This icy volcanism and minor tectonism appears to continue to the present. Active sulphur volcanic eruptions were captured on both Voyager spacecraft flyby missions and the surface of Io changed significantly in the few months between these two missions. Sulphur volcanism is extremely active to the present. The progression of increasing modern geological activity of the Galilean Moons from Callisto to Io corresponds to proximity to Jupiter and supports the interpretation that the primary source of energy for the geological activity on the Galilean Moons is tidal energy from their tidally locked orbits around Jupiter. Europa and Io may have originally had thick icy layers like Callisto and Ganymede, but these layers have been lost to space as a result of tidal heating.
Titan is the only body in the solar system that appears to have a primitive, unevolved atmosphere.
Its atmosphere is therefore very important as a starting point for studies of the evolutions of all other planetary atmospheres, including Earth. Unfortunately Voyager could not see through Titan's clouds. The Hubble Space telescope has imaged the surface of Titan in the infra-red portion of the spectrum and indicates a variable surface. Titan will next be visited in 2004 with a probe dedicated to descend into its atmosphere that will tell us much more about this important satellite and its atmosphere. 1 to 1 spin orbit coupled to Uranus Densities indicate a small rocky core with an icy mantle and crust.