Since 2012, NASA’s Curiosity rover has been exploring the nature and extent of ancient habitable environments in Gale Crater. The search for organic compounds and the characterization of their origins (prebiotic, biotic or abiotic) by using the Sample Analysis at Mars (SAM) investigation is a key part of the mission. So far, organic molecules originating from the sediments, including chlorinated hydrocarbons, sulfur-bearing organics, fragments of alkyl and aromatic compounds, and midlle-chain alkanes, have been identified in drilled rock samples (1, 2,3).
The wet chemistry experiments on the Sample Analysis at Mars (SAM) instrument on NASA’s Curiosity rover were designed to facilitate gas chromatography mass spectrometry analyses of polar molecules such as amino acids and carboxylic acids. A new report presents the results of such a successful wet chemistry experiment on Mars on sand scooped from the Bagnold Dunes with the N-methyl-N-(tert-butyldimethylsilyl) trifluoroacetamide derivatization agent. (4) No amino-acid derivatives were detected. However, chemically derivatized benzoic acid and ammonia were detected. Mass spectra matching derivatized phosphoric acid and phenol were present, as were several nitrogen-bearing molecules and as yet unidentified high-molecular-weight compounds. The origin of these compounds, including those that may be internal to the SAM background, is examined. This derivatization experiment on Mars has expanded the inventory of molecules present in Martian samples and demonstrated a powerful tool to further enable the search for polar organic molecules of biotic or prebiotic relevance.
Derivatized benzoic acid was detected with an abundance of 442 ± 82 pmol in the GC-chromatograms with a very good match. There are several possible paths to production of benzoic acid. Non-volatile organics such as benzoic acid or benzenecarboxylate salts, which are possible products from the oxidation of meteoritic organic matter, have been predicted to accumulate in the upper metre of the Martian surface with concentrations up to 500 ppm by weight. The benzoic acids may originate from toluene precursor, that if present on Mars, could be successively oxidized to benzylic-alcohol, benzaldehyde and finally benzoic acid.
Besides the benzoates, also nitrogen-containing compounds were detected, such as tert-butyldimethylsilyl)amine (derivatized ammonia), a N-bearing inorganic compound, as identified by its mass spectrum and retention time in the chromatogram obtained with the Chirasil-β Dex column* with an abundance of around 7 ± 1 nmol. Twelve further nitrogen-containing compounds were detected, namely derivatized ammonia, hydrogen cyanide, derivatized isocyanate, isocyanomethane, trifluoroacetonitrile, acetonitrile, propenenitrile, propanenitrile, isobutyronitrile, butenenitrile, dimethylaminoacetonitrile and benzonitrile.
*On the Chirasildex-beta-CD column:
In 1991 at the Riva del Garda Capillary chromatography Symposium, Professor Volker Schurig reported his first results using the unique capillary GC column coated with per-O-methylated- beta-CD bonded to polysiloxane elastomer.
In inclusion GLC permethylated cyclodextrins diluted in polysiloxanes were coated onto high resolution capillary columns which resolved well underivatized enantiomers – extending from highly polar carboxylic acids to apolar saturated hydrocarbons – between 0 and 150 degree C. The new column technology, which was soon commercially available (by Chrompack company as Chirasil-Dex®), considerably widened the scope of enantiomer analysis by GLC. Anchoring of beta-CD to a polysiloxane matrix yielded a polymeric chiral stationary phase. Its successful thermal immobilization to the vitreous capillary surface allowed to use CD stationary phases in supercritical fluid chromatography for enantioseparation.
- Szopa, C. et al. First detections of dichlorobenzene isomers and trichloromethylpropane from organic matter indigenous to Mars mudstone in Gale Crater, Mars: results from the Sample Analysis at Mars instrument onboard the Curiosity rover. Astrobiology 20, 292–306 (2020). doi: 10.1089/ast.2018.1908
- Freissinet, C. et al. Detection of long-chain hydrocarbons on Mars with the Sample Analysis at Mars (SAM) instrument. In Ninth International Conference on Mars abstr. no. 6123 (Lunar and Planetary Institute, 2019). https://www.hou.usra.edu/meetings/ninthmars2019/pdf/6123.pdf
- Eigenbrode, J. L. et al. Organic matter preserved in 3-billion-year-old mudstones at Gale Crater, Mars. Science 360, 1096–1101 (2018). DOI: 10.1126/science.aas9185
- Freissinet, C. et al. Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars. J. Geophys. Res. Planets 120, 495–514 (2015). https://doi.org/10.1002/2014JE004737
- Millan, M., Teinturier, S., Malespin, C.A. et al. Organic molecules revealed in Mars’s Bagnold Dunes by Curiosity’s derivatization experiment. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01507-9
- Schurig V.; Separation of enantiomers by gas chromatography. J Chromatogr A . 2001, 906(1-2):275-99. doi: 10.1016/s0021-9673(00)00505-7.