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NASA Educational BriefsFor theclassroom

AnEducationalPublicationof the V / I V A W V J I>^vx i 11 * iN a t i o n a lA e r o n a u t i c s ( M A S A - T M - 8 4 0 9 3 ) G A L I L E O ' SS 1 A B L E 1 S 5a n dSpaceA d m i n i s t r a t ij u P I T E E ' S G A L I L E A NS AT E L L I T E S (National

Aeronautics and Space Administration) 4 p

Galileo's 'Starlets'Jupiter's Galilean SatellitesOn January 7, 1610, the Italian astronomer GalileoGalilei discovered that "beside the planet (Jupiter),there were three starlets, small indeed, bu t verybright." Within 10 days, he discovered the fourth ofthe satellites thatare named in his honor.

These four large Jovian satellites are now namedlo, Europa, Ganymede, and Callisto. In recent yearsstudies from Earth, including those supportedbyNASA, added much to knowledge about them. NASA'sVoyagers 1 and 2 in their flights throughthe Joviansystem in 1979 enabled astronomers for the firsttime to viewand study these satellites at close range.What the scientists saw were bizarre objects that aredifferent not only from any other solar system bodybut also from one an other.

lo Is a Volcanic Isle in the Ocean of Space

Certainly, the star of the Voyager picture show waslo, on whose multi-colored surface were discover-ed more than a half dozen erupting volcanoes.This is far more than have been knownto erupt simul-taneously on Earth.

lo's eruptions appear to last longer than thoseonEarth. Of eight active volcano es discov ered in Vo y-ager 1 pictures taken in March 1979, six were stillerupting wh en Voyager 2 passed in July.

lo's volcanic eruptions also appearto be moreviolent than Earth's. Voyager measurements of thevelocities of materials fired from lo's volcanoes indi-cated speeds as high as 3600 kilometers (2240miles) pe r hour. Mount Aetna, Earth's most violentvolcan o, ejects material at about 180 kilom eters (112miles)per hour.

Witho ut a doubt, lo dethrones Earth as the mostvolcanically active bodyye t observed in our SolarSystem.

What Powers lo's Volcanoes?lo is a waterless rocky body about the size of ourMoon and,until the Voyager close-ups,was believedto be cold and dead like our Moon. Earth's volcanoesare believed to be generated by heat from decayingradioactive materialsin its interior.The major drivingforce of Earth's explosive v olcan oes is steam.

Because it has only about a sixth the mass of Earth,like our Moon, lo should have exhausted nearlyall of

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Unclas0 0 / 9 1 0 7 5 7 1

lo , Jupiter's volcanic moon.

the radioactivity thatit acquired from the originalsolar nebula approximately4Y2 billion years ago.Even if lo had acquired water and other light ele-ments (volatiles)at its birth, it appears to have lostnearly all of them. So, why are there active volca-noes on lo?

Most scientists believe that lo's heatis generatedby "tidal pumping". Tidal pumping may be explaineas follows:

If lo were Jupiter's only satellite, it would keep thesame face toward Jupiter just as the Moon doeswhile orbiting Earth. Jupiter's gravity wouldpull lo's

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crust upward about a hundred meters, which isapproximately as high as a 33-storybuilding. Thiscrustal bulge, or tide, wo uld be stationa ry.

However, as the Galilean satellites Europa andGanymede pass lo, their gravities create perturba-tions in the shape of lo'sorbit. These perturbationscause the amplitude, or height, of lo's tidal bulge torise and fall, generating treme ndous frictional heat inlo's interior.

What replaces Earth's steam as the major drivingforce of lo's volcanoes? Voyager detected sulphurdioxide gas coming from lo.

Scientists theorize that gases from molten sulphurdeep beneath lo's crust, me ltedby the frictional heat,provide the driving force for the satellite's volca-noes. Gaseo us sulphur erupting from lo's volcanoeswould cool quickly and condense on the surface.This material could account for the satellite's vividorange, yellow, red and black surface colors.

lo Particles May Scatter throughoutJovian System

lo's eruptions m ay send particles far and wide through-out the Jovian magnetosphere.The magnetosphereis the volume of space encompassed by a magneticfield. It traps or deflects charged atomic particles.Jupiter 's magnetosphere was detected by NASA'sPioneer and Voyager spacecraftas far out a s about 15million kilometers (9.3 million miles)on Jupiter's dayside and beyond Saturn's orbit, more than600 mil-lion kilometers (373 m illion miles) away,on Jupiter'snight side.

