Mg-sulfate can spectrally obscure siderite: Implications for martian surface carbonates

Rachel Yeaton Sheppard ^1,2* Damien Loizeau ² Abigail A Fraeman ³ Elizabeth Rampe ⁴ Cédric Pilorget ² Jean-Pierre Bibring ²

• ¹ Planetary Science Institute, Tucson, United States
• ² UMR8617 Institut d'Astrophysique Spatiale (IAS), Orsay, Île-de-France, France
• ³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States
• ⁴ Johnson Space Center, National Aeronautics and Space Administration, Houston, Texas, United States

The search for carbonates on the martian surface has been ongoing since the Viking missions. Recently the Curiosity rover observed carbonate in situ in Gale crater, however it is not visible from orbit. This study investigates the role of Mg-sulfate, one of the most common secondary minerals on Mars, in obscuring the spectral signatures of carbonates in orbital datasets. We collect spectral images of polyhydrated Mg-sulfate and siderite physical mixtures in various proportions exposed to a dry environmental chamber. We also collect spectral images at multiple timepoints to track the temporal evolution of the mixtures as the Mg-sulfate component dehydrates from 7H2O epsomite to ~2H2O X-ray amorphous forms, particularly focusing on how sulfate dehydration impacts the visibility of carbonate absorption bands at 2.3 and 2.5 µm. Our results reveal that Mg-sulfate can obscure the carbonate signature, especially the 2.3 µm band. These findings suggest that Mg-sulfate deposits may mask carbonates from orbital spectrometers like CRISM and OMEGA, implying that carbonate could be present in more locations on Mars than current orbital observations indicate.