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 ^(18)O/^(16)O and ^(13)C/^(12)C 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 · 1n ([^(18)O_(ice)/^(16)O_(ice)] / [^(18)O_(vapor)/^(16)O_(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^(13)C_(ice-vapor) ≤ 0.2‰) at temperatures ≥ 135 K and only modestly fractionated (Δ^(13)C_(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^(18)O 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 ^(18)O-depleted volatile reservoirs