If there's any evidence of ancient life on Mars, it's likely to be found near the surface, new research suggests - dramatically upping the chances that it'll be discovered by the Curiosity rover, set to land next month.
Complex carbon structures aren't easy to find, as they're vulnerable to cosmic radiation. But new research by Alexander Pavlov of the NASA Goddard Space Flight Center and his colleagues gives some suggestions about where to start looking, though analysis of the amounts of radiation that rock and soil are exposed to over time, and how deep that radiation penetrates.
The scientists report that there's almost no chance of finding these molecules in the first 0.8 inches of Martian soil. That top layer, they calculate, will absorb a total of 500 million grays of cosmic radiation over the course of one billion years - capable of destroying all organic material.
Look a little further down, though, and the odds improve. Just two to four inches beneath the surface, the amount of radiation reduces tenfold, to 50 million grays. Although that's still extreme, the team believes that simple organic molecules, such as a single formaldehyde molecule, could exist at this depth - and in some places, such as young craters, the complex building blocks of life could remain as well.
"Right now, the challenge is that past Martian landers haven't seen any organic material whatsoever," says Pavlov. "We know that organic molecules have to be there but we can't find any of them in the soil."
When evaluating the chances of survival for organic molecules, previous studies have mainly focused on the maximum depth, about five feet, that cosmic radiation reaches, as beyond that point organic molecules could survive, unharmed, for billions of years. However, drilling to this depth is currently too expensive to engineer for a Martian rover.
But the new study shows that simply sticking a shovel in the dirt could bear fruit. Not only could some simple carbon-containing molecules exist within four inches of the surface, some regions may have radiation levels far lower than 50 million grays near the surface - meaning that more complex molecules like amino acids could remain intact.
In order to find these molecules within the rover's two-and-a-half-inch drilling range, the scientists found the best bet is to look at craters that are no more than 10 million years old - unlike past expeditionary sites that mainly sampled from landscapes undisturbed for billions of years.
These relatively young craters exhibit freshly exposed rock and soil that was once deeper beneath the surface - and has been exposed for a short enough time that organic molecules wouldn't have been wiped out.
"When you have a chance to drill, don't waste it on perfectly preserved [landscapes]," says Pavlov. "You want to go to fresh craters because there's probably a better chance to detect complex organic molecules. Let Nature work for you."