In order to understand if life ever existed on Mars we need to not only look for evidence of possible ancient Martian organisms but also try and understand if there were ever environments on ancient Mars that would have been habitable for life. If Mars had existed as an extremely dry cold desert for its entire history then the chance of finding any evidence of living organisms would be extremely low. Fortunately exploration of the red planet has revealed that ancient Mars was dramatically different from the present day with evidence of flowing water and lakebeds forming in neutral benign waters.
The next step is to see if the chemistry that supports living organisms also existed. One of the most important elements for life is nitrogen as it is part of the structure of DNA and proteins. Nitrogen exists in the atmosphere in the chemical form N2, a molecule that has an extremely strong chemical bond that has to be broken before living organisms can incorporate it into biomolecules. This process is known as fixation and on Earth it is done primarily by nitrogen fixing organisms. It can also occur during volcanism, asteroid and comet impacts, or through lightning strikes. N2 is transformed into molecules such as ammonia (NH3) and nitrate (NO3).
Nitrate has been detected in Martian meteorites but until now it had never been detected directly on the surface. Ammonia is rapidly destroyed by ultraviolet radiation so it is highly unlikely any has been preserved on Mars, however nitrate is stable. At present nitrogen makes up about 2% of the Martian atmosphere but we know that early Mars’ atmosphere would have contained more. Nitrogen has stable isotopes known as nitrogen 14 and nitrogen 15. On Mars nitrogen 14 is depleted relative to nitrogen 15, this is because 14 is a lighter isotope that was lost to space more readily as the Martian atmosphere was stripped away by cosmic winds.
If nitrate is found on Mars then it is conclusive evidence that nitrogen fixation was taking place (remember that this can be a biological or non-biological process) and that a form of nitrogen that could be utilised by life existed on the planet. It is therefore very exciting news that Curiosity rover has detected molecules containing oxygen and nitrogen on Mars (you can read the new paper here). The rover studied materials from three sites; Rocknest, a wind-blown deposit, and two fine grained samples from a rock type known as mudstone, named as John Klein and Cumberland. The samples were heated and the gases that came off were analysed by a mass spectrometer. A mass spectrometer can measure the mass of the compounds present in the gas released and identify what they are. For light gases this can be quite tricky as many different species form compounds of similar mass but fortunately the compound NO can be distinguished. NO forms when nitrate breaks down during heating and it was present in all three samples. The amount of NO detected suggests values of 110-300 ppm nitrate in Rocknest, 70-260 ppm in John Klein and 330-1100 ppm in Cumberland. Chemical agents used to process samples within the rover could contaminate samples with nitrogen containing compounds but even the worst case scenario could only explain around a third of the NO measured in the samples.
The values for nitrate seen are consistent with models that suggest formation of nitrate during impacts early in Mars’ history, which created an ancient nitrate reservoir. The persistence of nitrate to the modern day is possibly not entirely good news. On Earth denitrifying organisms turn fixed nitrogen back into atmospheric N2. The fact that nitrate persisted in the Martian soil suggests that a nitrogen cycle comparable to that seen on Earth was not established on Mars. Simple early Martian life may well still have used this nitrate though. Another key question about the habitability of Mars has been answered, and answered positively. The search for life continues!