480 PART 4^ |^ THE SOLAR SYSTEM
as massive as a carbon dioxide molecule, and would not be lost
easily. In addition, argon is inert and cannot form compounds in
the soil. Th e 1.6 percent argon in the atmosphere of Mars is
evidently left over from an ancient atmosphere that may have
been 10 to 100 times denser than the present Martian air.
Finally, you should consider the interaction of the solar wind
with the atmosphere of Mars. Th is is not an important process
for Earth because Earth has a magnetic fi eld that defl ects the
solar wind. In contrast, Mars has no magnetic fi eld, and the solar
wind interacts directly with the Martian atmosphere. Detailed
calculations show that signifi cant amounts of carbon dioxide
could have been carried away by the solar wind over the history
of the planet. Th is process would have been most effi cient long
ago when the sun was more active and the solar wind was stron-
ger. However, you should also keep in mind that Mars probably
had a magnetic fi eld when it was younger and still retained sig-
nifi cant internal heat. A magnetic fi eld would have protected its
atmosphere from the solar wind.
Th e polar caps contain large amounts of carbon dioxide ice;
and, as spring comes to a hemisphere, that ice begins to vaporize
and returns to the atmosphere. Meanwhile, at the other pole,
carbon dioxide is freezing out and adding to the polar cap there.
Dramatic evidence of this cycle appeared when the camera
aboard the Mars Odyssey probe sent back images of dark mark-
ings on the south polar cap. Evidently as spring comes to the
polar cap and the sun begins to peek above the horizon, sunlight
penetrates the meter-thick ice and vaporizes carbon dioxide,
which bursts out in geysers a few tens of meters high carrying
dust and sand. Local winds push the debris downwind to form
the fan-shaped dark markings (■ Figure 22-14). Th ese dark
markings appear each spring but last only a few months as frozen
carbon dioxide returns to the atmosphere.
Although planetary scientists remain uncertain as to how
much of an atmosphere Mars has had in its past and how much
it has lost, it is a good example for your study of comparative
planetology. When you look at Mars, you see what can happen6050403020100
100 200 300 400 500 600 700Pluto (dwarf planet)Ceres (dwarf planet)MoonMars MercuryEarthUranusNeptuneSaturnJupiterVenusTemperature (K)H 2 O, NH 3 , CH 4H 2HeN 2 , O 2
Triton Titan CO 2Velocity (km/s)■ Figure 22-13
Loss of planetary atmosphere gases.
Dots represent the escape velocity and
temperature of various solar-system
bodies. The lines represent the typical
highest velocities of molecules of vari-
ous masses. The Jovian planets have
high escape velocities and can hold
onto even the lowest-mass molecules.
Mars can hold only the more massive
molecules, and the moon has such
a low escape velocity that even the
most massive molecules can escape.