INTERPRETING AQUEOUS ALTERATION ON EARTH AND ON MARS USING FIELD ANALYSES, LABORATORY EXPERIMENTS, AND MODELING

Although abundant evidence exists for aqueous alteration on Mars, the duration and characteristics of liquid water remain unconstrained. On Earth, interaction with liquid water is recorded in weathering profiles through soil, sediments and weathering rinds into less altered parent material, with field mineral weathering rates reflecting the dissolution rates and solubilities of different minerals. Laboratory dissolution experiments can therefore be used to interpret naturally weathered samples, and reactive transport modeling incorporating laboratory-measured kinetics can be used to quantitatively interpret kinetic processes occurring in the field. Mineral synthesis experiments can also help shed light on environmental conditions that result in different secondary minerals. Aqueous alteration on Mars can therefore be interpreted using field work on Earth, laboratory dissolution and synthesis experiments, and modeling.

Here we are using a combination of laboratory synthesis and dissolution experiments and reactive transport modeling to interpret the origin of Mawrth Vallis. Mawrth Vallis is one of many locations on Mars in which a sequence of Al-rich clay minerals overlies Fe, Mg-rich clay minerals, and possible formation mechanisms include weathering or a change in environmental conditions such as oxidation state. Dissolution experiments indicate that nontronite dissolves more rapidly than montmorillonite and kaolinite under very acidic conditions, suggesting that a mixture of these minerals would become enriched in montmorillonite and kaolinite over geological time. Dissolution of nontronite in brines occurs much more slowly than in dilute solutions, and reactive transport modeling of clay minerals relevant to Mawrth Vallis indicates the mineral sequences that occur and the duration of time required. Synthesis experiments indicate the importance of conditions such as oxidation state, pH, and water: rock ratio of reacting solutions to secondary mineral formation.