The Search for Fatty Acids on Mars: Results from the First In Situ Thermochemolysis Experiment at Gale Crater, Mars

The SAM instrument on the Curiosity rover can perform wet chemistry experiments in conjunction with pyrolysis gas chromatography mass spectrometry (py-GC-MS) analysis of surface samples. Two cups in SAM contain reagents for thermochemolysis with tetramethylammonium hydroxide [TMAH] in 25% in methanol. These wet chemistry experiments transform polar organic molecules, such as carboxylic and amino acids, into volatile derivatives that are more amenable to GC-MS analysis, and free bound components of larger macromolecules otherwise undetectable with GC-MS. Of particular interest to astrobiology are carboxylic acids, bound in macromolecules or monomers bound to mineral phases, which could derive from abiotic or biotic sources. These are rendered detectable by transformation into methyl esters. The fatty acids (FA) used in terrestrial biology are abundant constituents of bacterial and eukaryotic cellular membranes, bound in phospholipids and glycolipids. These exhibit distinct even-over-odd carbon chain length preference due to their characteristic biosynthesis. Carboxylic acids are also found in carbonaceous meteorites, such as Murchison; however, these populations favor shorter carbon chain lengths and show no carbon number preference due to abiotic single-carbon-addition reactions. These patterns will aid in differentiating the sources of FA detected on Mars.

Specific SAM experiments in the Cumberland mudstone have yielded interesting results including the detection of C₁₀ - C₁₂ alkanes, which may originate from carboxylic acids. The first thermochemolysis experiment will occur in 2020 in Glen Torridon. Laboratory thermochemolysis of clay-rich analog samples have demonstrated that FA would be detectable if well preserved or recently present (<2 Ga). The SAM thermochemolysis experiment represents the first opportunity to detect organics bound in macromolecules on Mars. Beyond the science from Curiosity, the results and analytical features of this experiment on SAM will inform future missions. For example, a version of the TMAH thermochemolysis experiment is also featured in the Mars Organic Molecule Analyzer (MOMA) instrument onboard the 2022 ExoMars rover. The SAM results will critically underpin the corresponding MOMA analysis of clay-rich samples from depths up to two meters.