Mars and Venus are therefore the only planets, besides the earth, on
which we may look for life; and in the case of Venus, the possibility is
very faint. But what about the moons which attend the planets? They
range in size from the little ten-miles-wide moons of Mars, to Titan, a
moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
3,000 miles in diameter. May there not be life on some of the larger of
these moons? We will take our own moon as a type of the class.