The case for life on Mars

摘要

There have been several attempts to answer the question of whether there is, or has ever been, life on Mars. The boldest attempt was the only ever life detection experiment conducted on another planet: the Viking mission. The mission was a great success, but it failed to provide a clear answer to the question of life on Mars. More than 30 years after the Viking mission our understanding of the history and evolution of Mars has increased vastly to reveal a wetter Martian past and the occurrence of diverse environments that could have supported microbial life similar to that on Earth for extended periods of time. The discovery of Terran extremophilic microorganisms, adapted to environments previously though to be prohibitive for life, has greatly expanded the limits of habitability in our Solar System, and has opened new avenues for the search of life on Mars. Remnants of a possible early biosphere may be found in the Martian meteorite ALH84001. This claim is based on a collection of facts and observations consistent with biogenic origins, but individual links in the collective chain of evidence remain controversial. Recent evidence for contemporary liquid water on Mars and the detection of methane in the Martian atmosphere further enhance the case for life on Mars. We argue that, given the cumulative evidence provided, life has and is likely to exist on Mars, and we have already found evidence of it. However, to obtain a compelling certainty a new mission is needed, one which is devoted to the detection of life on Mars.