Fixed nitrogen: Mars was once habitable

curiosity rover

Curiosity has found evidence of indigenous nitrogen in the form of nitrates in aeolian deposits. – Photo: NASA

The Mars rover Curiosity has for the first time found evidence of indigenous nitrogen in the form of nitrate in aeolian deposits and in two mudstone deposits on the red planet. This discovery has great implications for habitability and, “specifically for the potential evolution of a nitrogen cycle at some point in Martian history.”

The results were published a few days ago in the journal Proceedings of the National Academy of Sciences (PNAS).

At first, Curiosity found indirect evidence of water that was once present on Mars, and then found true indicators of water that existed as rivers and lakes. The evidences unequivocally showed the presence of fresh water that was neither acidic nor salty.

In December last year, Curiosity detected wafts of methane in the Martian air. On Earth, methane is largely produced by living organisms. The detections indicated that the gas is present at about 1 part per billion in the Martian atmosphere, or 4,000 times less than in Earth’s.

The detection of nitrate in aeolian samples and two mudstone samples drilled from a relict lakebed suggests “widespread atmospheric deposition” of nitrogen gas.

The detection of nitrate in samples of different kinds (aeolian deposits and mudstone) is quite likely due to nitrogen fixation to nitrate as a result of thermal shock either from impact or volcanic plume lightning. Immaterial of the route in which the nitrogen fixation had taken place, the very presence of fixed nitrogen would have facilitated the development of “primitive nitrogen cycle” on the Martian surface. In turn, this would have provided a “biochemically accessible source of nitrogen.”

Much like water, nitrogen is essential for life. After all, it forms the building blocks of larger molecules like DNA, RNA and protein. But nitrogen has to be fixed for it to take part in chemical reactions essential for creation of life. Both on Earth and Mars, atmospheric nitrogen is in the form of nitrogen gas (N2) where two nitrogen atoms combine and do not easily react with other molecules.

Earlier studies have found nitrogen gas to constitute around 2 per cent of the Martian atmosphere.

Curiosity detected the bulk of nitrogen in the form of nitric oxide. According to the scientists, the nitric oxide quite likely indicates a mineralogical sink for atmospheric nitrogen gas before being fixed as nitrate (NO3). The estimated abundance of nitrates in the aeolian sample is about 110-300 ppm and vary in the two mudstone samples — 70-260 ppm and 330-1,100 ppm.

Nitrogen has to be fixed to become biochemically available. On Earth, certain organisms carry out this process of fixing atmospheric nitrogen. On Mars, lightning and/or thermal shock would have played a role.

The scientists state that the presence of fixed nitrate suggests that the first half of the Martian cycle was established at some point. But in the absence of near-surface life in Mars after some 3.5 billion years ago, the cycle of releasing the fixed nitrogen to atmosphere did not take place. As a result, the fixed nitrogen accumulated as nitrate in the rocks. On Earth, nitrogen returns to the atmosphere by de-nitrification via biological activity.

In the Atacama Desert, organisms have little role to play in nitrogen fixation; the fixation takes place through an abiotic process. Since the hyperarid climate is not quite conducive for biological activity, the nitrogen fixed as nitrates is not lost to the atmosphere through the de-nitrification process.

Published in The Hindu on March 26, 2015