The potential of hydrated magnesium sulfates to preserve organics on Mars: spectroscopic analysis under simulated Martian UV exposure

To assess the astrobiological significance of Martian samples collected by NASA’s Mars 2020 Perseverance rover at Jezero crater, the primary criterion lies in identifying minerals with a high potential to preserve biosignatures and organic compounds. By abrading rock surfaces and analyzing the underlying material with its scientific payload, Perseverance examines their mineralogical and organic composition. The SHERLOC instrument [1], onboard Perseverance, found interesting Raman features in the organic spectral range, spatially co-located with sulfates in abrasion patches among the Jezero crater floor, likely originating from aromatic organic compounds [2], e.g. in the Polar bear spot on Quartier abrasion (Fig. 1).

Because of operational constraints, the abraded surfaces are left exposed to Martian environmental conditions for at least one Martian day (“1 sol,” equivalent to 24 hours and 38 minutes on Earth) before analysis with proximity science instruments. During this period, solar ultraviolet (UV) photons could modify or degrade potential organic matter within the abraded patches [3,4]. To shed light on the nature of these Raman features, it's crucial to assess whether aromatic organic compounds can be photostable under Martian-like UV irradiation for at least 1 sol and whether sulfates can play a photoprotective role towards them. In this study, we explored the molecular photostability by irradiating two carboxylic acids (phthalic acid and mellitic acid) that are metastable products of the generic oxidation of meteoritic organic compounds [5,6], and two PAHs (2,6-dihydroxynaphthalene and benzo[a]pyrene) that can be delivered by Interplanetary Dust Particles (IDPs) and (micro)meteorites [7,8]. Because of the sulfate spatially co-located Raman signals, the potential photoprotective effect of magnesium sulfate heptahydrated (epsomite), widespread at Jezero crater [9], is investigated. Therefore, we irradiated these organic molecules even when adsorbed or embedded on epsomite, and subsequently compared the results with those obtained from irradiation of the pure molecules. To simulate natural interactions in an early Martian aqueous environment, the Martian analog samples (molecule-mineral complexes) were prepared by suspending the mineral powder in an aqueous solution containing the molecule, followed by a desiccation process [10]. The Martian analog samples were characterized using Diffuse Reflectance Infrared Fourier Transform (DRIFT) collected by a Bruker VERTEX 70v FTIR interferometer to investigate molecule-mineral interactions. Subsequently, the irradiations were performed using a Newport Oriel 300W Xenon discharge lamp (spectral range 200-930 nm) interfaced with the Bruker VERTEX 70v interferometer allowing to study the kinetics of degradation live on the sample without removing it from the sample compartment. To estimate the organics Martian survivability, half-lifetimes were scaled on the Jezero Crater’s UV flux, calculated using the radiative transfer model COMIMART [12], which includes state-of-the-art dust radiative properties, and is fed with Mastcam-Z opacities[13]. The pure molecule irradiation shows half-lifetimes t_1/2 between ~ 0.5 and 7 sols of their molecular structures, regarding mostly the COOH functional groups for the two carboxylic acids [14], CH groups for benzo[a]pyrene and OH/CH groups for 2,6-dihydroxynaphthalene (Figure 2).

On the other hand, when these organic compounds are adsorbed/embedded on epsomite, no significant degradation is observed during UV irradiation, suggesting a photoprotective behavior of the mineral. Finally, photoproducts were detected. These intriguing findings support the key role of sulfates in Martian organic preservation and suggest that SHERLOC Raman signals spatially co-located with sulfates could potentially originate from aromatic organic compounds. Additionally, the photo-protective properties of sulfates may explain why the strongest SHERLOC Raman signals have been observed in association with sulfates rather than other minerals with photocatalytic properties that may degrade organics prior to SHERLOC analysis. These limitations of in-situ investigations highlight the need for Mars Sample Return (MSR) to accurately evaluate the organic content of the Martian samples collected by Perseverance.

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Acknowledgements: ASI/INAF Agreement 2023-3-HH.