Icy Moons of Jupiter, Saturn, and

Beyond

Rosaly Lopes, JPL

Galilean satellites: Io, Europa, Ganymede, and Callisto

Io: hot volcanoes, colorful surface, lots of sulfur, no impact

craters

Europa: icy crust, possible ocean underneath, few craters

Ganymede: largest of all, surface is bright (icy) and dark (rocky)

Callisto: many craters, impact basin 4,000 km diameter

(Valhalla)

Europa: cracks on an icy crust

Young, thin, cracked and ruptured ice shell probably moving slowly over a liquid

ocean. Cracks and ridges (linear brown features) can be 1000s of km long.

Chaotic terrain (mottled brown) exists where surface was disrupted and ice

blocks moved.

Internal structure of Europa

Artist’s view of Europa’s interior: thin or thick crust (M. Carroll)

Chaos regions may have formed when liquid water melted its way

through a thin (< 6 km) ice shell or formed through the interaction

of rising ice diapirs with the relatively thick (15-30 km) ice shell

Europa: interplay of surface colors and ice structures

Small region (Conamara Chaos, 70 km across) shows fine ice particles

(white/blue) from an impact cover some of the surface. Reddish-brown surface

has been altered by minerals carried by water vapor released when the crust

was broken.

Europa: Chaos

regions may

have been

formed by the

movement of

trapped pools

of water in the

ice (Schmidt

et al., 2011)

Ganymede in enhanced color

Bright, grooved terrain and dark, furrowed older terrain

Galileo instruments detected atmosphere and magnetic field

Callisto: a cratered world

Small bright spots in global view are large craters in medium resolution view.

Local view shows smooth dark materials that cover much of the surface.

Close up views show evidence of erosion, probably due to sublimation of

volatiles from the surface ice.

Saturn on July 2008

A

B

C

C D F

Rings in detail never before seen

Rings: small

particles (microns

to meters) form

clumps

Rings in forward scattering

we are here

The E-ring is created by Enceladus

Icy Moons

• The Saturn system has 62 confirmed satellites, most are

small and far away from the planet.

• Titan is by far the largest

• There are 8 major other satellites: Mimas, Enceladus,

Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe

Rhea diameter 1528 km (955 mil)

Saturn’s second largest moon

Image from August 2007

Last Rhea Flyby

9 March 2013

North polar regions

Graben

(extensional faulting)

indicates ice crust

pulled apart

Surface Temperature

-210 C (-281 F)

Density 0.93 g cm-3

Michael Carroll’s Enceladus-Earth size comparison

Saturn’s Moon Enceladus: small, but active

Cassini Enceladus fly-by:

July 2005

Showed south polar “tiger

stripes”

Flyby at 168 km (105 mi)

Enceladus diameter 504

km (315 mi)

Thermal data (CIRS)

Avg Earth

87 mW/m2

Enceladus

South Polar Terrain

250 mW/m2

Tiger Stripes

13,000 mW/m2

Planetary Heat Flow

Yellowstone

2500 mW/m2

Enceladus Jets and Plume

UVIS detected 4 high density water vapor jets.

Jet sources < 300 x 300 m, water molecules, safe for Cassini close flybys of

plumes (August 2008 flyby c/a 50 km!)

UVIS stellar occultation (Zeta

Orionis), October 2007

GEYSER COMPOSITION (Waite et al. 2006; Hansen et al., 2006)

H2O 91 ± 3 % wt.

CO2 3.2 ± 0.6 % wt.

N2 4 ± 1 % wt.*

CH4 1.6 ± 0.4 % wt.

CO < 0.9 % wt

NH3, HCN, C2H2, C3H8 < 0.5 % wt. (i.e., detected)

*Inferred from a combination of INMS and UVIS data

• Gas: mostly water vapor, with other gaseous molecules mixed in (e.g. CO2, N2, CH4,

CO, C2H6, NH3)

• Many ice particles (particularly close to Enceladus) contain sodium, potassium, and

carbonates, which may indicate they originate in an ocean deep below the icy crust

• Plumes may be material escaping through surface cracks from an internal salty ocean

or lake

• Cosmic Dust Analyzer on Cassini showed sodium within ice grains in the E ring and

plumes, consistent with ocean hypothesis (Postberg et al. 2009)

• Ground based observations showed no sodium gas in vapor cloud (Schneider et al.,

2009) – not consistent with rapid boiling near surface

• Alternatively, ice along cracks may sublime or melt, followed by escape of water vapor

and icy particles

• Plumbing still now known: near surface reservoirs or is material boiling more slowly

over larger areas?

