Surface Science Constraints to Regolith Models

Introduction: Much of what we know about airless bodies in the solar system comes from observation of their surface or surface processes. From observations come hypotheses or models that, in favorable cases, can be constrained by additional observations on returned samples. Here, we show that other useful constraints arise from the extant knowledge of surface science processes occurring in other fields of research and technology. Surface charging: Exposure to the interplanetary radiation environment produces electrical charging of surfaces by electron ejection (by photons, ions or electrons) or by implantation (electrons, ions). While the charging of dust can be inferred by observation with dust sensors we do not have direct information on surface potentials of regoliths. It has been proposed that lunar transient events, such as the lunar horizon glow, are caused by levitation of charged grains from the surface [1,2]. Other hypotheses include electrical effects of rock fracture and venting of sub-surface gas. While electrostatic levitation has been observed under special laboratory conditions [3], it is not clear that it can occur on the Moon or on asteroids [4]. Grain charging can, rather, result in enhanced adhesion, through induced image forces with neighboring grains. Levitation requires the grain and grains below it to have charge of the same polarity and of enough magnitude to overcome adhesion forces and gravity. Charging of heterogeneous surfaces depends on location , since electron emission yields depend on the microscopic properties of the surface grain(s) and the retention of the charge on the electrical conductivity of the surroundings. Heterogeneity in composition should result in inhomogeneous surface charging, with local electric fields, even if the net charge of the surface is zero. This, in turn, will cause increased adhesion. Differential charging is seen in the laboratory in pho-toelectron emission experiments, which measure the electrostatic potential of the ejected photoelectron. Effects of electric fields on ion migration: Ions, such as Na, in glasses drift under electric fields, and therefore their surface concentration can be enhanced or depleted by negative or positive surface charging. [5] The surface concentration is of paramount importance in determining whether a particular atom can contribute to the exosphere by sputtering or photode-sorption [6]. Thus, in addition to its possible role of charging in the injection of dust into the lunar exo