At approximately 500 kilometres in size, Vesta is the largest known asteroid in the Solar System. Like its numerous companions in the Main Asteroid Belt, it is made of the 'primordial matter' of the Solar System. A new study published in Nature Astronomy concludes that Vesta was exposed to an extensive impact series of large rocky bodies much earlier than previously assumed. This suggests that the entire inner Solar System, and thus the rocky planets, was affected by such an early 'bombardment'. As such, this observation also provides important insights into the early development of our Earth. This is the conclusion reached by an international research team with the participation of geoscientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), the University of Heidelberg, the Free University of Berlin and the Berlin Natural History Museum.
As part of the study published today, Wladimir Neumann from the DLR Institute of Planetary Research and the Institute of Geosciences at the University of Heidelberg carried out numerous calculations to model Vesta's thermal evolution. This made it possible to better narrow down the time period in which the early impacts occurred. "In order for the material of the impacting bodies to be mixed into the rocky mantle of young Vesta in a reasonably homogeneous way, the mantle must have been hot enough and circulate convectively, driven by internal heat," Neumann explains. "Our models have shown that this is only true for impacts within the short time span of around 4.56 to 4.50 billion years ago – almost immediately after the formation of the planets in the inner Solar System."
Rare meteorites provide important evidence
Until now, it was assumed that the main phase of this bombardment occurred several hundred million years later, around the time that some of the large impact craters were formed on the Moon. However, for Earth's moon, and probably also for the terrestrial planets, this study indicates that the main mass of this bombardment reached the planets very soon after their formation, as occurred with Vesta.
This finding is also based on analyses of meteorites in terrestrial collections whose parent body is almost certainly Vesta – the so-called HED meteorites. The acronym HED is derived from the first letters of a subgroup of rare stony meteorites, the howardites, eucrites and diogenites. This group shows similarities to magmatic rocks on Earth. Due to their chemical composition, they must have come from an already differentiated planetary body in which heavy, metallic elements accumulated in a core that was surrounded by a lighter rock mantle and an even lighter crust, and in which magmatic processes caused changes.
Planetary bodies continued to grow as a result of bombardment
Numerical simulations and investigations using data on Vesta collected by NASA's Dawn spacecraft in 2011 and 2012 reveal a new picture of the chronology of collision history in the early Solar System. The Earth-like planets in the early Solar System initially grew via the clumping together of tiny, adhering dust grains. Then, in their final stage, by impacts of increasingly larger rocky bodies. This is also true of the asteroid Vesta. During the growth process, Vesta increasingly heated up during the early phase of its development, resulting in the formation of a near-surface ocean of molten silicate rock and a liquid metallic core below.
Over time, other bodies struck Vesta's crust, causing material to be hurled into space and transported into the inner Solar System. Thus, rock debris from Vesta occasionally reached Earth as meteorites. Chemical analyses of these meteorites have shown that even after Vesta's core formed, further cosmic impacts changed the composition of the asteroid's crust and mantle. "However, this supply of material was much greater during the early phase than afterwards" explains Harry Becker from the Free University of Berlin (FU Berlin), one of the authors of the study. Vesta was struck by at least two very large bodies from the Main Asteroid Belt, as evidenced by two impact basins several hundred kilometres in size at the south pole, which were discovered using a camera developed by DLR and the Max Planck Society on board the Dawn spacecraft.
Earth also had a magma ocean and a blazing hot atmosphere
Furthermore, the impacting bodies apparently did not originate in today's asteroid belt, as previously assumed. They originated in the inner Solar System, just like the terrestrial planets. "For Earth, this once again underlines the significance of an early hot phase with a magma ocean that was continuously replenished by large impacts. During this time, Earth's first atmosphere was red-hot for many millions of years. Only much later were oceans of water able to form as the hot water vapour cooled and rained down," explains Kai Wünnemann from the Natural History Museum and FU Berlin.
The research work at Heidelberg University was funded by the Klaus Tschira Foundation. The contributions from Berlin and Münster are part of the Collaborative Research Centre Transregio TRR 170 'Late Accretion onto Terrestrial Planets' which is funded by the German Research Foundation (DFG). The international study also involved scientists from the Macau University of Science and Technology (Macau), the Université de Nice Sophia-Antipolis (France), the University of California, Davis and the University of California San Diego (both USA), the University of Bayreuth, the Planetary Science Institute in Tucson (USA), and the Institute of Planetary Research of the German Aerospace Center (DLR).
M.-H. Zhu, A. Morbidelli, W. Neumann, Q.-Z. Yin, J.M.D. Day, D.C. Rubie, G.J. Archer, N. Artemieva, H. Becker, K. Wünnemann: Common feedstocks of late accretion for the terrestrial planets. Nature Astronomy (30 September 2021), https://doi.org/10.1038/s41550-021-01475-0