SKY Saturns Bounty

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OBSERVING MaysSun, Moon &PlanetsOBSERVINGSun, Moon &Planets

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Mays Amazing Planet TrioThe three brightest planets form an extremely tight group at dusk.

Sun, Moon & PlanetsSaturn is well placedfor viewing almostall night throughout May. And a remark-

able planetary drama unfolds in the lastthird of the month: Venus, Jupiter, andMercury converge into a very tight knot

low in the evening twilight.

D U S KMay begins with Jupiterhigh and Venus

low in the west-northwest shortly aftersunset. Venus sets in bright twilight, just

hour after the Sun. Jupiter, a huge to Venuss upper left, remains visible longafter the sky is dark. But Jupiter appears

about lower each evening, while Venuscreeps a little higher, as they head toward

conjunction on May th.Venus and Jupiter form lovely configu-

rations with the crescent Moon on May

, as shown below.Mercuryis invisible for the first half of

May, passing through superior conjunc-tion behind the Sun on May th. But it

joins the evening planet scene just eightdays later.

On May th, Jupiter, Venus, and Mer-cury form a line less than long. Mer-

cury is only about high a half hour aftersunset, so you may need binoculars to spot

it even though it shines at magnitude ..Venus is about upper left of Mercury,and Jupiter upper left of Venus.

The line shortens each evening asMercury appears higher and Jupiter lower.

From May , Venus, Jupiter, andMercury form a trio a temporary

gathering of three celestial objects that fitswithin a circle or less wide. The eventtakes place fairly low in bright twilight,

but its readily visible because all threeplanets are very bright. Mercury fades

just a trace, from magnitude .to .,while Venus and Jupiter hold steady at

magnitude .and ., respectively. Forskywatchers around latitude north, allthree planets are more than above the

west-northwest horizon a half hour aftersunset throughout the trio.

Venus and Mercury appear closesttogether on May th, with Mercury /

upper right of Venus. The trio is mocompact on the evening of May th

the Americas, when the three planewithin a circle less than wide. T

is also when Jupiter and Mercury arclosest. On May th, Venus and Jupare side by side apart with Merc

less than /above them. Then on

th, Venus and Jupiter, the two bri

planets, are only apart.Compare the disks of the three p

in a telescope while theyre a trio: Juwide and fully lit, Venus widabout % lit, while Mercurys gibbo

disk alters subtly from .and %

.and % lit. Try to catch the pla

immediately after sunset, while theyhigh as possible above the horizon.

All good things must come to an

On May st the three planets are agin a diagonal line, but now its expan

ing Venus is lower right of Meand Jupiter lower right of Venu

Jupiter, falling out of the gathering, setting less than an hour after the S

minutes after sunset

Aldebaran

Capella

Betelgeuse

Venus

Jupiter

MoonMay

Moon

May

MoonMay

Looking West-Northwest

Use binoculars forvery young Moon!

Dusk, May

Capella

Mercury

Venus

Jupiter

Looking Northwest

Dusk, May minutes after sunset

Tau

Tau

A U R I G A

These scenes are

always drawn for

the middle of No

America (latitude

north, longit

west); Europ

observers should

move each Moon

symbol a quarte

the way toward t

one for the previ

ous date. In the

East, move the M

halfway. The blu

scale bar is abou

the width of you

at arms length.

clarity, the Moon

shown three tim

actual apparent


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Fred SchaafTo see what the sky looks like at any given time and date, go toSkyandTelescope.com/skychart.

SkyandTelescope.c

Jupiter

Neptune

Uranus

Pluto

Saturn

Marchequinox Sept.

equinox

Decembersolstice

June solstice

Mars

Earth

Sun

Mercury

Venus

ORBITS OF THE PLANETSThe curved arrows show each planets m

during May. The outer planets dont chan

enough in a month to notice at this scale

See skypub.com/mayplanetsforan animation of the planets changing

configurations.

D U S K T H R O U G H

M O S T O F N I G H TSaturnwas at opposition on April th,

so in May it remains bright, bigger thanusual in telescopes, and visible almost

all night long. The planet shrinks anddims just a little, ending May at magni-tude +.. But it climbs high earlier in

the night. At dusk on May st Saturn isalready well up in the south-southeast,

shining about one-third the way up thesky. Saturn retrogrades out of Libra into

easternmost Virgo this month. Its separa-tion from Spica decreases from to .

