Experimental constraints on the stable-isotope systematics of CO 2 ice/vapor systems and relevance to the study of Mars

Abstract

Variations in the isotopic compositions of oxygen, hydrogen, and carbon in the near-surface environment of Mars are likely influenced by condensation, evaporation, and sublimation of major volatile species (H 2O, CO 2). We present here an experimental study of the fractionations of 18O/ 16O and 13C/ 12C ratios between CO 2 ice and vapor at conditions relevant to the present near-surface of Mars; these experiments constrain isotopic variations generated by the current Martian CO 2 condensation/sublimation cycle. Oxygen-isotope fractionation between ice and vapor (Δ ice-vapor = 1000 · ln ([ 18O ice/ 16O ice] / [ 18O vapor/ 16O vapor]) varies approximately linearly vs. 1/T between temperatures of 150 and 130 K (from 4.2 and 7.5 ‰, respectively). Carbon isotopes are unfractionated (Δ 13C ice-vapor ≤ 0.2‰) at temperatures ≥ 135 K and only modestly fractionated (Δ 13C ice-vapor ≤ 0.4‰) at temperatures between 135 and 130 K. Martian atmospheric volumes that are residual to high extents of condensation (i.e., at high latitudes during the winter) may vary in δ 18O by up to tens of per mil, depending on the scales and mechanisms of ice/vapor interaction and atmospheric mixing. Precise (i.e., per mil level) examination of the Martian atmosphere or ices could be used as a tool for examining the Martian climate; at present such precision is only likely to be had from laboratory study of returned samples or substantial advances in the performance of mass spectrometers on landers and/or orbital spacecraft. Oxygen-isotope fractionations accompanying the CO 2 condensation/sublimation cycle may play a significant role in the oxygen-isotope geochemistry of secondary phases formed in SNC meteorites, in particular as a means of generating 18O-depleted volatile reservoirs.