488 PART 4^ |^ THE SOLAR SYSTEM
huge bulge on the side of Mars (■ Figure 22-22). Some of the
oldest lava fl ows on Mars are in the Th arsis rise, but it also con-
tains some of the most recent lava fl ows. It has evidently been a
major volcanic area for most of the planet’s history.
Th e valley networks found in the southern highlands were
formed during the Noachian period when water fell as rain or
snow and drained down slopes. To keep the water liquid required
a higher temperature and higher atmospheric pressure than is
present on Mars today. Violent volcanism could have vented
gases, including more water vapor that kept the air pressure high.
Th is may have produced episodes in which water fl owed over the
surface and collected in the northern lowlands and in the deep
basins to form oceans and lakes, but it’s not known how long
those bodies of water survived. A planetwide magnetic fi eld may
have protected the atmosphere from the solar wind.
Because Mars is small, it lost its internal heat quickly, and
atmospheric gases escaped into space. Th e Hesperian period
extended from roughly 3.7 billion years ago to about 3 billion years
ago. During this time massive lava fl ows covered some sections of
the surface. Most of the outfl ow channels date from this period,
which suggests that the loss of atmosphere drove Mars to become a
deadly cold desert world with its water frozen in the crust. When
volcanic heat or large impacts melted subsurface ice, the water could
have produced violent fl oods and shaped the outfl ow channels.
Th e history of Mars may hinge on climate variations. Models
calculated by planetary scientists suggest that Mars may have once
rotated at a much steeper angle to its orbit, as much as 45°. Th is
could have resulted in a generally warmer climate and kept more
of the carbon dioxide from freezing out at the poles. Th e rise of
the Th arsis bulge could have tipped the axis to its present 25° and
cooled the climate. Some calculations suggest that Mars goes
through cycles similar to Earth’s Milankovitch cycles (see
Chapter 20) as its rotational inclination fl uctuates, and this may
cause short-term variations in climate, much like the ice ages on
Earth.
Th e third period in the history of Mars, the Amazonian
period, extended from about 3 billion years ago to the present
and was mostly uneventful. Th e planet has lost much of its inter-
nal heat, and the core no longer generates a global magnetic fi eld.
Th e crust of Mars is too thick to be active with plate tectonics,
and consequently there are no folded mountain ranges on Mars
resembling the ones on Earth. Th e huge size of the Martian vol-
canoes clearly shows that crustal plates have not moved on Mars.
Volcanism may still occur occasionally on Mars, but the crust has
grown too thick for much geological activity beyond slow ero-
sion by wind-borne dust and the occasional meteorite impact.
Planetary scientists cannot tell the story of Mars in great
detail, but it is clear that the size of Mars has infl uenced both its
atmosphere and its geology. Neither small nor large, Mars is a
medium-sized world, with characteristics intermediate between
small Mercury and Earth’s moon on one hand, and large Venus
and Earth on the other.■ Figure 22-22
The Tharsis rise is a bulge on the side of Mars consisting of stacked lava
fl ows extending up to 10 km (6 mi) high and over a third the diameter of
Mars. It dominates this image of Mars, with a few clouds clinging to the four
largest volcanoes. This image was recorded in late winter for the southern
hemisphere. Notice the size of the southern polar cap. (NASA/JPL/Malin Space
Science Systems)
SCIENTIFIC ARGUMENT
Why doesn’t Mars have coronae like those on Venus?
This argument is a good opportunity to apply the principles of
comparative planetology. The coronae on Venus are caused by ris-
ing currents of molten magma in the mantle pushing upward under
the crust and then withdrawing to leave the circular scars called
coronae. Earth, Venus, and Mars have had signifi cant amounts of
internal heat, and there is plenty of evidence that they have had
rising convection currents of magma under their crusts. Of course,
you wouldn’t expect to see coronae on Earth; its surface is rapidly
modifi ed by erosion and plate tectonics. Furthermore, the mantle
convection on Earth seems to produce plate tectonics rather than
coronae. Mars, however, is a smaller world and must have cooled
faster. There is no evidence of plate tectonics on Mars, and giant
volcanoes suggest rising plumes of magma erupting up through the
crust at the same point over and over. Perhaps there are no coronae
on Mars because the crust of Mars rapidly grew too thick to deform
easily over a rising plume. On the other hand, perhaps you could
think of the entire Tharsis bulge as a single giant corona.
Planetary scientists haven’t explored enough planets yet to
see all the fascinating combinations nature has in store. But it
does seem likely that the geology of Mars is typical of medium-size
worlds. Of course, Mars is not medium in terms of its location. Of
the Terrestrial planets, Mars is the farthest from the sun. Build a
scientifi c argument to analyze that factor. How has the location
of Mars affected the evolution of its atmosphere?