It has been proposed that viruses are likely to be
encountered on other life-bearing planets. Efforts to discover current or past
life on Mars is an active area of research. On 24 January 2014, NASA reported
that current studies on the planet Mars by the Curiosity and Opportunity rovers
will now be searching for evidence of ancient life, including a biosphere based
on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as
well as ancient water, including fluvio-lacustrine environments (plains related
to ancient rivers or lakes) that may have been habitable. The search for
evidence of habitability, taphonomy (related to fossils), and organic carbon on
the planet Mars is now a primary NASA objective.
(Wikipedia)