"Right now the challenge is that past Martian landers haven't seen any organic material whatsoever," study lead author Alexander Pavlov, of NASA's Goddard Space Flight Center in Greenbelt, Md., said in a statement. "We know that organic molecules have to be there, but we can't find any of them in the soil."
The researchers report that the chances of finding organic molecules roughly 0.8 inches (2 centimeters) below the surface are close to zero. The top layer of the Martian surface has absorbed so much cosmic radiation over the past billion years that all organic material is likely to have been destroyed, the scientists said. Past rovers on Mars collected and analyzed only loose soil from the topmost layer of the Martian surface. [7 Biggest Mars Mysteries]
Yet only inches deeper — within reach of Curiosity — simple organic molecules could still exist, the researchers said.
Even if Curiosity detected these molecules, the discovery wouldn't necessarily mean ancient life existed on Mars. Simple organic molecules could have originated from other sources, such as meteors and interplanetary dust particles, the researchers said.
Complex organic molecules, such as those made up of 10 or more carbon atoms, would be more reliable indicators of past life on the planet, since they could closely resemble building blocks of life as we know it. These structures, however, would be much harder to find, and they would have been more vulnerable to the radiation that mercilessly bombards the Red Planet.