HYDROGEN AND DEUTERIUM IN THE THERMOSPHERE OF VENUS - SOLAR-CYCLE VARIATIONS AND ESCAPE

A strong solar cycle variation in hydrogen and deuterium densities is observed in the nightside thermosphere of Venus when Pioneer Venus Orb iter (PVO) measurements made during the first three Venus years of the mission are compared with those made during the preentry phase of the mission. Solar maximum conditions prevailed during the former period, while near solar minimum conditions occurred during the latter. Prono unced density enhancements in H of 6.5 times and D of 4 times are obse rved in the nightside bulge region as solar activity decreased from ma ximum to near-minimum values. We attribute the buildup of H and D to a reduction in the escape fluxes as solar activity decreased, a behavio r that is consistent with the known properties of the dominant escape processes due to the charge separation electric field (E) and charge e xchange (CE). Application of this simple concept leads to expressions for the H and D escape fluxes which relate solar cycle variations of t he respective bulge densities and escape fluxes to the dayside source fluxes. Planet averaged escape fluxes in the ranges 1.23 x 10(7) cm(-2 ) s(-1) < <(Phi)>(e) (H) less than or equal to 1.45 x 10(7) cm(-2) s(- 1) and 1.61 x 10(5) cm(-2) s(-1) less than or equal to <(Phi)>(e) (D) less than or equal to 2.15 x 10(5) cm(-2) s(-1) are obtained by this m ethod without specifying any particular escape mechanism. Considering the uncertainties in the measured parameters, these flux ranges are in reasonable agreement with the escape fluxes derived from the specific processes, E and CE. When fractionation without or with an external s ource of water (e,g., comets) is applied to the range of possible esca pe fluxes, a fractionation factor f = 0.44 is obtained and an ancient reservoir of water equivalent to the range 125 m to 570 m of liquid un iformly distributed on the surface or 3.7% to 17% of a full terrestria l ocean is derived. When the specific escape processes E and E + CE ar e considered, the f values are 0.15 and 0.1 and the magnitudes of the reservoirs are lower, having a range of equivalent depths of 4.1 m to 36.8 m, respectively.