The plot shows the simulated flux distribution of escaping H+ H2+ and O+ ions at Mars 3.5 Gyr ago. The fluxes are shown through a plane perpendicular to the Sun-Mars line at a distance x = -2 downstream of Mars (all scales are in Martian radii). The electric field points into the +y direction and causes the north-south asymmetry, while the interplanetary magnetic field is in the xy-plane. Since the gyroradius of oxygen ions is relatively large, most of the O+ flux is found in a region with y > 3 and therefore not seen in the Figure.
Planetary atmospheres prove to be a very efficient shield for X-rays: At the exception of very hard X-rays (l < 20 A) that go deeper into the atmosphere, the longer wavelength are absorbed in the atmospheric upper layers. This high altitude absorption results in an energy deposition at low density levels that may enhance the thermal and non-thermal escape processes already known to be driven by the early high EUV radiation. The contribution of X-rays to the photochemistry of palaeoatmospheres, will be investigated in the future.