sizes areabout a factor of 10 smaller than theterrestrial microbes that are commonlyrecognized as bacteria. Still, typical cellsoften have appendages that are general-ly quite small—in fact about the samesize as these features observed withinALH84001. It may be possible thatsome of the features are fragments orparts of larger units within the sample.

ALH84001’s numerous ovoid andelongated features are essentially identi-cal in size and morphology to those ofso-called nanobacteria on Earth. So farlittle study has been devoted to nano-bacteria or bacteria in the 20- to 400-nanometer range. But fossilized bacte-ria found within subsurface basalt sam-ples from the Columbia River basin inWashington State [see “Microbes Deepinside the Earth,” by James K.Fredrickson and Tullis C. Onstott; Sci-entific American, October 1996]have features that are essentially identi-cal to some of those observed in theovoids in ALH84001.

ALH84001 was present on Mars 4.5billion years ago, when the planet waswetter, warmer and had a denser atmo-sphere. Therefore, we might expect tosee evidence that the rock had been al-tered by contact with water. Yet therock bears few traces of so-called aque-ous alteration evidence. One such pieceof evidence would be clay minerals,which are often produced by aqueousreactions. The meteorite does indeedcontain phyllosilicate clay mineral, butonly in trace amounts. It is not clear,moreover, whether the clay mineral


formed on Mars or in the Antarctic.Mars had liquid water on its surface

early in its history and may still have anactive groundwater system below thepermafrost or cryosphere. If surface mi-croorganisms evolved during a periodwhen liquid water covered parts ofMars, the microbes might have spreadto subsurface environments when con-ditions turned harsh on the surface. Thesurface of Mars contains abundant ba-salts that were undoubtedly fracturedduring the period of early bombard-ment in the first 600 million years ofthe planet’s history. These fracturescould serve as pathways for liquid wa-ter and could have harbored any biotathat were adapting to the changing con-ditions on the planet. The situation hasan analogue on Earth, where thin gapsbetween successive lava flows appear toserve as aquifers for the movement andcontainment of groundwater contain-ing living bacteria.

Organisms may also have developedat hot springs or in underground hydro-thermal systems on Mars where chemi-cal disequilibriums can be maintainedin environments somewhat analogousto those of the mineral-rich “hot smok-ers” on the seafloor of Earth.

Thus, it is entirely possible that if or-ganisms existed on Mars in the distantpast, they may still be there. Availability

of water within the pore spaces of a sub-surface reservoir would facilitate theirsurvival. If the carbonates within ALH-84001 were formed as early as 3.6 bil-lion years ago and have biological ori-gins, they may be the remnants of theearliest Martian life.

The analyses so far of ALH84001 areconsistent with the meteorite’s carbon-ate globules containing the vestiges ofancient microbial life. Studies of themeteorite are far from over, however.Whether or not these investigationsconfirm or modify our hypothesis, theywill be invaluable learning experiencesfor researchers, who may get the op-portunity to put the experience to usein coming years. We hope that in 2005

a “sample-return” mission will belaunched to collect Martian rocks andsoil robotically and return them toEarth two and a half years later. To takeoff from the Martian surface for the re-turn to Earth, this revolutionary mis-sion may use oxygen produced on theMartian surface by breaking down car-bon dioxide in the planet’s atmosphere.

Through projects such as the samplereturn, we will finally begin to collectthe kind of data that will enable us todetermine conclusively whether lifecame into being on Mars. This kind ofinsight, in turn, may ultimately provideperspective on one of the greatest scien-tific mysteries: the prevalence of life inour universe.

Further Reading

Mars. Edited by Hugh H. Kieffer, Bruce M. Jakosky, Conway W.Snyder and Mildred S. Matthews. University of Arizona Press, 1992.

What We Have Learned about Mars from SNC Meteorites.Harry Y. McSween, Jr., in Meteoritics, Vol. 29, No. 6, pages 757– 779; November 1994.

Search for Past Life on Mars: Possible Relic Biogenic Activi-ty in Martian Meteorite ALH84001. David S. McKay et al. inScience, Vol. 273, pages 924–930; August 16, 1996.

Microbes Deep inside the Earth. James K. Fredrickson and TullisC. Onstott in Scientific American, Vol. 275, No. 4, pages 68–73; Oc-tober 1996.

Water on Mars. Michael H. Carr. Oxford University Press, 1996.Destination Mars: In Art, Myth and Science. Jay Barbree andMartin Caidin, with Susan Wright. Penguin Studio, 1997.

Information on meteorite ALH84001 and other SNC meteorites isavailable at http://www-curator.jsc.nasa.gov/curator/antmet/antmet. htm on the World Wide Web.

The Authors

EVERETT K. GIBSON, JR., DAVID S. MCKAY, KATHIETHOMAS-KEPRTA and CHRISTOPHER S. ROMANEK weremembers of the team that first reported evidence of past biologicalactivity within the ALH84001 meteorite. Gibson, McKay andThomas-Keprta work at the National Aeronautics and Space Ad-ministration Johnson Space Center in Houston, Tex.; Romanek, aformer National Research Council postdoctoral fellow at theJohnson center, is with the department of geology and the Savan-nah River Ecology Laboratory at the University of Georgia. Gib-son, a geochemist and meteorite specialist, and McKay, a geologistand expert on planetary regoliths, are senior scientists in the John-son center’s Earth Sciences and Solar System Exploration Division.Thomas-Keprta, a senior scientist at Lockheed Martin, is a biolo-gist who applies electron microscopy to the study of meteorites,interplanetary dust particles and lunar samples. Romanek’s spe-cialty is low-temperature geochemistry and stable-isotope massspectrometry. Gibson can be reached via [email protected]

WATER FROST, probably only micronsthick, covers parts of red, rocky Martiansoil in a photograph taken by the Viking 2lander in May 1979. The image was seenas further evidence that water exists on thesurface of the planet, albeit in solid form.

NAS

A JE

T PR

OPUL

SION

LAB

ORAT

ORY


COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC

Documents

Transcript of COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC