Life on Mars is possible but unlikely, with no evidence found by Viking and Phoenix landers. Methane in the Martian atmosphere suggests the presence of underground methanogenic bacteria, but the search for life on Mars remains uncertain.
Of all the planets in the solar system other than Earth, life on Mars seems the most likely, although the chance is small, probably less than 1%. Astronomers have speculated about this possibility ever since they saw “channels” on Mars with the first telescopes, as well as glimpsing the presence of ice caps. The flurry of excitement that the “canals” of Mars generated upon their discovery gave birth to the popular culture notion of Martians. Eventually it became obvious that these channels were natural formations, but speculation about life on Mars continued nonetheless.
The direct search for life on Mars began in the 1970s when Viking landers were sent there, with their primary mission being analysis of the Martian soil for microorganisms. They found none, including no organic molecules. A more recent lander, Phoenix, arrived at Mars in 2008 with more sensitive equipment and repeated tests, finding nothing. Extensive testing will likely have to wait until human visitation and long-term colonization of Mars, which may not happen before 2050 or more.
While no surface life on Mars was detected in these missions, these results are not definitive. Some scientists have used similar techniques to test for life in Antarctica’s McMurdo Dry Valleys, which are known to possess microorganisms, and come back negative, suggesting that different approaches will be needed to fully verify the Viking and Phoenix findings. Regardless, these tests would have missed life beneath the surface. In 2007, it was calculated that life based on DNA or RNA would not be able to survive closer than 7.5 meters (25 feet) below the surface due to the high influx of cosmic rays. Therefore, the most probable location of life on Mars is still unchecked.
The most definitive evidence we currently have that there may indeed be Martian microorganisms beneath the surface is the observation of methane in the Martian atmosphere. On Earth, methane almost always comes from a biological source, but other processes can create methane, usually volcanism or hydrothermal activity. Some other rare non-biological processes can create methane, such as serpentinization. The presence of methane in the Martian atmosphere indicates that something is continuously producing it, as methane breaks down rapidly in the Martian sun. Mars’ lack of volcanism or hydrothermal geysers makes it unlikely that the methane was generated this way. So, it appears that the source is some rare non-biological process or underground methanogenic bacteria.
Ultimately, the answer is that we are still uncertain.
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