Global Mars-solar wind coupling and ion escape

Loss of the early Martian atmosphere is often thought to have occurred due to an effective transfer of the solar wind energy through the Martian induced magnetic barrier to the ionosphere. We have quantified the coupling efficiency by comparing the power of the heavy ion outflow with the available power supplied by the upstream solar wind. Constraining upstream solar wind density n(sw), velocity v(sw), and EUV intensity I-EUV/photoionizing flux F-XUV in varying intervals reveals a decrease in coupling efficiency, k, with solar wind dynamic pressure as k proportional to p(dyn)(-0.74 +/- 0.13) and with F-XUV as k proportional to F-XUV(-2.28 +/- 0.30). Despite the decrease in coupling efficiency, higher F-XUV enhances the cold ion outflow, increasing the total ion escape rate as Q(F-XUV) = 10(10)(0.82 +/- 0.05)F-XUV. The discrepancy between coupling and escape suggests that ion escape from Mars is primarily production limited in the modern era, though decreased coupling may lead to an energy-limited solar wind interaction under early Sun conditions. Plain Language Summary We used data from the Mars Express spacecraft to study the rate at which the Martian atmosphere escapes the planet as charged particles (ions) and flows out to space to be carried away by the solar wind, an important process in the evolution of the Martian atmosphere. Our analysis shows that the main driver for the escape rate is ionizing radiation from the Sun which produces more ions that can be removed. Counterintuitively, the increased ion production still better shields the atmosphere from the energy carried by the solar wind; however, very little energy is required due to the low gravity binding the atmosphere to Mars. Comparing with similar studies at heavier Venus hints that gravity is an important factor determining the solar wind's influence on how planets evolve.