Investigating the stability of aromatic carboxylic acids in hydrated magnesium sulfate under UV irradiation to assist detection of organics on Mars

Abstract

The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard the Mars 2020 Perseverance rover detected so far some of the most intense fluorescence signals in association with sulfates analyzing abraded patches of rocks at Jezero crater, Mars. To assess the plausibility of an organic origin of these signals, it is key to understand if organics can survive exposure to ambient Martian UV after exposure by the Perseverance abrasion tool and prior to analysis by SHERLOC. In this work, we investigated the stability of organo-sulfate assemblages under Martian-like UV irradiation and we observed that the spectroscopic features of phthalic and mellitic acid embedded into hydrated magnesium sulfate do not change for UV exposures corresponding to at least 48 Martian sols and, thus, should still be detectable in fluorescence when the SHERLOC analysis takes place, thanks to the photoprotective properties of magnesium sulfate. In addition, different photoproduct bands diagnostic of the parent carboxylic acid molecules could be observed. The photoprotective behavior of hydrated magnesium sulfate corroborates the hypothesis that sulfates might have played a key role in the preservation of organics on Mars, and that the fluorescence signals detected by SHERLOC in association with sulfates could potentially arise from organic compounds.

Keywords: Aromatic organic compounds; FTIR spectroscopy; Mars 2020 Perseverance mission; Signs of biological activity; UV irradiation.

Figure 1

IR and Raman spectra comparison for pure phthalic acid, $10$ wt$\%$ phthalic acid adsorbed on hydrated magnesium sulfate and epsomite blank in (a) IR $8000-3500{\text{cm}}^{-1}$ spectral range; (b) IR $3500-500{\text{cm}}^{-1}$ spectral range; (c) Raman $65-2000{\text{cm}}^{-1}$ spectral range. In panels (a,b) only adsorbed phthalic acid bands (black spectrum) with shifts greater than the resolution of the instrument ($>4{\text{cm}}^{-1}$) with respect to the pure phthalic acid (red spectrum) are shown. In panel (c) the shifts of the hydrated magnesium sulfate (black spectrum) with respect to the epsomite blank $463{\text{cm}}^{-1}$ and $982{\text{cm}}^{-1}$ bands (blue spectrum) are highlighted. Legend: ${\nu }_s$ symmetric stretching vibrations; ${\nu }_{\mathit{as}}$ asymmetric stretching vibrations; ${\delta }_p$ in-plane bending vibrations; ${\delta }_{\mathit{op}}$ out-of-plane bending vibrations.

Figure 2

IR and Raman spectra comparison for pure mellitic acid, $10$ wt$\%$ mellitic acid adsorbed on hydrated magnesium sulfate and epsomite blank in (a) IR $8000-3500{\text{cm}}^{-1}$ spectral range; (b) IR $3500-500{\text{cm}}^{-1}$ spectral range; (c) Raman $65-2000{\text{cm}}^{-1}$ spectral range. In panels (b) the partial redshift of the $1153{\text{cm}}^{-1}$ band due to interaction between mellitic acid and hydrated magnesium sulfate via carboxyl groups is shown. In panel (c) a partial blueshift of the $982{\text{cm}}^{-1}$ band due to the mineral dehydration is shown. Legend: ${\nu }_s$ symmetric stretching vibrations; ${\delta }_p$ in-plane bending vibrations$.$

Figure 3

Pure phthalic acid half-lives degradation values $(\text{sol})$ according to Patel et al. UV flux. A $4\text{sol}$ threshold value can be identified between the bands mainly assigned to the COOH carboxyl group (red) and the bands mainly assigned to ring C–H bands (blue) degradations. Legend: $\nu$ stretching vibrations; ${\delta }_p$ in-plane bending vibrations; ${\delta }_{\mathit{op}}$ out-of-plane bending vibrations.

Figure 4

Pure phthalic acid $1261{\text{cm}}^{-1}$ $(7.9\mathrm{\mu }\text{m})$ band increase: (a) IR spectra changing during irradiation experiment; (b) curve fit for the $1261{\text{cm}}^{-1}$ band.

Figure 5

Pure mellitic acid half-life degradation values $(\text{sol})$ according to Patel et al. UV flux for the bands mainly assigned to the COOH carboxyl group. Legend: $\nu$ stretching vibrations; ${\delta }_p$ in-plane bending vibrations; ${\delta }_{\mathit{op}}$ out-of-plane bending vibrations.

Figure 6

Fit model (red for pure mellitic acid and green for adsorbed mellitic acid) regarding the formation of the photoproduct bands for (a) pure mellitic acid $3776{\text{cm}}^{-1}$; (b) adsorbed mellitic acid $1261{\text{cm}}^{-1}$; (c) pure mellitic acid $1959{\text{cm}}^{-1}$; (d) adsorbed mellitic acid $1959{\text{cm}}^{-1}$; (e) pure mellitic acid $1383{\text{cm}}^{-1}$; (f) adsorbed mellitic acid $1383{\text{cm}}^{-1}$.

Figure 7

Intramolecular pathway for the formation of benzenehexacarboxylic acid-trianhydride from mellitic acid proposed by Stalport et al. (adapted from Stalport et al.).