Biology
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0203h&link_type=abstract
American Astronomical Society, DPS meeting #28, #02.03; Bulletin of the American Astronomical Society, Vol. 28, p.1058
Biology
Scientific paper
Recently, a debate has begun over the temperature of formation for the secondary Mg and Fe,Ca carbonate deposits found within ALH84001. With substantial implications for exobiology and Martian crustal-atmospheric coupling, it is imperative to re-exam the conditions of carbonate formation from a broader planetary perspective. Based on the extensive fractionation of delta-13C found within carbonates in ALH84001 (+ 41 per mil), it has been concluded that carbonates represent exchange of atmospheric volatiles with the Martian crust. Previously, it has been shown that sputtering loss of CO2 and H2O from the Martian atmosphere (as well as other nonthermal escape mechanisms) can distinctly fractionate oxygen and carbon isotopes in the remaining atmosphere. Therefore, we re-examined oxygen and carbon isotope thermometry calculations including the effect of isotopic fractionation of the atmospheric reservoirs by nonthermal escape mechanisms. Using the thermometry equations established previously, we found that as isotopic enrichment of the atmosphere increased (for both oxygen and carbon) due to escape, the temperature required for forming the carbonates increased. For example, if the carbonates formed from water in equilibrium with an atmosphere with delta-18O of 20 per mil, the temperature of formation could increase by as much as a factor of 3. It should be noted that the possibility for post-deposititional isotopic exchange may make interpretation of the oxygen thermometer ambiguous. However, the temperature increase is supported by the carbon isotope calculations which indicate that the temperature of formation lies between 200 to 600 degrees C depending on the isotopic fractionation in the atmosphere (between 25 and 35 per mil). Thus consideration of the atmospheric environment upon carbonate formation and isotopic equilibrium is consistent with higher temperatures than cited previously.
Hutchins Kevin Sean
Jakosky Bruce M.
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