Earth aside, Mars is the most-studied planet in the solar system. Over the past half century a fleet of probes has photographed it, prodded it, sniffed it and blasted it with lasers. Most of the data those probes have gathered suggest Mars is biologically and geologically dead. Most, but not all. Over the past decade or more, several groups of scientists claim to have detected methane in the planet's atmosphere, using observations both from orbiting spacecraft such as Mars Express, a European probe, and from Earth-bound telescopes. Some have reported that the methane comes and goes in vast plumes, which cover much of Mars's surface.
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