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)

My first conference talks, two very different experiences

I flew all the way out to California last week to present work at the COSPAR (Committee of Outer Space Research) conference in Pasadena. 

I had been accepted to give two talks which was both pretty exciting and terrifying as I had somehow avoided ever giving a talk at a conference until now. Poster presentations, which I have done many of, are way more chilled (in my experience). You stand by your poster for a few hours and if anyone is interested they come and find you for a one-on-one chat, and there’s usually free beer to help the science flow. A talk on the other hand is (for me) a much more stressful proposition. Standing up in front of your peers, which may include eminent scientists who may ask horrifically complicated questions at the end, or may even just stand up and denounce your work to the whole audience (this dick-move is unfortunately quite a common occurrence). As such, I was quite nervous about the whole thing.

Both of my talks were about very different subjects. One for a project that is only a minor part of my job and I am in no way an expert on the subject matter – I was just the compiler of a large group’s work; the other on my latest research, which I’m pretty psyched to tell people about. So I felt somewhat worried about screwing up the first and not doing justice to the work of actual experts, but pretty good and excited about the latter.

What went down, however, was the complete opposite of my expectations.

In first talk, which was the one I was panicking about, went surprisingly well. I was presenting the chapter we have been writing for the Planetary Protection of the Outer Solar System (PPOSS) project. This is a report on how we can improve future organic contamination control for missions to the icy moons of Jupiter and Saturn.

I was basically arguing that organic molecules (from plastics, oils, grease, etc.) on spaceflight hardware pose as great a threat to our attempts to detect life in the outer solar system as microbial hitchhikers (the current main target for planetary protection efforts). This is because, if we have a ‘dirty’ instrument we may only detect the ‘dirt’. In a normal environment this is often not such an issue as we can usually recognise contaminants. However, we do not understand much about the environments of the icy moons, especially the radiation levels, and how that will affect the contamination molecules, the molecules which could be evidence of extra-terrestrial life (what we call ‘biomarkers’), and any other non-biological organic molecules that are present on the surface. Basically we’re looking for a needle in a haystack, except we don’t know what the needle is made out of, nor what the haystack is made out of, if we don’t have a clean instrument we’ll never figure this out.

Titan, one of the Icy Moons of Saturn has a very complicated organic chemistry. To have any chance of understanding it we need to not contaminate our analyses with organic molecules from Earth as they may be greatly altered by the complex radiation environment and unrecognizable for what they are by the time they get there (image credit: NASA)

This becomes a planetary protection issue as if we detect our own contamination and mistake it for evidence of extra-terrestrial life everybody will get really excited and future missions will waste a lot of time and money trying to find out more about these imaginary aliens. Conversely, the contamination levels could be so high that they mask a real life signal – in this scenario we would lose our interest in looking for life on the moon, maybe never checking again and instead only sending ‘dirty’ non-life detection missions to look at other aspects of the moon. This could permanently contaminate the moon and destroy any chances of detecting that life there in the future. Both of these scenarios are pretty bad for planetary protection whose main goal is to avoid jeopardizing the search for extra-terrestrial life.

Despite the fact that I was delivering this talk first thing in the morning, telling a room packed full of real planetary protection experts (I’m just a confused geologist remember) how to do their job it went down pretty well. It triggered questions and discussion on whether organic contamination control is a planetary protection issue, exactly what we were trying to achieve.

Me doing science (credit ESF-Science Connect)

After the success of the first talk I was feeling pretty confident for my second where I was going to be presenting what I actually know about – my own research on the interactions between organic matter and minerals on Mars. This was mid-afternoon on the Thursday, what you’d expect to be the perfect slot: not too early or late so people haven’t woken up or have shut down, not right at the start or end of the week so people haven’t arrived yet or already left and not just before or just after lunch so that people aren’t too hungry or in their post-lunch daze.

In this talk I was presenting the findings in our latest paper, this will shortly be available open access but is currently behind a paywall here. I talked about how we’d calculated the minimum about of organic matter there would need to be in a Martian sample for a rover to be able to detect it despite the presence of problematic minerals (using current techniques). I then showed how this worked with samples from the closest environment we have to Mars on Earth, the Atacama Desert, and then applied this new knowledge to explain why organic matter has suddenly been found on Mars after 40 years of trying.

Nobody cared.

