FRACTIONATION OF HYDROGEN AND DEUTERIUM ON VENUS DUE TO COLLISIONAL EJECTION

The fractionation factor f is important for interpreting the current e scape fluxes of H and D on Venus and how the D/H ratio has evolved. Th e escape flux is currently governed by the two processes of charge exc hange and collisional ejection by fast oxygen atoms. Using a best-fit parameterized equation for the O-H scattering angle phase function, mo re accurate branching ratios for the oxygen ion dissociation and inclu ding the effects of the initial energy and momentum of the ions and el ectrons, as well as for the hydrogen and deuterium gas, we have reanal yzed the collisional ejection process. Our analysis produces improved values for the efficiency of H and D escape as a function of the ionos pheric temperature. From our results we propose the reduction of the h ydrogen flux for collisional ejection from 8 to 3.5 x 10(6) cm-2 s-1. Assuming that collisions leading to escape occur mostly in the region between 200 and 400 km, the revised D/H fractionation factor due to co llisional ejection is 0.47. where previously the process had been cons idered completely discriminating against deuterium escape (or f approx imately 0). The resulting deuterium flux is 3.1 x 10(4) cm-2 s-1, roug hly 6 times the flux due to charge exchange, making collisional ejecti on the dominant escape mechanism for deuterium on Venus.