Amino acid preservation under mars surface UV irradiation protected by martian regolith simulants.

The preservation of amino acids in the immediate martian surface is cut short by the intense UV irradiation it suffers. Most organic matter in and amino acids in particular are significantly degraded in short periods of time. For instance, glycine has been described to have a half-life of 250 hours aprox. under noon irradiation conditions (1). These very harsh conditons are imposed by UV as the principal degrading agent (2,3). More recent studies have further described the preservation of amino acids in a wide array of substrates, from clays to sulphatic minerals (4) as well as in the presence of oxychlorine salts (5), and it has been determined that many substrates can preserve amino acids successfully, be it though adsorption or UV attenuation, as is the case for clays and sulphates, respectively. Perchlorates do also hinder this preservation, but not to a point at which their recovery is impossible.

The main issue with these studies is that the amino acidic analysis has been carried through chromatographic techniques, which although extremely powerful, add steps and require sample extraction. To avoid this, we attempted to quantify the preservation of amino acids through raman spectroscopy. This approach is less sensitive than the previous ones, particularly in heterogeneous mixtures, but it allows for direct analysis of the surface of the samples, and is an instrument present in most current and future missions to Mars (6–9). To assess the feasibility and usefulness of raman spectroscopy for the identification of amino acids, we spiked five amino acids (His, Tyr, Phe, Met and Leu) into four regolith simulants (Olivine, MGS-1, MMS-2 and rock dust from Río Tinto, an acidic river in southern Spain) at a 1% w/w proportion. We then pelleted the mixtures and irradiated them in the UVB range for three days, or an accumulated dose of 40000 kJ/m², that is, around 110 martian sols.

Aromatic amino acids, like tyrosine, histidine and phenylalanine, are extremely sensitive to UV damage. They also display the strongest raman signals. The raman signal in of these three amino acids in the irradiated samples was completely lost. Methionine, an S-bearing amino acid, appeared to be extremely sensitive to substrate damage and no raman bands could be reliably observed under any condition. Leucine, an aliphatic amino acid, did show a decrease in signal after irradiation, but it retained some signal, the only amino acid to do so. We also observed differences in preservation in different substrates, as unreactive ones like olivine and MGS-1 favoured the preservation and detection of raman bands, while MMS-2 and PRT, which are significantly more oxidative, generally masked or degraded most signals, even in unirradiated samples. Finally, we also observed an intense, 7-15 fold increase in the background fluorescence of the regolith simulants after irradiation. This further hindered the detection of organic raman bands and may be an issue in the interpretation of future raman spectra.

1. Ten Kate IL, Garry JRC, Peeters Z, Foing B, Ehrenfreund P. The effects of Martian near surface conditions on the photochemistry of amino acids. Planet Space Sci. 2006 Mar 1;54(3):296–302.

2. Garry JRC, Loes ten Kate I, Martins Z, Nørnberg P, Ehrenfreund P. Analysis and survival of amino acids in Martian regolith analogs. Meteorit Planet Sci. 2006;41(3):391–405.

3. Ten Kate IL, Garry JRC, Peeters Z, Quinn R, Foing B, Ehrenfreund P. Amino acid photostability on the Martian surface. Meteorit Planet Sci. 2005;40(8):1185–93.

4. Dos Santos R, Patel M, Cuadros J, Martins Z. Influence of mineralogy on the preservation of amino acids under simulated Mars conditions. Icarus. 2016 Oct 1;277:342–53.

5. Liu D, Kounaves SP. Degradation of Amino Acids on Mars by UV Irradiation in the Presence of Chloride and Oxychlorine Salts. Astrobiology. 2021 Jul 1;21(7):793–801.

6. Caffrey M, Boyd K, Gasway D, McGlown J, Michel J, Nelson A, et al. The Processing Electronics and Detector of the Mars 2020 SHERLOC Instrument. IEEE Aerosp Conf Proc. 2020 Mar 1;

7. Abbey WJ, Bhartia R, Beegle LW, Deflores L, Paez V, Sijapati K, et al. Deep UV Raman spectroscopy for planetary exploration: The search for in situ organics. Icarus. 2017 [cited 2023 Dec 1];290:201–14.

8. Rull F, Maurice S, Hutchinson I, Moral A, Perez C, Diaz C, et al. The Raman Laser Spectrometer for the ExoMars Rover Mission to Mars. https://home.liebertpub.com/ast. 2017 Jul 1 [cited 2024 Mar 21];17(6–7):627–54.

9. Tomba JP, Pastor JM. Confocal Raman Microspectroscopy: A Non-Invasive Approach for in-Depth Analyses of Polymer Substrates. Macromol Chem Phys. 2009 Apr 2 [cited 2024 Mar 21];210(7):549–54.