Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23f..07t&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P23F-07
Other
[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6295] Planetary Sciences: Solar System Objects / Venus, [6296] Planetary Sciences: Solar System Objects / Extra-Solar Planets
Scientific paper
For many years, it has been suggested that the composition of Venus' atmosphere, notably its abundance of CO2, is controlled by gas-solid reactions at its surface. However, the suggested reaction for Venus atmosphere, CaCO3 + SiO2 = CaSiO3 + CO2, cannot act as a buffer - the pressure-temperature trajectory of the reaction and that of the atmosphere (a dry adiabat) do not provide buffering capacity. Instead, perturbations to T or P(CO2) would produce catastrophic expansion or collapse of the atmosphere. This instability can be generalized to all devolatilization reactions that produce a radiatively active gas in a planetary atmosphere dominated by such gases, and gives a simple thermochemical criterion for whether a reaction could buffer such an atmosphere. Simple decarbonation reactions fail this criterion, implying that the abundance of CO2 in a CO2-dominated atmosphere cannot be buffered by chemical reactions with the surface. The same inference holds for the abundance of H2O in an H2O-dominated (steam) atmosphere (e.g., exoplanet GJ 1214b), and for the abundance of methane above a methane-hydrate ice surface (e.g., a Titan-like exoplanet). Buffering of minor gases is more likely; the proposed mineral buffer reaction for SO2 in the Venus atmosphere (FeS2 + CO2 = Fe3O4 + SO2 + CO) passes the thermochemical criterion, as does a reaction involving Ca sulfate (CaSO4 + CO = CaCO3 + SO2). These inferences can be generalized to extrasolar Venus-like planets - those with surfaces hot enough to permit rapid chemical reactions between solids and gas, yet cool enough that their atmospheres contain (are dominated by) gases active in thermal radiative equilibria. On the other hand, it seems likely that abundances of minor atmospheric gas species can be buffered by crust-atmosphere chemical reactions. So, minor species in exoplanet atmospheres may be particularly important for constraining the chemical characteristics of their surfaces.
Bullock Mark Alan
Treiman Allan H.
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