Salts and soils on Mars

PI: Bonnie Light

Key issues for Mars exploration concern the origin of aqueous alteration materials and what they mean for the evolution and habitability of Mars. We propose to use data analysis and modeling to constrain the salt chemistry of the soil measured by Phoenix in the context of soil chemistry measured by the Viking Landers (VLs), Mars Pathfinder (MPF) and the two Mars Exploration Rovers (MERs). The major questions we seek to address are: (1) What is the quantitative composition of salts at the Phoenix site? (2) Is this consistent with the composition of soils at other lander sites? (3) What does the composition of salts indicate about the origin of the soil? (4) Assuming Phoenix soils went through past aqueous processing and evaporation or freezing, what salts would be expected, what is their chemical and physical expression, and can we identify these in lander data? We propose three main tasks to address these questions.
In Task 1, our goal is to deduce best estimates of the salt composition of Phoenix soil. First, we will reexamine the noisy wet chemistry lab (WCL) aqueous solution data to give bounds on the solution chemistry, while also using dissolution kinetics to estimate the probable grain size of salts. Second, we will model evaporite or ‘freezite’ salts produced when the Phoenix-measured aqueous solutions are evaporated or frozen in the numerical model FREZCHEM, which we have updated to include perchlorate chemistries. Third, by knowing evaporite and freezite minerals, we will solve the inverse problem of deducing the salts in the Phoenix soil before dissolution.
In Task 2, we will consider the composition of a hypothesized global soil unit using the data of all past landers in light of the results from Phoenix. We will construct mass balance models of the soil based on the assumption that gains and losses of elements would have arisen from reactions with carbonic, sulfuric or perchloric acid, as well as soil oxidation from photochemical oxidants. The models will estimate additions of volcanic volatiles that bear upon historical rates of volcanic outgassing. To provide a check on the possible endmember minerals used in our soil models, we will also conduct numerical weathering calculations of Martian basalts using a range
of assumptions.
In Task 3, we will compare Phoenix and MER microscope image data to ‘freezites’ formed experimentally as well as evaporites formed from the same solutions. Our main goals will be to compare the morphology (crystal shape and growth orientation) and particle size distribution of the salts with the data. Our studies will include salts that are likely to have been at the Phoenix and MER-B sites. We will also compare microscale structure formed from incongruent melting when freezites dissolve upon warming.
Our results will include a quantitative estimate of the salt mineralogy of the Phoenix soil and probable grain sizes, soil models for a hypothesized global unit, and estimated amounts of volatiles added. All these bear upon the origin of Martian soil and the history of volcanic outgassing. Also, our results from Task 3 will aid in the interpretation of the microscopic nature of the soil observed at Phoenix and the outcrops imaged at MER-B, as well as future observations on Mars.