D A W NUranus, in Pisces, and Neptune, in

Aquarius, are highest at dawn; see skypub.com/urnepfor finder charts.

Pluto, in Sagittarius, is highest aroundthe beginning of morning twilight; next

months issue will contain finder charts.Marsrises too soon before the Sun to

be observed.

S U N A N D M O O N

The Sunundergoes an annular eclipseon May . The eclipse is visible mostly

over the Pacific, but the path of annularity

crosses northwestern Australia, and thepartial phase is visible in most of Mela-

nesia, Australia, and New Zealand. Seeskypub.com/mayeclipsesfor details onthis and the lunar eclipse described below.

The Moonundergoes an extremelyslight penumbral eclipse on the American

night of May , probably undetect-able even by the most sensitive scientific

instruments. But theres a consolation thatnight: the full Moon occults the double

star Beta Scorpii in the southeastern

and elsewhere; see page .The Moon is a very thin crescent

lower left of Venus on May th very

in the west-northwest a half hour afsunset. It passes Jupiter on the th

th. The waxing gibbous Moon is jto the right of Spica on the America

evening of May

st and near Saturnthe nd.

Capella

Mercury

VenusJupiter

Looking Northwest


Tau

Tau

A U R I G ACapella

Mercury

VenusJupiter

Looking Northwest


Tau

Tau

A U R I G A

Mercury

Venus

Jupiter

Looking Northwest


Tau

Tau

A U R I G A


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5 Sky & Telescope

These questions, and more, motivated the development

of NASAs Cassini spacecraft, which was launched in 1997

and entered Saturnian orbit in July 2004. Along with map-

ping methane lakes on Titans surface, catching Enceladus

spewing icy geysers, and tracking storms in Saturns atmo-

sphere, Cassini has conducted extensive studies of the ring

system and its environment. These observations have shed

new light on the nature of the ring particles, how they in-

teract with one another and with the planets moons, and

the role of fine dust within the ring system. And, as always

with exploration, Cassinis revelations are prompting us

to ask even more questions, as mysteries we didnt know

about come to our attention.

Saturns main rings are divided into the outermost A

ring, the dense and bright B ring, and the inner, more deli-

cate C ring. The Cassini Division between the A and B rings

is not empty but contains tenuous material analogous to

the C ring. The F ring is a wild and woolly narrow loop just

outside the A ring, while three dusty, diaphanous rings are

designated D (inside C), G (outside F), and E (outside G).

Moons and Disks

Saturns rings provide our only way to get up close and per-sonal with an astrophysical disk. Astronomers have found

many nearby stars encircled by young gas disks or slightly

older dust disks; presumably our solar system once looked

similar to these. We can learn much about disks from re-

mote observations and theoretical models, but only at Sat-

urn can we directly observe disk processes in detail.

Of particular interest is a disks interaction with a mas-

sive object a moonlet or moon in Saturns case, a plan-

etesimal or planet around a star. Cassini has shed light on

such processes in a number of ways.

Two moons, each several kilometers wide, orbit inside

gaps in the outer A ring: Pan in the 320-km-wide Encke

Division and Daphnis in the 35-km-wide Keeler Gap. Much

as planetesimals must do in a disk around a young star,

Pan and Daphnis gravitationally perturb the orbits of the

smaller ring particles that pass by them, creating scalloped

patterns in the gap edges and generating wakes that propa-

gate into the ring.

The gap edges have turned out to be considerably more

complex than theorists predicted. Cassini sees the wavy

Last September, Cassini

spent 12 hours in Saturn

shadow and used that

opportunity to capture

the rings brilliantly back

lit by sunlight. Because

the camera was viewing

the systems unlit side,

opaque regions like the

ring look dark, while mo

tenuous rings of small

dust particles look brigh

The ring segment in

shadow (at left) appears

in silhouette against the

planets faintly illumi-

nated night side.

