Asteroid Surface Geophysics Naomi Murdoch Institut Sup´ erieur de l’A´ eronautique et de l’Espace (ISAE-SUPAERO) Paul S´ anchez University of Colorado Boulder Stephen R. Schwartz University of Nice-Sophia Antipolis, Observatoire de la Cˆ ote d’Azur Hideaki Miyamoto University of Tokyo The regolith-covered surfaces of asteroids preserve records of geophysical processes that have oc- curred both at their surfaces and sometimes also in their interiors. As a result of the unique micro-gravity environment that these bodies posses, a complex and varied geophysics has given birth to fascinating features that we are just now beginning to understand. The processes that formed such features were first hypothesised through detailed spacecraft observations and have been further studied using theoretical, numerical and experimental methods that often combine several scientific disciplines. These multiple approaches are now merging towards a further understanding of the geophysical states of the surfaces of asteroids. In this chapter we provide a concise summary of what the scientific community has learned so far about the surfaces of these small planetary bodies and the processes that have shaped them. We also discuss the state of the art in terms of experimental techniques and numerical simulations that are currently being used to investigate regolith processes occurring on small-body surfaces and that are contributing to the interpretation of observations and the design of future space missions. 1. INTRODUCTION Before the first spacecraft encounters with asteroids, many scientists assumed that the smallest asteroids were all monolithic rocks with a bare surface, although, there had been a few articles suggesting possible alternative sur- face properties and internal structures (e.g., Dollfus et al. 1977; Housen et al. 1979; Michel et al. 2001; Harris 2006). Given the low gravitational acceleration on the surface of an asteroid, it was thought that regolith formation would not be possible; even if small fragments of rock were cre- ated during the impact process nothing would be retained on the surface (e.g., Chapman 1976). However, the NASA Galileo, NEAR-Shoemaker (hereafter simply NEAR) and the JAXA Hayabusa space missions revealed a substantial regolith covering (951) Gaspra, (243) Ida, (433) Eros (Sulli- van et al. 2002; Robinson et al. 2002) and (25143) Itokawa (Fujiwara et al. 2006). In addition to finding each of these bodies to be regolith-covered, there is strong evidence that this regolith has very complex and active dynamics. In fact, it was due to the NEAR observations of Eros that the local gravity was first understood to be of importance to asteroid surface processes (Robinson et al. 2002). The importance of gravity for regolith dynamics was emphasised even fur- ther when the first images were received from the Hayabusa probe. Over the course of these space missions and others a wide range of geological features have been observed on the surfaces of asteroids and other small bodies such as the nu- cleus of comet 103P/Hartley 2 (Thomas et al. 2013). How- ever, we do not have direct access to the properties of the granular material that led to these features. Although con- stitutive equations exist for granular interactions on Earth, the inferred scaling to the gravitational and environmental conditions on other planetary bodies such as asteroids is currently untested. Understanding the dynamics of gran- ular materials in the small-body gravitational environment is vital for the interpretation of their surface geology and is also critical for the design and/or operation of any device planned to interact with their regolith-covered surfaces. Regolith was originally defined as “a layer of fragmented debris of relatively low cohesion which overlies a more co- herent substratum” (Shoemaker et al. 1968), although, this definition runs into difficulties when there is no clear inter- face separating the fragmented debris and the coherent sub- strate (Robinson et al. 2002). Here we will use the term regolith to describe, in general terms, the “loose uncon- solidated material that comprises the upper portions of the asteroid” (as defined in Robinson et al. 2002). However, we note that self-gravitating aggregates like Itokawa, often referred to as “rubble piles” (Richardson et al. 2002), are composed of rubble - boulders of the order of tens of me- tres and less - held together by gravity and cohesive forces instead of being a monolithic body (Fujiwara et al. 2006). 1 arXiv:1503.01931v1 [astro-ph.EP] 6 Mar 2015