2010 Annual Science Report
University of Hawaii, Manoa Reporting | SEP 2009 – AUG 2010
Mars Bulk Composition and Aqueous Alteration
The bulk composition of Mars, including its total inventory of water, is central to understanding how Mars and the other inner planets formed. Comparison between the abundances of water and volatile elements in Mars, Earth, and Moon are particularly important to understand the source of water to the Earth. Martian bulk composition is also crucial to elucidating the processes involved in the initial differentiation into core, mantle and crust, and to the subsequent geologic evolution of the crust. Unraveling and quantifying the details of aqueous alteration on Mars is central to assessing the planet’s habitability and much of its geologic evolution. It also bears on determining Martian bulk composition and the source of planetesimals that accreted to form Mars.
Orbital gamma ray spectrometry shows that the Martian surface has a mean Cl/K ratio of 1.3 ± 0.2, indistinguishable from the ratio in CI chondrites (1.28). Although Cl and K fractionate by magma degassing and aqueous processing, during igneous partial melting both elements are highly incompatible. Thus, if the surface Cl/K reflects the bulk crustal value, then the mantle, hence primitive silicate Mars, also has a roughly CI ratio. Martian meteorite data indicate that Cl/Br is also approximately chondritic, suggesting that elements that condensed in the nebula between ~1000 K (K and Cl) to ~ 500 K (Br) are uniformly depleted in Mars at about 0.6 x CI chondrite concentrations. Mars clearly does not contain 0.6 x CI levels of H2O, which would be ~6 wt%, indicating that Mars was constructed by planetesimals rich in volatile elements, but not in water.
We have also launched an extensive study of aqueous alteration products in Martian meteorites, including direct measurements of D/H in weathered areas. The goal is to develop criteria for distinguishing terrestrial from Martian weathering, and to define the compositions of water solutions on Mars. So far we have made detailed studies of weathering veins in the Antarctic nakhlite MIL 03346 and are preparing papers on that work and integrating it into previous analyses of trace element concentrations in weathering veins. Our dataset, along with those of previous studies, indicates the Martian fluid which flowed through MIL 03346 had a composition largely controlled by the weathering of basalt. Fluid heterogeneity was caused by a strong dependency upon the surrounding microenvironment. In-situ data from the Mars Exploration Rovers, mineral identifications from orbit, and element measurements by the Mars Odyssey Gamma-Ray Spectrometer are consistent with Martian aqueous alteration being driven by solutions containing SO3 and Cl.