Escape of4He and fast O atoms from Mars and inferences on the4He mixing ratio

Abstract

The dependence of the escape flux of fast O atoms from the upper atmosphere of Mars on the concentration of O and 4He is derived. The escape flux is computed by using the observed electron concentration profile, by assuming the ion composition to be largely O2+, and by assuming an exospheric altitude for a pure CO2 exospheric composition. The dependence of the fast O escape flux on the concentrations of O and 4He is then derived by computing the change in exospheric altitude resulting from addition of these constituents. An O/CO2 ratio of 0.03 at the ionospheric peak is found to substantially reduce the O escape flux computed assuming a pure CO2 exosphere, whereas an O/CO2 ratio as small as 0.005 does not significantly affect the escape rate. This range of the O/CO2 ratio is that inferred from Mariner ultraviolet results. A 4He mixing ratio in the lower atmosphere exceeding 2 × 10−4 would also suppress the escape flux of O. The concentration of H is too small to affect the loss of O. The escape flux of O atoms will produce an escape flux of 4He by collision. If the O escape flux is unsuppressed, the resulting 4He escape flux exceeds that produced by thermal escape and probably that produced by plasma expansion escape into the solar wind. Lower and upper bounds on the 4He mixing ratio in the lower atmosphere are estimated to be 10−9 and 10−3, respectively.