DOES MARTIAN SOIL RELEASE REACTIVE HALOGENS TO THE ATMOSPHERE?

On Earth halogens are remobilized by both aqueous and atmospheric processes. Recent work^1,2 further suggests that atmospheric processes can affect rock/sediment geochemistry in highly arid evaporative settings. Accordingly, we assess the consistency of halogen variation in Martian soil -- at Gusev and Meridiani -- with terrestrial analogs such as volatilization of surficial bromide in Sabkhas³ and in polar environments at the polar sunrise⁴. We also examine whether the S/Cl ratio may be more variable in the Martian soil profile than suggested previously⁵. In addition to constraining the volatility of Cl, this reveals the extent to which Martian soils are compositionally uniform.

Bromine concentrations increase from the surface to the subsurface roughly in the order Br > S > Cl, consistent with higher Br/Cl, lower S/Br, and higher S/Cl ratios in the subsurface. In addition, nearly all subsurface variability in Br can be modeled by Mg and S variations alone. Our observations suggest the loss of surficial Br to the atmosphere, driven either by UV photolysis³ or by chemical oxidants⁶. We expect our ongoing laboratory program, with experimental evaporation of UV-exposed bromide-bearing salt solutions, to discriminate among models. Where present, reactive Br is known to eliminate O₃⁷, which in turn could affect the distribution of perchlorates⁸, current models of H₂O₂ production⁹, and atmospheric ozone¹⁰variation.

We observe a vertically variable S/Cl ratio and correlation suggesting that bulk soil, which includes material at decimeter depth, may bear a S/Cl mass ratio in the 4 - 5 range. This contrasts with the smaller 1.8¹¹ – 3.6 ratio and strong correlation in surface soil. Such variability at depth seems consistent with differential vertical mobility of the two elements, driven primarily by S increase at depth, not volatility of Cl.

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