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Total Atmospheric Mass
Figure 2 gives a listing of the fractional contribution of each
of several gases to the total atmospheric mass of each body.
Comparison of these two tables explains why Venus, Earth, and Mars,
with their low escape velocities, have very low concentrations of
lighter elements. We don't expect to find, and we don't find, very
much hydrogen or helium on Venus,
Earth, or Mars because these light elements have relatively high
probable velocities in relation to the
escape velocities for these planets and there are no sources of light
elements on these planets. For more massive molecules, lower most
probable velocities mean that escape is less likely, For instance, for
CO2 (M=44) at the effective temperature of the earth, Vo (CO2) = 310
m/s or 0.31 km/s. The time for carbon dioxide to leave the planet is
long compared to the age of the earth, which is about 4-5 billion
years.
