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  • Does martian soil release halogens to the atmosphere?

Detailed statistical examination of Cl, Br, and S distributions, in martian soil profiles at Gusev Crater and Meridiani Planum, indicates decreasing Br abundance and weakening Br–S association towards the sur- face. All three elements decrease towards the surface in the order Cl < S < Br. Furthermore, Br variability decouples from potential cations such as Mg at the surface relative to the subsurface. These observations support a relative loss of surficial Br compared to S and Cl, all highly mobile elements in aqueous environments.We propose that Br may have converted preferentially to gas phases (e.g., BrO), driven either by UV photolysis or by chemical oxidants. Such volatilization pathways may in turn impart a global sig- nature on Mars by acting as controls on oxidants such as ozone and perchlorates. S/Cl mass ratios vary with depth (~4–5 in the subsurface; 1.8–3.6 on the surface) as well, with a strong correlation of S and Cl near the surface but more variable at depth, consistent with differential vertical mobility, but not vol- atilization of Cl. Elevated S/Cl in subsurface soil also suggests that the ratio may be higher in bulk soil – a key repository of martian geologic and climatic records – than previously thought.

Trench profile at Meridiani. Labels identify specific trenches, BB: Beagle Burrow; PT: Peanut Trench; TR: Trench. Suffixes identify location at trench site, S: Surface, W: Wall, and F: Floor. We propagated highly conservative numerical uncertainties from reported values for Br and Cl at 2 standard errors, shown as shaded rectangles. Consequently, the values may be more precise than they appear. Note the difference across floor, wall, and surface with highest ratios at the floor sampling the deepest. The ratio also varies roughly linearly with Br, indicating a relatively uniform Cl content. Excerpted from Figure 7 by Karunatillake et al. [2013].

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