One of the less understood phases in the history of the Solar System is that of the Solar Nebula, when the young Sun was surrounded by its circumstellar disk of gas and dust. The Solar Nebula was a relatively short phase, lasting no more than 10 Ma, yet across this timespan planetesimals and the first generation of planetary embryos were forming across the Solar System and, in some cases, they were differentiating due to the decay of short-lived radionuclides. At the same time, the giant planets were forming in the outer Solar System, capturing the nebular gas before its dispersal. Exploring this ancient time, however, is a task made difficult by the fact that most present planetary bodies either formed after this ancient phase (e.g. the Earth and the Moon) or underwent a long phase of active geological evolution that caused their resurfacing (e.g. Mars). According to recent results in the study of the Howardite-Eucrite-Diogenite suite of meteorites (Bizzarro et al. 2005; Schiller et al.
formed, differentiated and possibly solidified in the Solar Nebula and, therefore, it occupies a unique position in the whole Solar System to explore such ancient times. The arrival of the Dawn mission to Vesta on July 2011 and the data that the Dawn spacecraft is already supplying on the asteroid offer us a possibility to gain a deeper insight into the time of the formation of the giant planets and to constrain the evolution of the Solar Nebula. However, the first estimates of the cratering record on Vesta seem to indicate that crater counting cannot give information about times earlier than 4 Ga ago (Neukum et al. 2011). As a consequence, a different approach has to be used to investigate the ancient past of this asteroid.
The Jovian Early Bombardment
It was recently showed that the formation of Jupiter triggered a phase of primordial bombardment in the inner Solar System due to the appearance of orbital resonances with the giant planet in the asteroid belt and the scattering of planetesimals from the outer Solar System (Turrini, Magni & Coradini 2011; Turrini, Coradini & Magni 2012). This Jovian Early Bombardment (JEB in the following) was brief yet violent due to a more abundant population of planetesimals residing in the asteroid belt at the time. Across the JEB, collisional erosion played a more important role than catastrophic disruption in determining the fate of the planetesimals. Moreover, the intensity of the JEB depends on the extent of the migration of Jupiter and on the size-frequency distribution of the planetesimals populating the Solar Nebula. Different scenarios for the formation of Jupiter (e.g. the standard scenario or the "Grand Tack" scenario recently proposed by Walsh et al., 2011) and the planetesimals (e.g. their
formation in a
quiescent or turbulent Solar Nebula) would thus imply different intensities of the JEB. In particular, it was found that the JEB could have caused either the (partial or complete) erosion of the primordial crust of Vesta or the destruction of the asteroid, depending on the considered scenario (Turrini et al. 2012). As a consequence, assessing and searching possible signatures left on Vesta by the JEB can allow us to gather information on the Solar Nebula and the process of planetary formation in the Solar System.
The goals of the project
This project will assemble a team of experts on dynamics, impact physics, meteorites, geophysics and thermal evolution of asteroids to assess the timescale of the geophysical evolution of Vesta and the possible signatures (e.g maria or cryptomaria, surface erosion, cratering record) left by the JEB on its surface. While independent, the team will work in strict collaboration with the scientific team of the Dawn mission, with the goal of producing a reference frame for the interpretation of the data collected by the Dawn spacecraft and, ultimately, to use Vesta as a probe into the origins of the Solar System.