CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 479
If you could visit Mars you would fi nd it a reddish, airless,
bone-dry desert (■ Figure 22-12). To understand Mars, you can
ask why its atmosphere is so thin and dry and why the surface is
rich in oxides. To fi nd those answers you need to consider the
origin and evolution of the Martian atmosphere.
Presumably, the gases in the Martian atmosphere were
mostly outgassed from its interior. Volcanism on Terrestrial
planets typically releases carbon dioxide and water vapor, plus
other gases. Because Mars formed farther from the sun, you
might expect that it would have incorporated more volatiles
when it formed. But Mars is smaller than Earth, so it has had
less internal heat to drive geological activity, and that would lead
you to suspect that it has not outgassed as much as Earth. In any
case, whatever outgassing took place occurred early in the plan-
et’s history, and Mars, being small, cooled rapidly and now
releases little gas.
How much atmosphere a planet has depends on how rapidly
it loses gas to space, and that depends on the planet’s mass and
temperature. Th e more massive the planet, the higher its escape
velocity (see Chapter 5), and the more diffi cult it is for gas atoms
to leak into space. Mars has a mass less than 11 percent that of
Earth, and its escape velocity is only 5 km/s, less than half
Earth’s. Consequently, gas atoms can escape from it much more
easily than they can escape from Earth.
Th e temperature of a planet’s atmosphere is also important.
If a gas is hot, its molecules have a higher average velocity and are
more likely to exceed escape velocity. Th at means a planet near
the sun is less likely to retain an atmosphere than a more distant,
cooler planet. Th e velocity of a gas molecule, however, also
depends on the mass of the molecule. On average, a low-mass
molecule travels faster than a massive molecule. For that reason,
a planet loses its lowest-mass gases more easily because those
molecules travel fastest.
You can see this principle of comparative planetology if you
plot a diagram such as that in ■ Figure 22-13. Th e data points
show the escape velocity versus temperature for the larger objects
in our solar system. Th e temperature used in the diagram is the
temperature of the gas that is in a position to escape. For the
moon, which has essentially no atmosphere, this is the tempera-
ture of the sunlit surface. For Mars, the temperature that is
important is that at the top of the atmosphere.
Th e lines in Figure 22-13 show the typical velocities of the
fastest-traveling examples of various molecules. At any given
temperature, some water molecules, for example, travel faster
than others, and it is the highest-velocity molecules that escape
from a planet. Th e diagram shows that the Jovian planets have
escape velocities so high that very few molecules can escape.
Earth and Venus can’t hold hydrogen, and Mars can hold only
the more massive molecules. Earth’s moon is too small to keep
any gases from leaking away. You can refer back to this diagram
when you study the atmospheres of other worlds in later
chapters.
Over the 4.6 billion years since Mars formed, it has lost
some of its lower-mass gases. Water molecules are massive
enough for Mars to keep, but ultraviolet radiation can break
them up. Th e hydrogen escapes, and the oxygen, a very reactive
element, forms more oxides in the soil—the oxides that make
Mars the red planet. Recall that on Earth the ozone layer protects
water vapor from ultraviolet radiation, but Mars never had an
oxygen-rich atmosphere, so it never had an ozone layer. Ultraviolet
photons from the sun can penetrate deep into the atmosphere
and break up molecules. In this way, molecules too massive to
leak into space can be lost if they break into lower-mass
fragments.
Th e argon in the Martian atmosphere is evidence that there
once was a denser blanket of air. Argon atoms are massive, almost■ Figure 22-12
Mars is a red desert planet, as shown in this true-color photo made by the Rover Opportunity. The rock outcrop is a meter-high crater wall. After taking
this photo, Opportunity descended into the crater to study the wall. Dust suspended in the atmosphere colors the sky red-orange. (NASA © 1995–2007
by Calvin J. Hamilton)Visual-wavelength image