The Retention of Atmospheres
- Two factors determine how well a planet can hold on to an
- Gravity: The stronger the surface gravitational pull
of a planet, the harder it is for gas molecules to escape.
The temperature of a gas is indicative of the
speed at which the gas molecules are moving.
- In a hot gas the atoms move quickly, and thus can
escape a planet's gravitational pull more easily.
- The Moon and Mercury do not have atmospheres because they
are both small and have high surface temperatures because the are
close to the
- Both of these objects outgassed atmospheres early in their
history, but the gas was lost because the gas molecules moved
quickly enough to escape.
- Pluto is smaller than the Earth's Moon, yet it
has an atmosphere because it is far from the Sun and thus very
cold. The gas molecules move slowly and cannot escape.
Revised November 24, 1997