Even if there is no life present on Mars, that planet should have organic molecules on its surface, due to impact delivery by asteroids and comets, which are known to contain organic carbon-bearing molecules. However, organics have not been detected by any landers sent to look for these materials---not by the Viking landers, nor by the more recent Phoenix lander. Organics are detected by heating soil samples in an onboard oven, and looking for their signature in the gases released during heating.
However, Douglas Ming (Johnson Space Center) and colleagues note that the Martian surface contains perchlorate salt, which releases oxygen when heated. Ming's experiments also show that this released oxygen might then burn up any organics present in the soil samples. So if Ming's thinking is correct, this might explain why the Martian landers have not detected organics on Mars---their detection methods might have been burning up the organics that they seek. For more information, see this New Scientist article, as well as Ming's LPSC abstract.
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