Cratering on Asteroids Simone Marchi Southwest Research Institute Clark R. Chapman Southwest Research Institute Olivier S. Barnouin The Johns Hopkins University Applied Physics Laboratory James E. Richardson Arecibo Observatory Jean-Baptiste Vincent Max-Planck Institute for Solar System Reasearch Impact craters are a ubiquitous feature of asteroid surfaces. On a local scale, small craters puncture the surface in a way similar to that observed on terrestrial planets and the Moon. At the opposite extreme, larger craters often approach the physical size of asteroids, thus globally affecting their shapes and surface properties. Crater measurements are a powerful investigation means. Crater spatial and size distributions inform us of fundamental processes, such as asteroid collisional history. A paucity of craters, sometimes observed, may be diagnostic of mechanisms of erasure that are unique on low-gravity asteroids. By-products of impacts, such as ridges, troughs, and blocks, inform us of the bulk structure. In this chapter we review the major properties of crater populations on asteroids visited by spacecraft. In doing so we provide key examples to illustrate how craters affect the overall shape and can be used to constrain asteroid surface ages, bulk properties, and impact-driven surface evolution. 1 1. INTRODUCTION Until the space age, craters had been observed on a single astronomical body, the Moon. It was only with the analysis of the first lunar samples, however, that it finally became clear that the vast majority of lunar craters (and a recognizable minority of terrestrial craters) were caused by cosmic impacts and were not generally of volcanic or other endogenic origin (e.g., Wilhelms, 1993). Three decades later, when the Galileo spacecraft flew past Gaspra, then a few years later past Ida, craters were found on asteroids. During the subsequent two decades, spacecraft flybys and dedicated orbital missions have recorded crater populations on many additional asteroids. While hypervelocity impact by asteroids and comets or by their debris will produce impact craters on any solar system body with a solid surface, there is a fundamental difference between craters on small bodies and those on larger planets and satellites: instead of cratering on semi-infinite surfaces, asteroidal cratering occurs on smaller bodies generally with minimal gravity. So the ejecta from an impact explosion travel far and often escape into independent orbits around the Sun, becoming individual small asteroids. Collisional fragmentation and cratering are major evolutionary processes for asteroids since the earliest epochs of solar system history and learning about the visible record of surficial cratering can provide vital clues about their evolution and interactions with the space environment. Cratered terrains provide snapshots of collisions that occurred eons ago, and in turn, inform us about the origin of the impactor populations that have shaped the surfaces of all but the most geologically active bodies. Moreover, craters excavate deep to reveal underlying layers, perhaps differing from surface materials, while some of the escaped material can eventually lend on the Earth as meteorites. The fundamental observable property of a crater is its size, and the fundamental property of a population of craters concerns the ratio of the number of small craters to large craters, that is the size- frequency distribution (SFD). As with crater populations on larger planets and satellites, there are additional factors that interfere with a direct inference of the projectile population from the observed crater SFD. These include saturation of craters (the maximum number of craters that can be accommodated on a given surface), formation of secondary craters (craters made by impact of ejecta rather than from the primary cosmic projectile impacts), as well as often size-dependent processes that erase craters or alter their morphology (downslope mass-wasting, pit-formation by volatile release, etc). Crater SFDs are also affected by the properties of the target material (e.g., hard rock, rubbly megaregolith, icy or volatile-rich material); these can vary not only spatially across the target body's surface but also in the vertical dimension, so that scaling of impactor size to crater size may actually vary across the surface and with impactor size. Furthermore, energetic collisions may drastically alter the bulk properties of asteroids, and scramble their surfaces by producing surface features such as troughs, ridges, and grooves. All these issues may initially manifest themselves as problems due to our limited knowledge of asteroid properties, but, if one regards them as potentially decipherable challenges, they may eventually enable crater studies to reveal many properties of asteroid interiors, surfaces, and geological processes. In this chapter, we attempt to summarize the most up-to-date understanding of asteroidal cratering processes, emphasizing presentation and interpretation of the more recent spacecraft data (e.g., from Vesta, Lutetia, and Itokawa), while also updating interpretations of earlier results from Gaspra, Ida, Mathilde, Eros, and some smaller targets of opportunity. 2 2. CRATER STATISTICS The identification of impact craters is a challenging process. One may think that craters should resemble nice, sharp bowls, but observations of craters on terrestrial bodies and asteroids readily show that this is naive. In reality, cratered landscapes evolve over time under various forces, such as cratering itself, mass wasting, and other endogenic geological processes, which especially hamper our ability to identify old, degraded craters. Furthermore, there is no unanimously accepted standard procedure to map craters, and researchers need to rely on their own bag-of-tricks. For instance, when a group of experienced mappers were given the same image from which to count craters, it was found that the results could differ by a factor of two (Robbins et al., 2014). In this section we introduce the topic of crater statistics, and chiefly the primary diagnostic tool of crater size-frequency distributions (SFDs). Further, we discuss how crater SFDs can reveal important processes that modify or alter the production population of craters − that is, the crater SFD per unit time that results from the mainly asteroidal projectile population − and are a powerful tool to infer relative and absolute ages of various terrains along with aspects of their bulk mechanical properties. 2.1. Crater Size-Frequency Distributions In this section we review crater SFDs of asteroids visited by spacecraft. As mentioned above, the identification of impact craters can be cumbersome, particularly when they are degraded and heavily modified by post-formation processes. In addition, oddly shaped asteroids often show large facets that sometimes are interpreted to be the result of impact sculpting or to be a consequence of their rubble pile structure. Here we take the approach of showing selected examples from the various asteroids to illustrate key processes, rather then presenting a global compilation of every measured crater SFD. In doing so we opt to show crater SFDs from selected references along with some newly measured crater SFDs, and remind readers that there are additional, and sometimes different, counts in the published literature (e.g., Schmedemann et al., 2014). An important and often neglected factor that makes cratering on asteroids different from cratering of terrestrial body surfaces, is the fact that the physical sizes of visited asteroids vary by more than three orders of magnitude. As a result, craters form and evolve under very different conditions. Here we start our discussion with the largest bodies and continue with smaller asteroids. 2.1.1. Large asteroids. Vesta is the largest asteroid so far visited by a spacecraft (Russell et al., this volume). The NASA/Dawn mission orbited Vesta for more than one year gathering images of 98% of the surface. The large surface and coverage makes Vesta the best example so far to study cratering on a large asteroid. In addition, Vesta formed within a few Myr after the first solar system solids (e.g., McSween and Huss, 2010), implying that its surface has been subject to extensive cratering throughout nearly all of solar system evolution. As anticipated, the surface of Vesta exhibits an extremely diverse set of crater populations. The significant population of craters (~10-15) larger than 50 km including a few old degraded structures witnesses the heavy collisional history, recorded primarily in the northern hemisphere (Marchi et al., 2012a). The southern hemisphere, on the contrary, has been obliterated by the two largest impact structures, the ~400-km Veneneia and ~500-km Rheasilvia basins. As a result the overall spatial distribution of craters is rather heterogeneous and shows a marked north-south asymmetry. This is easily seen in the global crater distribution, and in the resulting global average crater density (Fig. 1). The formation of Veneneia and Rheasilvia had major effects on the whole surface, as manifested by the extensive troughs and voluminous ejecta blanketing (Schenk et al., 2012; Buczkowski et al., 2012; Yingst et al., 2014). Mapping of these and other geological features led to the 3 development of a well-defined time-stratigraphic system (Williams et al., 2014). In this system, the youngest epoch − Marcian − begins with the time of formation of the freshest of the large craters, the ~70 km Marcia. The second