Annu. Rev. Earth Planet. Sci. 2000. 28:281–304
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MARS 2000
Arden L. Albee
Division of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, California 91125; e-mail: aalbee@cco.caltech.edu
Key Words
Mars, Mars Global Surveyor
Abstract
Twenty years after the Viking Mission, Mars is again being scrutinized
in the light of a flood of information from spacecraft missions to Mars, the Hubble
Space Telescope, and the SNC meteorites. This review provides an overview of the
current understanding of Mars, especially in light of the data being returned from the
Mars Global Surveyor Mission. Mars does not now have a global magnetic field, but
the presence of crustal anomalies indicates that a global field existed early in Martian
history. The topography, geodetic figure, and gravitational field are known to high
precision. The northern hemisphere is lower and has a thinner and stronger crust than
the southern hemisphere.
The global weather and the thermal structure of the atmosphere have been moni-
tored for more than a year. Surface-atmosphere interaction has been investigated by
observations of surface features, polar caps, atmospheric dust, and condensate clouds.
The surface has been imaged at very high resolution and spectral measures have been
obtained to quantify surface characteristics and geologic processes. Many questions
remain unanswered, especially about the earliest period of Mars’ history.
INTRODUCTION
Observed from the earliest days of humankind as a wandering star in the sky,
Mars was a god to many civilizations, its menacing red color associated with
blood and war. Mars has long been a part of human history, thought, and fiction.
Eventually, optics revealed two tiny moons, an atmosphere, clouds and dust
storms, and polar caps that diminished and expanded periodically during the two-
year Mars year. The nineteenth-century fascination with Mars was stimulated by
the idea that life might exist there and that humans might eventually visit the
planet. Early in this century, it was widely believed that advanced civilizations
had developed on Mars, and that linear features seen by many observers were
canals built to carry water from the polar caps to parched red equatorial deserts.
The perception of a planet hospitable to life changed dramatically with the
advent of the Space Age. Beginning with the 1962 launch of the Soviet Union’s
Mars-1 spacecraft, 16 U.S. and Soviet missions had flown by, orbited, or landed
on Mars prior to 1980. Before the current missions, most of what we know about
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Figure 1
Overview of the planned mission of Mars Global Surveyor.
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Figure 2
The unanticipated elliptical mission of Mars Global Surveyor. The orbit period
steadily decreases with time during the aerobraking, but the orbit remains constant with a high-
er periapsis during the science phasing periods.
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Figure 3
Map of the global topography of Mars as determined by the
Mars Orbiter Laser Altimeter (Zuber et al 1999). The full-scale map has
about 5-m accuracy with 118-km resolution. Major geographic features are
labeled in Figure 5.
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Figure 4
Map of the gravity field of Mars as determined by tracking of the Mars
Global Surveyor spacecraft (Tyler et al 1999, Zuber et al 1999). The full-scale
map has about 10-mGal accuracy with 220-km resolution. Major geographic fea-
tures are labeled in Figure 5.
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