Mars's life-friendly past just got friendlier. Using samples previously collected by the NASA rover Curiosity, scientists have discovered evidence of nitrates in Martian rock: nitrogen compounds that on Earth are a crucial source of nutrients for living things.

The findings, published in the journal Proceedings of the National Academy of Sciences, lend further support to the idea that the Red Planet, now barren and dry, could once have hosted habitable environments.

Although planetary scientists have been on the hunt for organic carbon the type of carbon-containing molecules that could be used and produced by living things nitrogen also plays an essential role in life as we know it, said lead author Jennifer Stern, a planetary geochemist at NASA Goddard Space Flight Center in Greenbelt, Md.

For example, nitrogen is a key component of nucleobases that make up RNA and DNA, and of amino acids, the building blocks of proteins.

People want to follow the carbon, but in many ways nitrogen is just as important a nutrient for life, said Stern, a science team member for the Mars Science Laboratory mission, as Curiositys mission is formally known. Life runs on nitrogen as much as it runs on carbon.

So the scientists examined data from three samples processed by the Sample Analysis at Mars, or SAM, instrument, which is part of a formidable laboratory in Curiositys belly. They looked at samples pulled from three spots near its landing site: aeolian deposits from Rocknest and mudstone deposits from John Klein and Cumberland.

These sites were all visited during a detour from Curiositys main mission, which was to drive to Mt. Sharp, the 3-mile-high mound in the middle of Gale Crater whose clay-rich layers looked like an ideal spot to search for signs of past habitable environments. Going off course was a risk, but it paid off; the John Klein and Cumberland mudstones have previously turned up a smorgasbord of chemicals and water-altered minerals that would have made it a potentially prime place for life, if it ever existed on the Red Planet. Now, this fresh analysis of the nitrogen compounds in these rocks further strengthens that idea.

The rock samples were cooked in SAMs oven and the resulting gases were analyzed. The researchers found a significant amount of nitric oxide, a compound that, before it was cooked, probably came from nitrates, Stern said.

What were detecting is nitric oxide, but we know from lab experiments that when we heat up nitrates, they break down in a predictable way, Stern said. And thats why we think these are nitrates.

The researchers had to carefully subtract out the amount of contamination that would be coming from the rover itself, to make sure they were not getting a false signal.

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NASA's Curiosity rover finds fresh signs of ingredients for life on Mars