Sorry Venus, Mars is still where it's at!

 All of the excitement in the astrobiological community of late has been on the detection of phosphine in the clouds of Venus and how this may be a ‘biosignature’ of microbial life in the ‘habitable’ environment of the Venereal (pretty sure that’s the correct term…) clouds. While this detection is pretty damn cool, I do think the detection of a simple molecule that can also be produced from volcanoes and lightning (which we know exist on Venus) or from some other weird high temperature geochemistry (we know sod-all about Venus really) is getting a little over-hyped (in the same vein as Curiosity’s 2018 ‘life’detection).   

Can always rely on the Daily Express...

Now what is worth getting excited about is the Curiosity rover finally carrying out a TMAH experiment this month (after nearly 8 Earth years on the martian surface). For non-organic geochemists; TMAH, or tetramethylammonium hydroxide, is one of the two derivatisation agents carried by SAM (Sample Analysis at Mars, Curiosity’s onboard chemistry laboratory), the other being the even more god-awfully named MTBSTFA or N-methyl-N-(tert-butlydimethylsilyl)trifluoroacetamide.

Thanks to previous experiments by Curiosity, we now know that organic molecules are certainly present on Mars, they may exist as complex macromolecules, and their detectability is affected by the presence of various minerals on the martian surface. The usual technique for detecting organic matter on Mars, thermal decomposition (pyrolysis), is a bit of a blunt instrument, as using heat to release organic molecules from the sediments basically blows them apart (especially in the presence of oxidising salts), destroying structural information and making it difficult to establish their provenance. Because of this, so far, we have only detected simple organic molecules on Mars with much speculation, but little evidence, to their source (reminder – organic molecules, whilst they are the building blocks of life, they can also be produced by many abiotic processes). In contrast, derivatization agents are a really useful tool in detecting and understanding organic molecules as they can liberate organic molecules of interest from macromolecular matter or (potentially reactive) mineral surfaces less destructively and at lower temperatures.

A particularly interesting class of molecules in the search for life on Mars are the fatty acids. Unlike most other (potential) biomolecules fatty acids survive well under harsh environmental conditions over geological timescales (pretty important as Mars’s most habitable conditions were over 3.5 billion years ago) and can contain much information suggestive of their source. Fatty acids, as the name suggests are the main breakdown products of lipids or fats. Abiotic processes (such as hydrothermal processes) primarily produce short fatty acid molecules, whereas life tends to use longer fatty acid chains with even carbon numbers as essential components of cell membranes. The specific lengths and saturation states of these longer fatty acids can also provide clues as to the type of life they came from, bacteria, algae, higher plants all leave behind specific distribution patterns of fatty acid chain lengths. Hopefully I'll be posting a more in-depth discussion of why fatty acids are a good target in the search for martian life shortly as we're trying to get a couple of papers published on the subject...

Oleic acid, an 18 carbon singularly unsaturated fatty acid

However, these potential biosignatures are notoriously tricky to detect as fatty acids (a) are ‘sticky’ and so are hard to get off mineral surfaces in the first place; (b) on heating they break down pretty easily to ‘boring’ alkenes/alkanes which don’t preserve much information about their source; and (c) even if you liberate them from the mineral surface intact they are a polar molecule so the gas chromatograph-mass spectrometer’s (GCMS) detectors only ‘see’ them if present in large quantities (unlikely on Mars).

If a TMAH derivatisation step is applied to the samples before pyrolysis, however, the fatty acids can be liberated from the macromolecules/mineral phases at a lower temperature. Heating in the presence of TMAH hydrolyses organic matter, freeing the fatty acids (and other bound molecules) and also methylates (adds a methyl -CH₃) to polar functional groups, including the carboxylic group of the fatty acid molecule. The methylation makes the fatty acid (or other polar molecules) less polar and more volatile, making them more amenable to detection in the GCMS. Here's an open access paper on this technique being used on Mars analog samples if you want (significantly) more detail.

The easier to detect methyl ester of oleic acid

This is exciting as this experiment will be our best chance so far of detecting more complex organic molecules, work out where they are from and ultimately find evidence of life on Mars (maybe). Unfortunately we'll have to wait months to find out the results as the scientists involved will have to carry out all sorts of experiments to validate the rover's findings (especially if they find something that looks particularly interesting).

Further into the future, the ExoMars Rosalind Franklin rover, scheduled for launch in 2022, will have the ability to carry out other derivatisation pyrolysis experiments which work at temperatures down to 250 or 140 depending on the exact technique used. It will also have a laser desorption unit which will liberate organic molecules through millisecond laser pulses, a non-destructive technique. Both of these abilities should preserve more structural information and be less likely to suffer mineral surface effects than any experiments we have been able to do on Mars with Curiosity or any of the previous landed missions.