Solar wind interaction with early Mars and atmospheric evolution

Abstract

First results from Phobos 2 encounter give large escape rates of O + ions (∼ 2×10 25 ions s −1) from the Martian ionosphere suggesting that direct solar wind interaction with the ionosphere can be an important source of loss of atmosphere over geologic times /1/. However, recent evidence from SNC meteorites and magnetostrophic balance principles show that early Mars had a surface equatorial field of about 0.05 gauss which has since decayed. This represents an increase by a factor of 1.6x10 4 for the magnetic pressure (P B) in early Mars over the present. The corresponding solar wind dynamic pressure (P SW) turns out to be about twice the present value. Thus P B in early Mars would far exceed P SW and the solar wind interaction would be magnetospheric (Earth like) rather than ionospheric (Venus like). Nevertheless ionospheric outflow could still be very significant from the point of view of atmospheric evolution. By analogy with earth, large outflows of ions (∼10 9 ions cm −2 s −1 for O +, ≳ 10 8 cm −2s −1 for H + etc.) through the polar regions could contribute significantly to the atmospheric loss. Indeed some similarities with earth's outflow have been noted by Lundin et al.