The Reflection of Solar Wind Ions from Airless Bodies: Laboratory Analysis of Backscattered Energetic Neutral Atoms from Powdered Targets

When solar wind ions interact with the regolith of airless bodies, a significant fraction neutralize and backscatter as energetic neutral atoms (ENAs). ENA imagers onboard Chandrayaan and IBEX observed that ~20% of the solar wind protons are reflected as energetic neutral hydrogen atoms. Laboratory measurements will be presented on the energy and angular distribution of energetic neutral atoms (ENA) reflecting from powered targets. Mass selected H+ and He2+ pulsed ion beams were used to simulate the solar wind impacting an Apollo 17 breccia powered sample. A low energy ion, 0.1 to 5 keV, impacting a surface loses energy by ionization, secondary electron emission, excitation, and kinetic/potential sputtering. When the straggling of these low energy ions becomes comparable to the penetration depth, the result is backscattering of ions and ENAs. A Quantar® imaging detector has been used to determine the angular distribution and yield of ENA's. Time-of-flight analysis has been performed to resolve the ENA kinetic energy distribution of this complex multi-scattering process.