Identification and physical properties of craters on Asteroid (2867) Steins

We report on the physical properties of the craters of Asteroid (2867) Steins based on an analysis of images obtained with the OSIRIS instrument during the Rosetta flyby that took place on 5 September 2008. To compensate for the limited spatial resolution of the images and unfavorable illumination conditions, we applied three different filtering techniques designed to enhance the craters detection. The selected images covered 44% of the overall surface of the asteroid and 42 craters were detected. We calculated their diameter and depth using a forward modeling procedure that combines a global shape model with an idealized semi-hemispherical representation of the craters, further incorporating photometric properties to generate simulated images. The resulting images were then compared to the original images. Crater diameters range from 150 to 2100 m with depth-to-diameter ratios of 0.04-0.25, a wide range consistent with other small bodies. A striking dichotomy was observed between the two sides of the asteroid reflected by both a ratio of 3.6 between the spatial density and very different power exponents of the size distribution functions, -3.3 versus -1.5. Altogether these peculiar properties most likely reflect the particular history of Steins, in particular the impact that created the largest crater, Diamond, and extensively damaged the original Steins. A combination of various processes, including destruction, erasure, direct or indirect infill by seismic shaking, and even the YORP effect can explain the observed physical properties of the craters on Steins. A few craters may be drainage basins connected to a hypothetical fault, itself a product of the Diamond event. (C) 2012 Elsevier Inc. All rights reserved.