Enceladus Plumes

A: Salty water boils explosiverly. Unlikely because sodium gas would have been detected

B: Slow evaporation. Unlikely as conduit would become clogged by sodium left behind

C: Salty particles stored near surface, incorporated in plume by gases. Hard to dislodge old ice

grains from walls.

D: liquid water stored near surface, salinity increases as water evaporates. Plausible.

E: liquid water salty, comes from ocean in contact with rocky core. Water evaporates slowly into a

pressurized chamber, from which water and ice particles (inc. sodium) escape along narrow

fissures. Plausible.

Enceladus: summary

• NASA’s Cassini spacecraft has

observed plumes of material

escaping from Saturn’s small icy

moon, Enceladus

• The plume is mostly water vapor,

with tiny ice particles and other

gaseous molecules mixed in (e.g.

CO2, CH4, C2H6)

• The plume supplies ice particles to

one of Saturn’s rings

• Some ice particles contain salt,

which may indicate they originate in

an ocean deep below the icy crust

Image mosaic of Enceladus taken by Cassini,

showing individual plumes of gas and ice

escaping from the surface. The plumes

extend 100’s of km into space from the ~500

km diameter moon.

Michael

Carroll’s

view of

plumes on

Enceladus

Earth: Submarine volcanism

Most of the Earth’s

volcanoes are under

the ocean, such as in

the East Pacific Rise

Iapetus: A

Strange Icy

World

If this “Belly Band” were on Earth

to scale it would be:

• 20 time higher than Mt. Everest

• 2.3 times longer than the Andes

Iapetus' Albedo Contrast

ISS, 10 Sept. 2007

Darker areas are warm enough that ice sublimes and then is re-deposited

in the bright regions

Dark areas have different composition (more CO2)

One Moon Coats its Neighbor in Dust

• The trailing face of Saturn’s moon

Iapetus is ~10 times brighter than its

leading face

• For 300 years, astronomers debated

whether the cause was internal (e.g.

eruption of dark material on one face)

or external (e.g. debris from a nearby

impact)

• The discovery of a giant ring around

Saturn and close-up Cassini images

confirm an external cause: dust

particles coat one side and drive ice to

the other by sublimation Saturn’s moon Iapetus has a dark leading side,

while its polar regions and trailing side are

bright. The dividing line follows a pattern like the

stitching on a baseball.

• Impactors strike one of several distant

dark Saturn moons (such as Phoebe),

supplying a ring of dark particles that

orbit Saturn ‘backwards’, like Phoebe

• Sunlight pushes the ~10 micron

particles inward over thousands of

years

• Particles collide with Iapetus and

other inner moons, making their

leading face slightly darker

• The darkened ice absorbs more

sunlight, warms up, and sublimes,

recondensing as bright frost on the

trailing side and poles

A Ring Creates Iapetus contrast

Dust from backwards-orbiting (captured)

Phoebe coats the leading side of Iapetus

Sun-warmed dust causes sublimation,

driving ice to the poles and trailing side

Phoebe

Trailing

Leading

Dust Iapetus

The Big Picture: the Phoebe ring

• Planetary moons can be “painted”

at a global level by external causes

such as dust and even charged

particles

• Dark dust and bright ice can

segregate on a moon’s surface, as

sun-warmed dust drives ice to

brighter, icier regions

• New telescopes and instruments

keep discovering new phenomena:

The “Phoebe ring” is the largest

and most distant from its parent

body

Artist’s conception of huge ring around Saturn,

discovered at IR wavelengths by the Spitzer Space

Telescope. The inset indicates scale by showing

an enlarged ground-based IR image of Saturn.

Hyperion

26 Sept. 2005

514 km flyby

Dark Material in crater floors

Image of the Year (2005): color view of

Dione from October 11, 2005, flyby

• TIME magazine’s 2005 best “Editor’s Choice Picture of the Year”

• MSNBC’s best “Space Photo of the Year”

• Aviation Week and Space Technology’s “Best of the Rest” contest

Dione’s Bright “Wisps”

revealed to

be fractures

Methone at 4500 km on May 20, 2012

Janus at 28,000 km on March 27, 2012

Saturn’s Bizarre Small Moons

And further out…

Neptune’s Triton: cryovolcanism

Michael Carroll’s view of geysers on Triton

Pink nitrogen ice melts directly into gas leaving strange shapes

Where next? New Horizons will

arrive at Pluto on 14 July 2015…

This is the most detailed view to date of the entire surface

of the dwarf planet Pluto, as constructed from multiple

NASA Hubble Space Telescope photographs taken from

2002 to 2003.