Rover detects ancient organic material on Mars – and it could be trace of past life

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The Curiosity rover on Mars has been busy.
NASA/JPL-Caltech/MSSS

By Monica Grady, The Open University

It was to a great fanfare of publicity that researchers announced they had found evidence for past life on Mars in 1996. What they claimed they had discovered was a fossilised micro-organism in a Martian meteorite, which they argued was evidence that there has once been life on the Red Planet. Sadly, most scientists dismissed this claim in the decade that followed – finding other explanations for the rock’s formation.

While we know that 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 new results from NASA’s Curiosity rover, including the discovery of ancient organic material, have revived the hope of doing just that. Understandably, the authors of the two papers, published in the journal Science, are very careful not to make the claim that they have discovered life on Mars.

While the 1996 discovery has never been verified, it hasn’t ever been conclusively disproved either. What the study has done, though, is to propel the search for life on Mars higher up the list of international space exploration priorities – giving space agencies ammunition to argue for a coordinated programme of missions to explore the Red Planet.

Mineral veins on Mars seen by Curiosity.
NASA/JPL-Caltech/MSSS

Curiosity is the latest rover to trundle across the gritty sands of Mars. It has been tacking across the floor of Gale Crater on Mars for five years, returning stunning images of Martian landscapes, with vistas opening up to show 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 of time. Such reactions could create enough energy to feed microbial life.

Ancient rocks

One of the papers reports the discovery of low levels of organic carbon in mudstones from Gale Crater. This might 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, analysed by the SAM instrument on Curiosity, 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 discovered on Mars is rich in sulphur, which would have also helped to preserve it.

However, the environment in which the mudstones were deposited – a 3.5-billion-year-old lake bed – 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 causes living matter mainly from plants and microbes to degrade – produce an insoluble material known as kerogen.

Excitingly, the material discovered 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 surface of Mars.

At this point, we simply don’t know whether 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, then 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: ESA’s ExoMars rover, scheduled for launch in two years’ time.

Mysterious methane

The second paper investigates a problem that has been disturbing Mars scientists for several years: the abundance of methane in Mars’ atmosphere. Earth-based telescopes, spacecraft orbiting Mars and now Curiosity, have measured episodic sudden increases in the background methane content.

While this might be taken as a signature of biological activity – the main producers of methane on Earth are termites and bovine gut bacteria – non-biological mechanisms, such as weathering of Martian rocks or release from ancient ice, are possible too.

Gale crater on Mars.
NASA/JPL-Caltech/ASU/UA

The new results represent the longest systematic record of atmospheric methane, with measurements taken regularly over five years. What the authors have found is a systematic variation in methane concentration with season, with the highest concentrations occurring at the Gale Crater towards the end of the northern summer. This is the period when the southern icecap – which freezes carbon dioxide 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 that methane could be trapped at depth, gradually seeping to the surface. Here it is retained by the soil until the temperature increases sufficiently to release the gas.

However the paper states that, despite this, there “remain unknown atmospheric or surface processes occurring in present-day Mars”. While the authors do not specify biology as one of those unknown 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 just started recording data.

So, what can we conclude after reading these two papers? That even with the superb instrument array carried by Curiosity, and detailed modelling and interpretation of the results, we are still left looking for evidence of life on Mars. Is it a romantic yearning to discover that we do have companions within the solar system (even if they are likely to be very small and uncommunicative)? Or is it that our theories of how life arose on Earth cry out to be verified by a “second genesis”?

The ConversationWhatever the reason, there is still much to be discovered on Mars. Luckily, a series of missions planned well into the next decade will help us make those discoveries. These include the return of Martian samples to Earth, where we can carry out even more detailed analyses than Curiosity.

Monica Grady, Professor of Planetary and Space Sciences, The Open University

This article was originally published on The Conversation. Read the original article.

Our rover could discover life on Mars – here’s what it would take to prove it

 

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Mars seen by Viking.
NASA / USGS

Claire Cousins, University of St Andrews

Finding past or present microbial life on Mars would without doubt be one of the greatest scientific discoveries of all time. And in just two years’ time, there’s a big opportunity to do so, with two rovers launching there to look for signs of life – Mars2020 by NASA and ExoMars by the European Space Agency and Roscosmos.

I am helping to develop one of the instruments for the ExoMars rover, which will be Europe’s first attempt to land a mobile platform on the red planet. It will also be the first rover to drill into the martian crust to a depth of two metres.

But the rover will not be the first to look for evidence of life. The Viking landers sent by NASA in the 1970s carried experiments designed to so. They were ultimately unsuccessful, but provided a wealth of information about Mars’ geology and atmosphere that comes in handy now. In fact, exploration over the last half-century has shown us that early Mars was once a dynamic and potentially habitable planet.

