Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p32a04j&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P32A-04 INVITED
Other
5400 Planetology: Solid Surface Planets
Scientific paper
The lighter isotope of H in the martian atmosphere escapes more readily to space than does the heavier D, so that loss to space leaves the atmosphere enriched in D. The observed enrichment in D/H thus is an indicator of the degree of loss. As the H comes primarily from water, it informs the discussion of volatile and climate history. In order to understand the meaning of the enrichment, we need to understand (i) the initial D/H incorporated into the planet at its origin, (ii) the history of outgassing of water to the surface, (iii) atmospheric chemistry and dynamics that results in supply of the upper atmosphere with D and H from H2O in the bulk atmosphere, (iv) current loss rates to space, and (v) the present-day atmospheric D/H ratio. In addition, the D/H ratio can be affected by the exchange of water between the atmosphere and non-atmospheric reservoirs, including the polar caps, the regolith, and the crust, both by diffusion and driven by groundwater circulation (perhaps in hydrothermal systems). There is convincing evidence for the existence of each of these non-atmospheric reservoirs, but only limited information on the history of exchange. The system appears to be sufficiently complex that any attempt to describe it as a two- or three-component system is doomed to failure. Despite this, there are some conclusions for which a compelling case can be made: (i) Enrichment of D/H requires loss of substantial quantities of H to space, with water providing the source. (ii) Improvements in our understanding of the initial and present-day D/H, and the photodissociation of water and supply to the upper atmosphere have changed the quantitative interpretation from a decade ago; as a result, the time-integrated enrichment factor is substantially less than had been previously thought, and the resulting fraction of water lost is less. Roughly 2/3 of the exchangeable water must have been lost. (iii) The unknown time-dependent exchange of water with the polar caps and the crust makes further interpretation difficult. Further, more detailed interpretation is probably not warranted without direct measurements of the isotopic compositions of exchanging reservoirs such as groundwater and polar cap ice, such that an accurate picture of the time-dependent interaction of water reservoirs can be adequately constrained.
Jakosky Bruce M.
Leshin Lauri
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