Inner Structure of Asteroid Revealed by Simulation

Models are regularly used to boost the significance of image and measurement data from space missions and help to understand our solar system. The simulation of a double impact that occurred on the proto-planet Vesta one billion years ago enabled scientists to describe the inner structure of the asteroid without even taking samples - although that can still be done in the future for definitive prove. The joint research from EPFL and Bern University, both in Switzerland, and researchers from France and the United States is on the cover of Nature this week.

Today, a 45-meter asteroid dubbed 2012 DA14 is drawing a lot of attention, since it will fly very close to the earth - even below the geosynchronous orbit that hosts thousands of satellites. Hopefully without harm, and it will soon be forgotten.

But for the longer-term studies, Asteroid Vesta is in the focus of astronomers and planetologists. With 500 km in diameter, Vesta is one of the three largest objects of the asteroid belt that formed simultaneously to the solar system we live in, four billion years ago. It is considered a “protoplanet” or a precursor of a planet. Moreover, it is the only known asteroid to have an earth-like structure – with a core, mantle and crust.

Employing 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 two large asteroids over a billion years ago. The models confirm that the protoplanet owes its botched, elliptical shape to these collisions and that they obviously scarred its surface structure.

Importantly, the simulations enables a better understanding of the evolution of the solar system by providing detailed conclusions about the composition and properties of Vesta’s interior. «Based on the sort and distribution of this material at the surface of the asteroid, we are able to precisely reconstruct the various inner layers of Vesta down to a depth of 100 km», explains Philippe Gillet, head of EPFL’s Earth and Planetary Science Laboratory.

The modelling was enabled in the first place by precise measurements of NASA’s «Dawn» probe, which in 2007 began its space voyage into the solar system’s past. From 2011 on, it closely orbited Vesta for one year. Images and analysis from within the visible range provided information on the asteroid’s topography and the composition of the minerals that are visible on its surface. This lead to the discovery that the crater, first observed by Hubble, at Vesta's south pole is actually composed of two partially overlapping basins.

Based on this data, the computer simulations calculated 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», rejoices Jutzi. “It also proves that the method we used to determine the composition of Vesta’s inner layers are accurate, adds Gillet. The simulation does show that some of the debris were ejected from a depth of 100 kilometers.”