ExoMars prototype rover.
Mike Peel/wikipedia, CC BY-SA

While it is not completely impossible that life could exist on Mars today, ExoMars is primarily focused on looking for extinct life. Because there’s a risk it could contaminate the planet with microbes from Earth, it is not allowed to go near the sites where we think it’s possible that microbes could exist today.

Chemofossils are the best bet

On Earth, life constantly unfurls around us, leaving its mark on our planet every day. There are, however, a number of factors to contend with when looking for life on Mars. The first is that the lifeforms we are looking for are single-celled microorganisms, invisible to the naked eye. This is because life on Mars is unlikely to have progressed any further down the evolutionary path. This is actually not so strange – Earth itself was a world of single-celled life for two billion years or more.

Another issue is that the life we’re looking for would have existed three or four billion years ago. A lot can happen in that time – rocks preserving this evidence can be eroded away and redeposited, or buried deep beyond reach. Luckily, Mars does not have plate tectonics – the constant shifting about and recycling of the crust that we have on Earth – which means it’s a geological time capsule.

Because we are looking for evidence of long-dead microorganisms, the hunt for bio-signatures lies in the detection and identification of organic “chemofossils” – compounds that are left behind by the decomposition of life. These are different to organic compounds delivered to planets on the backs of meteorites, or those, such as methane, that can be produced by both geological and biological processes. No single compound will prove life once existed.

Rather, it will be distinctive patterns present in any organic compounds discovered that betray their biological origin. Lipids and amino acids, for example, are fundamental components to living things, but are also found in certain meteorites. The difference lies in finding evidence that shows a process of selection. Lipids left behind by degraded cell membranes will likely have a limited size range, and comprise an even number of carbons. Similarly, amino acids naturally exist in both left-handed and right-handed forms (like gloves), but for some reason life only uses the left-handed ones.

It is also possible for microorganisms to produce visible fossils in the rock record. When conditions allow, microbial mats (multilayered communities of microorganisms) can become interspersed with fine sediment, producing characteristic morphological structures in rocks that form subsequently. However, the specific environmental conditions required for this mean such deposits are unlikely to be discovered by a rover exploring just one small region of a whole planet.

Microbial mat on Earth.
Alicejmichel/wikipedia, CC BY-SA

So, the best bet will be looking for organic compounds, a task which falls to the Mars Organic Molecule Analyser (MOMA) – the largest instrument in the ExoMars rover payload.

One intriguing finding from the Viking landers was the absence of detectable organic compounds at the martian surface. This was unexpected – many organic compounds are found throughout the solar system that do not form through biological activity. Subsequent missions revealed that a combination of harsh chemistry and intense radiation effectively remove much of the organic material from the surface of Mars, regardless of its origin.

But more recently NASA’s Curiosity rover has begun to find some simple organic compounds, hinting at what may lie beneath. By analysing samples brought up from below the surface, MOMA will have a better shot at finding those organic biosignatures that have survived the ravages of time.

Confusing contamination

Before any search for biosignatures even begins, however, ExoMars will first need to find the right rocks. The landing sites shortlisted for the mission have, in part, been chosen based on their geological characteristics, including their age (more than 3.6 billion years old).

Panorama of Mars taken by the Opportunity rover.
NASA/JPL-Caltech/Malin Space Science Systems

If MOMA identifies organic molecules within the samples brought up by the drill, one of the first things will be to establish whether they are the result of contamination by any rogue Earth-based organics. While ExoMars is looking for alien life, it is designed to look for life that is based on the same fundamental chemistry as life on Earth. On one hand, this means highly sensitive instruments like MOMA can be designed that target biosignatures that we have a good understanding of, and therefore increase the likelihood that ExoMars will be a success.

The downside is that these instruments are also sensitive to life and organic molecules on Earth. To ensure terrestrial organic or microbiological stowaways are minimised, the rover and its instruments are built and assembled inside ultra clean rooms. Once on Mars, the rover will run a number of “blank” samples, which will show what, if any, contamination may be present.

Ultimately, finding strong evidence of extinct life on Mars, whether it be chemofossils or something more visible, will be just the first step. As with most scientific discoveries, it will be a gradual process, with evidence building up layer by layer until no other explanation exists. If the NASA Mars2020 rover also finds similarly tantalising evidence, then these discoveries will represent a step change in our understanding of life in general. And, while incredibly unlikely, it is of course possible that ExoMars chances upon some living martian microorganisms.

The ConversationWhether ExoMars hits the jackpot remains to be seen, but at the very least it will mark a new beginning for the search for life on Mars.

Claire Cousins, Research Fellow in Planetary Science, University of St Andrews

This article was originally published on The Conversation. Read the original article.