It is diffic ult to im agine the size of Jupiter's ma gnet-osphere. From Earth, Jupiter looks to the n aked eyelike a bright star. If its magnetosphere were illumi-nated, it would be twice the apparent size of ourMoon.

Voyager detected particles o f sulphur an d oxyg en,believed to be from lo, in a doughnut-shaped cloudof ions (electrically charged atoms) that wobblesaround Ju piter a t the distance of lo'sorbit.

The ions may follow magnetic field lines from theorbiting cloud or "torus" not only to the outer limitsof the magnetosphere but also inward to Jupiter'snorth and south poles, causing the brilliant auroraspictured by V oyage r, lo's eruptions mayalso be thesource of the faint ring discovered around Jupiter byVoyager.

Although some measurements indicate that lo'seruptions are kicking three metric tons of materialdaily into the torus, lo is not rapidly blowing itself intooblivion. It has been calculated that, at the currentrate, lo will lose less than a tenth of one percen t of itsmass in the next billion years.

Global Ice Cap May Cover Europa

Europa has the smo othest surface seen on anysolarsystem body. No large scale physicalrelief moun-tains, valleys, or extensive meteorite craters marsits surface.

Longer linear structures resembling cracks in theshell of a hardboiled egg crisscross this incrediblysmooth surface . They are probably shallow cracks in

what scientists believeis an icycrust.Europa's density suggests that it may be 80 pe

cent silicate (rock) and 20 percent water. Infarstudies and the observed high albedo (brightness )Europa indicate that its surfac e is co vere d by ice.

Analyses of data relative to Europa and othinformation suggest a possible scena rio for Europearly history. This scenario would also help explthe absence of extensive cratering on the surfaand the creation o f the linear structures.

Tidal heatingof Europa is about a tenth of that olo. It mayhave delayed the complete freezingof layer of water covering Europa. For a time, thEuropa was covered by an ocean of water witha thiice crust over its surface. Most of the craters resuing from the primeval solar system period of hebombardment probably were erasedby the creepinof the ice ove r this moon's surface.

Progressive freezing of the liquid water layerresulted in expansion of the icy crust which crackeproducing linear fracture pa tterns. The cracks wquickly filled w ith water, we lling up from b elow.risingwater frozeas it reached the surface.

When Europa's icy covering was deep and stro

enough to preserve meteorite craters, the heaviesbombardment by meteorites had already occured.received onlythe final, low intensity, meteorite bombardment. Most of the resulting craters have belost to ice creep. As a result, Europa's smo oth surfais now lightly marked with scattered small, relatishallow craters.

The Four Faces of Ganymede

Ganymede showed four different kindsof surfacareas to Voyager spacecraft cameras. Each typeosurface may represent a stage in the history of thifascinating satellite.

On e is an enorm ous multi-ringed structure believto be the remaining scar of the impact of a giganmeteorite som e4 to 4M> billion years ago. Du ring thperiod in our solar system's developm ent, mountasized meteorites rained upon the nascent planeand their sa tellites: Whydidn't such an impact leavevast deep basin with a high cen tral peak and concetric rings of tall mountains as on the Mo on, M ars,Mercury?

Mars, Mercury, Earth, and the Moon, are largesilicate bodies. Although oceans cover about thrquarters of Earth's surface, water makes up onlabout two hundredths of one percent of its com potion. Their structuresand composition enable the

Earth, Moon , M ars, and M ercury to support hvertical relief in the form of mountains, valleys, deep craters.

Ganymede's density suggests it is about ha lf wateSpectroscopic studies indicate that Ganym ede's crumay be mostly water in the form of ice mixed wsilicates. Such a crust is unable to support higvertical relief because creep o r flow of the ice erassuch structures.

The multi-ringed area represents the most ancieface of Ganymede. Another faceis characterized bynumerous shallow impact craters a few tens of-kilmeters across. This meteorite bom bardmen t

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believed to have followe dthe one that produced thevast ring structures.As a result, this surfacemay beconsidered youngerthan the other surface.

Voyager photographs reveal yet another face ofGanymede. This surface has only about atenth themeteorite crater density of the previous one and,therefore, is considered the younger surface.

The interesting aspect of this surface is its complexof closely spaced shallow grooves. Geologists saythe complex reminds them of parallel faultingonEarth. This could mean that for a time there waswidespread fracturing of sections of Ganymede'scrust. Such an active crust would be caused by amolten interior. The molten interior might have beenheated by radioactive materials, bytidal pumping ason lo, or by a comb ination of the two.

