Detecting crater ejecta-blanket boundaries and constraining source crater regions for boulder tracks and elongated secondary craters on Eros

Abstract

We present results of a numerical model of the dynamics of ejecta emplacement on asteroid 433 Eros. Ejecta blocks represent the coarsest fraction of Eros' regolith and are important, readily visible, "tracer particles" for crater ejecta-blanket units that may be linked back to specific source craters. Model results show that the combination of irregular shape and rapid rotation of an asteroid can result in markedly asymmetric ejecta blankets (and, it follows, ejecta block spatial distribution), with locally very sharp/distinct boundaries. We mapped boulder number densities in NEAR-Shoemaker MSI images across a portion of a predicted sharp ejecta-blanket boundary associated with the crater Valentine and confirm a distinct and real ejecta-blanket boundary, significant at least at the 3-sigma level. Using our dynamical model, we "back track" the landing trajectories of three ejecta blocks with associated landing tracks in an effort to constrain potential source regions where those blocks were ejected from Eros' surface in impact events. The observed skip distances of the blocks upon landing on Eros' surface and the landing speeds and elevation angles derived from our model allow us to estimate the coefficient of restitution, ε, of Eros' surface for impacts of 10-m-scale blocks at approximately 5ms -1 impact speeds. We find mean values of ε of approximately 0.09-0.18. © The Meteoritical Society, 2012.