Reexamination of Global Carbonate Abundances Using Tes Data

Introduction: The recent discovery of carbonaterich outcrops in the Nili Fossae region of Mars using data returned by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument [1] as well as indications of 2-3% carbonate in alkaline soils at the Phoenix landing site [2] have brought into question the hypothesis that much of martian history has been dominated by an acidic weathering regime. Carbonate minerals were previously detected by the Thermal Emission Spectrometer (TES) at the 2-5% level in the globally homogenous martian fines [3]. This detection was confirmed by the Spirit Mini-TES instrument [4]. Until the recent CRISM detection, however, carbonates had not been detected in outcrop form on Mars, leading to several hypotheses regarding their absence [5-6]. A focus of recent studies has been the possibility of carbonate breakdown by incident UV radiation. In this scenario, carbonate minerals would be dissociated, releasing CO2 into the atmosphere and leaving behind oxide or, perhaps hydroxide remnants. Laboratory studies of carbonate breakdown under martian conditions have yielded conflicting results [7-8]. Previous global mapping of carbonate mineralogy using TES data [9] utilized only two mineral endmembers: calcite (CaCO3) and dolomite ((Ca,Mg)CO3). In this study, we searched the TES data set for evidence of carbonate, hydrous carbonate, and carbonate decomposition products at 8 ppd using an expanded mineral library including siderite (FeCO3), magnesite (MgCO3), lime (CaO), portlandite (Ca(OH)2), periclase (MgO), brucite (Mg(OH)2), hydromagnesite (Mg5(CO3)4(OH)2·4H2O, dypingite (Mg5(CO3)4(OH)2·5(H2O), monohydrocalcite (CaCO3·H2O), and nesquehonite Mg(HCO3)(OH) ·4H2O. Methods: Sample selection and preparation. Siderite and magnesite samples were acquired from Wards Natural Science, and hydromagnesite, nesquehonite, and dypingite samples were acquired from Excalibur Mineral Corporation. A sample of monohydrocalcite was loaned to us by R. J. Reeder. Reagent-grade powders of portlandite, periclase, and brucite were acquired from Fisher Scientific. Lime was produced by heating reagent-grade calcite overnight at 875 °C. All samples were powdered and pressed into compact pellets for specular reflectance measurements. Specular reflectance measurements were carried out on a Nicolet 6700 FTIR spectrometer fitted with an FT-30 specular reflectance accessory at Stony Brook University. Mid-IR spectra (400-4000 cm) were acquired using a DLaTGS detector with a KBr window and a KBr beamsplitter. Far-IR spectra (100-600 cm) were acquired using a DLaTGS detector with a polyethylene window and a Nicolet Solid Substrate beamsplitter.