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.
1. Outermost of Saturn's major moons, ~ 1500 km diameter.
2.
The leading face of Iapetus in its orbit has an extremely low albedo;
otherwise the moon is fairly typical. The cause of this low albedo is
unknown, perhaps dust from Phoebe, a small moon farther out.
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.
A450 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.
7. 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.