K and Cl concentrations on the Martian surface determined by the Mars Odyssey Gamma Ray Spectrometer: Implications for bulk halogen abundances in 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 × CI chondrite concentrations. Mars clearly does not contain 0.6 × CI levels of H2O, which would be ∼6 wt%, indicating that Mars was constructed by planetesimals rich in volatile elements, but not in water.