AKA Is There Life On Mars In Dorset?
Gale Crater, Ancient Mars/ St Oswolds Bay, Dorset; the similarities are remarkable....
So despite the global pandemic shutting down the lab and having to spend weeks in a Singaporean quarantine centre with coronavirus himself, Jonny has managed to get the lab’s only 2020 paper accepted. It’ll be a few weeks until the final version is online but an open access pre-proofed version of the accepted manuscript can be found here (link). As it’s a quite a long paper and we’re all busy trying to survive the end times, we’ve tried to write a accessible summary here to get the main points across….
Around 4-3.5 billion years ago (the Late Noachian to Hesperian periods) the surface of Mars was a much more habitable place than it is today. Increased volcanic activity and a protective active magnetic field maintained Mar's atmosphere. This provided a global warming effect, allowing liquid water to be stable on the martian surface (at least some of the time). Rivers flowed, valleys were formed, and lakes were filled. This is around the same time that life evolved on Earth, and if it also evolved on Mars, or hitched a ride there from Earth via meteorite, it may well have flourished under these conditions. Therefore, our best chance to find evidence of ancient martian life will be in the sediments deposited in this period.
As well as providing an atmosphere, those volcanoes injected large volumes of sulphur dioxide (SO2) into the atmosphere and made the waters quite acidic. This encouraged the deposition of sedimentary sulphate minerals and iron oxides. On Earth, acidic groundwaters containing dissolved sulphates bubble up in a few places to produce sulphur streams and precipitate sulphate salts, such as jarosite, alongside iron oxides, such as haematite and goethite. These streams provide handy analogues for ancient habitable martian environments, especially as some can be found as nearby as Dorset!
In a previous paper (link), Jonny already established that organic biosignature molecules (the ‘fingerprints’ of life) are concentrated within the iron-rich phases of the sulphur stream environment and that they may be detected by techniques similar to the capabilities of current and future Mars Rover missions.
The new work takes this a step further, to see what would be detectable after the nearly 4 billion years that have passed since this most habitable period of Mars. During that time the sediments will have been buried, heated and subsequently uncovered; any sediments that have not been buried on Mars will have had all their interesting organic molecules destroyed by cosmic and solar radiation so there's no point looking in those. We know that the sediments at Gale Crater that Curiosity has been poking at were at one point buried to at least 1.2 km depth. Increased pressure and temperature, when coupled with potentially reactive mineral surfaces, may destroy or at least alter, the evidence of life (organic biosignature molecules) we are searching for.
Sediment samples were collected from 2 sulphur streams in Dorset, at St. Oswald’s Bay and Stair Hole. These sites are known to be inhabited by weird extremophile microbial life, including acidophilic (acid loving) algae and microbial mats of phototrophic purple sulphur bacteria (they use sunlight to make energy out of sulphur), which thrive in these harsh conditions.
The sulphur stream, green acidophillic algae and purple sulphur bacteria are easy to spot so should be easy to detect their 'biosignatures'
After being freeze-dried, these samples were artificially matured using hydrous pyrolysis. In this technique millions to billions of years of burial and low temperature heating can be replicated within a few days by using higher temperatures to speed up the reactions (Arrhenius equation). After this any surviving soluble organic matter was extracted with solvents, as in reality this would be lost due to the actions of percolating fluids through the sediment, so we only want to look at the insoluble fraction left behind. To see what effects the sulphates and iron oxides in the sediments had on the detectability of the organic material during the analysis step these were removed using strong acid and alkali washes to dissolve them away for half the samples.
'Bomblets' and pressure vessel 'bomb' used for hydrous pyrolysis, billions of years of burial in one weekend!
After all this preparation, the samples were analysed by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). This technique is similar to the main way that all Mars missions have looked for organic matter in martian sediments/rocks. By heating up the sample organic matter is liberate and volatilised into fragments that can be separated and identified (more details here). This technique, whilst being the simplest way of detecting insoluble macromolecular material (of the type we may expect to be left behind after billions of years of burial) the heating encourages reactions between the organic matter and reactive/oxidising mineral surfaces, which has been a problem in the past (hopefully we’ll have a paper looking at this in more detail out very soon); hence the acid/alkali treatment to remove these phases in this study.
Unsurprisingly, the sulphur stream samples that had not been artificially matured were found to produce a wide range of organic compounds, consistent from those generated from the pyrolysis of microbial mats from similar environments. The shear amount of organic matter in the sample was able to overcome any issues in detection due to the presence of the sulphates and iron oxides in the samples. However, samples that received the acid/alkali wash still produced a greater abundance and variety of organic molecules than those which did not. Many of the organic compounds detected are diagnostic of biology and have the potential to be used as biosignatures. Some can be related to bacteria, with markers of both anaerobic and aerobic metabolisms present, while others indicate an input of woody higher plant material.
Pyrolysis-GCMS data showing what organic molecules may be detected from the same sulphur stream sample with different pre-treatment regimes, before and after hydrous pyrolysis and before and after acid treatment
After artificial maturation/diagenesis, organic matter detectability decreased markedly with increased hydrous pyrolysis temperature as organic matter was degraded. It was not until the sulphates and iron oxides were dissolved away that we could see anything interesting, i.e. nothing diagnostic of life was detected without the acid/alkali treatment! After they were removed it could be observed that many biosignature compounds did, in fact, survive the maturation process, although some were lost or altered.
So, why’s this interesting at all? Well it demonstrates that if we only use bog standard thermal decomposition (pyrolysis) techniques to look for organic matter in martian sediments which are rich in sulphates and iron oxides then we’re going to miss out on a lot of stuff, we just won’t see biosignatures that are there! Sulphates and iron oxides are yet another barrier to organic matter detection by thermal extraction strategies and should be avoided where possible.
Issues with this may have already happened, the simple organic compounds detected by Curiosity in 2018 (link) look rather similar to what was detected in the un-acid/alkali washed samples and were detected in mudstones with high sulphate and iron oxide contents! If these reactive minerals could have been removed in a pre-treatment step prior to analysis, who knows how much of a greater variety of organic information would have been unlocked, perhaps even the first compelling biosignatures of ancient life on Mars? 
Simple organic compounds already detected on Mars, but what information was lost due to reactive minerals?
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