An exospheric perspective of isotopic fractionation of hydrogen on Venus

Data from Pioneer Venus provide a basis for estimating current escape rates of H and D from Venus, but there is no consensus in the existing estimates. In an attempt to clarify the effects of exospheric dynamics on escape, models of the distributions of H and D in the upper thermosphere and exosphere of Venus, and mutually consistent distributions of H+ and D+, have been calculated for several scenarios at solar maximum. The model results indicate that the combined (neutral and ion) planet average escape fluxes for H and D are 4.7×107 cm−2 s−1 and 1.4×105 cm−2 s−1, respectively. Allowing for uncertainties in the modeling procedure, the exospheric escape flux of H is inexplicably greater than current estimates of its thermospheric counterpart derived from Hunten's limiting flux theorem.