The end of my talk and the customary ‘I’d be happy to answer any questions’ was met with glazed expressions and silence. I half expected a tumbleweed to blow down the central aisle of the conference hall. Normally in these situations the chair of the session will have a question prepared, but even they were unable to hide their disinterest.

As an early career researcher, giving my second ever conference talk, on something I spent months working on, this was pretty crushing. I could only scurry back to my seat and, somewhat shell-shocked, watch the rest of the afternoon’s session.

What had I done wrong, it had all seemed to go smoothly from my end?

As the rest of the session unfolded it all became clear. It was not that I had delivered bad science, I had delivered the wrong science for the crowd’s interest. While I am primarily a lab rat, doing experiments to try to understand the results coming back from the Mars rovers, everybody else in the session worked on the satellites orbiting Mars. They were all interested in atmospheric gas measurements or photographs of surface landforms which is what all the other talks were about. This was, for once, not my fault. I should never have been given a talk in this session. While it was titled, ‘Mars Science Results’ and so should have been suitable, because of the dominance of orbital data it was not a diverse enough audience. This was on the session organisers.

Despite this realisation this pretty much ruined the rest of the conference for me, it was just too much of a downer after the way I’d built it all up in my head beforehand – I am NOT a confident public speaker in the slightest so had really had to psyche myself up.

This dependence on the right audience being present seems to be the major crucial thing to get something good out of a conference presentation of any sort. I’ve also had this with poster sessions in the past. I stood around for hours with no one interested enough to come up for a chat next to a poster at the European Geophysical Union conference a few years ago. I was presenting some my PhD research on high resolution palaeoclimate reconstruction based on the chemistry of coral skeletons. Everybody else in the session was doing things with water or plant chemistry – but we were all using ‘isotopes for novel environmental studies’ or whatever the title of the session was. Dead sessions like this are excruciating, you’re almost praying for the crazy ‘scientist’ who’s had a few too many at the free bar to come up and discuss his latest theory with you – there’s often one. Other poster sessions I’ve had great discussions which have led to ideas to improve the work I’m presenting or have created ideas for new projects. Although sometimes I just make a tit of myself, while intimidated and slightly star struck, in front of the top scientists in my field (although as long as they leave with a copy of my latest paper its all good…right?).

The issue seems to be that you can’t really gauge what it’s going to be like when you submit your abstract – the session titles and descriptions are always so vague. I guess if this happens you just have to shrug it off and just take it as a good practice run for the next time you have a more interested crowd. It’s not put me off anyway, now I really need to pull my finger out and write that AGU abstract, hopefully the crowd there will be better…

The System Works....Occasionally

I've said some pretty nasty things about Reviewer 2 in the past, they always seem intent on screwing us over in some way or another. However, we've just had a paper accepted for publication and this manuscript's journey through the system has finally (after 3 prior publications) shown me how the system is supposed to work.

My previous manuscript had a rather rocky journey to publication, with numerous rejections from editors (too specialist interest) and having to appeal the decision of a particularly arsey Reviewer 2 who suggested rejection even after we'd done all they'd asked - months of extra experimentation.

This time, the reviewers - especially Reviewer 2 - picked apart the gaps and weaknesses in the manuscript that were mostly in there as we forget that outside our tiny lab other people have other ways of thinking about things so we need to explain everything:

'please elaborate...'

'how do you justify using this technique/sample...?'

'why did you not do it this way rather than the way I'd do it...?',

They also found a (rather glaring) omission, we'd been using carbon monoxide/carbon dioxide ratios detected by the GC-MS as a quick proxy for the survival of excess organic carbon on combustion - but had offered no proof that this actually worked other than theoretically, oops!

And they used their expertise to suggest how we could fix these problems to make the study better, all written in the sort of way that suggested they were genuinely interested in our findings and wanted to help.

Yes, this resulted in extra lab time to do a few more experiments (and the odd bit of swearing at the mass spectrometer), but the extra work gathered greatly strengthened the manuscript - and I made a real pretty new figure with the new data. 

When it came back from its second round of review Reviewer 1 was happy and Reviewer 2 had a few very minor comments, the worst of which just needed me to delete a sentence where I'd slipped into what could be construed as somewhat wild over speculation about some of the Mars data.

I genuinely mean the thanks to the two (anonymous) reviewers in the acknowledgements this time, they really did make the science better.

This is how the peer review system is supposed to work.

Don't be a dick.