The beginning of this year seemed to be a good time for our group
getting papers accepted and the third of those that snuck in is now online in
Astrobiology:
Once again, we’ve been looking at the troublesome effects of perchlorates
on the detection of organic molecules, only this time we’ve used the Atacama
Desert as a stand-in for Mars.
I’ve discussed the ‘perchlorate problem’ on Mars numerous
times on this blog as it’s what I’ve been working on for the last couple of
years (although not any more, watch this space). In short, oxygen-rich salts
(of which perchlorate is probably the most problematic) are present in the
Martian soil, we also expect there to be organic matter present (note, organic
does not necessarily mean biological, see previous rant). With Mars lander missions, such as
Curiosity, we attempt to detect the organic matter in the soil by heating the
samples up in an oven to break down large molecules into smaller (more
volatile) fragments that we can detect. However, those salts also break down
when heated, releasing oxygen. This oxygen causes the organic matter to combust
and any interesting organic molecules ‘burn up’ and are lost as carbon dioxide
and carbon monoxide gases. This explains the difficulty that Mars missions have
had in detecting organic matter on the surface, there has only been a single
successful detection, which I’ll come back to later.
Mars today is a hyper-arid environment. While there may have been
flowing rivers, lakes and even seas on the surface in the past, nowadays any
liquid water anywhere near the surface is very rare. This allows the build-up
of perchlorates and other salts which are highly soluble and would otherwise be
washed away, which is why they are rare on Earth. This is where the Atacama Desert comes in as, recent floods notwithstanding, this is one of the
driest places on Earth and so is one of the few places where perchlorates are present
in the soil in significant amounts. This, combined with the low abundance of
organic matter in the desert soil, due to its inhospitability to most life,
makes the Atacama a (relatively) easy option for testing out ideas about Mars.
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| A rather Martian looking dawn in the Atacama Desert |
The whole premise of this study can essentially be boiled down to: if
these perchlorates are so soluble, can we just wash them out of our samples to
allow us to detect the trace amounts of organic matter that we previously could
not detect?
The answer, it turns out, is yes.
When the desert soil samples were initially analysed, by heating them in
a similar fashion to what is carried out on Mars, showed little or no evidence
of any organic matter being present.
Sub-samples of those soils were then well washed in very pure water,
filtered and then dried. Unsurprisingly, analysis of the water showed that it
had dissolved most of the soluble salts from the soils and it did not appear to
have washed away any organic matter (which is mostly insoluble in water).
Once dried, analyse of these leached (washed) soil samples now allowed
the detection of a variety of organic molecules. The molecules detected were
indicative of the presence of cynobacteria (algae) that are known to be able to
grow even in the dry desert.
This was, all-round, a pretty good result!
This potential for the problematic salts to be washed away has some
pretty exciting implications for our search for organic matter on Mars. If we
want to get around this ‘perchlorate problem’ we can either:
1. Wash our Martian samples with water. This, however, introduces a
whole host of issues. Do we take water to Mars, it’s pretty heavy and we risk
creating a nice, wet habitable environment for any Earth microbes that have hitched a ride, a major issue for planetary protection. Do we produce water on
Mars by melting water-ice or extracting it from hydrated minerals, again, this
would probably upset the mission’s Planetary Protection Officer (yes, this is a
real job).
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| Buried ice exposed on steep slopes could be a useful water source |
2. Go look for areas with evidence of ‘recent’ water activity on Mars
were the salts will already have been leached away for us. This is the ‘easy’
option and what may have already happened accidentally. A rock unit called the
lower Murray mudstone is the one place on Mars where evidence of complex
organic matter has been found so far, co-incidentally this unit also has one of
the lowest concentrations of perchlorate yet measured on Mars. There is
evidence that, after the mudstones were deposited and buried, fluids flowed
through the rock. These fluids could have leached away any soluble salts
originally present, leaving being the insoluble organic matter, making it
easier to detect. Areas with evidence of current or more recent water activity,
such as above near-surface aquifers and near exposed and melting water-ice
could also be promising areas to check out, however, these will also present
planetary protection issues if there is liquid water available to support life.
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| Mineral veins show evidence of fluid flow, these fluids may have 'washed away' the soluble salts |
This was actually a project that Wren had been trying to get published
for a while now and the whole organic matter: perchlorate ratio paper we
published last year actually originated as a response to reviewer’s comment to
one of the early drafts of this current work. Happily, the two studies agree
with each other, and NASA’S detection of organic matter which was announced
while we were working on the re-write, pretty nicely (which is always good).
