Mars: Surface and Interior 317
little support from subsequent investigations by the general
science community.
The most recent stage of Mars exploration started in 1997
with the landing ofMars Pathfinderon Chryse Planitia. This
has been followed by a series of long-lived missions that
have precisely determined the topography and gravity field
and returned a vast amount of imaging and spectral data.
In addition, as of this writing, the two rovers,Spiritand
Opportunity, were relaying from the surface data that
showed definitive evidence of water-lain sediments and
aqueous alteration.
2. General Characteristics
2.1 Orbital and Rotational Constants
The martian day is almost the same as a day on Earth, but the
year is almost twice as long (Table 2). Because its rotational
axis is inclined to the orbit plane, Mars, like the Earth,
has seasons. But the Mars orbit has significant eccentricity.
This causes one pole that tilts toward the Sun at perihelion
to have warmer summers than the other pole. At present,
the south has the warmer summers, but, because of a slow
change in the direction of tilt of the rotational axis and a
slow change in the orientation of perihelion, the hot and
cold poles change on a 51,000-year cycle. The eccentricity
also causes the seasons to have significantly different lengths
(see Table 2). At present the Marsobliquityis similar to
the Earth’s. Yet the Earth experiences only minor changes
in obliquity, while the obliquity of Mars changes chaotically,
ranging from a low of 0◦to a high in excess of 60◦.
At low obliquities, the atmosphere thins as most of the
CO 2 in the atmosphere condenses on the poles. At high
obliquities, the water ice polar caps dissipate, and ice con-
denses at lower latitudes.2.2 Surface Conditions
Mars has a thin atmosphere that provides almost no ther-
mal blanketing. As a result, temperatures at the surface
have a wide diurnal range, controlled largely by latitude,
the reflectivity of the surface, and the thermal properties
of the surface materials. Typically, surface temperatures in
summer at latitudes± 60 ◦range from 180 K at night to
290 K at midday but can range more widely if the sur-
face consists of unusually low-density, fine-grained mate-
rial. However, these temperatures are somewhat deceiving
because, at depths of a few centimeters below the surface,
temperatures are at the diurnal mean of 210–220 K. At the
poles in winter, temperatures drop to 150 K at which point
CO 2 condenses out of the atmosphere to form the seasonal
cap. The atmospheric pressure at the surface ranges from
about 14 millibars in the bottom of the Hellas basin to about
3 millibars at the top of the tallest volcanoes, and it changes
annually as a result of formation of the polar caps. Winds are
typically a few meters per second but there may be gusts up
to 50 m/s. Dust devils and local dust storms are common,
and almost every year regional or global-scale dust storms
occur.TABLE 2 Earth and Mars: General Characteristics ComparedEarth MarsMean equatorial radius (km) 6378 3396
Mass (× 1024 kg) 5.98 0.624
Mean distance from Sun (10^6 kg) 150 228
Orbit eccentricity 0.017 0.093
Obliquity 23.5◦ 25.2◦
Length of day 24 h 24 h 39 m 35 s
Length of year (Earth days) 365.3 686.9
Seasons (Earth days)
Northern spring 92.9 199
Northern summer 93.6 183
Northern fall 89.7 147
Northern winter 89.1 158
Atmosphere 79% N 2 , 21% O 2 95% CO 2 ,3%N 2 ,2%Ar
Surface pressure (mbar) 1000 7
Mean surface temperature (K) 288 215
Surface gravitational acceleration (cm/s−^2 ) 981 371
Moons 1 2