Letter to the Editor

A large crater as a probe of the internal structure of the E-type asteroid Steins

¹ Earth and Planetary Sciences Department, University of California, 1156 High Street, Santa Cruz California 95060, USA
² University of Nice-Sophia Antipolis, CNRS UMR 6202 Cassiopée, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France e-mail: michelp@oca.eu
³ University of Bern, Physikalisches Institut, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 26 October 2009
Accepted: 8 December 2009

Abstract

Context. The detection of a large crater (2 km in diameter) on Steins, a diamond-shape asteroid with a diameter of only about 4.6 km, was a large surprise given the size of the asteroid itself. Steins belongs to the rare class of E-type asteroids, which had not been observed by any spacecraft before the Rosetta mission of the European Space Agency (ESA) in 2008.

Aims. We demonstrate that this large crater places constraints on both the internal structure of Steins and its age based on crater counting.

Methods. Numerical simulations of impacts were performed to reproduce the large crater, assuming four different initial internal structures of the asteroid: monolithic with or without microporosity, and rubble pile with or without microporosity.

Results. To reproduce this crater, Steins must be either a rubble pile, which also contains microporosity, or a monolithic body with or without microporosity. The lack of porosity in meteorite analogues favors a structure without microporosity, which, according to our results, must have been monolothic. Moreover, the asteroid would be transformed into a rubble pile structure as a result of the crater formation, allowing it to be reshaped in its current shape by the YORP spin-up thermal effect.

Conclusions. A combination of modeling and observations of surface features can thus serve as a probe of the internal structure of a small body and constrains its age estimate. Since the surface was totally refreshed when the large crater was formed, crater counting on Steins indicates the time that has passed since this impact event occurred.