Outer Galilean Moons
Callisto: • Moderately dark
surface• Heavily cratered• No magnetic field• ~1/4 of interior is
ice (by mass)
Ganymede:• Moderately bright
surface• Weak magnetic
field and evidence for a denser core
• Perhaps melted in past to allow for differentiation
Inner Galilean Moons
Europa:• Bright surface• Absence of craters –
young surface• Possible mag. Field
and denser core• Surface ice is a thin
layer (100’s of meters) floating on H2O ocean (!) – requires a source of heating
Io:• Rocky interior• Orbits inside Jupiter’s
magnetosphere• Io is tidally heated and
volcanically active (predicted in 1979 and discovered by Voyager 1 in same year)
• Surrounded by a yellow Na-cloud
Titan
• 1944 – Kuiper discovers a thick atmosphere• Atmosphere is a reddish, featureless haze; mainly
N2 with 10% CH4 and smog:
• Psurf=1.6PE and Tair=93 K (-292oF)
• Only satellite with a thick atm.:
– CH4 rain, snow, and ice?
– Pools of liquid N2 and CH4
• 2004 – Cassini probe called Huygens parachuted into Titan’s atmosphere
An Ocean Below Enceladus’ Icy Crust?• 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.
• Plumes may be material escaping through surface cracks from an internal salty ocean or lake
• Alternatively, ice along cracks may sublime or melt, followed by escape of water vapor and icy particles
• Many scientists find the salty ocean model most convincing, but others favor combinations of alternative explanations
What Process Creates the Plume?
Left: Enceladus may have a salty subsurface ocean that releases material to space through cracks in the moon’s icy shell. Right: The walls of icy cracks in the surface may melt or sublime, venting gas and icy particles to space.
The Big Picture
• Enceladus is surprisingly active for such a small body - likely a consequence of tidal heating
• Future flybys of Enceladus by Cassini may help to resolve whether Enceladus joins the growing “club” of solar system bodies believed to have oceans
• If Enceladus has an ocean, then it contains all of the ‘‘ingredients’’ known to be important for life: liquid water, molecular building blocks, and energy
Image of Enceladus showing the ‘tiger stripes’ region in the southern hemisphere, where the plumes originate
Tiger stripes
The Impact at MimasDiameter ~ 390 kmHuge crater from impact, almost big enough to shatter this
moon
Estimate:
Binding Energy of Mimas
Kinetic Energy of Collider
Mass and Size of Collider
m v M
D
H2O
Moons of UranusUmbriel and Oberon• Heavily cratered• Large cracks
Ariel and Titania• Lighter cratering• Resurfacing by flows of volcanic water (!)• Some regions of Ariel are featureless - very recent!• System of deep cracks (tidal heating effects?)
Miranda• Rolling and cratered terrain• Some fractures• 20 km cliff - takes 10 min to fall (in contrast to 1 min as at Earth)
Triton• Triton is the only large moon with a retrograde
orbit (possibly a captured body?)• Tidal bulges on Neptune lag instead of lead Triton
Orbit is degradingWill eventually shatter as nears Roche Limit (in
~109 years to make a ring to rival Saturn’s)• Composition: 75% rock, 25% ice• Atm.: thin N2
• Polar Capts: N and CH4 ices• Evidence for resurfacing and internal activity,
even geyser eruptions of N gas, leading to sooty plumes