I just saw the excellent film Cosmic Collisions at the rose Center in NYC with my 14 year old. The film does a wonderful job of explaining how the Moon formed from the Earth. But it raised this question I couldn’t answer: Why do we think the Moon didn’t form an atmosphere and some form of life but Earth did given that the Moon was cosmically next door to the Earth.
There are two requirements for a planet to have an atmosphere: it must acquire the gas and it must keep it in. The Moon is just not large enough to hold on to an atmosphere (less than 2 percent the mass of the Earth). The Moon and Earth lost most of the gas that was presents at the time of the giant Moon-forming collision, but the debris that came together from the Moon was especially well stripped, even of water that was chemically bound to the rock. Subsequently, however, the Earth and Moon both had the opportunity to acquire a new atmosphere from collisions with comets and asteroids. The Earth had enough gravity to retain this new atmosphere, but the Moon did not. On the Moon, the water and gas from the collisions dissipated almost immediately, since the gravity is so low. Thus today we have a substantial atmosphere on Earth, while the Moon has none and is also exceedingly dry. Incidentally, Mars (with about 10 percent the mass of the Earth) is an intermediate case; it once has a substantial atmosphere, but much of that has been lost, rendering Mars a barren world, but not nearly so barren as the Moon. The mass of the planets is thus at least as important than the distance from the Sun in determining whether it will be habitable.