In many ways, moons are smaller versions of the terrestrial
planets. Some, like Earth’s Moon, are made of rock. Others,
especially in the outer Solar System, are mixtures of rock
and water ice, with densities intermediate between the
two. A few are made almost entirely of ice. Only two moons
(Ganymede and Titan) are
larger in diameter than
Mercury, and the smallest
known moons are smaller
than a large metropolitan airport. Although most moons
are airless, one has an atmosphere denser than Earth’s, and
several have very low-density atmospheres