Superimposed here and there upon the groovedareas are smooth places with few or no impactcraters. Consequently, these are assumed to be newersurfaces.

The four faces of Ganymede are believed to havebeen formed within the first billion years of thesatellite's existence and to have since been froz en intheir present position.

The Ancient Face of CallistoCallisto apparentlyhas the oldest surface of any ofthe Galilean satellites; probably as old as our Moonan d the planet Mercury. Cratersare crowded shoul-der-to-shoulder across its surface.Vast multi-ringedstructures like those seen on Ganymede are alsovisibleon Callisto. One e ncom passes nearly a quarterof the satellite's surface .

Callisto is only alittlesmaller than Ganym ede. LikeGanymede, its surface appears to be covered mostlyby a mixture of rock and of water in the form of ice.Like Ganymede, its density suggests a compositionthat is half water and half rock. There may be morewater or other volatilesin Callisto's composition thanin Ganymede's because Callisto appears to belessdense than Ganym ede.

Like Ganymede, Callisto's surface has only low

relief. This may support the argument that Callistohad a soft crust early in its history. Low relief mayalso be the consequence of creep or flow in the icycrust.

Apparently Callisto cooledand hardened in depthmuch more rapidlythan Ganymede, preserving thescars of the primeval meteorite bombardments. Sci-entists do not know why this occurred, because thesizes and compositions of Ganymede and Callistoare so similar. Observers also speculate about whyCallisto's surface bears no signs of ancient, exten-sive fracture systemsas on Ganymede. One sugges-tion is that tidal pumping of Ganymede's crust con-tributed the additional heat that made Ganymedeevolve differently from Callisto. Callisto is aboutwice as far as Ganymede from Jupiter. Thus,tidaleffects would beless pronounced.

Unique Solar System Bodies

The Galilean satellites do not resemble each other,or any other body in the solar system , lo is the m ostvolcanic object in the solar system and has theyoungest surface. Europa, Ganymede, and Callistopossess substantial qua ntitiesof water. Their crustsmay be all or mostly ice. lo has no water. Europa hasthe smoothest surface of any body in the solarsystem, lo may have a tenuo us atmo sphere fed by itsvolcanic eruptions.The others are engulfed in a dark,frigid vacuum. Ganymede shows signs of tectonicactivity in the past. Callisto's surface is the oldest ofthe Galilean s atellitesand probably amongthe mostancient in the solar system. All of thesatellites arewithin Jupiter's intense lethal radiation region.As aresult, the possibility of life on or even mannedexploration of their surface is negligible. Exploringthese bodies will continueto be the task of sophisti-cated unmanned spacecraft and perhapsrobot sur-face machines.

Ganymede is larger than the planets Mercury andPluto. It is the largest satellite in the solar system.Callisto is nearly as large as Mercury, lo and Europaare about the size of our Moon.

The Galilean Satellites

Name Diameter Distance from Jupiter(Kilometers*) (Kilometers)

lo 3632 422,000Europa 3126 671,400

Ganymede 5276 1,071,000

Callisto 4820 1,884,000

* Toconvert kilometersto statute miles,multiplythem by 0.6214.**Timefor one revolutionaround Jupiter

Period**(Day: hr: min: sec:)

1: 18: 27: 33/23: 13: 13: 42

7: 3: 42: 33

16: 16: 32: 11.2

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Questions and Activities for the Classroom

1. Obtain from your libraryand read "The StarryMessenger", a translationof Galileo's a ccountofhis astronom ical studies.

2. Find in your library a description of the m ytholog-ical characters Jupiter, lo, Europa, Ganymede,an d Callisto.How did thepractice of nam ing solarsystem bodies for mythological characters begin?

3. Why is loconsidered the mo st geologica lly activebody in the Solar System?

4. Plan an d conduct experim ents with sulphur, show-ing any change of color with alterationsin tem-perature, in molten, gaseous states, etc.

5. What is tidal pumping? Explainhow this maypower lo's volcanic erruptions. S tudy and explainthe possible effects of tidal pumpingon Ganymedeand Europa.

6. What evidence suggests that material fromlo isscattered throughout the Jovian system?

7. Whatis the "torus" encirclinglo?

8. Wha t are the cu rrent theories about the crustevolutionsof Europa, Ganym edeand Callisto?

9. What is meant by the term "tectonics"as appliedto bodies in our Solar System? On which of tGalilean satellitesdo changes involving tectoniphenomena appear to have occurred?

10. Map the locationsof satellites of Jupiter you areable to see through a telescope viewing theover a 10-day period.

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