Mega-ejecta on asteroid Vesta

摘要

Asteroid 4 Vesta, sometimes called the "smallest terrestrial planet", will be orbited next July by NASA's Dawn mission. This will be the first time a small planet is visited by a spacecraft, and novel geological structures are expected. A key issue regarding Vesta (mean diameter 530 km) is to what extent its geology is dominated by the ～460 km diameter impact basin on its southern hemisphere. We model the basin's formation using a very high resolution 3D smooth-particle hydrodynamics simulation to establish some of the major impact-related aspects of Vesta's geology. The goal is to provide a framework for interpreting anticipated observations of landforms. A collision of this magnitude (a ～50 km diameter impactor at ～5 km/s) exposes many deep strata from within the planet and offsets the center of mass by ～10 km from the center of figure. Vesta spins every 5.3 hr, so that a hemispheric-scale impact evolves in a non-inertial frame, and deposits variably-shaped, multiply-folded and abruptly-terminated ejecta sequences of regional scale. Since little of this ejecta would have been molten, these massive sequenced deposits could be mistaken in images of Vesta for other geologic forms such as thrusts and folds. Detailed mapping, and the piecing-together of mega-ejecta via impact models, will enable an informed understanding of the interior geology of Vesta.