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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. 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 Mars 2, the first spacecraft to land on Mars and the two
Viking
landers in 1976 (left). Ending a long 20 year gap, Mars
Pathfinder landed successfully on Mars on
1997 July 4 (right).
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 -55 C, Martian surface temperatures range widely from as little as -133 C, at the winter pole to almost 27 C on the dayside during summer.
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 (right).
- 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.
The
southern hemisphere of Mars is mostly ancient cratered highlands
(left) somewhat similar to the Moon.
In contrast, most of the northern hemisphere consists of plains which
are much younger.
Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent 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 no evidence of current volcanic activity, however. But there is new evidence from Mars Global Surveyor that Mars may have had tectonic activity in its early history, making comparisons to Earth all the more interesting!
There
is very clear evidence of erosion in many places on Mars
including large floods and small river systems (right). 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 and about 4 billion years ago.
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. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.
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%).Very strong winds and vast dust storms on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.
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 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 (left) since its
carbon dioxide layer never completely disappears. There may also be
water ice hidden below the surface at lower latitudes.
The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). 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.
Exciting as this is, it is
important to note while this evidence is strong it by no means
establishes the fact of extraterrestrial life.
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.
Mars has two tiny satellites which orbit very close to the
surface.
Distance Radius Mass Satellite (000 km) (km) (kg) Discoverer Date --------- -------- ------ ------- ---------- ---- Phobos 9 11 1.08e16 Hall 1877 Deimos 23 6 1.80e15 Hall 1877
... Sun ... Earth ...
Moon ... Mars ... Phobos
... Jupiter ... 
