Evolutionary impact of sputtering of the Martian atmosphere by O+ pickup ions



Nonthermal processes such as dissociative recombination of ionospheric molecules are known to lead to loss of atmospheric constituents (N, O, C) at Mars where the gravitational potential is easily overcome by the energy imparted to the product atoms. Moreover, observations of escaping planetary ions on the PHOBOS-2 spacecraft showed that the solar wind is presently scavenging significant amounts of both oxygen and molecular species as it flows past the planet. Because both the sun and the atmosphere of Mars have changed over time, the evolutionary importance of these processes cannot be estimated by simply multiplying the contemporary loss rates by the solar system age. Models of these loss mechanisms must include consideration of the evolution of the solar EUV intensity and solar wind and their effects. Here we describe calculations of solar wind-induced loss rates for evolving solar and atmospheric conditions like those described by Zhang et al. [1992a], but including sputtering of the Martian atmosphere by reentering O+ pickup ions. The inclusion of the sputter loss increases by about 30% the cumulative, estimated loss of oxygen to that in ∼ 50 m of water (global surface depth) over the last ∼3.5 billion years, when contemporary loss mechanisms are thought to have become dominant. More significant is the result that these ions also sputter CO2 and its fragments in substantial amounts. That integrated loss is equivalent to ∼ 0.14 bar atmospheric CO2 pressure, of the order of some estimates of Mars' early atmospheric inventory.