Two other hypotheses offer nonbiological explanations. These new molecules would have rained down to the surface and now could be preserved with their distinct carbon signature in Martian rocks. It involves ancient bacteria in the surface that would have produced a unique carbon signature as they released methane into the atmosphere where ultraviolet light would have converted that gas into larger, more complex molecules. The biological explanation Curiosity scientists present in their paper is inspired by Earth life. Their hypotheses are drawn partly from carbon signatures on Earth, but scientists warn the two planets are so different they can't make definitive conclusions based on Earth examples. In a report of their findings to be published in the Proceedings of the National Academy of Sciences journal on January 18, Curiosity scientists offer several explanations for the unusual carbon signals they detected. From a report: While the finding is intriguing, it doesn't necessarily point to ancient life on Mars, as scientists have not yet found conclusive supporting evidence of ancient or current biology there, such as sedimentary rock formations produced by ancient bacteria, or a diversity of complex organic molecules formed by life. After analyzing powdered rock samples collected from the surface of Mars by NASA's Curiosity rover, scientists today announced that several of the samples are rich in a type of carbon that on Earth is associated with biological processes.

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