Facts about Mars....
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%). 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.
South
Polar Cap:
Early telescopic observations revealed that Mars has permanent
ice caps at both poles; they're visible even with a small
telescope. We now know that they're composed of water ice and
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. ESA's Mars Express has shown that a similar layer of water
ice exists below the southern cap as well. 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).
Mars
by HST: 
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 (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.
On
1996 Aug 6, David McKay announced what they thought might
be evidence of ancient Martian microorganisms in the meteorite
ALH84001. Though there is still some controversy, the majority
of the scientific community has not accepted this conclusion. If
there is or was life on Mars, we still haven't found it.
Large,
but not global, weak magnetic fields exist in various regions of
Mars. This unexpected finding 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' 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. Mars is a difficult but rewarding target for an
amateur telescope though only for the three or four months each
Martian year when it is closest to Earth. Its apparent size and
brightness varies greatly according to its relative position to
the Earth. There are several Web sites that show the current
position of Mars (and the other planets) in the sky. More
detailed and customized charts can be created with a planetarium
program.
Mars'
Satellites
Mars has two tiny satellites which orbit very close to the
Martian 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
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