(The Red Planet)

HEMISPHERE             ATMOSPHERE              PICTURES     

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 hiatus, 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 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.

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.

HEMISPHERE

The southern hemisphere of Mars is predominantly ancient cratered highlands (left) 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. 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 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 very long ago; the
age of the erosion channels is estimated at about nearly 4 billion years. (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.)

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.

ATMOSPHERE

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. 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 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 (left) 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 (right) 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. ( more details from STScI)

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.

FIRST ORGANIC COMPOUNDS

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.
(left?)
Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact
of extraterrestrial life. There have also bee several contradictory studies published since the McKay
paper. Remember, "extraordinary claims require extraordinary evidence." Much work remains to be
done before we can be confident of this most extraordinary claim.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars
Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since
disappeared. This may have important implications for the structure of Mars's interior and for the past history of its atmosphere
and hence for the possibility of ancient life.

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

PICTURES