Backscattered solar wind protons by Phobos

We report Mars Express (MEX) measurements showing that the regolith‐covered Phobos surface backscatters 0.5%–10% of the impinging solar wind protons. Backscattered protons with an energy corresponding to 30%–40% of the solar wind energy injected in the flow are picked up by the solar wind and may cause field disturbances and wave activity. Since a similar backscattering of solar wind protons was observed and reported at the Moon, the backscattering of the solar wind particles is a typical physical process for regolith‐covered bodies in space, even though the regolith is highly porous and it has been thought that almost 100% of the solar wind protons are implanted.