Negative Ions at the Lunar Surface (NILS)

• Negative Ions at the Lunar Surface (NILS) is a research topic focused on the in-situ detection and analysis of H⁻ ions near the Moon’s surface.
• The Chang’e-6 NILS experiment provided the first dayside measurements of H⁻ ions, informing models of dust transport and electrostatic interactions.
• Findings from NILS guide engineering designs by optimizing sensor placement, dust mitigation, and modeling of lunar surface charging.

Negative Ions at the Lunar Surface (NILS) refers to the detection and analysis of near-surface negative hydrogen ion (H⁻) populations on the Moon, enabled by the NILS experiment aboard the Chang’e-6 lander. This experiment provided the first in-situ evidence of dayside H⁻ ions within the lunar surface sheath, folding this component into the broader framework of dust transport, surface charging, and engineering design for lunar operations. Instrumentation metrics and detailed spectra are developed further in companion works (Canu-Blot et al. 2025; Wieser et al. 2025); the following distills NILS-specific results and models from the review by Turyshev et al. "Lunar Dust: Formation, Microphysics, and Transport" (Turyshev, 11 Nov 2025).

1. Experimental Overview and Instrumentation

The NILS experiment, deployed on Chang’e-6, is cited as providing the first in-situ detection of a dayside near-surface H⁻ population. The review by Turyshev et al. references, but does not reproduce, the specifics of the NILS sensor head—including geometric configuration, aperture, analyzer design, energy-per-charge measurement range, spectral resolution, or calibration protocols. These are documented in companion instrument papers (Canu-Blot et al. 2025; Wieser et al. 2025). The review indicates that H⁻ ions are identified in the near-surface sheath during dayside illumination, but does not present differential fluxes, energy distributions, or detailed detection methodology.

2. Observational Results: H⁻ Abundance and Vertical Distribution

Sec. 6.C of the review characterizes the near-surface negative ion layer, with the following empirically measured properties:

• Backscattering Efficiency: $(2.5^{+1.2}_{-0.8})\%$ of incident solar-wind protons are charge-exchanged and backscattered as H⁻ within the dayside sheath.
• Local Number Density: $n_{0}(\mathrm{H}^-)\simeq 1.8\times 10^{-1}$ cm$^{-3}$ at the surface during dayside.
• Vertical Scale Height: $H_n\simeq 10$ km, constrained by rapid photodetachment of H⁻ in sunlight.
• Temporal/Spatial Coverage: Observed population is restricted to the dayside; no persistent nightside H⁻ layer is reported.

Quantitative dependencies on solar incidence angle, regolith composition, or time-resolved energy spectra are not provided in the review and must be sourced from companion NILS publications.

3. Physical and Electrostatic Modeling

The negative ion population is interpreted via generic rate and sheath models:

a) Negative Ion Formation/Detachment

The temporal evolution of $n_{H^-}$ obeys: $$\frac{dn_{H^-}}{dt} = k_{\mathrm{attach}}\, n_e\, n_{H} - k_{\mathrm{detach}}\, n_{H^-}$$ where $k_{\mathrm{attach}}$ parameterizes charge exchange or electron attachment (proton $\rightarrow$ H⁻), and $k_{\mathrm{detach}}$ describes photodetachment processes.

b) Sheath Potential Structure

The sheath potential as a function of height $z$ is given by: $$\phi(z) = \phi_0 \exp\left(-\frac{z}{\lambda_D}\right)$$ with Debye length $\lambda_D$ ranging from 3–30 cm near terminators, shadow boundaries, and PSR rims (Table VII), and $\phi_0$ the reference surface potential.

c) Charge Relaxation

Charge-neutralization timescale is described by: $$\tau_{\rm relax} = \frac{\varepsilon_0 \varepsilon'}{\sigma(T,\rho)}$$ with $\sigma$ the regolith conductivity (spanning $10^{-13}$ to $10^{-17}$ S m$^{-1}$, Table V), yielding $\tau_{\rm relax} \sim 10^2$–$10^6$ s across varying thermal and porosity conditions.

d) Dust Grain Electrostatics and Transport

When dust grain adhesion is overcome, a grain of radius $a$ and potential $\phi_{g}$ possesses charge $q = 4\pi\varepsilon_0 a \phi_g$. The sheath provides a vertically integrated electric field $\mathcal{E} \approx E_0 \lambda$. The resultant ballistic hop height and equilibrium hover height are: $$h_{\mathrm{ball}}(a) = \frac{3\varepsilon_0\phi_g}{\rho_p g_{\rm Moon}}\, a^2\, \mathcal{E}$$

$$h_{*}(a) = \lambda \ln \Bigl(\frac{3\varepsilon_0 \phi_g E_0}{\rho_p g_{\rm Moon} a^2}\Bigr)$$

No detailed functional form for $\phi(z)$ beyond the exponential is provided.

