Recall from Chapter 7 that the terrestrial planets are less
massive than the giant planets and therefore have weaker
gravity. Giant impacts by large planetesimals early in the
history of the Solar System may have blasted away some
of the terrestrial planets’ primary atmospheres because,
with their relatively weak
gravity, these planets lack
the ability to hold light
gases such as hydrogen
and helium. When the supply
of gas in the protoplanetary disk ran out, the primary
atmospheres of the terrestrial planets began leaking back
into space. How can gas molecules escape from a planet?
Recall from Chapter 4 that all it takes for any object—from a
molecule to a spacecraft—to escape a planet is for the object
to reach a speed greater than the escape velocity and be
pointed in the right direction. Intense radiation from the
Sun—which is the primary source of thermal, or kinetic,
energy in the atmospheres of the terrestrial planets—raises
the temperature and thus the kinetic energy of atmospheric
molecules enough that some escape. The kinetic energy of
any object is determined by its mass and its speed. Hotter
molecules have higher kinetic energies than do cooler
molecules and therefore move faster and are more likely to
escape.