Even though their scales and origins differ, spiral arms arise in bothSaturns rings and galaxies. Resonant effects from larger, more dista

moons generate spiral density waves in the rings, where particles b

up in a rhythmic progression. Here we see these waves in the A r in

pringworld revelations

E ring

G ringF ring

Encke Division

(Pan)

C ringD ring

B ringA ring

Cassini

Division


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edges of the Encke Division fluctuate in both frequency

and amplitude; they never completely die out. Many other

ring edges, including those of the A ring, the B ring, and

the Keeler Gap, have more elaborate shapes than expected.

The cause of this complexity remains unclear. Tracking it

down may lead to the discovery of small moonlets nearby,

or it may correct a deficiency in our understanding of gap-

clearing processes.

Larger moons outside the main rings raise spiral density

waves at ring locations that are in resonance with a moons

orbit. Resonances are akin to pushing a swing on a play-

ground. If you push the swing indiscriminately some-

times when the swing is moving forward, sometimes when

its moving backward, sometimes at the top of its arc,

sometimes in the middle then the swing will not go veryhigh and nothing very interesting happens. But if you push

in step with the swings natural frequency, then the swing

will go increasingly higher and higher often to the de-

light of a child.

Moons can have a similar effect on ring particles. For

example, when a chunk of rock or ice completes six orbits

in precisely the interval it takes a moon to orbit five times,

each moon passage will produce the same push on it. As

those pushes add up, the particles orbit becomes elon-

gated, moving in and out from the perspective of other

(well-behaved, circular-orbiting) objects. When millions of

ring particles in the same region become perturbed in this

way, they generate a spiral density wave that propagates

away from the disturbance.

These density waves follow the same physics that give

rise to spiral arms in galaxies, though their origins are dif-

ferent. Measuring the height, shape, and wavelength of

these waves can yield information about the rings surface

density (mass per unit area) and the mass of the perturbing

moon, as well as put limits on the rings vertical thickness.

Cassini results indicate that the A rings surface density is

highest around the Encke Division, falling offboth inward

and outward. Density-wave measurements also limit the

vertical thickness of the Cassini Division to a few meters at

most, and the inner A ring must be less than 10 to 15 me-

ters (30 to 50 feet) thick.

Zebras in the A Ring

Individual disk particles are always trying to gather into

larger particles, while tidal forces try to pull those aggre-

gations apart. Saturns A ring is very close to the planets

Roche limit, the distance at which this eternal tug of war

is in balance. As particles constantly clump together and

shear apart, zebra-like patterns called self-gravity wakes

emerge in the A ring.

These structures werefirst inferred from Voyager stud-

ies of a curious A-ring property a viewer sees the ring

The small moons Pan and Daphnis orbit in the Encke Division and Keeler

Gap, respectively, and sweep away the material that would otherwise

fill these gaps. A: Pan is the small dot inside the Encke Division. The

Keeler Gap is at upper right. B: Pans gravity shapes this clumpy ringlet

in the Encke Division. C: Pans gravitational perturbations generate

scalloped features on the edge of the gap. D: Daphnis gravity sculpts a

sharp saw-tooth pattern in the gap edge, rather than graceful scallops.

A

B

C

D


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surface toward a light source. Cassini does

the same thing when its on the side of Saturn

opposite the Sun especially when it slips

into the planets shadow to highlight the

backlit dust.

The F rings constant activity as a result

of its two sheperd moons churns up huge

amounts of dust and makes it the brightest

component of the rings when backlit. Other

prominent features are the innermost D ring

and the outermost G and E rings, which are

nearly invisible from other viewing angles.

The D ring, seldom studied as it sits sand-

wiched between the main rings and Saturns

cloudtops, is unexpectedly dynamic. The

most prominent ringlet seen by Voyager

has moved inward by 200 km and dimmed

considerably, one of very few confirmed

changes in the 25 years between spacecraft

encounters. The outer D ring not only appears warped like

a corrugated tin roof, but the corrugation also continues

to wind itself tighter with time. By determining the wind-

ing rate and extrapolating backward, researchers haveconcluded that the corrugation began as a simple vertical

displacement around 1984, perhaps caused by an asteroid

or comet impact (January issue, page 18).

