|Rotational Period||24h37m (similar to Earth)|
|Obliquity||24 degrees (similar to Earth)|
|Atmosphere||0.007 bar (surface pressure 0.7% of Earth's)|
|No free oxygen|
|Traces of water vapor|
|Between that of Earth and Moon (FeS core?)|
Because of these generally Earth-like characteristic, Mars has been the most heavly explored planet.
Mars has two satellites,
Deimos. Both are probably captured bodies, possibly from the
asteroid belt. Phobos is tidally locked so that it keeps the same face
toward Mars at all times. It also has an orbital period of 0.32 days,
less than the rotation period of Mars. Hence it is tidally unstable;
it is falling toward Mars. The tidal lifetime of Phobos is about 50
million years, so that there may be a new impact feature on Mars in
the future. However, before it hits the surface of Mars, it may
possibly be torn apart by tidal stresses. If so, Mars will join the
gas giants in having a planetary ring.
The southern highlands are characterised by high crater densities , indicating that they are old terrain similar to the lunar highlands. There are several multiring basins, including Hellas , which at nearly 2000km diameter is larger than the Caloris basin on Mercury or the Imbrium basin on the Moon. There are a number of volcanoes associated with Hellas. Like Mercury, the highlands on Mars have plains with lower crater densities; some of the plains are volcanic in origin. Unlike Mercury the highlands do contain shield volcanoes, and more unusual, large channels that appear to have been carved by flowing water. There are even dendritic systems similar to those seen on Earth.
The northern plains are similar to the ocean basins on Earth. Some, near the Tharsis volcanic regions, consist of volcanic flows similar to maria on the moon. They are lightly cratered and hence probably young. Other regions are more heavily cratered, but the crater densities are generally much lower than in the highlands. There are indications that standing water once existed on portions of the lowlands. In Candor Chasma, in the Valles Marineris, one finds material that resembles sediment. The Chryse Planitia, where Viking 1 landed, has been shaped by catastrophic floods flowing down from the highlands to the south. There are large volcanic structures in the lowlands, some the size of continents on Earth. The most prominent is the Tharsis region, where the largest volcano in the solar system, Olympus Mons (seen here in perspective) is found.
Mars exhibits a number of large volcanic areas, including the Hellas region, associated with a giant impact, the Elysium Planitia, and the Tharsis region, the youngest of the three. The volcanic features are usually divided into three types:
Valles Marineris is a continent sized canyon extending eastward from the Tharsis volcanic field to the global escarpment marking the transition to the older highlands. It follows a series of faults believed to have been produced by the upwelling that formed the Tharsis volcanoes. The canyon walls clearly show the effects of massive landslides.
The terrestrial planets Venus, Earth, and Mars (and possibly Mercury)
most likely all started with dense H and He atmospheres collected from the
solar nebula. Because their surface gravities are low, they lost this
primary atmosphere. The atmospheres we see around the larger
terrestrial planets today are called secondary
atmospheres. They consist of less volatile gasses such as
CO2 or N2, which have molecular weights
substantially greater than those of H and He. This gas probably was
outgassed by volcanoes from the interiors of the planets, but some
fraction may have come from accretion of comets. The initial
atmospheric pressure on the terrestrial planets is estimated to be
about 80 times that of the Earth's current atmosphere.
|Trace gasses (Ar)||100%||90||<1%||1.6%|
What happened on Mars?
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