The Galilean Satellites 

Moons of Jupiter, discovered by Galileo Galilei in 1610.
Easy to see with small telescope or even binoculars.

Basic Properties:
 

 
Mass (MMoon)
Radius (RMoon)
Jupiter Distance (km)
Density (gm/cm3)
Io
1.2
1.1
422,000
3.5
Europa
0.65
0.90
671,000
3.0
Ganymede
2.0
1.5
1,071,000
1.9
Callisto
1.5
1.4
1,884,000
1.8
(The next largest moon, Amalthea, is only 135 km x 78 km in size.)
 

The outer moons (further away than the Galileans) are much smaller, have eccentric orbits, and orbit inclined or retrograde. What does this suggest about their origins?
 

The Galilean satellites are basically small planets. They all exist at the same distance from the Sun, where the equilibrium temperatures are cold enough that condensing objects should be a mix of rock and lots of volatile ices, with very little interior activity.

But as we shall see, the Galilean objects have wildly different properties from one another. Why might this be true?