The E ring, flowing from and enveloping Enceladus, is

Saturns largest ring, though its also the faintest and hard-

est to see when not backlit. Cassini has confirmed that it is

composed entirely of very fine water-ice crystals, condensed

after having been spewed from geysers near the moons

south pole (S&T: March 2006, page 38).

Arguably Saturns most mysterious ring is the faint and

dusty G ring, which occupies the void between the main

rings and the E ring. Rings A through D form a unit, E

clearly originates with Enceladus, and F is confined by the

moons Prometheus and Pandora. By contrast, the G ring

lacks any known parent moon or reason for existing. But

Cassini has found a vital clue: an arc taking up some 10% of

the rings circumference. The arc is five times brighter than

the rest of the ring and turns out to be in a resonance with

the moon Mimas, which orbits Saturn six times while the

arc completes seven roundtrips. Only Neptunes rings have

been previously observed to have persistent arcs, and there

also a resonance is probably responsible for preventing

particles from spreading around the rings circumference.

Could the arc be hiding the G rings parent moon or per-

haps a belt of parent moonlets?

Kicking Up DustThe main rings also sport a few locations that are rich in

dust. These regions stand out in the largely dust-free ex-

panse, so they merit special attention.

In the Encke Division, the moon Pan shares its space

with several dusty ringlets. Although less chaotic than the

F ring, these contain a number of kinks and arcs that orbit

along with Pan. Yet they are not fully in lockstep with Pan;

Cassini has observed these ringlet clumps moving over a

time scale of months, some changing direction when they

encounter Pan and others apparently destroyed. Scientists

The B ring, like the A ring, is rifewith tightly wound radial structure.

But unlike the A rings spiral density

waves, most of the B rings features

do not correlate with known reso-

nance locations, so the origin of

the structure remains unexplained.

The difficulty of studying the B ring

has been compounded by its tight

packing of particles, with so little

space between them that Voyagers

stellar- and radio-occultation ex-

periments failed to penetrate it.

Cassinis high-gain radio antenna

overcame this obstacle in 2005,

sending its powerful signal directly

through the B ring to Earth. Data

analysis is ongoing, but preliminary

results indicate fine structure even

in the densest regions of the B ring,

which block as much as 99% of light

passing through them.The B ring is also famous f

spokes, ghostly radial markin

seen by ground-based obser

and later confirmed by Voya

1 and 2. The spokes appear s

denly and remain intact for m

longer than Keplerian orbita

should allow, since interior s

particles should orbit faster t

terior ones. Cassini failed to o

a single spoke during its first

at Saturn, a disappointment

up to seasonal effects. Spoke

to appear predominantly du

Saturnian spring and autum

sunlight strikes the ring at a

glancing angle.

Spokes finally made their

reappearance in September

and theyve been seen more

whenever Cassinis viewing g

etry is favorable. Still, the spo

observed so far have been w

few in number compared to

imaged by the Voyager spac

198081; scientists anticipat

gaining strength with the aping 2009 equinox.

The spokes formation pro

remains unclear, though Sat

powerful magnetic field is al

certainly involved in maintai

their radial aspect. The imag

team hopes that high-speed

will eventually catch spokes

act of forming, leading to a r

tion of the mystery.

Left: Voyagers 1 and 2

spokes galore in Satu

ring, probably becaus

a favorable Sun angle

cording to the laws of

ity, these mysterious

striations should imm

ately break apart bec

inner spoke particles

Saturn faster than ou

ones. Below: Cassini i

these spokes, which e

pronounced shearing

September 28, 2006.

number and intensity

spokes should increas

the next few years as

viewing geometry im

The B Ring: Land of Spokes

Sky & T

NASA/

JPL


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Just beyond the main rings

is a narrow, tortured ribbon

known as the F ring. This

structure is hemmed in on

either side by the shepherd

moons Prometheus and Pan-

dora, which average 102 and

84 kilometers (63 and 52

miles) across, respectively.

Each of the two moons ap-

pear to push ring material

away from it by scattering

nearby particles. As a result,

this ring maintains an aver-

age width of only 1,500 km.

