Four and a half billion years ago, dust particles in a giant, dusty gas cloud combined to form increasingly large clumps. These collided, aggregated and thus grew into planets. Between the planetary orbits of Mars and Jupiter, however, hundreds of thousands of smaller fragments remained. They formed the so-called asteroid belt and hardly changed their composition since then. Asteroids thus contain an inestimable amount of information on the origin of our solar system. In research, particular attention is paid to an asteroid called Vesta: with a diameter of around 500 kilometres, it is one of the three largest asteroids and considered to be a protoplanet. Moreover, it is the only known asteroid to have an earth-like structure - with a core, mantle and crust.
Using a three-dimensional computer simulation, Martin Jutzi from the Center for Space and Habitability (CSH) at the University of Bern has now accurately reconstructed how Vesta collided with other asteroids twice over a billion years ago. The models show that the protoplanet owes its elliptical shape to these collisions and that they also scarred its surface structure. The simulations also enable detailed conclusions on the composition and properties of Vesta's interior to be drawn for the first time, which helps us to better understand the evolution of the solar system.
After all, the formation of planets largely depends upon collisions between celestial bodies. "Our method facilitates especially informative analyses of image and measurement data from space missions", stated Martin Jutzi. The study, which was conducted in collaboration with researchers from the EPFL, France and the USA, features as the cover story inNature.
Previously, observations with the Hubble Space Telescope provided initial evidence of a giant crater at Vesta's south pole. Then, in 2007, NASA's "Dawn" probe began its space voyage into the solar system's past. Starting in the summer of 2011, it closely orbited Vesta for one year. Images from within the visible range as well as other measurement data provided information on the asteroid's topography and the composition of the minerals that are visible on its surface. It became apparent that the crater observed by Hubble at Vesta's south pole is actually composed of two partially overlapping basins.
Based on this information, the computer simulations by Martin Jutzi's team now demonstrate exactly how two consecutive impacts of celestial bodies led to the formation of the observed overlapping basins, which almost span the entire southern hemisphere of Vesta. The models show the size (66 and 64 kilometres in diameter), velocity (5.4 kilometres per second) and the impact angle of the bodies that collided with Vesta. This reveals a lot about the properties of the objects that were near the protoplanet a billion years ago.
The final images of the simulations closely resemble the shape and topography of Vesta's southern hemisphere as observed by the Dawn mission. The models even accurately reproduce the spiral-shaped structures inside the youngest crater which are visible on images from the Dawn mission. "This shows how reliable our method is", rejoiced Martin Jutzi. The researchers assume that the models also provide information about previously hidden features of Vesta. For instance, the simulations reveal that the material exposed by the impacts comes from depths of up to 100 kilometres. "Based on the sort and distribution of this material we are able to precisely reconstruct the various inner layers of Vesta", explained Philippe Gillet, Direktor des Earth and Planetary Science Laboratory der EPFL.
"The fact that we can now look inside such protoplanets makes entirely new perspectives in the research on the history of our solar system possible", stated Martin Jutzi.
M. Jutzi, E. Asphaug, P. Gillet, J.-A. Barrat, W. Benz are the authors of the study titled "The structure of the asteroid 4 Vesta as revealed by models of planet-scale collisions", which apprears inNature, 14 February 2013, in print.