Latest (catchy titled) paper summary: Perchlorate‐Driven Combustion of Organic Matter During Pyrolysis‐Gas Chromatography‐Mass Spectrometry: Implications for Organic Matter Detection on Earth and Mars

Credit: JGR: Planets

Our latest paper is now out and openly accessible to read here. I’d rather like you to read the full version so we can justify the £2500 it cost to remove the subscriber-only paywall and make it open access. However if you CBA to plough through all the technical jargon (there’s not too much I hope) here’s a little (hopefully) more accessible (and shorter) summary of what we found out.

As you probably know if you’ve read any of my other posts, my research is mostly on the interactions between organic matter and minerals on Mars, with the overall aim of trying to figure out why we’re having such a hard time detecting organic matter on the Martian surface. If you’re new here then what you need to know first is that organic matter, in this sense, does not necessarily have anything to do with life (whatever recent headlines have said). Organic molecules are just molecules that contain carbon, they are the building blocks of life BUT can also be produced from non-biological processes – even in deep space.

We know that organic matter should be widespread on the Martian surface as it will be delivered there by meteorites, comets and interplanetary dust particles. There may also have occurred processes in Martian history that created organic molecules on Mars itself (although this is less certain), and if there is, or has, ever been life on Mars then that would also leave behind molecular traces of itself. Even if the only source of organic matter to the Martian surface is from outer space, and the high radiation environment of the Martian surface (there’s very little atmosphere for protection) breaks that down somewhat, the constant delivery of organic matter over billions of years should have left something behind we can detect.

Oddly, though, despite trying since the Viking life detection mission back in the mid ‘70s, we have failed to detect any evidence of complex organic matter until very recently (more on that later). All attempts to look for organic matter in the Martian rocks and soil have used thermal decomposition – heating up the sample in an oven so that anything in there breaks down into small enough molecules to be identified by the on-board instruments. However, mostly, they only detected (at best) very simple, small, chlorinated organic molecules along with carbon dioxide and carbon monoxide gases. Not as interesting as we’d hoped.

Simple, chlorinated organic molecules found on Mars previously

The discovery of perchlorate in the Martian soil by the Phoenix lander in 2008 gave an answer to this puzzle of the missing organic matter. Perchlorate is an ion made up of 1 chlorine and 4 oxygen atoms, when you heat this up it breaks down to produce lots of oxygen – it basically explodes! This makes it useful for rocket fuel (ammonium perchlorate was used as the propellant for the space shuttle rocket boosters) but not so useful when you’re looking for tiny amounts of organic matter which are in the same soil/rock sample. When you heat up a sample that contains both perchlorate and organic matter the oxygen release causes the organic matter to combust (burn away) and be lost as carbon dioxide and carbon monoxide – not helpful for analysis.

We therefore wanted to find out the true potential of this reaction for destroying evidence of organic matter to answer the question: How much organic matter, relative to perchlorate, do we need in a sample to be able to detect it? If we could answer this, maybe we could figure out where, if anywhere, on Mars could have a ratio more favourable for detection.

To do this I made up lots of different mixtures of magnesium perchlorate and charcoal, (charcoal is superficially similar to the organic matter in meteorites, but much more accessible) and flash heated these samples to see what gases they gave off. By analysing the relative proportions of carbon dioxide and carbon monoxide given off it was possible to rapidly see the extent of combustion. If there was too much perchlorate/too little organic matter in a sample then all of the organic matter’s carbon would be saturated by oxygen - complete combustion - and only carbon dioxide would be given off. If there was too little perchlorate/too much organic matter for this to occur then it would be the oxygen that would saturate and not all of the carbon would be combusted, this incomplete combustion would produce carbon monoxide as well as carbon dioxide and there would, in theory, also be surviving organic molecules that were not burned away. The point at which we started producing carbon dioxide was termed the critical ratio.

If we have too much perchlorate and not enough organic matter (subcritical ratio) then there is enough oxygen produced to saturate all the carbon so the organic matter is fully combusted to carbon dioxide. If we have enough organic matter and less perchlorate (supercritical ratio) then we run out of oxygen, not all of the carbon can be saturated so we have partial combustion, producing carbon monoxide as well and organic molecules can survive to be detected. 

It turned out that this critical ratio was about 11 times as much charcoal as magnesium perchlorate. And by doing some maths we figured out that this meant (as the charcoal is a little over 50 % carbon) that on Mars you’d need about 6 times as much organic carbon than magnesium perchlorate in a sample to be able to detect it.

While this paper was out in review, there was a big NASA announcement (link to paper and summary). They’d finally found evidence of complex organic matter on Mars, but only in two samples. This suggested to us that for some reason these ‘new’ (they were analysed in 2015, it just took a long time to get the work published) samples must have an organic carbon/perchlorate mixture above our critical ratio and all the other samples previously analysed must have one below the critical ratio. Thankfully for us, someone had made and published those measurements of perchlorate levels in all of the analysed samples. It turned out that, yes, the samples that showed evidence of complex organic matter had about a tenth of the perchlorate content as the samples that only simple chlorinated molecules were detected in – kind of nicely proving our point for us, thanks NASA!

More complicated organic molecules that have recently been detected on Mars

This demonstrated that the perchlorate levels on Mars are variable and that this is important. If we want to find any more organic matter to increase our knowledge of extra-terrestrial organic chemistry, or even for life detection, we must look where the perchlorate levels are reduced. Perchlorates are highly soluble, this is why they are rare on Earth except in hyper-arid regions such as the Atacama Desert, so we should be looking for areas of recent water activity where they may have been washed away.

Where should Curiosity look next? (Credit: NASA)