Lunar surface: Dust dynamics and regolith mechanics

Abstract

[1] The lunar surface is characterized by a collisionally evolved regolith resulting from meteoroid bombardment. This lunar soil consists of highly angular particles in a broad, approximately power law size distribution, with impact-generated glasses. The regolith becomes densified and difficult to excavate when subjected to lunar quakes or, eventually, manned and unmanned activity on the surface. Solar radiation and the solar wind produce a plasma sheath near the lunar surface. Lunar grains acquire charge in this environment and can exhibit unusual behavior, including levitation and transport across the surface because of electric fields in the plasma sheath. The fine component of the lunar regolith contributes to the operational and health hazards posed to planned lunar expeditions. In this paper we discuss the mechanical response of the regolith to anticipated exploration activities and review the plasma environment near the lunar surface and the observations, models, and dynamics of charged lunar dust.

The Active Asteroids

“…In the absence of a discharging current, the potential can rise to the energies of the most energetic photons (10 3 V, or more; cf. De & Criswell 1977), but, in practice, the measured dayside potential is about +10 V (Colwell et al 2007). On the unilluminated side, or in shadows, the surface becomes negatively charged as a result of the greater flux of solar wind electrons (the solar wind is electrically neutral, but the low-mass electrons travel much faster than solar wind protons, leading to a net flux of electrons onto the surface).…”

“…The typical energy of solar wind electrons is ∼10 eV, leading to a negative potential of order 10 V. Near the terminator, the same processes lead to positive and negative charging of the surface, but on much smaller spatial scales corresponding to the scale of the local topography, leading to locally high electric field gradients. Near a shadow edge, the photoelectrons produced in sunlight can travel to and stick in unilluminated regions, causing local electric field gradients estimated at E ∼ 10 to 100 V m −1 (Colwell et al 2007;Farrell et al 2007). The positive, sunlit surface attracts a cloud of electrons, effectively neutralizing the gradient on length scales 1 m. The electrostatic processes that move dust particles on the Moon presumably operate also on the asteroids.…”

Activity in Geminid Parent (3200) Phaethon

“…Electrostatic forces offer another mechanism to remove dust. Evidence from the Moon shows that dust particles are periodically lifted from the surface at speeds ∼1 m s −1 by steep electric field gradients that develop in the terminator regions (de & Criswell 1977;Colwell et al 2007). Strong electric field gradients should be present on any low-conductivity surfaces exposed to the solar radiation and the solar wind.…”

“…Particles smaller than a c experience radiation pressure forces larger than their weight and can be stripped from the nucleus if they are briefly detached from the surface and the net force vector acting upon them aims away from the nucleus. The latter condition is naturally satisfied at the terminator, where electric field gradient forces are the largest (Colwell et al 2007). Equation (6) is approximate in the sense that we have assumed a prolate nucleus (b = c) whereas a triaxial approximation may be more accurate.…”

Neutral solar wind generated by lunar exospheric dust at the terminator

“…The most compelling evidence for this came from the Lunar Ejecta and Meteorites (LEAM) experiment deployed on the surface of the Moon by Apollo 17 astronauts [Berg et al, 1976]. LEAM was designed to detect very small fluxes of hypervelocity (>10 km s À1 ) impacts from interplanetary and interstellar dust, but instead detected significantly larger fluxes of slower moving ($100 m s À1 ) highly charged dust grains of lunar origin [Berg et al, 1976;Colwell et al, 2007]. The limited observations of the lunar ''dust-plasma'' environment suggest that it is most active at the terminator, therefore we focus our attention on dust-created neutral solar wind near this region.…”