Romain Canu-Blot

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 constructed a semi-analytical model for the scattering of ions of hundreds of electron volts (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 used 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 a good agreement with the NILS data. We find that a precipitating solar wind proton has a roughly 22 _−6.1 ^+4.9 % chance of scattering from the lunar surface in any charge state and 8.1 _−3.9 ^+7.9 % chance of sputtering a surface hydrogen atom. The resulting ratio of scattered to sputtered hydrogen flux is η ^sc /η ^sp = 1.5 _−1.1 ^+1.5 for a proton speed of 300 km/s. We found 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 estimated 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.

Apart from positive ions and electrons, negative ions are expected in various astrophysical environments. However, they have never been detected on the Moon until the Chang’E-6 mission. The NILS instrument onboard Chang’E-6 lander is the first dedicated instrument for detecting negative ions beyond Earth and has successfully obtained H ⁻ spectra on the lunar surface, providing an unprecedented opportunity to investigate their origin and distribution. Here, we present a positive correlation between the H ⁻ spectra and solar wind parameters, which provides direct evidence for the generation of negative ions from solar wind–surface interaction. Combined with Monte Carlo simulations, we predict a thin dayside H ⁻ layer and a long nightside H ⁻ tail, which can contribute to the lunar plasma environment, especially during an extreme solar wind density event. These findings greatly improve our understanding of the generation and distribution of negative ions on the Moon and other airless bodies.

Many different kinds of waves occur in the ionised coma of a comet, and these waves span a wide range of temporal and spatial scales. For example, ESA's Rosetta spacecraft, which spent to years near comet 67P/Churyumov-Gerasimenko from 2014 to 2016, detected singing comet waves, steepened magnetosonic waves, ion Bernstein waves, ion acoustic waves, and lower hybrid waves.Using data from the Rosetta Plasma Consortium of the Rosetta mission, we present a study of waves near the electron plasma frequency that we identify as Langmuir waves (Gunell et al. 2025, doi:10.1051/0004-6361/202555043). These Langmuir waves were observed when the comet was near perihelion. During this period a diamagnetic cavity had developed around the nucleus, and outside this cavity steepened magnetosonic waves were observed. Significant Langmuir wave activity was detected only in the environment outside the diamagnetic cavity, where simultaneously the steepened magnetosonic waves were observed. We suggest a possible generation mechanism for the Langmuir waves and a scenario through which energy can be transferred from the large and slow scale of the steepened waves to the small and fast scales of the Langmuir waves.

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.

Context . Solar wind interacts with the atmospheres of planets, moons, and comets. One of the important interaction processes is charge exchange. High charge-state heavy ions in solar wind are prone to charge exchange with typical atmospheric constituents due to their large cross-sections. These processes are important for remote observations of comets through X-ray emissions. Aims . We survey the ESA/Rosetta data for in situ observations of multiply charged oxygen ions and estimate the neutral outgassing rate of comet 67P/Churyumov-Gerasimenko through the fluxes of heavy ions such as O ⁶⁺ and O ⁵⁺ . Methods . We used data from the Ion Composition Analyzer on board the Rosetta spacecraft to obtain oxygen ion fluxes. We found the expected signal to be weak and therefore applied careful data filtering. We then adapted a pre-existing model to calculate the outgassing rate of the comet. Results . We find that multiply charged oxygen ions are mostly found post perihelion. The estimated outgassing rate is Q _O ^th = 1.87 ⋅ 10 ²⁸ × R _h ^−5.00 [s ⁻¹ ], with R _h being the heliocentric distance. Conclusions . Our outgassing estimates agree well with estimates from previous studies based on observations of helium ions. Our work extends these previous studies through the use of a completely independent dataset with coverage until the end of the Rosetta mission and by taking the varying neutral gas composition into account. The ratio of oxygen-to-helium fluxes we determined also matches previous measurements. Our comparison to ROSINA neutral gas measurements shows that heavy solar wind ions, such as oxygen, can be used to retrieve the neutral gas activity of the comet when direct neutral gas observations are not available.

In the plasma environment of a comet, waves are generated on vastly different temporal and spatial scales. Wave observations were carried out during the cometary flybys in the 1980s and 1990s as well as by the Rosetta spacecraft which accompanied comet 67P/Churyumov-Gerasimenko between 2014 and 2016. Waves are thought to contribute to the transfer of energy in the ionised coma. One of the fundamental plasma waves observed in space is the Langmuir wave, which appears at or above the electron plasma frequency. The Mutual Impedance Probe of the Rosetta Plasma Consortium (RPC-MIP) recorded frequency spectra of electric field fluctuations in the cometary plasma, and we used these spectra in order to detect and identify Langmuir waves. Langmuir waves were found during the part of the Rosetta mission when the comet was less than 2.65-2.8 AU from the Sun. The Langmuir waves appear near, but always outside, the diamagnetic cavity boundary, in a region where, at much lower frequencies, steepened magnetosonic waves also are present.

