Evolutionary implications of a steady-state water abundance on Venus

Details Evolutionary implications of a steady-state water abundance on Venus Evolutionary implications of a steady-state water abundance on Venus

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993lpi....24..579g&link_type=abstract

In Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 2: G-M p 579-580 (SEE N94-16173 03-91)

Other

Abundance, Atmospheric Models, Hydrogen, Hydrogen Isotopes, Outgassing, Planetary Composition, Planetary Geology, Steady State, Venus (Planet), Venus Atmosphere, Water, Charge Exchange, Comets, Mass Spectrometers, Near Infrared Radiation, Planetary Radiation, Volcanoes

Scientific paper

In 1987, Grinspoon proposed that the data on hydrogen abundance, isotopic composition, and escape rate were consistent with the hypothesis that water on Venus might be in steady-state rather than monotonic decline since the dawn of time. This conclusion was partially based on a derived water lifetime against nonthermal escape of approximately 108 years. Others have questioned this conclusion. De Bergh et al. found H2O lifetimes of greater than 109 years. Donahue and Hodges derived H2O lifetimes of 0.4 - 5 x 109 years. The most sophisticated analysis to date of near-IR radiation from Venus' nightside reveals a water mixing ratio of approximately 30 ppm. Recent re-analysis of Pioneer Venus Mass Spectrometer Data are consistent with a water abundance of 30 ppm. Hodges and Tinsley found an escape flux due to charge exchange with hot H(+) of 2.8 x 107 cm-2 s-1. Gurwell and Yung estimated an escape flux of 3.5 x 106 cm-2 s-1 from collisions with hot O produced by dissociative recombination of O2(+). Brace et al. estimated an escape flux of 5 x 106 cm-2 s-1 from ion escape from the ionotail of Venus. The combined estimated escape flux from all of these processes is 3.7 x 107 cm-2 s-1, suggesting a lifetime against escape for water of less than 108 years. A recent estimate of H escape flux employing a different ionospheric model and using Pioneer Venus reentry data to estimate the response of the escape flux to the solar cycle finds a somewhat lower escape flux of 1.4 x 107 cm-2 s-1, suggesting a water lifetime closer to 2 x 108 years, significantly less than the age of the planet. Large uncertainties remain in these quantities, yet the data suggest that a source of water more recent than primordial sources is required and that a steady-state is likely. To obvious candidates for this source water are cometary impact and volcanic outgassing. Other aspects of this investigation are discussed.

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