The porosity of the upper layers of regolith is key to the interaction of an airless planetary body with precipitating radiation, but it remains difficult to characterize. One of the effects that is governed by regolith properties is Energetic Neutral Atom (ENA) emission in the form of reflected and neutralized solar wind protons. We simulate this process for the surface of the Moon by implementing a regolith grain stacking in the ion-solid-interaction software SDTrimSP-3D, finding that proton reflection significantly depends on the regolith porosity. Via comparison with ENA measurements by Chandrayaan-1, we derive a globally averaged porosity of the uppermost regolith layers of urn:x-wiley:00948276:media:grl65047:grl65047-math-0001. These results indicate a highly porous, fairy-castle-like nature of the upper lunar regolith, as well as its importance for the interaction with impacting ions. Our simulations further outline the possibility of future regolith studies with ENA measurements, for example, by the BepiColombo mission to Mercury.
