¹E. Bisceglia, ^1,2G. Micca Longo, ^1,2,3S. Longo
¹Dipartimento di Chimica, Università degli Studi di Bari, via Orabona 4, I-70126 Bari, Italy
²CNR-NANOTEC, Bari section, via Amendola 122/D, I-70126 Bari, Italy
³INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy e-mail:

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Reference
Bisceglia E, Longo GM, Longo S (2016) Thermal decomposition rate of MgCO3 as an inorganic astrobiological matrix in meteorites. International Journal of Astrobiology (in Press)
Link to Article [http://dx.doi.org/10.1017/S1473550416000070]

¹G.M. Marion, ²D.C. Catling, ³J.S. Kargel, ⁴J.K. Crowley
¹Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA.
²Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA.
³Department of Hydrology & Water Resources, University of Arizona, Tucson, AZ 85721, USA.
⁴P.O. Box 344, Lovettsville, VA 20180, USA.

On Mars, evidence indicates widespread calcium sulfate minerals. Gypsum (CaSO4•2H2O) seems to be the dominant calcium sulfate mineral in the north polar region of Mars. On the other hand, anhydrite (CaSO4) and bassanite (CaSO4•0.5H2O) appear to be more common in large sedimentary deposits in the lower latitudes. The tropics are generally warmer and drier, and at least locally show evidence of acidic environments in the past. FREZCHEM is a thermodynamic modeling tool used for assessment of equilibrium involving high salinity solutions and salts, designed especially for low temperatures below 298 K (with one version adapted for temperatures up to 373 K), and we have used it to investigate many Earth, Mars, and other planetary science problems. Gypsum and anhydrite were included in earlier versions of FREZCHEM and our model Mars applications, but bassanite (the CaSO4 hemihydrate) has not previously been included. The objectives of this work are to (1) add bassanite to the FREZCHEM model, (2) examine the environments in which thermodynamic equilibrium precipitation of calcium sulfate minerals would be favored on Mars, and (3) use FREZCHEM to model situations where metastable equilibrium might be favored and promote the formation or persistence of one of these phases over the others in violation of an idealized equilibrium state.

We added a bassanite equation based on high temperatures (343 to 373 K). A Mars simulation was based on a previously published Na-Ca-Mg-Cl-SO4 system over the temperature range of 273 to 373 K. With declining temperatures, the first solid phase under equilibrium precipitation is anhydrite at 373 K, then gypsum forms at 319 K (46 °C), and epsomite (MgSO4•7H2O) at 277 K. This sequence could reflect, for example, the precipitation sequence in a saturated solution that is slowly cooled in a deep, warm aquifer.

Because FREZCHEM is based on thermodynamic equilibrium, a crude approach to problems involving metastable equilibria is available by removing phases that may have kinetically inhibited formation. Removing anhydrite allows bassanite to precipitate at 373 K, followed by gypsum at 351 K (78 °C), and epsomite at 277 K. Removing anhydrite and gypsum allows bassanite to form from 373 to 273 K. But bassanite formation from warm to cold temperatures does not seem appropriate for Mars and Earth.

An explanation for spatial patterns of gypsum, anhydrite, and bassanite on Mars and Earth could be past environmental differences. Anhydrite and bassanite are favored near Mars’ equator with higher temperatures, along with drier, more saline, and more acidic environments. Gypsum would be favored at the lower temperatures in the Mars polar region with wetter, lower salinity, and less acidic environments. On Earth, Ca-sulfate would likely over time largely finish re-precipitating as the more insoluble gypsum. But Mars was not in long-term moderate climates compared to Earth that strongly influenced the dominance of gypsum on Earth. So while temperature and water/acid environments for CaSO4 minerals on Mars may have been a major factor for these precipitations, the short-term moderate climates on Mars may also have influenced the prevalence of higher soluble CaSO4 species in the lower Mars latitudes.

Reference
Marion GM, Catling DC, Kargel JS, Crowley JK (2016) Modeling Calcium Sulfate Chemistries with Applications to Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.016]
Copyright Elsevier

¹Renato dos Santos, ^2,3Manish Patel, ⁴Javier Cuadros, ¹Zita Martins
¹Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
²Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
³Space Science and Technology Division, Rutherford Appleton Laboratory, Harwell, Oxfordshire, UK
⁴Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK

The detection of organic molecules associated with life on Mars is one of the main goals of future life-searching missions such as the ESA-Roscosmos ExoMars and NASA 2020 mission. In this work we studied the preservation of 25 amino acids that were spiked onto the Mars-relevant minerals augite, enstatite, goethite, gypsum, hematite, jarosite, labradorite, montmorillonite, nontronite, olivine and saponite, and on basaltic lava under simulated Mars conditions. Simulations were performed using the Open University Mars Chamber, which mimicked the main aspects of the Martian environment, such as temperature, UV radiation and atmospheric pressure. Quantification and enantiomeric separation of the amino acids were performed using gas-chromatography-mass spectrometry (GC-MS). Results show that no amino acids could be detected on the mineral samples spiked with 1 μM amino acid solution (0.1 μmol of amino acid per gram of mineral) subjected to simulation, possibly due to complete degradation of the amino acids and/or low extractability of the amino acids from the minerals. For higher amino acid concentrations, nontronite had the highest preservation rate in the experiments in which 50 μM spiking solution was used (5 μmol/g), while jarosite and gypsum had a higher preservation rate in the experiments in which 25 and 10 μM spiking solutions were used (2.5 and 1 μmol/g), respectively. Overall, the 3 smectite minerals (montmorillonite, saponite, nontronite) and the two sulfates (gypsum, jarosite) preserved the highest amino acid proportions. Our data suggest that clay minerals preserve amino acids due to their high surface areas and small pore sizes, whereas sulfates protect amino acids likely due to their opacity to UV radiation or by partial dissolution and crystallization and trapping of the amino acids. Minerals containing ferrous iron (such as augite, enstatite and basaltic lava) preserved the lowest amount of amino acids, which is explained by iron (II) catalysed reactions with reactive oxygen species generated under Mars-like conditions. Olivine (forsterite) preserved more amino acids than the other non-clay silicates due to low or absent ferrous iron. Our results show that D- and L-amino acids are degraded at equal rates, and that there is a certain correlation between preservation/degradation of amino acids and their molecular structure: alkyl substitution in the α-carbon seem to contribute towards amino acid stability under UV radiation. These results contribute towards a better selection of sampling sites for the search of biomarkers on future life detection missions on the surface of Mars.

Reference
dos Santos R, Patel M, Cuadros J, Martins Z (2016) Influence of mineralogy on the preservation of amino acids under simulated Mars conditions. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.029]
Copyright Elsevier