The Viking landers found conditions on the surface of Mars unlikely to
support life as we know it.  The mass spectrometer found too little
carbon, which is the basis for organic molecules.  The chemistry is
apparently highly oxidizing as well.  Some optimists have nevertheless
argued that there still might be life on Mars, either below the
surface or in surface regions not sampled by the landers, but most
scientists consider life on Mars quite unlikely.  Evidence of surface
water suggests, however, that Mars had a wetter and possibly warmer
climate in the past, and life might have existed then.  If so, there
might still be remnants (either living or fossil) today, but close
examination will be necessary to find out.

More recently, McKay et al. have invoked biological activity to
explain a number of features detected in a meteorite from Mars.  See
<URL:http://www.fas.org/mars/> for additional information.

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