Chapman: Cratering on Asteroids 315 Cratering on Asteroids from Galileo and NEAR Shoemaker Clark R. Chapman Southwest Research Institute Asteroidal craters provide information about not only the geology of asteroids but also the populations of impactors. Significant statistics on crater densities as a function of diameter have been obtained from spacecraft imaging of Gaspra, Ida (and its moon), Mathilde, and Eros. At spatial scales larger than ~100 m, the saturated crater populations on Ida and Eros are similar. Gaspra is undersaturated with such craters, exhibiting a “steep” power-law production function. Mathilde is uniquely dominated by huge craters with diameters similar to the body’s radius. The NEAR Shoemaker camera sampled craters and boulders down to just a few centimeters in size on Eros. At spatial scales smaller than 10 m, craters are extremely rare on Eros and boulders are several hundred times more prevalent. These data are best understood if small projectiles are unexpectedly rare in the asteroid belt (forming few small craters and fragmenting few boul- ders); armoring of the surface by boulders, seismic shaking, and levitated dust may also con- tribute to the unexpected nature of the surface layer of Eros. 1. INTRODUCTION focus on the smaller-scale impacts that affect localities but not the bulk geophysical properties of these bodies; neces- As airless, solid bodies traveling in interplanetary space, sarily, impact cratering affects the variety of surface mor- asteroids are cratered by smaller asteroids and meteoroids as phologies and geological structures generally, as amplified is our own Moon. However, since asteroids are small, com- by Sullivan et al. (2002). paratively distant objects, groundbased telescopic observa- Asteroid surfaces also serve as witness plates, recording tions had been inadequate to resolve even the shapes of these impacts of objects too small to be seen by other techniques. bodies, let alone details of their surface geology such as im- For example, the three fly-by spacecraft imaged craters as pact craters. That changed beginning on October 29, 1991, small as tens to hundreds of meters in diameter, recording when the Galileo spacecraft obtained images of the 18-km- the impacts of bodies meters to tens of meters in size. While long, S-type, main-belt asteroid 951 Gaspra from as close as the number of such objects in near-Earth space is approxi- 5300 km (Veverka et al., 1994). On August 28, 1993, Galileo mately known from observations of rare, large bolides in acquired even higher-resolution images of the larger, S-type, the Earth’s atmosphere and from the Spacewatch Survey main-belt asteroid 243 Ida (Belton et al., 1996) and discovered (see Jedicke et al., 2002; Morbidelli et al., 2002), such small its moon Dactyl. Since then, NEAR Shoemaker has flown objects cannot be detected in the much more distant asteroid past and imaged a larger, C-type, main-belt asteroid, 253 belt or Trojan clouds. NEAR Shoemaker’s imaging of Eros Mathilde (Veverka et al.,1999). It then spent a year orbit- reveals craters as small as a couple of centimeters (actually, ing and mapping the surface of the S-type, Earth-approach- the virtual absence of such craters), providing insights about ing asteroid 433 Eros (see special NEAR Shoemaker issue still smaller interplanetary projectiles. of Icarus, January 2002). A few geological features like im- Impact cratering controls the attributes of asteroid sur- pact craters have also been identified in recent years from faces, which will affect potential human operations at aster- delay-Doppler radar imaging of several chiefly small, Earth- oids (mining asteroids for resources, attaching devices for approaching asteroids (cf. Benner et al., 2001; Ostro et al., purposes of deflection, etc.). Because of the low-gravity 2002), and a large crater has been identified from Hubble fields of asteroids, regolith processes are very different from Space Telescope imaging of 4 Vesta (Thomas et al., 1997). those well-studied on the Moon, leading to the possibility Studies of asteroidal cratering elucidate many fundamen- (actually observed, in the case of Eros) that surface prop- tal issues in planetary science. Asteroids have been geologi- erties on a human scale may differ radically from our lunar- cally inert since formative epochs; ever since, impacts have influenced expectations. Associated with regolith evolution, been the dominant process that has shaped their forms, of course, are issues related to the exposure of the optical sizes, and surface geology. Other chapters in this volume surfaces of asteroids (and meteorites derived from materi- focus on the large-scale interasteroidal collisions that cata- als once near the surface) to micrometeorites, solar-wind strophically disrupt them, creating fragments that form particles, etc.; these “space-weathering” processes alter the families and asteroidal satellites and generally modulate the optical properties of asteroids, presenting a challenge to asteroid size distribution. Such collisions also shape the telescopic, remote-sensing studies of asteroids (see Clark interior structures of asteroids (e.g., “rubble piles”). Here I et al., 2002). 