Miranda (left) - a moon of Uranus and Gaspra (right) - a small asteroid
The inner planets have no moons, but Mars has two tiny ones and both are thought to be captured asteroids. The Gas and Ice Giant Planets each have around 20 moon and, other than those already described, they are similar in nature - rock and ice bodies in deep-freeze. Although some of the larger moons, particularly around the Ice Giants, may display cold volcanism as does Triton, none offers good habitats for life. The asteroids must be considered to be in a similar situation, though they receive more sunlight than the distant moons and it is known that some contain interesting chemicals associated with the precursors of life.
If
life exists on any of these distant bodies it is almost certainly in a
dormant state and must have been carried to its current home hitching
a ride on space debris. Alternatively, if life started in space,
long before the formation of the Solar System and found a place to dwell
in extended hibernation, then it may exist deep inside some of the minerals
that make up these odd scraps of the Solar System.
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