But neither the ring nor

the shepherd moons main-

tain circular orbits. From

the F rings perspective Pro-

metheus and Pandora movein and out and up and down

in a constant dance. These

motions should hamper the

moons ability to shepherd

the ring between them, yet

it remains confined. As a

result, the rings central core

contains countless knots an

kinks, as it is worked and

reworked by the moons. The

inner moon, Prometheus, oc

casionally dips into the ring

outskirts, each time carving

a narrow channel that shear

away downstream. Pro-

metheus will penetrate the

rings core in 2009, an event

that promises even more

spectacular fireworks.

Cassini images have fur-

ther revealed that parallel

strands in the F ring con-

nect with one another into

a tightly wound one-armspiral, possibly trailing away

from a collision between a

small moonlet and the ring

core.

A: The narrow F ring lies just outside the main rings

and is confined by the small shepherd moons Pro-

metheus (shown here) and Pandora. B: The inces-

sant in-and-out, up-and-down dance of Prometheus

and Pandora (shown here) relative to the ring grav-

itationally sculpts clumps and kinks in the rings

core. C: Narrow channels carved by Prometheuss

in-and-out motion shear away downstream.

D: A spiral arm winds all the way around Saturn.

F is for Freewheeling

9 Sky & Telescope

A

B

C

D

CoreSpiral Arm

pringworld revelations


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SOLID GROUN

Huygens landed

plain, but the sm

rounded rocks an

soil suggest th

was recently wet

knowing what aw

Huygens, ESA pl

equipped the pro

land safely in liqu

solid ground. Th

ground rocks are

of water ice and

about the size of

We suspect that methane rainfall drives the chan-

nel flows. But how does this rainfall vary by season and

latitude? And does it simply rain on Titan, or do its clouds

unleash torrential downpours? Although we havent seen

any lightning yet, Titans storms evolve much as thunder-

storms do on Earth. Low cumulus clouds rapidly grow invertical extent until they reach the tropopause and then

shrink presumably as they dump a methane-ethane

rain onto Titans surface.

As on Earth, thunderstorms are localized in a few

preferred spots. One hovers over Titans south pole, which

is now experiencing late-summers constant sunlight.

Another is a thin band centered at south, where solar

heating pushes air upward in much the same way that the

Intertropical Convergence Zone drives water to Earths

tropical rain forests.

But why was the ground beneath Huygens moist,

when Cassini doesnt see clouds anywhere near the equa-

tor? Perhaps we dont yet know how to predict where rainwill fall. Huygens found lots of turbulence to miles

up, conditions that could produce precipitation despite

the absence of clouds. Confirmation of this ghost rain

came from Hawaiis Keck Observatory, which found

liquid methane to be more concentrated in this same

atmospheric layer. Still, its not yet clear what drives this

drizzle, or whether it occurs all over Titan or just at equa-

torial latitudes.

RIVERBEDS Left:Cassinis radar instrument has

found dry riverbeds al l over Titan. The channels

come in all sizes and in both smooth and rough

textures. They were presumably carved by liquid

hydrocarbons running downhill. Right: As Huygens

parachuted to Titans surface, its descent cameraimaged dark channels flowing into what appears to

be a dry lakebed.The channels are currently dry, bu

they indicate recent fluvial activity fed by rainfall.

NASA / JPL / ESA

UNIVERSITY OF ARIZON

NASA / JPL

NASA/JPL/ESA/UNIVERSITYOFARIZONA


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30 December 2008

Splish, SplashHuygens didnt see any surface puddles because, we

realize, it landed in Titans equivalent of a vast deser

There are big pools of liquid on the surface but th

in the polar regions. Cassini first spotted clusters of

polar patches in , and theyve tantalized our sci

team ever since.

Initially the evidence for true hydrocarbon lakes

circumstantial. They appear really dark in both rada

scans and infrared images. The radar result is consi

with nearly mirror-smooth surfaces that reflect Cass

radar emissions away from the spacecraft and out in

space. The infrared darkness implies that clear liqui

extends so far down that photons of light are absorb

before they can scatter offsuspended particulates.

