478 PART 4^ |^ THE SOLAR SYSTEM
A hundred years of speculation have raised high expectations for
Mars. If there were intelligent life on Mars and its representatives
came to Earth, they would probably be a big disappointment to
the readers of the tabloids.
The Atmosphere of Mars
If you visited Mars, your fi rst concern, even before you opened
the door of your spaceship, would be the atmosphere. Is it
breathable? Even for the astronomer observing safely from Earth,
the atmosphere of Mars is of major interest. Th e gases that cloak
Mars are critical to understanding the history of the planet.
Th e air on Mars is 95 percent carbon dioxide, with a few
percent each of nitrogen and argon. You probably noticed that
this is quite similar to the composition of Venus’s atmosphere.
Th e reddish color of the Martian soil is caused by iron oxides
(rusts), and this is a warning that the oxygen humans would
prefer to fi nd in the atmosphere is locked in chemical com-
pounds in the soil. Th e Martian atmosphere contains almost no
water vapor or oxygen, and its density at the surface of the planet
is only about 1 percent that of Earth’s atmosphere. Th is does not
provide enough pressure to prevent liquid water from boiling
into vapor. Water can exist at the Martian surface only as ice or
vapor.
Although the air is thin, it is dense enough to be visible in
photographs (■ Figure 22-11). Haze and clouds come and go,
and occasional weather patterns are visible. Winds on Mars can
be strong enough to produce dust storms that envelop the entire
planet. Th e polar caps visible in photos are also related to the
Martian atmosphere. Th e ices in the polar caps are frozen carbon
dioxide (“dry ice”) with frozen water underneath.■ Figure 22-11
The atmosphere of Mars is evident in this image made by the Hubble Space
Telescope. The haze is made up of high, water-ice crystals in the thin CO 2
atmosphere. The spot at extreme left is the volcano Ascraeus Mons, 25
km (16 mi) high, poking up through the morning clouds. Note the north
polar cap at the top. (Philip James, University of Toledo; Steven Lee, University of
Colorado, Boulder; and NASA)■ Figure 22-10
(a) Early in the 20th century, Percival Lowell mapped canals over the face of Mars and concluded that intelligent life resided there.
(Lowell Observatory) (b) Modern images recorded by spacecraft reveal a globe of Mars with no canals. Instead, the planet is marked by cra-
ters and, in some places, volcanoes. Both of these images are reproduced with south at the top, as they appear in telescopes. Lowell’s
globe is inclined more nearly vertically and is rotated slightly to the right compared with the modern globe. (U.S. Geological Survey)a b VisualVisual-wavelength image