Jupiter

Europa is an ivory, gray, and brown world with a water ice surface. Few craters are visible, and the surface is relatively smooth, but dissected by global networks of fractures. Some areas of Europa (bright plains) have a high density of fractures, whereas other areas (mottled terrains) have less obvious fractures. Icy "lava" from below Europa's ice crust may have flowed out and resurfaced parts of the mottled terrains. This suggests that processes have been (and may be still) at work to renew and re-coat the surface of Europa.

Even more exciting is evidence suggesting that Europa may possess the right combination of conditions to sustain life. Based on data from Galileo, paleontologists now believe that the satellite features liquid (or at least slushy) oceans beneath its icy crust, warmed by volcanic activity, geysers, and other thermal outflow generated by tidal stretching and squeezing (like Io, on a smaller scale). In particular, the surface in many areas appears fractured and segmented, indicating ice floes moving over liquid water. In conjunction with organic compounds, the water and heat may have created a biologically viable environment, similar to the hydrothermal vents that on Earth have been shown to support organic activity.

Europa has but five clearly defined impact craters, but there are many other impact-crater-like features called crater palimpsests, which look like flat, circular, brown spots that may be craters filled by water lavas. The largest of these features is called Tyre Macula, and is 62 mi (100 km) in diameter.

It is hypothesized that Europa has a solid iron-sulfur core, a solid silicate mantle, an asthenosphere of silicates, ice, and water, and a cryosphere (ice crust) of water ice that is approximately 62 mi (100 km) thick.

Ganymede, the largest of the moons, is an ivory and grey water-ice world. Impact craters are visible on the surface, but there is a mixture of both dark heavily cratered (older terrains) and sparsely cratered but highly fractured regions (bright and grooved terrains). This suggests that part of the surface is very old (heavily cratered regions) and part has experienced resurfacing since its formation (bright and grooved terrains). Ganymede has a density that suggests a silicate rock core surrounded by a mantle and cryosphere somewhat like that of Europa (but perhaps not as warm). Ganymede has a large, very old impact crater basin that has been partially covered by ice lavas, known as Memphis Facula (about 311 mi [500 km] in diameter).

Callisto is the outermost of the Galilean satellites. Callisto is another gray water-ice world, but it is darker than the Europa and Ganymede. Impact craters are visible over the entire surface, suggesting that little resurfacing has occurred. The density of craters on Callisto is higher than on Earth's moon, suggesting most of the surface is very old. There is a huge, multi-ring impact crater basin on Callisto called Valhalla, which has 25 concentric rings extending out to a diameter of 621 mi (1,000 km). Valhalla may be the largest impact crater in the solar system. Callisto has an internal core of silicate rock and a mantle and cryosphere like the other Galilean satellites.

Several other small satellites of Jupiter have been imaged (e.g., Sinope, Amalthea, and Thebe) and these appear to be captured chondritic asteroids. This lends credence to the theory that all irregularly shaped (non-Galilean) satellites of Jupiter are captured asteroids of one kind or another.