The Negative Ions at the Lunar Surface (NILS) instrument [1] is the first instrument to measure negative ions directly on the lunar surface. Carried onboard the Chang’e-6 mission to the lunar far-side (South Pole-Aitken basin) in June 2024, the instrument produced 346 minutes of data. The instrument uses an electrostatic elevation scanning system and energy analyzer to measure the direction- and energy-resolved negative ion flux. The instrument field-of-view spans from the surface to space in 16 linearly-spaced angular steps. The instrument response, integrated over the 8 steps viewing the surface, is shown in Fig. 1 alongside the scattering function for energetic neutral hydrogen atoms (ENAs) previously derived from orbital observations [2].Figure 1: Hydrogen ENA scattering function from Schaufelberger et al., 2011 along with the projected summed NILS response function.NILS data provide direct constraints—at the emission source—on the energy and angular distributions of scattered and sputtered negative hydrogen ions. A detailed mathematical model of the instrument is used within a Bayesian inference framework to retrieve the scattering and sputtering functions. Finally, these NILS-derived functions are compared with orbital measurements of scattered solar wind protons and hydrogen ENAs.[1] Canu-Blot, R., Wieser, M., Kérényi, M. et al. The Negative Ions at the Lunar Surface (NILS) Instrument on the Chang’E-6 Mission. Space Sci Rev 221, 38 (2025). https://doi.org/10.1007/s11214-025-01162-w[2] A. Schaufelberger et al., “Scattering function for energetic neutral hydrogen atoms off the lunar surface,” Geophysical Research Letters, vol. 38, Art. no. 22, Nov. 2011.

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 an electrons in the energy range of 3 eV/q to 3 keV/q with a mass resolution of 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. The power consumption is on average 2.7 W during nominal operations.

Mission description The Negative Ions at the Lunar Surface (NILS) instrument (Fig. 1) is the first instrument to measure negative ions directly on the lunar surface. Launched aboard the Chinese Chang’E-6 sample-return mission on May 3, 2024, the instrument will land on the lunar far-side (South Pole-Aitken basin) in early June 2024. During the surface phase of the lander, the NILS instrument will be active, measuring negative ion and electron fluxes from the lunar surface. Scientific objectives When the solar wind interacts with the lunar surface, a portion of it is reflected back instead of being absorbed. Previous studies have provided evidence of neutral hydrogen atoms [1] and solar wind protons [2] being reflected from the surface. However, to date, no measurements of negative ions have been conducted. It remains unclear whether a significant population of negative ions exists near the lunar surface. The NILS instrument, the first-ever dedicated negative ion instrument flown beyond the Earth, will address this gap by performing in-situ measurements directly on the lunar surface. Instrument description The NILS instrument is a 9th generation instrument of the SWIM family [3]. NILS uses a surface-interaction based time-of-flight system combined with an electrostatic analyzer and an electrostatic elevation scanning system to measure the direction-, energy-, and mass-resolved negative ion flux on the surface. A electromagnetic electron suppression system allows to switch between negative ion and electron measurements. NILS can separate the main solar wind components as well as negative sputtered oxygen ions and heavier species. Key performance parameters are shown in Table 1.Table 1: Instrument performancesFigure 1: NILS flight modelAt the Europlanet Science Congress 2024, we present and interpret the first data obtained and discuss the implications of the observed negative ion flux on the lunar near-surface plasma environment. References [1] M. Wieser, S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M. B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura. First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms. Geophys. Res. Lett., 37(5), 03 2010.[2] Y. Saito, S. Yokota, T. Tanaka, K. Asamura, M. N. Nishino, M. Fujimoto, H. Tsunakawa, H. Shibuya, M. Matsushima, H. Shimizu, F. Takahashi, T. Mukai, and T. Terasawa. Solar wind proton reflection at the lunar surface: Low energy ion measurement by map-pace onboard selene (kaguya). Geophys. Res. Lett., 35, 12 2008.[3] M. Wieser and S. Barabash. A family for miniature, easily reconfigurable particle sensors for space plasma measurements. Journal of Geophysical Research: Space Physics, 2