Mars has several interesting surface features. Olympus Mons, the largest mountain in the solar system, rises 24 km from the surrounding plain. Tharsis, a huge bulge on the Mountain's surface, is about 4000 km across and 10 km high. Valles Marineris, a system of canyons 4000 km long and from 2 to 7 km deep, and Hellas Planitia, an impact crater in the southern hemisphere over 6 km deep and 2000 km indiameter, are just a couple of the odd features of Mars. Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown.
The interior of Mars is believed to have a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. At present, Mars appears to lack active plate tectonics. There is no evidence of current volcanic activity. There is very clear evidence of erosion in manyplaces on Mars, including large floods and small river systems. At some time in the past there was clearly some sort of fluid on the surface. Liquid water is the obvious fluid but there are other possibilities. There may have been large lakes or even oceans at one time, but now all the water and liquid is frozen at the poles.
Mars has a very thin atmosphere composed mostly of the tiny amount ofremaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasionally engulf the entire planet for months. Mars has permanent ice caps at both poles composed mostly of solid carbon dioxide (dry ice).
Mars has two tiny satellites; Phobos and Deimos. Phobos is the larger and innermost of Mars' two moons. Phobos is closer to its primary than any other moon in the solar system,less than 6000 km above the surface of Mars. It is also one of the smallest moonsin the solar system. Hall discovered it in 1877. Phobos is doomed, for it's orbit is so close to Mars that it is gradually being pulled in and will eventually crash into the planet's surface or break up into a ring. Deimos is the smaller and outermost of Mars' two moons. It is the smallest known moon in the solar system. It was also discovered by Hall in 1877.
Aliens in science fiction often come from Mars, and many scientists are convinced that there is or was life on Mars, though most likely it consited of organisims like bacterias and other microscopic organisms. On Aug 6, 1996, David McKay announced the first identification of organic compounds in a Martian meteorite. The authors further suggest that these compounds, in conjunction with a number of other mineralogical features observed in the rock, may be evidence of ancient Martian microorganisms. This, however, has not been proven.
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