Other solid bodies of the solar system also diplay round craters. On the large ice-covered moons of Jupiter, the return to equilibrium is much more pronounced, because ice sags and flows much more readily than rock. In the centuries after Galileo's discoveries, the Moon was extensively studied by astronomers using telescopes. When a star was eclipsed by the Moon, it vanished suddenly and its light showed no refraction or absorption by an atmosphere. The location of that point allows astronomers to deduce the mass of the Moon, and from that, the pull of the Moon's gravity.
Gravity is important for the retention of an atmosphere.
It holds an atmosphere down, while heat is what can make it escape. Heat is atomic or molecular motion. In a hot solid or liquid, it can be viewed as a shaking motion of atoms or molecules around their average position, like the rustling of leaves in a wind. The higher the temperature, the more vigorous the motion, until the material boils or evaporates, at which point its particles shake loose altogether. According to that distribution, a few molecules always move fast enough to escape, and if they happen to be near the top of the atmosphere, moving upwards and and avoiding any further collisions, such molecules would be lost.