Erosion rate of lunar soil under a landing rocket, part 1: identifying the rate-limiting physics (2403.18583v1)

Abstract: Multiple nations are planning activity on the Moon's surface, and to deconflict lunar operations we must understand the sandblasting damage from rocket exhaust blowing soil. Prior research disagreed over the scaling of the erosion rate, which determines the magnitude of the damage. Reduced gravity experiments and two other lines of evidence now indicate that the erosion rate scales with the kinetic energy flux at the bottom of the laminar sublayer of the gas. Because the rocket exhaust is so fast, eroded particles lifted higher in the boundary layer do not impact the surface for kilometers (if at all; some leave the Moon entirely), so there is no saltation in the vicinity of the gas. As a result, there is little transport of gas kinetic energy from higher in the boundary layer down to the surface, so the emission of soil into the gas is a surprisingly low energy process. In low lunar gravity, a dominant source of resistance to this small energy flux turns out to be the cohesive energy density of the lunar soil, which arises primarily from particles in the 0.3 to 3 micron size range. These particles constitute only a tiny fraction of the mass of lunar soil and have been largely ignored in most studies, so they are poorly characterized.

• The paper reevaluates lunar erosion scaling and identifies kinetic energy flux as the critical parameter determining soil removal rates.
• It highlights that ultra-fine particles (0.3–3 μm) significantly enhance soil cohesion, influencing erosion despite their low mass contribution.
• Reduced gravity experiments show a low-energy, non-saltation erosion process, guiding improvements in landing site selection and spacecraft design.

Erosion Rate of Lunar Soil Under a Landing Rocket: Understanding Rate-Limiting Physics

The paper "Erosion Rate of Lunar Soil Under a Landing Rocket, Part 1: Identifying the Rate-Limiting Physics" by Philip T. Metzger presents a comprehensive analysis of the underlying physics influencing soil erosion rates during lunar landings. The paper is motivated by the operational challenges posed by abrasive lunar soil particles that are mobilized by the exhaust plumes of landing spacecraft. These particles can cause significant damage to spacecraft hardware and subsequent lunar operations, necessitating an accurate predictive model for soil erosion to inform landing site decisions and engineering designs.

Key Findings

1. Reevaluation of Erosion Rate Scaling: The paper challenges previous assumptions about erosion rate scaling, which had disagreements on whether erosion rates were proportional to various factors such as shear stress, momentum flux, or kinetic energy flux. Metzger provides evidence supporting the kinetic energy flux at the bottom of the laminar sublayer as the critical parameter influencing erosion rates.
2. Influence of Particle Size and Cohesion: The research highlights the importance of ultra-fine particles, particularly those within the 0.3 to 3 μm range. Despite their minuscule mass fraction within the lunar soil, these fines significantly contribute to the soil's cohesive energy density, which resists the erosive force of the rocket exhaust. This insight underscores that erosion on the lunar surface is a low-energy process, contrary to assumptions that larger particles play a more dominant role.
3. Mechanics of Erosion without Saltation: The paper elucidates that, unlike terrestrial erosion processes involving saltation, lunar erosion due to rocket exhaust involves limited transport of kinetic energy across the boundary layer. Ejected particles do not typically return to the surface, reducing energy transfer through collisions, and necessitating consideration of molecular scale processes for energy transfer.
4. Experimental Observations: Through reduced gravity experiments and observations of past lunar landing imagings, the paper identifies distinct patterns of crater formation, characterized by an inner region showing direct soil lifting by rocket plume and an outer region shaped by gravitational avalanches. These experiments corroborate the low-energy nature of the soil erosion and indicate that increased mass loading in the boundary layer does not directly elevate erosion rates.

Implications

The findings of this paper have significant implications both theoretically and practically. Theoretically, the research advances our understanding of the mechanics of erosion under reduced gravity conditions and contributes a new model of energy transfer mediated through molecular diffusion in laminar sublayers, rather than through turbulent flows or saltation. This challenges existing models, such as Roberts' hypothesis, by demonstrating that soil erosion is governed by energy flux rather than shear stress.

Practically, this refined understanding can assist in designing mitigation strategies for spacecraft landings to minimize damage by predicting more accurate erosion rates. The emphasis on cohesive properties of ultra-fines suggests new directions for simulating and characterizing lunar soil in laboratory settings to predict erosion more reliably.

Future Directions

The paper marks a first step toward developing a robust model of lunar soil erosion but acknowledges the necessity of comprehensive experimental campaigns to validate these findings. Future work is expected to focus on:

• Conducting experiments across a range of gravitational environments to refine the understanding of particle dynamics and energy fluxes.
• Further characterizing the submicron fines in both terrestrial similitude experiments and using in situ lunar data.
• Investigating the effects of electrostatic properties on erosion rates, considering the presence of charged rocket exhausts during lunar missions.

Ultimately, these advancements will support international lunar exploration efforts, potentially shaping policies and operational protocols to safeguard ongoing and future missions against the erosive effects of rocket landings.