Solar cycle variations in H + and d + densities in the Venus ionosphere: Implications for escape

The hydrogen ion concentrations recently observed on Venus, near solar minimum, by the Ion Mass Spectrometer on the Pioneer Venus Orbiter in the anti‐solar sector (22:00–02:00 LST) of the ionosphere are more than an order of magnitude less than those previously observed at solar maximum. This strong solar cycle variation has a profound effect on the escape of hydrogen (and deuterium) from Venus; almost all escape occurs during solar maximum. After adjustment for solar cycle variation, a planet‐averaged hydrogen escape flux of 0.6–1.4 × 10 7 cm −2 s −1 is obtained along with a large deuterium fractionation factor of 0.1–0.14. These results suggest at least two plausible scenarios for the evolution of water on Venus: (1) Water vapor on Venus may be approaching a steady state if the escape flux is balanced by endogenous or exogenous sources of water. The source of water must be highly fractionated, with a D/H ratio differing by less than an order of magnitude from the present ratio of 2.4 × 10 −2 , thus precluding low D/H water from comets, asteroids or a mantle reservoir. (2) The present day D/H ratio of 2.4 × 10 −2 could be established by Rayleigh fractionation of an early low D/H water reservoir if the escape flux was sufficiently large in earlier times. An early water endowment at least 340 times today's abundance, equivalent to 4.2 to 14 m of liquid water on the surface, would be needed.