The satellites discovered by Galileo with his small telescope form a small, `mini' solar system around Jupiter. They each have special characteristics related to their formation process, but have the following traits in common:

1) all orbit Jupiter
2) they all are tidally locked to Jupiter
3) they all have radii larger than our Moon
4) the inner moons have densities higher than outer moons (implies that Jupiter was much warmer in the past, such that the moons formed near Jupiter have less of the volatile elements such as CO₂ and H₂O)

Io:

Io is the innermost world, closest to Jupiter and can be classified as one of the most unusual moons in our solar system. Its unique properties include:

• surface of Io is a mottled yellow, reds and blackish-browns = sulfur compounds, where different colors correspond to regions with different temperatures. The mean temperature on Io's surface is about -143 C (-230 F).
  

  A hot spots associated with a volcanic features measure about 17 C (60 F). Scientists believe the hot spots may be lava lakes, although the temperature indicates the surface is not molten.

• thin patchy atmosphere of SO₂
  

• active plumes (not volcanos since there is no mantle), geysers of molten sulfur and SO₂. The mechanism for heating Io's core is tidal friction from Jupiter. Jupiter's gravitational field is much stronger than, say, the Moon's on the Earth. The tides pull and squeeze the core of the moons around Jupiter producing mechanical friction. This friction is turned into heat, which makes the inner moons much more active than the outer moons. When liquid sulfur on surface flows into vents and makes contact with the hot interior, there is an explosive conversion into gas producing the geyers.
  

• Io is composed primarily of rocky material with very little iron. Io has the ``youngest surface'' in the Solar System, constantly changing, as the above two images demonstrate. The one on the left is from Voyager, the one of the right is from the Galileo probe, 20 years later.
  

• no impact craters, which means that the surface of Io is very young, less than a million years old = lots of erosion

Europa:

Europa is the next world out from Io and is considered a strong candidate to find primitive life. Its characteristics are:

• icy world with high albedo, smooth surface with only a few hills/mountains and craters, meaning the surface is young and renewing itself
  

• large long cracks on the surface, due to ice plate tectonics floating on water mantle
  

• visible surface is actually H₂O ice mixed with rock (dirty icefield) about 100 km thick

Ganymede:

Ganymede is much less dense than Europa or Io. Its interesting characteristics are:

• very diversified surface with dark regions, valleys, mountains, evidence of past tectonic activity and lots of vertical relief

• low mean density implies a very deep ice mantle, about 1000 km thick
  

• white craters and rims = impact exposes mantle ice
  

• ghost craters = smoothed by ice flow

• thin oxygen atmosphere, possible aurora at its poles (magnetic field?)

Callisto:

Callisto is the outermost of the 4 primary satellites:

• similar surface to Ganymede but with 10 times more cratering, which means less tectonic activity, cooler interior
  

• little vertical relief = craters destroyed landscape = weak surface material (low density)
  

• large impact basins

Summary of Galilean Moons:

• mini-Solar System model where the closer moons are rocky, high density and the outer moons are icy, low density
  

• geological activity decreases with distance from Jupiter, due to the decreasing tidal friction from Jupiter on interior of moons - this has a marked influence on the appearance of their surfaces

• little or no atmospheres
• high icy component to crust of all moons

Titan:

Titan is the largest satellite of Saturn, unique in its methane atmosphere:

• detected to have spectral lines of methane in 1944
  

• heavy atmosphere (1.4 times the Earth's) with structure in the form of haze - reddish color from organic molecules formed by breakdown of CH4 by cosmic rays

• composition of atmosphere is 90% N₂, 7% CH₄, 2% Ar and traces of ethane, acetylene, ethylene, propane and hydrogen cyanide (i.e. smog)

• Titan's surface temperature averages around -178C (-289F)

• the temperature and pressures are near the triple point of methane (CH₄) -> methane rain, snow, ice, clouds, rivers, etc.

• Earth's environment is near the triple point of water - water serves as the solvent for organic chemicals to interact = life. Titan is an excellent place to search for new forms of life based on methane as a solvent

Above is a moasic of three frames taken by the lander as it entered Titans atmoshere. It shows detail of a high ridge area including the flow down a major river channel (liquid methane river) into what appears to be a "wet" plain.

The above image shows the boundary between lighter-colored uplifted terrain and drainage channels into the darker lower areas. This picture was taken from an altitude of 8 kilometers.

Above is a color image from the surface of Titan. Rocks near the bottom are pebble-sized, objects in the middle of the frame are meters in size. The surfce is darker than expected, consisting of a mixture of water and hydrocarbon ice. There is evidence of erosion indicating fluid activity in the recent past.