Abstract/Summary

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for , and pick up ions of about and about , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up ions give O atom loss rates in the order of about . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in and densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated ion outflow on the Venus nightside of about due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.