Poles
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 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 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. 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. 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.