Constraining Source Crater Regions for Boulder Tracks and Elongated Secondary Craters on Eros

Introduction: Crater ejecta launched from the surface of small, rapidly-rotating, and highly-elongated and irregularly-shaped bodies are subjected to a complex dynamical process. Dynamical models [1,2] of reaccretion of impact ejecta on asteroids provide important tools for a detailed investigation of the distribution of blocks and finer regolith across Eros’ surface as revealed by the NEAR MSI instrument. In particular, ejecta blocks and/or other ejectablanket units might be linked back to specific source craters, yielding valuable information on physical properties of Eros (e.g., regolith structure and target strength) and constraining various aspects of impact cratering in low-gravity environments (e.g., ejecta mass/speed distributions and amount of retained ejecta). A few good examples of boulders with tracks (Fig. 1) can be found on Eros [3]. None of these appear to be the result of simple downslope rolling, suggesting that the boulders were emplaced directly at the termination of their sub-orbital, impact-induced trajectories. There are also several examples of small, oblong craters. Although some of these craters do not display obvious terminal boulders, their morphologies and small sizes are suggestive of an origin by oblique, secondary impact. If these are indeed secondary craters, their parent impactors may: (1) have skipped far enough downrange so that their association with the secondary crater is not readily apparent; (2) be below the limit of image resolution; (3) be buried below the terminus of the oblong crater; or (4) have been composed of a clod of ejecta too weak to survive reimpact onto Eros’ surface. These boulder tracks and possible secondary craters contain clues to the terminal trajectories of their parent impactors. In the case of boulders with tracks, the back-azimuth of the boulder’s terminal trajectory relative to Eros’s surface is unambiguous. For elongated secondaries lacking grossly asymmetric crater rims or not obviously blocked by high terrain or obstacles to one side or another, the incoming trajectory is at least constrained to two azimuths 180o apart. Without detailed knowledge of the state of compaction or cohesive strength of the regolith, track or secondary crater morphology alone is unlikely to be particularly diagnostic of the terminal trajectory elevation angle or impact speed. However, knowledge of only the terminal trajectory azimuth may allow some constraints to be placed on candidate source-crater regions for terminal boulders or secondary impactors. This in turn can be used to study the process of primary impact-crater excavation and to place additional constraints on regolith and subsurface material properties by providing ‘ground truth’ on boulder size/ejection speed relationships.