Mars is the fourth planet from the Sun and the seventh largest:

Orbit:	227,940,000 km (1.52 AU) from Sun
Diameter:	6,794 km
Mass:	6.4219e23 kg

Mars (Greek: Ares) is the god of War. 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.

Mars has been known since prehistoric times. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including the two Viking landers in 1976. Ending a long 20 year hiatus, three new spacecraft will arrive at Mars in 1997. 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. Overall, the Viking landers found that Martian temperatures vary from 150 K (-220 F) to 295 K (70 F).

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.

Except for Earth, Mars has 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.

- 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. 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). Recently, some scientists have begun to question whether the abrupt elevation is real in the first place. Mars Global Surveyor should resolve the issue. 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. The lack of a global magnetic field indicates that Mars' core is probably solid. 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).

Like Mercury and the Moon, Mars appears to lack active plate tectonics; there is no evidence of horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes.

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly water on the surface There may have been large lakes or even oceans. But it seems that this occurred only briefly. (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.)

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining 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 occasion engulf the entire planet for months. Although its atmosphere is mostly carbon dioxide (like Venus'), the greenhouse effect on Mars is strong enough to raise the surface temperature by only 5 degrees (K).

Mars has permanent ice caps at both poles composed mostly of solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. It's not known if a similar layer of water ice exists below the southern cap since its carbon dioxide layer never completely disappears. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers.

The Viking landers performed experiments to determine the existence of life on Mars. The results were negative. Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars. A small number of meteorites (the SNC meteorites) are believed to have originated on Mars.

On 1996 Aug 6, David McKay et al 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.

Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact of extraterrestrial life. Remember, "extraordinary claims require extraordinary evidence." Much work remains to be done before we can be confident of this most extraordinary claim.

Mars has no global magnetic field.

When it is in the nighttime sky, Mars is easily visible with the naked eye. Its apparent brightness varies greatly according to its relative position to the Earth.

Planet Profile

Mass: 6.42 x 10^23 km
Diameter: 6787 km
Mean density: 3940 kg/m^3
Escape velocity: 5000 m/s
Average distance from Sun: 1.524 AU
Rotation period (length of day in Earth days): 1.026 Days
Revolution period (length of year in Earth days): 686.98 Days
Obliquity (tilt of axis in degrees): 25
Orbit inclination (degrees): 1.85
Orbit eccentricity (deviation from circular): 0.093
Maximum surface temperature: 310 K
Minimum surface temperature: 150 K
Visual geometric albedo (reflectivity): 0.15
Highest point on surface: Olympus Mons 24 km above surrounding lava plains

Atmospheric components
95% carbon dioxide
3% nitrogen
1.6% argon

Surface materials
basaltic rock and altered materials