I. Satellite formation and their interiors
A. Most major satellites appear to have formed in situ about their respective planets:
1. Primary evidence for this is that all the major satellites except Trition orbit in the same direction as the planet and in the planet's equatorial plane.
2. Consequently, like the solar system we generally expect the satellites to decrease in density outward from the planet they orbit. However, this really only seems to hold for the Gallilean satellites.
B. Potential sources of heating in icy satellites.
1. Accretional heating.
10 - 50% of the kinetic energy of accreting bodies is retained by the satellite.
2. Radiogenic heating:
a. Possibly 26Al, but it is likely that the icy satellites accreted too late to be affected by this element's decay.
b. Normal radioactive elements (235U, 238U, 232Th, and 40K):
i. Probably could not produce enough heat in bodies with <600 km radius to produce internal melting.
ii. For larger moons, it is conceivable that the interior could heat up enough to differentiate while the surface remained unaffected.
iii. The largest moons may have completely differentiated.
3. Tidal heating
a. Until a moon's rotation becomes synchronous with its orbital period tidal forces continually flex it , generating heat.
b. Some satellites (especially Io and Europa) are in noncircular orbits permanently or for brief times due to interactions between satellites. Elliptical orbits cause changes in the tidal forces that flex the planet and produce heat.
C. A sampling of potential density structures:
1. Completely undifferentiated ice+rock mixture.
2. Complete differentiation to a rocky core covered by ice.
3. The interior differentiates but the outer layer remains an ice+rock mixture. Thus, outward from the center the layers are: rock, ice, ice+rock mixture.
4. Same as 3, except the ice+rock mixture is gravitationally unstable and overturn occurs, leaving ice as the external layer. Thus, outward from the center the progression is rock, ice+rock, ice.
D. Impact cratering in the outer solar system.
1. There are no observational constraints on impactor flux in the outer solar system. Thus, absolute age-dating of a surface is quite unreliable.
2. Some of the reasons for different crater production rates on the icy satellites versus the terrestrial planets:
a. The population of impactors is different:
i. In the outer solar system there are probably much fewer (if any) rocky impactors.
ii. Cometary impactors from the Oört Cloud strike at much lower velocities.
iii. The flux rate of comets is much different.
iv. There is a population of impactors from within the planetary system.
b. The behavior of ice (as a target material) is different from rock in the cratering process; thus the same size craters on the icy satellites and the terrestrial planets could be produced by impactors of substantially different energies and masses.
3. The mechanisms for erasing (resurfacing) craters are much different in the outer solar systerm. In particular, ice flows much more easily than rock, and it is possible that some craters have simply viscously relaxed (flowed away) with time.
E. Additional general notes:
1. When water freezes it expands; however, under different pressure-temperature conditions ice occupies several different crystal phases, some very dense. Thus, planetary cooling can, over time, produce tectonic features and ice volcanism associated with planetary contraction and/or expansion.
2. At the low temperatures and surface gravities in most of the outer solar system, ice and ice+rock mixtures can behave in similar fashion to volcanic materials.
A survey of the outer planet satellites:
The Galilean Satellites are the four largest moons of Jupiter, and are so named because Galileo discovered them with an early telescope. Their discovery and the evidence that they orbited Jupiter (and not the Earth) helped to prove that the Earth was not at the center of the universe. In order outward from Jupiter, they are
I. Io
1. Nearest Galillean satellite to Jupiter.
2. Similar in size and mass to Earth's moon.
3. No ice, and not a single impact crater on the surface.
4. Surface is covered by volcanoes, and several erupting plumes were spotted when Voyager and Galileo spacecraft went by. In fact, noticeable changes had occurred in the 15-year interval between the two spacecraft visiting the planet.
5. Tidal interaction with Europa (the next moon out) keeps the orbit elliptical; consequently, the moon is continually being flexed and heated.
Europa
Next moon out from Io, a little smaller than Io
Looks like a white ball with a lot of linear features on it.
Has only a handful of craters on it; surface is young.
The mean density indicates most of Europa is rock, it's probably fully differentiated.
The upper layers of the planet may be ice over a layer of fluid water.
The linear features are often dark, suggesting extrusion of subsurface material contaminated with rock.
