Loss of water from Venus. I. Hydrodynamic escape of hydrogen

A one-dimensional photochemical-dynamic model is used to study hydrodynamic loss of hydrogen from a primitive, water-rich atmosphere on Venus. The escape flux is calculated as a function of the H₂O mixing ratio at the atmospheric cold trap. The cold-trap mixing ratio is then related in an approximate fashion to the H₂O concentration in the lower atmosphere. Hydrodynamic escape should have been the dominant loss process for hydrogen when the H₂O mass mixing ratio in the lower atmosphere exceeded ～0.1. The escape rate would have depended upon the magnitude of the solar ultraviolet flux and the atmospheric euv heating efficiency and, to a lesser extent, on the O₂ content of the atmosphere. The time required for Venus to have lost the bulk of a terrestrial ocean of water is on the order of a billion years. Deutrium would have been swept away along with hydrogen if the escape rate was high enough, but some D/H enrichment should have occurred as the escape rate slowed down.