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Title:
Martian Volatiles: Isotopic Composition, Origin, and Evolution
Authors:
Bogard, D. D.; Clayton, R. N.; Marti, K.; Owen, T.; Turner, G.
Affiliation:
AA(Planetary Sciences SN, NASA Johnson Space Center, Houston, TX 77058, USA), AB(Enrico Fermi Institute, University of Chicago, 5640 S. Ellis, Chicago, IL 60637, USA), AC(Chemistry Department, University of California San Diego, La Jolla, CA 92093-0317, USA), AD(Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, HI 96822, USA), AE(Department of Earth Sciences, University of Manchester M13 9PL, UK)
Publication:
Space Science Reviews, v. 96, Issue 1/4, p. 425-458 (2001). (SSRv Homepage)
Publication Date:
04/2001
Origin:
KLUWER
Bibliographic Code:
2001SSRv...96..425B

Abstract

Information about the composition of volatiles in the Martian atmosphere and interior derives from Viking spacecraft and ground-based measurements, and especially from measurements of volatiles trapped in Martian meteorites, which contain several distinct components. One volatile component, found in impact glass in some shergottites, gives the most precise measurement to date of the composition of Martian atmospheric Ar, Kr, and Xe, and also contains significant amounts of atmospheric nitrogen showing elevated ^15N/^14N. Compared to Viking analyses, the ^36Ar/^132Xe and ^84Kr/^132Xe elemental ratios are larger in shergottites, the ^129Xe/^132Xe ratio is similar, and the ^40Ar/^36Ar and ^36Ar/^38Ar ratios are smaller. The isotopic composition of atmospheric Kr is very similar to solar Kr, whereas the isotopes of atmospheric Xe have been strongly mass fractionated in favor of heavier isotopes. The nakhlites and ALH84001 contain an atmospheric component elementally fractionated relative to the recent atmospheric component observed in shergottites. Several Martian meteorites also contain one or more Martian interior components that do not show the mass fractionation observed in atmospheric noble gases and nitrogen. The D/H ratio in the atmosphere is strongly mass fractionated, but meteorites contain a distinct Martian interior hydrogen component. The isotopic composition of Martian atmospheric carbon and oxygen have not been precisely measured, but these elements in meteorites appear to show much less variation in isotopic composition, presumably in part because of buffering of the atmospheric component by larger condensed reservoirs. However, differences in the oxygen isotopic composition between meteorite silicate minerals (on the one hand) and water and carbonates indicate a lack of recycling of these volatiles through the interior. Many models have been presented to explain the observed isotopic fractionation in Martian atmospheric N, H, and noble gases in terms of partial loss of the planetary atmosphere, either very early in Martian history, or over extended geological time. The number of variables in these models is large, and we cannot be certain of their detailed applicability. Evolutionary data based on the radiogenic isotopes (i.e., ^40Ar/^36Ar, ^129Xe/^132Xe, and ^136Xe/^132Xe ratios) are potentially important, but meteorite data do not yet permit their use in detailed chronologies. The sources of Mars' original volatiles are not well defined. Some Martian components require a solar-like isotopic composition, whereas volatiles other than the noble gases (C, N, and H_2O) may have been largely contributed by a carbonaceous (or cometary) veneer late in planet formation. Also, carbonaceous material may have been the source of moderate amounts of water early in Martian history.
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