Expert: Courtney Seligman - 9/10/2011

Question
Titan, one of Saturn's many moons, has an atmosphere, but our own Moon does not. What is the explanation?

Answer
One reason is that Titan is ten times further from the Sun, and as a result, far colder. This allows its gravity to compress gases toward its surface more than if it were closer to the Sun and warmer. If it were moved to the orbit of the Moon, the gases in Titan's outer atmosphere would expand into space and be lost, and eventually, all of its atmosphere would disappear. In fact, eventually most of Titan itself would disappear, as much of its mass is ices which would boil away and be lost, in the same way as its atmosphere.

An equally important reason is that Titan is much bigger than our Moon, and has a larger escape velocity. Even if our Moon were moved to the orbit of Titan, its smaller size would make it incapable of holding onto an atmosphere. However, that factor could be overcome by moving it still further from the Sun, so that its atmosphere was even colder than that of Titan. Pluto has a very thin atmosphere at times (when it is closest to the Sun, and warmest, so that the ices on its surface can evaporate), and it is much smaller than our Moon; so if our Moon were near the orbit of Neptune or Pluto, it could also hold on to an atmosphere.