ISOTOPIC FRACTIONATION OF HYDROGEN IN PLANETARY EXOSPHERES DUE TO IONOSPHERE-EXOSPHERE COUPLING - IMPLICATIONS FOR VENUS

Planetary D/H fractionation factors for ionosphere-exosphere coupling through D-H+, H-D+, and H-H+ interactions have been computed for a variety of ion and neutral temperatures. These results strongly support differential escape of H relative to D, in stark contrast to the almost indiscriminate nature of collisional ejection by hot O atoms. The competitive effects of these processes is most important on Venus, where differential escape has created a hundredfold enhancement of the D to H ratio. It is argued that, since exospheric equilibrium concentrates neutral hydrogen in the night hemisphere where ion temperatures are maximum, the creation of fast hydrogen by charge exchange flourishes. On the other hand, hot O is produced by dissociative recombination of O2+ on the day side of the planet, in a region where hydrogen is depleted and thermospheric O is the primary collision partner. Oxygen exosphere models show that O-O collisions suppress the high energy component of the hot O distribution by more than a factor of 10. More important, the ballistic trajectories of fast O atoms that reach the nighttime reservoir of exospheric hydrogen favor downward scatter of D and H rather than their escape. Owing to the severe limits placed on the effectiveness of collisional ejection, it is concluded that the differential escape of D and H from Venus is determined by charge exchange interactions.