The lake hypothesis reached its splash point last

December, when Cassinis Visual and Infrared Map

Spectrometer (VIMS) got a good look at a conspicuo

dark region near the south pole known as Ontario LVIMS analyzed the features reflectivity between a

microns, infrared wavelengths at which the atmosph

transparent. A handful of absorption lines match th

expected for liquid ethane finally, we had our lon

sought smoking gun for fluid-filled reservoirs (No

ber issue, page ).

Close-ups of Ontario Lacus from that flyby also re

what may be mudflats and a surrounding bathtub ri

LAND OF LAKES Left and above:Cassinis radar has rev

numerous flat, smooth features, mainly at high northern la

tudes, which scientists have interpreted as lakes. This view

been confirmed by recent spectral analysis. Titan and Eart

the only bodies in the solar system to have liquid bodies o

surface. The colors in the left image represent radar reflect

not what youd see.Above lef t:Cassini imaged Ontario La

in near-infrared light. This feature is similar in size and sha

Lake Ontario, and is located near Titans south pole. Recen

tral observations have confirmed the presence of liquid eth

Titan

NASA

/JPL/S

PACESCIENCEINSTITUTE

NASA/JPL


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17

Titan

spectra suggest some kind of organic composition. Imag-ine mountains of coffee grounds hundreds of feet high!

The global maps built from Cassinis dozens of close

Titan flybys reveal that these sand seas straddle the

equator and cover about % of Titans surface. By com-

parison, dunes cover only % of the desert world Mars

and only % of Earths dry land.

Much Ado About MountainsIf you had taken a poll of planetary scientists before Cas-

sini reached Saturn, few would have bet that the orbiter

would discover mountains on Titan. But the orbiter has

found not just isolated peaks, but entire ranges. Most are

between ,and ,feet

high, similar to the elevations

of the Appalachians in the

eastern U.S.

Given that the mountains

consist of ice, these some-

what modest peaks are rather

substantial, and their heights

help us to constrain the nature

of Titans crust. Some ranges

might be large blocks ejected

by large impacts. Others form

true ranges, tens of miles wideand hundreds long. Mountain

chains are ubiquitous on Earth,

thanks to plate tectonism, but theyre rare elsewhere in the

solar system. Some kind of crustal upheaval within Titan

must have created these mountain chains and may still

be building them though the cause remains unknown.

The set of mountains at south, near a dark region

called Senkyo, may be large enough to influence Titans

weather. As winds blow over these peaks, the risingair cools and generates clouds, and ground-based o

ers have noticed that thunderstorms preferentially g

at this latitude. Nearer the equator, winds deviate ar

mountains and fashion dune crests that resemble s

lines around a raindrop.

With all of this churning, blowing, and precipita

geologists wouldnt expect to see many impact crate

Titan and, in fact, theyre surprisingly rare. The

that we can identify are being eroded away, covered

or both. In reality, Titan has probably experienced ju

as many large impacts as have its sister moons, such

heavily cratered Rhea. But geologic activity has eras

all the old scars over the intervening eons and has n

removed more recent ones.

Despite these leaps in knowledge, our exploratio

Titan remains in its infancy, roughly matching wha

knew about Mars following Mariner s pioneering

more than years ago. Cassini remains in excellen

health, and were hoping the mission continues to fi

gaps in our understanding until it runs out of prop

or funding, whichever comes first.

After that, the future exploration of Titan is an o

book. Scientists are looking to bundle an orbiter, lan

and hot-air balloon on a single mission that could b

mounted a couple of decades from now. But given Tmany similarities to Earth, its not hard to imagine

astronauts ice picks in hand chipping away at

icy outcrop and scooping up samples of Titanian co

grounds for analysis.

Now an assistant professor of physics at the University

Idaho,Jason Barneshas worked with Cassinis VIMS for three years.

MOUNTAIN RANGESAbove:Cassinis radar has found numer-

ous mountain ranges that are similar in scale to the Appalachians

in eastern North America. Right:This composite VIMS image

from Cassini reveals a -mile-long range just south of the equa-

tor. The origin of these structures remains unknown.

AN ABODE FOR LIFE?

Titans surface abounds with organicmolecules and water ice, but its frigidtemperatures offer bleak prospectsfor life. At Titans F (C)surface temperatures, chemicalreactions slow to a crawl, limitingthe ability of complex molecules toform. But Titans interior is warmenough to sustain liquid water. Giventhe plethora of lifes building blockson Titan, scientists cannot rule outthe possibility that the moon harborsbiological activity deep underground.

