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| 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.
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| 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.
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| 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!
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| 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.
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| Where should Curiosity look next? (Credit: NASA) |
We also provide analytical services and laboratory services to our customers. 1-ethyl-2,3-dimethylimidazolium perchlorate
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