I recently speculated that spacecraft both orbiting and sitting upon Mars may have already detected signs of life. In particular, some spacecraft have detected signs of methane:
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In 2004 the European Space Agency probe Mars Express detected the presence of methane in the atmosphere of Mars. Methane can be produced geologically (and Mars is not short on volcanoes), or biologically. (Though media reports of that observation got a bit out of hand.) Either way, this is an important observation and research on the source of this methane is still ongoing.