Paper Summary: Pyrolysis of carboxylic acids in the presence of iron oxides: Implications for life detection on missions to Mars

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.

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.  

Steric acid (left) and Oleic acid (right)

We mixed these fatty acids into a variety of inorganic minerals, we used quartz as a ‘control’ sample, as this mineral is known to not be very reactive, and tested the iron oxides haematite and magnetite; the iron oxyhydroxide goethite and the iron hydroxide ferrihydrite. All of these minerals have been directly detected or inferred to be present at the surface of Mars.

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).

Iron oxides promote the fragmentation & transformation of fatty acids into molecules more normally indicative of abiotic macromolecular matter
 

The inability to distinguish between low concentrations of ‘fresh’ biologically derived molecules and ancient abiological matter in the presence of iron minerals is a serious problem for life detection efforts at Mars, however our work did suggest a potential solution. Quartz had very little effect on the breakdown of the fatty acids, however, quartz-rich sediments are not a good environment for preserving organic matter over geological time as they are actually not reactive enough, iron-bearing minerals are much better because the organic matter binds to its surface, providing some protection. Out of all the iron-bearing minerals we tested the ‘least bad’ was haematite, this means that, on Mars, we should be looking for organic matter in sediments that have been subjected to conditions where haematite is the most stable form of iron. Haematite is the most stable iron-bearing phase under oxidising and acidic conditions, especially at higher temperatures, and there are numerous localities on Mars where we have evidence (from mineral veins) that hydrothermal fluids fitting this description flowed through the rocks while they were buried. At these localities, any of the more reactive iron-bearing phases will have already been replaced by haematite, which based on some of the experiments Jonny did for his PhD thesis, shouldn’t negatively affect the preservation of any organic matter adsorbed onto those minerals.

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…

Merry Christmas!