NASA / JPL


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Saturn Moons

18

PlanetIce Worldsof theRinged


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Cassini-Huygenswas billed as amission to Saturn and Titan. But

it has also thrilled us with unex-pected discoveries and amazing imagesfrom Saturns mid-size moons: Mimas, En

adus, Tethys, Dione, Rhea, Hyperion, Iapetand Phoebe. Though far smaller than Titan

iceball is a unique world with interesting geOne of the greatest surprises has been

Enceladus. Only Saturns sixth-largest moo

has proven to be astonishingly active. Wategeysers rocket hundreds of kilometers into

Its potential for liquid water and perhaps evlife provokes discussion of a dedicated mis

ranking tiny Enceladus among such tantalworlds as Titan and Europa. Meanwhile, Cahas resolved the nearly -year-old myster

why Iapetus, Saturns outermost mid-size mis black as coal on one face and icy on the o

Touring Saturn

Cassini is bound by gravity to travel in elliparound Saturn. The moons orbit the giant p

on near-circular paths with speeds, sizes, ainclinations often quite different from CassMission navigators shift the spacecrafts or

tour the Saturnian system using frequent flof Titan, the only moon massive enough to

vide a gravity assist.To study the other moons, Cassini must

precious fuel to position itself in the right pat the right time to meet one in its orbit. Thtargeted flybys usually involve an approach

to within ,kilometers (,miles), an

sometimes as low as km. Other non-targopportunities present themselves through orbital juxtapositions, but generally Cassinapproach nearer than ,km to condu

ful science.Cassinis moon observations thus come

rare, concentrated bursts of data. Before Caarrived, Enceladus had already been marke

SkyandTelescope.co

NASAs Cassini mission has solved l

standing mysteries about Saturns ic

but raised new ones in their place.

EMILY LAKDAWALLA

SATURNS SWARM OF BEESThe Ringed Planet has

known moons, most

of which are only a few kilometers across. In these images from NASAs Cassini

orbiter, we see several of the planets mid-sized satellites, objects a few hundred

to ,km across. The icy moon Enceladus, a prime target for future exploration

because of its potential habitability for life, appears in all three of these images.

In the far left image, it appears to the right of center just above the ring plane. In

the image above, the moon appears at the far right, in the ring plane. And in the

image below, Enceladus appears just above and to the left of the rings.

UNLESS OTHERWISE INDIC ATED, ALL IMAGES: NASA / JPL / SPACE SCIENCE INSTITUTE; AUTHOR PHOTO: ERIK NELDER


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ticles were considered too small to threaten the spacecraftdespite the inevitability of high-speed collisions.

The fly-throughs have firmly established the connec-tion between the Enceladus plumes and the E ring. The

plumes are made of two components: gas and particles.The INMS found the gas to be primarily water vapor(%), mixed with smaller amounts of carbon dioxide,

nitrogen, methane, ammonia, hydrocarbons includingacetylene and propane, and possibly even argon. The gas

shoots out at a speed of many hundreds of meters per sec-ond. Since that is much higher than Enceladuss escape

velocity, it goes into Saturn orbit, populating the E ring.A large fraction of the particles, primarily ice, come out atmuch lower speed, so most re-impact Enceladus, giving it

its fresh, snowy countenance.If Enceladus has been venting at its current rate over

the age of the solar system, it would have lost % of itsmass. Enceladus is much denser than either of its neigh-bors, Mimas or Tethys, another possible indication that it

has lost a substantial quantity of lower-density water ice tothe E ring.

Using a novel technique to compensate for thespacecrafts high relative velocity (.km per second),

the imaging team was able to take sharp pictures of thesouth-polar terrain during the August and October ,

encounters. The resulting panoramic views revealed

a rumpled and fractured terrain, seamed with deep fis-sures, and covered everywhere with house-sized boulders

of ice. Many of the images were targeted to cover areaswhere CIRS had found the hottest temperatures, and

where triangulation had identified plume sources.Although the sulci are fresher than the surrounding

terrain, they were not obviously diff

erent near the mapped

plumes; the sulci appear uniform along their lengthimaging team has concluded that there is nothing s

about the location of a given plume source. Plumes emanate from any point along a sulcus, and they mmove with time. The sulci could be sites of crustal s

ing, like Earths mid-Atlantic ridge. Such tectonic awould explain the apparent youth of Enceladuss so

polar terrain.

