The idea that life can or even does exist on Mars has a long
history. In 1877 the Italian astronomer Giovanni Schiaparelli
claimed to have seen a planetwide system of channels. The
American astronomer Percival Lowell then popularized these faint
lines as canals and held them out as proof of a vast attempt by
intelligent beings to irrigate an arid planet. Subsequent
spacecraft observations have shown that there are no canals on
the planet, and various other alleged proofs of life on Mars have
turned out to be equally illusory. Not only are there no canals,
but dark areas once thought to be oases are not green, and their
spectra contain no evidence of organic materials. The seasonal
changes in the appearance of these areas are not due to any
vegetative cycle, but to seasonal Martian winds blowing sterile
sand and dust. Water probably occurs only as ice on or below the
surface or as trace amounts of vapor or ice crystals in the
atmosphere. The strongest evidence against the presence of life,
however, is the thinness of the atmosphere and the fact that the
surface of the planet is exposed not only to lethal doses of
ultraviolet radiationbut also to the chemical effects of highly
oxidizing substances (such as hydrogen peroxide) produced by
photochemistry.Perhaps the most fundamental and far-reaching
result obtained by the Viking landers is that the soil contains
no organic material (there is no reason to assume that the two
landing sites are not representative of Mars). Although small
amounts of organic molecules are continually being supplied to
the surface of Mars by carbonaceous meteoroids, apparently this
material is destroyed before it has a chance to accumulate. The
results of the soil analysis for organic molecules carried out by
the Viking landers provide no evidence for the existence of life.
A more difficult question is whether life ever existed on Mars,
given the strong evidence of climatic change and the indications
of a previously warmer, thicker atmosphere. Answering this
question will probably involve collecting carefully selected
subsurface samples and returning them to earth for detailed
analysis. U.S. National Aeronautics and Space Administration has
proposed a manned voyage to Mars early in the 21st century.
