Spectroscopy,Stability,and Thermodynamics Of Martian Calcium/Ferric Sulfates

Numerous occurrences of sulfate minerals have been identified on Mars by remote sensing and in-situ detection.This means there is a significant sulfur cycle among Martian gases,liquids,and solids.Martian sulfates may have played a key role in the weathering of Martian surface and subsurface materials,the cycling of metals,the cycling and storage of water,and the hydrological processes that occurred over the course of Mars’ history.Therefore,the study of Mars sulfates is important for our insights into the evolution of the Martian environment.The investigation of the spectral properties of sulfate minerals under Mars relevant conditions can provide the ground truths for Mars missions and help to constrain the kinds and distribution of sulfates on Mars;The study of the physicochemical properties of Martian sulfate minerals(structure,stability,and formation conditions of sulfates,etc.)under Mars simulated conditions,and the laboratory studies of their phase boundaries,thermodynamics,as well as the kinetics of sulfate minerals,can provide the critical knowledge and experimental constraints for interpreting the co-existence of sulfate minerals observed by remote sensing on Mars,Martian surface temperature,humidity,acidity,Martian paleoclimate,and Mars environmental evolution.Currently,the major sulfate minerals identified on Mars by remote sensing and in situ detection are calcium sulfates,iron sulfates,and magnesium sulfates.However,there are still many challenges for the exact identification of sulfates,the hydration degrees of sulfates,and phase stability of sulfates due to the lack of experimental data on those sulfate minerals.In this thesis,two major sulfate systems,calcium sulfates and iron sulfates,were selected for laboratory study,and the fundamental spectroscopic properties of calcium/iron sulfates were systematically investigated for the interpretation of spectra features obtained by different payloads on Mars missions,the reason of the abnormal high stability of y-CaSO4 from a hyperarid area on Earth and from Mars was studied by different microanalyses,phase boundaries among three ferric sulfates were determined under RH%-T space and their thermodynamic parameters were investigated.Thus,the main content of this thesis is divided into the following sections:(1)Spectroscopy of calcium/iron sulfates studied in the laboratory with remote sensing applications.The calcium sulfates and ferric sulfates have been identified at multiple sites on Mars by remote sensing and landing missions.The systematic spectroscopic investigation of calcium sulfates and ferric sulfates in the laboratory is important for interpreting the data from the Mars missions.In this study,the calcium sulfates and ferric sulfates with different hydration degrees were synthesized,and their Raman,VNIR,MIR,and LIBS spectra were collected that would promote our understanding the fundamental spectral properties of calcium sulfates and ferric sulfates.Their fundamental spectral database can provide references for remote sensing and in-situ detection of calcium sulfates and ferric sulfates on Mars.Raman spectra can be used for the phase identification and homogeneity analysis of the samples,and the analysis of the Raman vibrational activate modes based on the features,which contribute to the next step study of their stability and thermodynamic properties,etc.In addition,their Raman spectra will be the basis for the interpretation of the Raman data from Perseverance and ExoMars.The infrared activate modes of crystal were studied using the Mid-infrared spectroscopy(MIR)to assist identification and mapping deposits of calcium sulfates and ferric sulfates on Mars by the thermal infrared data.Visible near-infrared spectroscopy(VNIR)data were used to study the overtones and combinations of the fundamental vibrations of different water-bearing samples for interpretation of VNIR data(such as CRISM,Pancam,and Mars Mineralogical Spectrometer)from Mars remote sensing and in-situ detection.Laser-induced breakdown spectra(LIBS)of calcium sulfates and ferric sulfates will provide as spectral references for the analysis of the LIBS data from Mars(e.g.,ChemCam,SuperCam,and MarSCoDe).(2)Gamma-CaSO4 with abnormally high stability from a hyperarid region on Earth and from Mars.Ordinary γ-CaSO4 is a metastable calcium sulfate,while γ-CaSO4 from the hyperarid region on Earth and from Mars has been found with abnormally high stability.In this study,we used multiple microanalyses to characterize the chemical and structural properties of two such γ-CaSO4:one from Atacama soil(#10-d30)and the other from Martian meteorite MIL03346,168.Silicon was determined to be quasi-homogeneously distributed in Atacama yCaSO4,while both silicon and phosphorus were detected in Martian γ-CaSO4.We found the abnormally high stability of those γ-CaSO4 from hyperarid environments was due to the chemical impurities(Si or Si and P)which filled their structural tunnels and blocked the entrance of atmospheric H2O,with non-detectable structural distortion.We propose that the yCaSO4 with Si or Si and P impurities could have igneous origin or evaporative origin.Due to the extreme similarity in the structures of bassanite and γ-CaSO4,their XRD patterns are almost non-distinguishable;thus some martian "bassanite" minerals identified by Curiosity’s CheMin instrument at Gale crater can actually be γ-CaSO4.The structural tunnels in γ-CaSO4 would allow ions and ionic groups to fill,thus provide meaningful insights about the geological and geochemical processes experienced by it during the formation and transformation.The Raman spectrometer carried by the Perseverance and by ExoMars rovers will help the selection of samples enriched in γ-CaSO4 at Jezero Crater and Oxia Planum,which should be sampled for in-depth analysis on Mars and back to Earth.(3)Phase boundaries among three hydrous Fe3+sulfates and their implication for Mars.Ferric sulfates have been detected by orbital and rover missions at multiple locations on Mars.Their occurrences are determined by many factors(T,Eh,pH,RH%,and brine chemistry)and are very sensitive to variations of environmental conditions.In this study,the phase boundaries among the kornelite(Fe2(SO4)3·7H2O),paracoquimbite(Fe2(SO4)3·9H2O),and ferricopiapite(Fe4.67(SO4)6(OH)2·20H2O)were experimentally defined using well-controlled PH2o and temperature conditions,combined with the gravimetric measurements and laser Raman spectroscopy.The defined phase boundaries and thermodynamic parameters among three ferric sulfates were derived and can be used to complement the incompleted phase diagram of ferric sulfates and to predicate the other ferric sulfates’ phase boundaries.Furthermore,we found the values of the enthalpy changes and Gibbs free energy changes for each water of crystallization are-291.77±0.26 KJ/mol and-237.9±0.0 KJ/mol between paracoquimbite and ferricopiapite;and-287.66±0.02 KJ/mol and-238.6±0.0 KJ/mol between the kornelite and paracoquimbite.These values are well agreed with the results of other studies on various sulfates,which could shed light on the stability and phase transitions of ferric sulfates on Mars and on Earth.Studying the thermodynamic properties of ferric sulfates would help to understand the past and current Mars environment,the history of the water evolution,climate change on Mars,and to provide a better reference for subsequent Mars mission payload design and for keep sample stability during sample return.