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
Rings in detail never before seen C B C D A F
Rings: small particles (microns to meters) form clumps
Rings in forward scattering The E-ring is created by Enceladus we are here
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
Saturn s Moon Enceladus: small, but active Michael Carroll s Enceladus-Earth size comparison
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)
Planetary Heat Flow Avg Earth 87 mw/m 2 Enceladus South Polar Terrain 250 mw/m 2 Yellowstone 2500 mw/m 2 Tiger Stripes 13,000 mw/m 2
Enceladus Jets and Plume
UVIS stellar occultation (Zeta Orionis), October 2007 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!)
GEYSER COMPOSITION (Waite et al. 2006; Hansen et al., 2006) H 2 O 91 ± 3 % wt. CO 2 3.2 ± 0.6 % wt. 4 ± 1 % wt.* N 2 CH 4 1.6 ± 0.4 % wt. CO < 0.9 % wt NH 3, HCN, C 2 H 2, C 3 H 8 < 0.5 % wt. (i.e., detected) *Inferred from a combination of INMS and UVIS data
Enceladus Plumes Gas: mostly water vapor, with other gaseous molecules mixed in (e.g. CO 2, N 2, CH 4, CO, C 2 H 6, NH 3 ) 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?
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. CO 2, CH 4, C 2 H 6 ) 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 CO 2 )
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.
A Ring Creates Iapetus contrast Phoebe 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 Dust from backwards-orbiting (captured) Phoebe coats the leading side of Iapetus Dust Leading Iapetus Trailing Sun-warmed dust causes sublimation, driving ice to the poles and trailing side
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
Saturn s Bizarre Small Moons Janus at 28,000 km on March 27, 2012 Methone at 4500 km on May 20, 2012
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.