First measurements of the ionospheric plasma escape from Mars

Abstract

BEFORE the Phobos mission, the magnetosphere of Mars was relatively unexplored. Basically all data on the charged-particle environment of Mars had come from the Soviet Mars missions (Mars 2, 3, 4 and 5). Here we report the results of an ion-composition experiment on board the Soviet Phobos 2 spacecraft which allowed us to determine the loss of plasma from the ionosphere of Mars. Because Mars has a very small intrinsic magnetic field, the ion outflow had been expected to be similar to that of Venus. Surprisingly, there were many similarities between the ionospheric outflow from Mars and the Earth. The ion loss from Mars results from both ion pick-up that results from mass-loading of the solar wind in the Martian boundary layer and ionospheric O⁺ beams of energies up to several keV, possibly from upward acceleration processes similar to those observed above the Earth's auroral oval. A preliminary estimate of the ionospheric outflow from Mars indicates that the planet is losing oxygen at a rate of ∼3 x 10²⁵ ions s^-1. This corresponds to an evacuation of its present total atmospheric oxygen content (contained in CO₂ and O₂) in less than 100 million years.