4. Coupling to Dust Motion: Adhesion, Electrostatics, and Mitigation

The role of negative ions in dust dynamics is addressed by blending grain electrostatics with inter-grain adhesion—the Johnson-Kendall-Roberts (JKR) pull-off force. The threshold field for lifting a grain: $$E_{\mathrm{req}}(a) \gtrsim \Bigl(\frac{\rho_p g_{\rm Moon}}{3\varepsilon_0 \phi_g}\Bigr) a^2 + \Bigl(\frac{3 W R}{8 \varepsilon_0 \phi_g}\Bigr)\frac{1}{a}$$ with $W$ the work of adhesion, $R$ the effective grain radius, $\phi_g$ the grain potential, and $\rho_p$ the particle density (Table VI, Fig. 5). Across $a = 0.1$–$10\,\mu$m, adhesion dominates, i.e., typical sheath fields ($10^2$–$10^3$ V m$^{-1}$) cannot detach dust without mechanical pre-liberation.

A two-channel $k(T,\rho)$ model is used for conductivity-driven charging: $$k(T,\rho) = k_{\rm cond}(\rho) + A T^3, \quad k_{\rm cond}(\rho) = k_0 \left(\frac{\rho}{\rho_s}\right)^{\ell}$$ with $\ell \approx 2$–3, $A \sim 3.6\times10^{-12}$–$3.6\times10^{-11}$ W m$^{-1}$K$^{-4}$.

5. Engineering Implications and Operational Design

Engineering guidance based on NILS-driven insights and model outputs is summarized as follows:

• Sheath Mitigation Voltages: To avoid lofting of pre-liberated grains ($a \sim 1\,\mu$m), local fields should be constrained below $10^1$–$10^2$ V m$^{-1}$ (Table IX).
• Sensor Placement: Electrostatic field probes and H⁻ spectrometers should be co-located within $\leq 0.5$ m of the surface to probe the dayside sheath ($\lambda \sim$ 3–30 cm, $E \sim 10^2$–$10^3$ V m$^{-1}$, Table VII).
• Dust Mitigation: Due to $F_{\rm adh} \gg m g$, mechanical agitation or micro-impacts are necessary prior to any attempt at electrostatic dust removal.
• Material and Conductivity Selection: Subsystem design may invoke the $k(T,\rho)$ model for conductive/thermal coupling, with reference to Diviner/ChaSTE values for regolith.

6. Limitations, Open Questions, and Research Directions

Turyshev et al. provide an overview anchored by the NILS H⁻ detection, yet key instrument characteristics (sensor geometry, energy resolution, absolute calibration) and detailed H⁻ spectral distributions are absent and delegated to Canu-Blot et al. (2025) and Wieser et al. (2025). Dependence of H⁻ density on incidence angle, surface mineralogy, or time of day is not addressed. The modeling framework accommodates H⁻ dynamics generically but stops short of explicit rate-equation solutions or photodetachment energy/kinetics analysis.

A plausible implication is that the observed dayside H⁻ layer, with its $(2.5^{+1.2}_{-0.8})\%$ backscatter efficiency and $H_n \simeq 10$ km scale, introduces a significant negative charge component into near-surface lunar plasma and may influence dust-grain charge state equilibrium, especially during high solar activity. The absence of nightside detections suggests strong photodetachment limiting H⁻ persistence.

Further experimental work, including resolved energy spectra, angular distributions, and multi-local measurements, as anticipated in the cited instrument papers, will clarify the coupling between negative ions, sheath structure, and electrostatic dust transport in heterogeneous lunar terrain. The review by Turyshev et al. establishes a unified microphysics-to-engineering framework for landed and rover operations in light of the newly detected H⁻ component.