Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon

The solar wind continuously flows out from the Sun, filling interplanetary space and impinging directly on the lunar regolith. While most solar wind ions are implanted into the lunar dust, a significant fraction is expected to scatter back and be emitted as energetic neutral atoms (ENAs). However, this population has never been observed, let alone characterized. Here we show the first observations of backscattered neutral atoms from the Moon and determine that the efficiency for this process, the lunar ENA albedo, is ∼10%. This indicates that the Moon emits ∼150 metric tons of hydrogen per year. Our observations are important for understanding the universal processes of backscattering and neutralization from complex surfaces, which occur wherever space plasmas interact with dust and other small bodies throughout our solar system as well as in exoplanetary systems throughout the galaxy and beyond.