methane levels mars may help determine likelihood life mars ref name pnas ref ref name space ref according scientists low h sub sub ch sub sub ratios less approximately indicate life likely present active ref name pnas scientists recently reported methods detecting hydrogen methane extraterrestrial atmospheres ref name nature ref ref name wired ref complex organic compounds life including uracil cytosine thymine formed laboratory outer space conditions using starting chemicals pyrimidine found meteorites pyrimidine like polycyclic aromatic hydrocarbons pahs carbon rich chemical found universe ref name nasa ref rare earth
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