Mystery Solved: Mars Has Nitrates

The Curiosity rover answers a long-standing question.

Curiosity on Mars.jpg
“Hey, look what I found!” Curiosity racks up another discovery on Mars.

Nitrogen is an extremely important building block for life, and most biomolecules, from amino acids to DNA, contain it. The Martian atmosphere includes molecular nitrogen, just as Earth’s does. However, for life to access nitrogen directly from the atmosphere is very difficult. Certain organisms on Earth, such as cyanobacteria and some fungi, have found a way to fix nitrogen directly from the atmosphere, but it costs a lot of energy and is thought to be a relatively late evolutionary invention. Thus, it is doubtful whether any putative organisms on Mars could accomplish it, since they’re already exposed to many other environmental stresses.

A much more easily available source of nitrogen is nitrate, which is abundant on Earth. Until now, the search for nitrate in Martian soil had come up empty. But Jennifer Stern from NASA and her colleagues report in Proceedings of the National Academy of Sciences that they’ve finally found the missing nitrate, using the SAM (Sample Analysis at Mars) instrument on board the Curiosity rover. Or, more accurately, they detected a breakdown product—nitric oxide—that results when nitrate is heated. The new results suggest that nitrate occurs not just locally in certain types of Martian mudstones, but also globally, because it was found in windblown sediments, with total concentrations ranging between roughly 0.1 and 1 percent.

This doesn’t mean the nitrogen was part of a biological cycle. It could also have been produced from meteorite impacts, lighting strikes, or volcanic activity. But it does mean that nitrogen exists on Mars in a form that organisms would find easier to use.

The importance of this finding is evident in the title of a paper published a few years ago by Rocco Mancinelli and Amos Banin, who asked: Where is the nitrogen on Mars? They suggested that Martian soils should contain nitrate salts, which makes sense because Mars is even dryer than the Atacama desert in Chile, the only place on Earth where we find nitrate salts. Some scientists even suggested that NASA should look for nitrogen, rather than water or organics, as evidence of life in Martian soils. But finding nitrate on Mars had proven to be very difficult. Instruments on the 2008 Phoenix lander could not distinguish between nitrates and perchlorates, and the much larger signal from perchlorates—which was unexpected—masked any possible signal from the nitrates.

Curiosity used a different method to identify nitrates, and now we know that a critical building block for life, nitrogen, is present in a readily available form on Mars. Another piece of the puzzle falls into place.

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