Vesta’s Crust And The Tale Of The Birth Of The Solar System

Investigating the birth of the Solar System is a bit like investigating the crime scene of a “cold case,” where the passage of time, here just about four and a half billions of years, canceled most traces left by the events we are trying to reconstruct. Luckily for us, the Solar System plays fair and is not actively misleading us, but, at the same time, it does not leave around a lot of clues. As in real crimes, things can get a lot easier to disentangle if we have a reliable witness.

Vesta, one of the two asteroids visited by the NASA mission Dawn, has been one of our best witnesses since the ‘70s, when it was discovered that its infrared spectrum was uniquely matched by those obtained in laboratory for a family of meteorites, the Howardite-Eucrite-Diogenite group (also known cumulatively as HED), suggesting some sort of link between the two.

These meteorites are fragments of the volcanic surface of a body that underwent differentiation like the Earth, acquiring a layered internal structure where core, mantle, and surface are characterized by very different compositions. Later studies pointed out that this volcanic crust should be relatively thin and that in our collection of meteorites we don’t have samples of the underlying mantle of the body that originated the HEDs.

The fact that the volcanic crust of Vesta was still intact, as suggested by astronomical observations,  supplied a simple explanation for the lack of fragments of mantle material linked to the HEDs among the meteorites landed on Earth, and strongly indicated in Vesta the source of the HED meteorites.

Vesta’s volcanic crust was both thin and intact, which meant that our witness Vesta was telling us a  simple story: whatever the scenario we devised, it should not destroy its crust. This became a fundamental constraint for all studies made in the following 40 years. Thanks to the HEDs, moreover, Vesta was also telling us that this fundamental constraint encompassed almost all the life of the Solar System since its crust formed when the young Sun was still surrounded by a disk of gas and dust.

Given that our view of the history of the Solar System evolved since the ‘70s to include also violent scenarios where its planetary bodies were sculpted by bombardments caused by the formation of the giant planets in this circumsolar disk and by their later migration, the constraint set by Vesta’s crust was extremely powerful. Or so we thought.

When Dawn arrived to Vesta, it confirmed us that its crust is indeed made of the same material as the HED meteorites, confirming the genetic link between Vesta and HEDs. Dawn also confirmed that the crust is indeed still intact… including where two giant impacts excavated at depths greater than its total thickness and should have exposed the underlying mantle! This finding revealed that something was wrong in our picture and that our witness was not as reliable as we thought.

A possible explanation for this mismatch is that Vesta was once larger and had a thicker crust, but exactly how large and how thick is unknown. A larger crust means that Vesta could have lost much more material than we thought in the last 40 years and still be similar to the Vesta that we see today. A larger Vesta, moreover, has a stronger gravity, which means that a greater fraction of the initially lost material would be later recaptured by the asteroid.

This unforeseen obstacle severely limited the usefulness of the constraint provided by Vesta’s crust, allowing it to exclude only extreme scenarios where giant impacts would have globally damaged or even destroyed the asteroid. Our witness Vesta, however, had not finished telling us its tale.

Recent studies of the composition of the HED meteorites revealed that some of them presented over-abundances of water and of siderophile elements (i.e. elements showing affinity to metals and therefore expected to be dominantly segregated in Vesta’s metallic core) and provided upper limits to their global presence in Vesta’s crust.

In a study recently published on the journal Icarus and born in the framework of the project “Vesta, the key to the origins of the Solar System” funded by the International Space Science Institute in Bern, we set out to investigate whether this new piece of information could allow us to reassess Vesta’s role as a witness of the youth of our Solar System. Our reasoning was simple and relied on the fact that, alongside their destructive effects, impacts have constructive effects too.

The same impacts that strip away volcanic material from Vesta’s crust also deliver new material to it. The new material brought by impacts will always contain siderophile elements (as they are carried by both asteroids and comets) and in some cases also water (as it is carried by comets and some types of asteroids). We, therefore, investigated whether the upper limits to their presence provided by the HED meteorites could be used as new constraints.

In principle, the uncertainty on the thickness of Vesta’s crust makes it difficult to assess how much the impact-delivered water and siderophile elements would be diluted when mixed with Vesta’s original material. What we found, however, is that this uncertainty has negligible effects: the limits on the concentrations of these two materials revealed by the HED meteorites are so low that the constraints they provide work also in cases where the initial thickness of Vesta’s crust is a few times larger than what we originally thought.

As we point out in our study, these new constraints should be used in conjunction with the original one, not substitute it. The new constraints, in fact, work in all cases except one: when Vesta’s bombardment is dominated by violent, destructive events that remove large chunks of material from the crust without delivering neither water nor siderophile elements. This, however, is the kind of scenario where the classical constraint posed by the survival of Vesta’s crust still holds.

The joint use of the three constraints permits to adopt what can be called a “Sherlock Holmes” approach: all scenarios violating even one of the three constraints can be rejected as impossible, leaving only the most realistic scenarios. This approach is simple, but the joint use of the three constraints makes it very diagnostic.

In the proof-of-concept case study we used to test this methodology, where we focused on the effects of Jupiter’s formation and migration on Vesta’s crust, we found that we could reject three out of four of the migration scenarios we considered. Within each migration scenario, moreover, we found that this methodology is also sensitive to the size distribution of the primordial populations of comets and asteroids, indicating that large populations of bodies bigger than 10 km in size tend to produce worst fits to the data than populations dominated by smaller bodies.

These findings are described in the article entitled The late accretion and erosion of Vesta’s crust recorded by eucrites and diogenites as an astrochemical window into the formation of Jupiter and the early evolution of the Solar System, recently published in the journal Icarus. This work was conducted by D. Turrini from the Institute of Space Astrophysics and Planetology INAF-IAPS, V. Svetsov from the Russian Academy of Sciences, G. Consolmagno from the Specola Vaticana, S. Sirono from Nagoya University, and M. Jutzi from the University of Bern.

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