Chemistry on Mars: Making Perchlorates

Title: Photooxidation of Chloride by Oxide Minerals: Implications for Perchlorate on Mars

Authors: Jennifer D. Schuttlefield†, Justin B. Sambur†, Melissa Gelwicks†, Carrick M. Eggleston‡, and B. A. Parkinson*†§

Journal: Journal of the American Chemical Society

Affiliation: Department of Chemistry, Department of Geology and Geophysics and §the School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071, United States

In an article the Astrobiters might also be interested in, Schuttlefield et. al investigate possible chemical reactions in Mars’ atmosphere that might be contributing to the large concentrations (0.4 – 0.6 wt %) of perchlorate in the Martian soil.  This evidence provided by the Phoenix Mars Lander indicates a ratio of perchlorate to chloride of 8 to 4.  There are places on Earth like the Atacama Desert that have perchlorate concentrations of 0.03 – 0.6 wt %, but the ratio of perchlorate to chloride 1 to 500.

The origin of natural perchlorate on Earth is still being debated, but the authors hypothesized that the thin atmosphere on Mars could lead to photochemical reactions that are unlikely on Earth.  Therefore they explored photochemical heterogeneous reactions taking place on naturally occurring semiconducting materials.  The authors used anatase, rutile, and UV light to explore these reactions.  Although there is no evidence for these specific minerals on Mars there is strong evidence for titanium, one of the main components of these minerals.  Moreover, the thin atmosphere should allow for a great deal of UV radiation.

Upon submersion in a NaCl solution and UV radiation perchlorate was produced in levels nearing 4 ppm.  The authors note that the reaction occurs due to the highly oxidizing valence band holes created in the materials by the UV irradiation.

The authors admit that many of the reaction conditions are not similar to conditions on Mars, most notably the difference between the Martian temperature (-20 to -98 °C) and those of the aqueous solution of NaCl in the lab.   However the experiments are still relevant, because the high perchlorate concentrations may cause such a large freezing point depression as to allow liquid water on Mars.  Additionally, the reactions could occur in thin liquid films that are thought to exist on Mars.  Also, photochemical reactions do not need heat activation, so they could easily occur at the subzero temperatures on Mars.

This paper is an important addition to our understanding of the chemical reactions that occur naturally on other worlds.  The chemistry of planetary surfaces and atmospheres is a hot topic in astronomy now that observations of extrasolar planets allow us to constrain their conditions and composition.

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