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D. Grinspoon (Southwest Research Institute)
The enhanced deuterium-to-hydrogen (D/H) ratio in the Martian atmosphere (5.25 x terrestrial;1,2) has been interpreted as the signature of water loss based on the assumption that primordial Martian water had Earth-like D/H. Yung et al.(3)considered that atmospheric water may be in steady-state with a crustal source, but rejected this possibility because of the non-terrestrial D/H signature inferred for the source hydrogen. Watson et al (4) measured D/H in SNC meteorites and found evidence for two reservoirs: a fractionated reservoir with values clustered around the measured atmospheric value and an unfractionated reservoir with values clustered at roughly twice the terrestrial D/H value. They interpreted the latter group to represent mixing between primordial hydrogen with Earth-like D/H and the more enhanced atmospheric value. However, this unfractionated reservoir has D/H which is very similar to that measured in three Oort cloud comets (5,6,7). If we assume that this is the D/H value of crustal water on Mars and calculate the predicted D/H value of atmospheric H in steady-state with such a source, we find that the predicted value corresponds closely with both the measured atmospheric value and the fractionated reservoir measured in the SNCs. Thus a cometary source for crustal water on Mars can explain all of the measured values. If Mars formed relatively dry due to its small size, then a late accretional flux of cometary material could have provided 300 meters of water, while leaving Earth’s D/H relatively unperturbed. 1) Owen et al.(1988). Science, 240, 1767-1770. 2) Bjoraker et al. (1989). Bull. Am. Astron. Soc., 21, 991. 3) Yung et al. (1988). Icarus, 76, 146-159. 4) Watson et al.(1994). Science, 265, 86-90. 5) Balsinger et al.. (1995) J. Geophys. Res. 100, 5827-5834. 6) Bockelee-Morvan, D. et al. (1999) Icarus, in the press. 7) Meier et al. (1998). Science, 279, 842-844.
The author(s) of this abstract have provided an email address for comments about the abstract: david@sunra.colorado.edu