There appear to be some tectonic features reminiscent of Earth's plate tectonics.
Some unusual areas, called "chaos terrain" appear to show big chunks of the surface ice layer that were disrupted, floated around like icebergs, and then froze back into place. At least in these locations, the chaos terrain seems to give pretty clear evidence that liquid water was at or very near the surface at some point in time.
There is currently debate about whether Europa's surface ice layer is thin and underlain by an ocean, or perhaps just underlain by a convecting ice layer (ice flowing on long time scales, like Earth's mantle rocks). Some features, like certain impact craters, seem to only be supportable by a thick layer of ice. Other features, like the chaos terrain, seem to suggest liquid water is pretty near the surface, so there is a global subsurface ocean.
III. Ganymede
1. Third Galillean satellite out from Jupiter.
2. Largest moon in the Solar System, a little larger in diameter thatn Mercury. Surface gravity (143 cm s-2).
3. Half of the planet, known as the "dark terrain", is similar in nature to Callisto but with a factor of 3 less craters.
4. Other half, known as the "bright terrain", is noticeably younger than the dark terrain as determined from crater counts and crosscutting relationships.
a. Bright terrain has subparallel groove, usually parallel to the terrain boundary.
b. The boundary of the bright terrain is usually quite sharp, suggesting the boundary extends directly down for a few kilometers.
c. It does not appear that the remaining dark terrain was deformed during emplacement of the bright terrain.
d. Bright terrain displays a range of ages, but even the youngest is probably still ~3 billion years old.
e. Craters in each terrain type do not generally excavate material of the other terrain type, suggesting that one terrain type does not shallowly underlie another
5. Possible explanations for bright terrain:
a. Planetary expansion cause large graben that are filled in and perhaps overflowed by bright material.
b. Ganymede experienced some sort of tidal interaction way in its past similar in nature to Europa's.
IV. Callisto
1. Galillean satellite farthest from Jupiter.
2. Third largest moon in the Solar System: Mercury's size with 1/3
surface gravity
(125 cm s-2).
3. Highly cratered, apparently not much geologic activity since formation.
4. Very low surface albedo from buildup of rock dust from impactors.
5. Upper layer appears to be ice+rock mixture; planet's moment of inertia indicates it is not differentiated.
6. Fresh craters form higher albedo pockmarks on surface by exposing ice-rich material below rock-dust thin surface layer.
7. At the smallest scale of resolution, Callisto seems smoother on its surface than its brother Ganymede, perhaps because of some poorly understood sublimation process.
8. Large multi-ring craters have unique appearance. The largest of these, Valhalla, has the largest diameter of any crater in the solar system (4000 km) and has dozens of concentric rings. The nested appearance of the rings suggests that they formed by the impactor shattering a relatively thin rigid layer underlain by a more fluid layer.
Some other moons of the outer solar system, excluding Saturn's moons (covered by Katie and Abby)
A 1200 km diameter moon of Uranus that, like many icy satellites, has an extensive surface pattern of graben. Although every surface on the planet is old, in some places the graben are filled by relatively younger material. Some sort of planetary expansion during cooling is a possible explanation.
A 450 km diameter moon of Uranus with a set of 3 large, unique tectonic features called coronae (no relation to Venusain coronae). These features are large and roughly square collections of parallel ridges and troughs. Some suggested origins:
1. Sinker - a large dense object landed on the surface at low velocity and sank into the interior, leaving the coronae.
2. Riser - some sort of light blob rose through the interior to the surface.
3. The coronae are scars from when the moon was broken apart during an impact and then reassembled.
1. A large moon of Neptune, a little bit smaller than Earth's moon.
2. It orbits in a retrograde motion and is thought to be a captured planetesimal, similar in density to Pluto.
3. Has very few craters.
4. Has seasonal polar caps of fairly thin layers of frost.
5. Some sort of small volcanic plumes observed.
6. Two or three different terrain types. The most unique of these has been called the "cantaloupe terrain" because it looks similar to the surface of a cantaloupe.
One possible explanation of the cantaloupe terrain: large-scale overturn by viscous processes (diapirs) of a dense layer overlying a less-dense layer. How one gets a dense layer over a less-dense layer is unknown.