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Context. Solar wind ions backscattering is a fundamental plasma-surface interaction process that may occur on all celestial bodies exposed to the solar wind and lacking a significant atmosphere or magnetosphere. Yet, observations have been limited to the regolith-covered Moon and Phobos, one of the Martian moons.

Aims. We aim to expand our knowledge of the process to include comets by investigating the backscattering of solar wind protons from the surface of comet 67P/Churyumov-Gerasimenko.

Methods. We used one of the ion spectrometers on board ESA s Rosetta spacecraft to search for evidence of backscattered solar wind protons from the cometary surface. The signal of interest was expected to be very weak and several statistical treatments of the data were essential to eliminate any influence from background noise and instrumental effects. Due to limited knowledge of the signal location within the observed parameter space, we conducted a statistical analysis to identify the most probable conditions for detecting the signal.

Results. No significant solar wind backscattered protons were ever observed by the instrument. The statement applies to the large spectrum of observation conditions. An upper limit of the backscattered proton flux is given, as well as an upper limit of the backscattering efficiency of 9 A 104.

Conclusions. The surface of comet 67P/Churyumov-Gerasimenko distinguishes itself as a notably weak reflector of solar wind protons, with its backscattering efficiency, at most, as large as the lowest observed backscattering efficiency from the lunar regolith.

The Negative Ions at the Lunar Surface (NILS) instrument is a compact mass resolving negative ion and electron analyser flown on the Chinese Chang’E-6 mission to the Moon. NILS measures negative ions and electrons in the energy range of 3 eV/q to 3 keV/q with a mass resolution m/Δm of about 2. The mass resolution is sufficient to separate charge-converted solar wind protons and sputtered negatively charged atoms form the surface. An electro-magnetic electron suppression system allows to switch between electron and ion measurements. The fan-shaped field of view is divided into 16 discrete angular pixels that are scanned sequentially. For each viewing direction, an electron and an ion energy spectrum is acquired in 4.06 s. NILS has a mass of 919 g, excluding cables and multi-layer insulation. Power consumption is on average 2.7 W during nominal operations.

Context: Airless planetary bodies are directly exposed to solar wind ions, which can scatter or become implanted upon impact with the regolith-covered surface, while also sputtering surface atoms.

Aims: We construct a semi-analytical model for the scattering of ions of hundreds of eV and the sputtering of surface atoms, both resulting in the emission of negative ions from the lunar surface. Our model contains a novel description of the scattering process that is physics-based and constrained by observations.

Methods: We use data from the Negative Ions at the Lunar Surface (NILS) instrument on the Chang'e-6 lander to update prior knowledge of ion scattering and sputtering from lunar regolith through Bayesian inference.

Results: Our model shows good agreement with the NILS data. A precipitating solar wind proton has roughly a 22% chance of scattering from the lunar surface in any charge state, and about an 8% chance of sputtering a surface hydrogen atom. The resulting ratio of scattered to sputtered hydrogen flux is eta_sc / eta_sp = 1.5 for a proton speed of 300 km/s. We find a high probability (7-20%) that a hydrogen atom leaves the surface negatively charged. The angular emission distributions at near-grazing angles for both scattered and sputtered fluxes are controlled by surface roughness. Our model also indicates significant inelastic energy losses for hydrogen interacting with the regolith, suggesting a longer effective path length than previously assumed. Finally, we estimate a surface binding energy of 5.5 eV, consistent with the observations.

Conclusions: Our model describes the scattering and sputtering of particles of any charge state from any homogeneous, multi-species surface. Using NILS data, we successfully applied the model to update our understanding of solar wind interacting with lunar regolith, and the emission of negative hydrogen ions. 

The solar wind can interact directly with the surface of airless bodies like the Moon. The interaction causes sputtering of surface materials and solar wind ions are also partially backscattered to space. Particles leaving the surface can have any charge-state. At the Moon, backscattered or sputtered positive ions and energetic neutral atoms have been observed, but all attempts to find negative ions in electron measurements have failed so far. Here we present measurements by Chang’E-6 from the lunar farside revealing the existence of a layer of negative ions close to the lunar surface. We found that about  of the impinging solar wind protons charge exchange on the lunar regolith and are backscattered as negative hydrogen ions. The negative ion fraction is similar to the observed positive ion fraction1,3. We estimate a H− surface density of . On the dayside, the lifetime of negative hydrogen ions is short due to photodetachment10, confining them to a layer with a scale height of about 10 km. Such surface-bound layers or regions with negative ions should exist at any planetary object with a surface directly exposed to solar wind11,12, including low gravity bodies such as asteroids or comets.