CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 481
to the atmosphere of a medium-size world. Like its atmosphere,
the geology of Mars is probably typical of medium-size worlds.
The Geology of Mars
If you ever decide to visit another world, Mars may be your best
choice. Mars is much more friendly than the moon, Mercury, or
Venus. Th e nights on Mars are deadly cold, but a hot summer
day would be comfortable (Celestial Profi le 6). Mars has
weather, complex geology, and signs that water once fl owed over
its surface. You might even hope to fi nd traces of ancient life hid-
den in the rocks.
Spacecraft have been visiting Mars for almost 40 years, but
the pace has picked up recently. A small armada of spacecraft has
gone into orbit around Mars to photograph and analyze its sur-
face, and six spacecraft have landed. Two Viking landers touched
down in 1976, and three rovers have landed in recent years.
Rovers have an advantage because they are wheeled robots that
can be controlled from Earth and directed to travel from feature
to feature and make detailed measurements. Pathfi nder and its
rover, Sojourner, landed in 1997. Rovers Spirit and Opportunity
landed in 2004, carrying sophisticated instruments to explore
the rocky surface. Th e Phoenix probe landed in the north polar
region of Mars in 2008.
Photographs made by rovers and landers on the surface of
Mars, such as Figure 22-12, show reddish deserts of broken rock.
Th ese appear to be rocky plains fractured by meteorite impacts,
but they don’t look much like the surface of Earth’s moon. Th e
atmosphere of Mars, thin though it is, protects the surface from
the blast of micrometeorites that grinds moon rocks to dust.
Also, Martian dust storms may sweep fi ne dust away from some
areas, leaving larger rocks exposed.
Spacecraft orbiting Mars have imaged the surface and mea-
sured elevations to reveal that all of Mars is divided into two
parts. Th e southern highlands are heavily cratered, and the num-
ber of craters there shows that they must be old. In contrast, the
northern lowlands are smooth (■ Figure 22-15) and so remark-
ably free of craters that they must have been resurfaced no more
than a billion years ago. Some astronomers have suggested that
volcanic fl oods fi lled the northern lowlands and buried the cra-
ters there. Growing evidence, however, suggests that the northern■ Figure 22-14
(a) Each spring, spots and fans appear on the ice of the south polar
cap on Mars. (b) Studies show the ice is frozen carbon dioxide in a
nearly clear layer about a meter thick. High-pressure carbon dioxide gas
vaporized by spring sunlight bursts out of the ice in geysers. The gas
carries sand and dust hundreds of meters into the air. (NASA; Arizona State
University/Ron Miller)aa Visual-wavelength image Visual-wavelength imageb