Scattering and sputtering on the lunar surface; Insights from negative ions observed at the surface

Abstract

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.

Figures (15)

• Figure 1: Differential number flux of negative hydrogen ions, $\mathcal{J}^-_\mathrm{H}$, versus emission energy. Vertical bars show flux estimates, with thick and thin bars representing the 68% and 90% highest density intervals, respectively. Bar colour qualitatively reflects the signal significance, based on the Widely Applicable Information Criterion Watanabe2010 comparing models with and without hydrogen. Each panel corresponds to a specific emission polar angle interval $\beta$ (shown in the upper-right inset), where inward arrows indicate the average solar zenith angle. The energy-axis arrow marks the average undisturbed solar wind proton energy. Hatched regions indicate energies without data coverage. Lines show the median modelled flux of scattered (black dashed), sputtered (black dash-dotted), and total (solid red) negative hydrogen ions; the gray shading denotes the 68% highest density interval of the total modelled flux.
• Figure 2: Illustration of the solar wind impinging angles (orange) and emission angles (blue). An arbitrary angular emission profile is drawn, with dashed arrows representing possible emission directions. Both the Solar Zenith Angle (SZA) and the emission polar angle $\beta$ are defined relative to the surface normal. The Solar Azimuth (SA) angle is relative to the North direction, and the emission azimuthal angle $\phi$ is relative to SA. The total scattering angle $\Psi$ is the angle between the solar wind direction and the emission direction.
• Figure 3: Probability of negative ionization as a function of the perpendicular emission velocity, $v_\perp$, for 1 keV ($\approx 4.37\times 10^{5}\,\mathrm{m/s}$) hydrogen ions scattering off silicon. The mapping to the emission energy for different microscopic emission angles $\beta{'}$ (Section \ref{['sec:microscaleabeta']}) is shown below the figure. Data points (open circles) are taken from Maazouz_1998, with the best fit from Eq. \ref{['eq:P_expo_dependece']} shown as a dashed line and from Eq. \ref{['eq:negative_ionization_probability_silicon']} as a solid line.
• Figure 4: Schematized sputtering induced by light ions. A proton (filled red circle) directed toward the surface (1) rapidly neutralizes to an hydrogen atom (open circle); (2) penetrates the material and loses a fraction $\gamma_\mathrm{extra}$ of its energy; (3) undergoes a large-angle scattering on a surface atom (filled black circles); (4) creates an isotropic emission of hydrogen primary knock-on atoms (PKAs, open circles) while losing additional energy inelastically (yellow overlay); most knock-on hydrogen atoms do not escape the surface (crosses) while those near the surface (5) have an increased probability of escaping the surface; (6) charge-exchange transfers (green arrows) may ultimately lead to a negative charge state (filled blue circle).
• Figure 5: Energy distribution of hydrogen atoms sputtered by 300 km/s solar wind $\mathrm{H^+}$ and $\mathrm{He^{++}}$ ions impacting the lunar regolith (the energies of the projectiles are marked by an upward arrow). A surface binding energy of $U = 5\;\mathrm{eV}$ is assumed. Three models are shown: proton-induced sputtering without extra energy loss (solid line); proton-induced sputtering with extra energy loss (dashed line); and sputtering from both protons and alpha particles, assuming an alpha-to-proton ratio of 4% and no extra energy loss (red solid line).