There are no cows on Mars.
Of that, planetary scientists are certain, which leaves them puzzling over what could be producing methane gas detected in the thin Martian air. Methane molecules are easily blown apart by ultraviolet light from the Sun, so any methane around must have been released recently.
Could the gas be burbling from something alive? Cows, after all, burp methane on Earth. Other creatures, including a class of micro-organisms that live without oxygen, also produce methane.
The National Aeronautics and Space Administration could get some answers soon. On the launching pad at Cape Canaveral in Florida is a spacecraft, the Mars Science Laboratory, that is scheduled to lift off on Saturday and reach Mars next August. It will deliver an S.U.V.-size rover named Curiosity that carries an instrument that can detect methane in the air, and if it does, it will unleash new excitement about the prospect of life on Mars.
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