315 316 Asteroids III The holy grail of planetary cratering, of course, is to faces are likely to be dominated by craters formed during determine the relative and, especially, absolute ages of cra- their earlier residence in the asteroid belt. tered surfaces — to apply the principles of “interplanetary Crater populations on asteroids should roughly mimic the correlation of geologic time” (Shoemaker et al., 1963). Actu- size distribution of impacting projectiles an order of mag- ally, absolute chronology is difficult to disentangle from nitude smaller in size. That is, depending on impact veloc- other variables, including the poorly known physical attrib- ity, target size (which determines the importance of gravity), utes of asteroid surfaces. Nevertheless, there are broad vari- and target strength (especially for smaller impacts and im- ations in crater densities and morphologies among the four pacts on smaller asteroids, where strength dominates over asteroids studied, as well as from place to place on Eros, gravity), a projectile forms a crater roughly 10× its size. which permit interpretations that address the fundamental For all asteroids, one might simplistically expect that the natures of these bodies. size distribution of impacting projectiles (and thus the diam- This chapter reviews measurements and interpretations eter distribution of craters, or the “production function”) of crater populations on Gaspra, Ida, its moon Dactyl, Ma- would be everywhere the same. There is the possibility, how- thilde, and Eros. My emphasis is on the general attributes ever, that the quasi-steady-state size distribution is different of the crater populations and on the processes that (1) form within the Trojan clouds, or in the zone between the main the craters and (2) degrade/destroy such craters as well as belt and the Trojans. Pending understanding of the reasons other features on asteroid surfaces. Necessarily, I will also for differences in bias-corrected size distributions correlated touch on rock/boulder populations on asteroid surfaces, with taxonomic types (Zellner, 1979; Gradie et al., 1989), since their production and retention is intimately entwined one might expect there to be size-distribution differences with cratering. between crater populations in the inner and outer main belt. Such differences, however, are probably modest, so the first- 2. SOME GENERALITIES ABOUT order approach is to compare the cratering populations on ASTEROIDAL CRATERING different asteroids assuming the same production function. Contrasting with the case of cratering on semi-infinite I begin by establishing a qualitative, largely theoretical planetary surfaces, the physics change as craters from larger context in which to place the spacecraft observations of real impacts approach the sizes of finite-sized asteroids. Cata- asteroidal cratering that follow. Many of these precepts have strophic disruption is defined to result when the largest rem- been understood since the original classic papers on aster- nant following collision has less than half the mass of the oid collisions (e.g. Dohnanyi, 1971) and asteroid regoliths original body. Nevertheless, the outcomes of much-smaller (e.g., Housen et al., 1979). Other elements of this picture impacts are sensitive to finite body size. First, there is the are still evolving, especially as computers have enabled case where the shockwave penetrates throughout the body realistic simulations of the processes that fracture asteroids, and is, perhaps, reflected, fracturing much of the body’s form asteroid families, and influence dynamical evolution interior [see simulations by Asphaug et al. (1998, 2002)]. of asteroids. An unfractured original body is forever changed after its There are good reasons for expecting that impact crater- first fracturing collision; it responds to subsequent impacts ing on asteroids has, to first order, been similar for the last as a fractured, weaker body. Larger impacts may spall off 3–4 b.y. on nearly all asteroids from near-Earth asteroids sizable portions of the target body, in addition to creating (NEAs) out to the Trojans. In general, cratering rates within craters that approach or even exceed the radius of the body the main asteroid belt (and in the Trojan clouds) are 2 or- itself. Finally, still-larger impacts that would form craters ders