Even with the superb instrument array on the Curiosity rover (above), we are still left looking for evidence of life on Mars. Photo / AP
Researchers announced in 1996 to a fanfare of publicity they had evidence for past life on Mars. They claimed to have discovered a fossilised micro-organism in a Martian meteorite which they argued was evidence there had once been life on the Red Planet. Sadly, most scientists dismissed their claim.
While we know Mars was habitable in the past, the case demonstrates just how hard it will be to ever prove the existence of past life on its surface. But now results from Nasa's Curiosity rover, including the discovery of ancient organic material, have revived the hope, though, understandably, the authors of the two papers published in Science are very careful not to make the claim they have discovered life on Mars.
While the 1996 discovery has never been verified, it hasn't been conclusively disproved either. Curiosity is the latest rover to trundle across Mars' gritty sands. It has been tacking across the floor of Gale Crater for five years, returning stunning images of landscapes, with rocky outcrops seamed with mineral veins. Close up, the veins have the appearance and chemistry of material that has been produced by reaction of water with the rocks, at a time when water was stable at the surface for extended periods. Such reactions could create enough energy to feed microbial life.
One of the papers reports the discovery of low levels of organic carbon in mudstones from Gale Crater. This may not sound like much carbon, but finding it at all is a big deal, since organic material could be traces of decayed living matter.
The sediments come from just below the surface, where they have been shielded from most of the UV radiation that would break down organic molecules exposed on the surface. The organic material is rich in sulphur, which would have helped to preserve it.
However, the environment in which the mudstones were deposited - a 3.5-billion-year-old lakebed - would have been altered in other ways as the sediments settled and compressed to become rock. Over the intervening years, fluid flowing thought it would have initiated chemical reactions that could have destroyed the organic matter - the material discovered may, in fact, be fragments from bigger molecules. In rocks on Earth, such reactions, which cause living matter to degrade, produce an insoluble material known as kerogen.
Excitingly, the material on Mars is similar to terrestrial kerogen. But that doesn't necessarily mean it is biological in origin - it is also similar to an insoluble material in tiny meteorites that rain down on the planet's surface.
At this point, we simply don't know if the origin is biological or geological. But it is the preservation of the material that is important - if there is this much organic matter preserved close to the surface, there should be even better protected material at greater depths. What is needed to find more clues is a mission to Mars with a deep drill. Luckily there is one: the European Space Agency's ExoMars rover, scheduled for launch in two years.
The second paper investigates a problem that has disturbed scientists for several years: the abundance of methane in Mars' atmosphere. Earth-based telescopes, orbiting spacecraft and now Curiosity have measured episodic sudden increases in background methane content.
While this may be taken as a signature of biological activity - the main producers of methane on Earth are termites and cattle - non-biological mechanisms, such as weathering of Martian rocks or release from ancient ice, are possible too.
The new results represent the longest systematic record of atmospheric methane, with regular measurements over five years. What the authors have found is a systematic seasonal variation, with the highest concentrations of methane at the Gale Crater towards the end of the northern summer. This is when the southern icecap - which freezes CO2 out of the atmosphere, but not methane - is at its biggest, so enhanced methane is not unexpected. However, the abundances of methane measured are greater than models predict should occur, meaning we still don't know exactly how they are produced.
The team also found several spikes where methane abundance suddenly jumped to be higher than average during the year. The authors conclude that this must be related to surface temperature. They therefore suggest methane could be trapped at depth, gradually seeping to the surface. Here it is retained by the soil until the temperature increases enough to release the gas.
However the authors state that, despite this, there "remain unknown atmospheric or surface processes occurring in present-day Mars". While they do not specify biology as one of those processes, it remains an intriguing possibility.
This, to me, is a cue for further measurements - and fortunately we may know soon. ESA's Trace Gas Orbiter is now in place at Mars, and has started recording data.
So, what can we conclude from these two papers? That even with the superb instrument array on Curiosity, we are still left looking for evidence of life on Mars. Is it a romantic yearning to discover we do have companions in the solar system? Whatever the reason, there is still much to be discovered.
• Monica Grady, Professor of Planetary and Space Sciences, The Open University
- This article was originally published on The Conversation. Read the original article.