THE PLUMES OF ENCELADUS Cassini scientists use f

color images such as this to identify individual plumes wit

source regions on the tiger stripes (sulci). The plumes sho

hundreds of kilometers into space and include water-ice pa

water vapor, carbon dioxide, nitrogen, methane, ammonia

other gases. In other words, they contain the ingredients fo

Saturn Moons

ENCELADUS IN SILHOUETTE Cassini captured this dramatic view of Enceladus against Saturns night side on May , .

moons geysers are clearly visible against Saturns southern hemisphere, which is illuminated by sunlight reflecting offthe rings

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Is There Liquid Water?CIRS observations show that the vents are relatively hot,

at least kelvins (F). This is intriguingly close tothe lowest temperature at which water could melt in the

Saturn system kelvins but only if there is a lotof ammonia present, which would depress the watersmelting point. Yet INMS observed ammonia only during

the very closest of the flybys, and not in great abun-dance. And the Visual and Infrared Mapping Spectrom-

eter (VIMS) has found none on the surface.Since temperatures tend to increase with depth on any

body, there could be liquid water very close to Enceladusssurface, perhaps even just meters (feet) down. Itsmechanically possible for the boiling of that water to drive

the high-speed plumes.But theres another possibility that would leave Encel-

adus without near-surface liquid water. The plumes couldbe driven by the explosive decomposition of a form of ice

called clathrate, which has many gaps in its crystal latticethat could allow it to hold up to %, by weight, of othergases. This is the same percentage that was observed in

the gas component of the plumes.If tidal flexure causes Enceladuss sulci to open and

close over the course of an orbit, new cracks could exposeclathrates to the vacuum of space, whereupon they wouldexplode, driving the gas and entrained particles outward

at very high speed.The tidal forces that squeeze Enceladus could gener-

ate heat just by rubbing the two sides of the sulci againsteach other. Again, this would be a dry process, not requir-

ing near-surface liquid water; it could help to expose andheat clathrates, or could operate without any clathrates

being present. Cassinis future Enceladus flybys will be

TIGER STRIPES UP CLOSE In August (left) and October (right), Cassini swooped under Enceladuss south pole at close range,

giving scientists high-resolution views of the tiger stripes, which might behave much like spreading ridges on Earths ocean floors.

SkyandTelescope.co

directed in part at trying to distinguish among thesecompeting ideas.

Whether or not pockets of liquid water are driving thegeysers, there could still be liquid water much deeper

down, well below the source of the plume activity. Manyof the solar systems round icymoons (and also the dwarf

planets of the Kuiper Belt, andthe main-belt asteroid Ceres) are

theorized to have layers of liquidwater at some depth below their

frozen surfaces. Enceladus is small, but it has a lot ofheat. So its very likely that Enceladus has either a globalocean or a localized south polar layer between its icy crust

and its rocky core.The composition of Enceladuss plumes particularly

the presence of molecular nitrogen and hydrocarbonssuch as methane, acetylene, and propane hint thatcomplex catalytic chemistry may have taken place inside

the moon. Mineral-rich liquid water may have circulatedamong warm rocks below an insulating cap of ice, pro-

ducing nitrogen from ammonia, and hydrocarbons fromcarbon dioxide.

This chemistry could be taking place now, or couldhave happened long ago. Either way, there was probablyliquid water, heat, organic chemicals, and active chemis-

try the stuffof life. As a possible abode for past or pres-ent life, Enceladus has catapulted from being moderately

interesting to brief consideration as the prime target ofNASAs next flagship mission to the outer solar system.

NASA and ESA recently decided on a mission to visitJupiters moons, but the fact that diminutive Enceladus

was included among such exalted company is amazing.

To learn more about the Cassinand Saturns icy moons, visit ht

saturn.jpl.nasa.gov.


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