Dust Levitation on Asteroids

Abstract

In comparison to the lunar regolith, asteroidal regoliths appear to be deficient in dust and in agglutinates, i.e., in particles ≤100 μm across. On asteroid surfaces of high electrical resistiv_ity, such particles may be electrostatically levitated and the smaller size fraction may be preferentially lost. Two electrostatic field production mechanisms commonly considered for the Moon, where dust levitation has been repeatedly observed, are applied to asteroids. Fields generated by electron photoemission appear effective in levitating charged grains. As a surface element on a resistive asteroid rotates into and out of view of the sun, electrostatic levitation winnows its uppermost particulate layer. Depending on the asteroid's size, rotation rate, heliocentric distance, and other factors, levitated fines up to ∼10−2μm in diameter may escape directly by electrostatic acceleration. Particles 10−2–1 μm across lofted in the subescape regime may also be entrained to escape by solar wind drag and radiation pressure. Larger levitated particles remaining gravitationally bound to the asteroid (≥1–100 μm) are redistributed across its surface following local electrostatic and gravity gradients. Their migration is arrested when they settle in topographic traps and/or in perennially shadowed areas (e.g., craters, grooves). Electrostatic levitation may be an effective process of fine particle segregation and transport on asteroids, and might constitute a significant loss mechanism for their smallest particle size fraction during time intervals between large, regolith-dispersing impacts. The process may also contribute to forming coarse-grained near-surface charged “dustspheres” over the sunlit hemispheres of some asteroids. Possible evidence for fine particle levitation on various asteroids is discussed.