Mars Global Surveyor

The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this symbolism. ) The name of the month March derives from Mars. The first spacecraft to visit Mars was Mariner 4 in 1965.

Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures; they are still operating after more than a year on Mars. Three Mars orbiters (Mars Global Surveyor, Mars Odyssey, and Mars Express) are also currently in operation.

Mars’ orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars’ climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth. Mars has some of the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular: Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft. ) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high. Tharsis: a huge bulge on the Martian surface that 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 (top of page); Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter. Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. (None of this is visible in any detail with a telescope, even the Hubble Space Telescope; all this information comes from the spacecraft that we’ve sent to Mars. 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 (some speculate that they are due to a very large impact shortly after Mars’ accretion).

Mars Global Surveyor has produced a nice 3D map of Mars that clearly shows these features. The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth’s and a thin crust. Data from Mars Global Surveyor indicates that Mars’ crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north.

Mars’ relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide). Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth’s plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect.