Normally I wouldn't even dream of doing any work 2 days before Christmas, but this year is a bit different. We're stuck in London's Tier 4 Festive Hell and Charlotte and I ran out of conversational topics half way through June, so I thought I'd share this tiny bit of good news at the end of a terrible year, we’ve had another paper accepted! This one was initially submitted all the way back at the end of 2018 before we’d even heard of social distancing or Barnard Castle. I remember because it received its first rejection while I was at a real in-person conference (who knows when they’ll be back) in Scotland (we’re never allowed back there are we?).
The new paper is the snappily titled Pyrolysis of carboxylic acids in the presence of iron oxides:
Implications for life detection on missions to Mars. It is the second in a series
of papers we’re hoping to get out looking at how the presence of iron
containing minerals may have affected attempts to detect organic matter,
including life detection efforts, on Mars. You can read an open access version here.
I’ve discussed what organic matter is and why efforts to
find life on Mars revolve around its detection on here before. But briefly,
organic matter is made up of organic molecules, these are chemical compounds
that contain carbon (C) and hydrogen (H). The C-H backbone is very flexible and
reactive but also manages to be quite stable. This allows many different elements
and functional groups to join on, in various positions and shapes, to form a very
wide range of carbon-based molecules, some of which may be quite large and
complex. Organic molecules can form through just the action
of heat or radiation on inorganic carbon-bearing species (like carbon dioxide),
even in the depths of space. But the unique properties of organic molecules allows carbon-based chemistry to form the building blocks of all known life; as
biology takes simple organic molecules and uses them to build complex biomolecules. Many non-biological processes can synthesize surprisingly
complex organic molecules, however, certain structures and patterns are almost
statistically impossibly to be formed by random chemical interactions, they may
only be produced by the dedicated, enzyme controlled processes of life. If preserved in sediments, these biological structures
are known as organic biomarkers and, as pretty solid evidence for life, their
detection would be the ‘smoking gun’ of Mars life detection efforts.
We may expect to find biomarkers on Mars as around 3 to 4
billion years ago, around the same time life was evolving on Earth, Mars was a
much more pleasant place to be. It was warmer and wetter as it still had an
atmosphere, replenished by volcanic activity and protected by a stronger
magnetic field. There were rivers, lakes and even oceans. All of the ingredients
for life to evolve were present for millions, if not hundreds of millions, of
years. If life did evolve it would leave its molecular fossils, biomarkers, in the
sediments for us to detect today. Even if life didn’t evolve, there should
still be evidence of interesting prebiotic chemistry occurring due to
hydrothermal or magmatic processes (as we do see evidence of this in martian meteorites).
Landed Mars missions have yet to find any definitive biomarkers,
although they have detected a suite of small, simple organic molecules that
appear to be the fragmentation products of larger molecules (a macromolecule),
and some longer chain alkanes which have been suggested to be the breakdown
products of fatty acids. All attempts to find organic molecules on Mars so far
have used methods that rely on heating up samples to break down and volatilise organic
matter into smaller fragments that can be separated, detected and identified.
The problem is that that also heats up any inorganic minerals that are also
present, and make up the bulk of, the sample. Some of these minerals may be highly oxidised and
release oxygen on heating, essentially burning up any organic matter that is
also present in the sample. In the best case scenario this organic matter becomes overly fragmented and loses structural information diagnostic of its source, in the worst case it is totally lost
to analysis as it oxidises to carbon dioxide and carbon monoxide. This is what
has been blamed for the lack of conclusive detections so far as we know that
some salts (particularly perchlorates) have had this effect.
Another factor that has so far been less explored is the effect of
other, 'less reactive', inorganic minerals in the samples. Iron oxides are
widespread across the surface of Mars, they’re the reason it is the Red Planet after all.
Therefore, we wanted to look at the effects iron oxides could be having on
attempts to find and identify biomarkers in the martian sediments. We had
already had some clues that iron oxides may affect biomarker detection from
Jonny’s work that was published a few months ago. This showed that in natural
samples that were rich in both iron and organic matter, you had to remove the
iron-bearing minerals to be able to properly detect the organic molecules and fully identify the source of organic matter.
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| Mars is the Red Planet due to iron oxides at the surface (credit: NASA) |
To work out exactly what was going on we needed a MUCH simpler system than Jonny’s stream environment, so we made our own analogue samples to eliminate unknown variables.
We decided to use 2 mid-long chain length fatty (carboxylic)
acids, both containing 18 carbon atoms but in different saturation states. Oleic acid, an unsaturated fatty acid that is
a major component of vegetable oils, and stearic acid, a saturated fatty acid
that is found in many animal and vegetable fats. In this context unsaturated
and saturated mean whether the molecule contains any double carbon-carbon bonds
or not, which is the same meaning as when you talk about fats in food. Fatty
acids are useful molecules to look at as their chain length and saturation
state can provide a lot of information about their probable source: biological processes
select for longer chain lengths whereas non-biological processes are
statistically more likely to produce shorter chain lengths and unsaturated
molecules saturate over time, so a concentration of longer, unsaturated fatty
acids may be a good indicator of recent life.
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| Steric acid (left) and Oleic acid (right) |
We then analysed these fatty acid-mineral mixtures in a way
similar to that which is used to analyse samples at Mars; by heating them up in
an inert atmosphere and seeing what organic molecules were released, a
technique called pyrolysis-gas chromatography-mass spectrometry.
What we observed was that on heating the organic matter and
iron-bearing minerals reacted with each other. This altered the organic
products detected as the breakdown of the fatty acids was enhanced and the
products were transformed into other species, far less diagnostic of their
source. This led to a reduction in both the abundance and variety of products
detected, especially when lower (more realistic) concentrations of fatty acids
were used. A serious loss of diagnostic structural information meant that
the products of these fatty acids, which could have been indicative of life, were
pretty much indistinguishable to the expected breakdown products of abiotic, mature
macromolecular matter. Abiotic macromolecular organic matter is what has
already been inferred to have been detected at the surface of Mars and is the sort
of thing we would expect to detect there as it could be delivered by meteorites
(they’re full of the stuff).
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| Iron oxides promote the fragmentation & transformation of fatty acids into molecules more normally indicative of abiotic macromolecular matter |
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| Veins provide evidence of hydrothermal fluid flow |
So, in conclusion, iron oxides are going to be problematic
for the detection and identification of fatty acids on Mars. However, as they
are good for preservation of organic matter over geological time periods we can’t
just avoid them. Instead we have to target localities where haematite is the
most stable iron oxide as this seems to be the ‘least bad’.
I am currently in the process of submitting a follow-up
paper examining what effect these iron-bearing minerals have on the detection
of biomarkers from whole bacteria and we’re also looking at a few other Mars-relevant
sources of organic matter. Watch this space but they seem to cause very similar
problems for detecting those as well…
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| Merry Christmas! |
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