Geology Reference
In-Depth Information
CHAPTER 7
Mars
Mars has an atmosphere composed mostly of carbon
dioxide (95%), with minor amounts of nitrogen, argon,
oxygen, carbon monoxide, and water vapor. But with an
average surface pressure of only 6.5mbar, the atmosphere
is very
“
thin
”
in comparison with Earth
'
s atmosphere
-
about the equivalent of Earth
'
s atmosphere at an altitude
of about 130,000 feet (nearly 40 km) above the surface, or
four times higher than commercial jet
flights. While the
average surface temperature is
7.1 Introduction
Few objects in the sky hold the fascination in the public
mind as much as Mars. Easily seen with the naked eye, the
“
has been linked with various gods of war
through the ages. The late 1800s and early 1900s saw both
serious and not-so-serious writings on martian life, includ-
ing the presence of advanced civilizations, and culminating
in the infamous radio broadcast of H. G. Wells
'
s
fictional
War of the Worlds, in which martian spacecraft land on
Earth. This broadcast
lled many a family with terror as the
story unfolded with the destruction of whole cities.
Building on public support for the exploration of Mars,
the Red Planet has been visited by more spacecraft than
any other object except Earth
Red Planet
”
−
63 °C, the range of tem-
peratures is
−
133 °C to +27 °C.
Mars is the most Earth-like of all the objects in the Solar
System and exhibits evidence of processes related to run-
ning water, glaciers, wind, volcanism, and tectonic defor-
mation. However, unlike Earth, Mars preserves extensive
parts of its ancient record of impact cratering, while the
current cold, dry climate militates against there being
extensive liquid water on the surface. Despite the low-
density atmosphere, winds and associated aeolian pro-
cesses dominate the current environment. Although
Mars appears to lack apparent plate-style tectonics, as
noted below, there are suggestions that such activity
might have occurred early in its history.
Given its size, rocky composition, and the presence of
an atmosphere, Mars exhibits surface features that are
recognizable to most geologists and affords the opportu-
nity to study familiar processes under non-Earthly
conditions.
s Moon. Along with Europa
and possibly Titan, Mars is a favorable planet in the
exploration for possible present-day or past life.
With a diameter of 6,779 km and a mass of 6.4 × 10
23
kg, Mars gravity is 0.37 that of Earth. The total surface area
of Mars is just about equal to the land surface on Earth
above sea level. One Mars year is 686.98 Earth days, while
one Mars day is 24 hr, 39m, 35.2 s. Its present-day spin
axis is inclined 25.19° (slightly more than Earth), which
leads to distinctive seasons. The seasons are de
ned by
Mars
'
position in orbit and described by aerocentric lon-
gitudes (L
s
) of the Sun in degrees. L
s
is the angle between
the Mars
-
Sun line and the line of equinoxes. L
s
of 0° is set
at the martian equinox for the beginning of winter in the
northern hemisphere (L
s
0° to 90°), with northern spring
(L
s
90° to 180°), northern summer (L
s
180° to 270°), and
northern autumn (L
s
270° to 360°).
Mars wobbles on its spin axis, which means that at times
the poles receive substantiallymore sunlight in their respec-
tive summers and are proportionally much colder in the
winters than at other times. This is thought to lead to
martian ice ages on time scales of 50
-
100Ma as a function
of the wobble and could result in large climate changes.
'
7.2 Exploration
Although even the earliest telescopes were trained on
Mars, it was not until the late 1800s that attempts were
made to identify surface markings and relate them to
processes. The famous
of Italian astronomer
Schiaparelli were loosely translated to
“
canals
”
and
spurred interest on the part of Percival Lowell. This weal-
thy Bostonian became fascinated with Mars and built an
“
canali
”
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