Investigating the earliest and least known phases of the history of the solar system, when the young Sun was still enveloped by the disk of gas and dust where its planets began to form, is probably one of the most complex challenges in modern planetary science.

The celestial bodies formed at the time that survived intact to now are few and in the majority of cases their "memory" of the ancient processes that marked the birth of the solar system has been canceled or otherwise altered by the environments to which they were exposed or by the later events that shaped their evolution.

For decades the asteroid Vesta has been one of our most reliable witnesses of this ancient past: in particular, the survival of its thin volcanic crust to impacts provided a powerful constraint to how violent the solar system was in its youth. Recently, however, the data collected by NASA's Dawn mission, which is now coming to its end after successfully exploring also asteroid Ceres, raised the possibility that Vesta's memory may not be as good as we thought.

On one hand, the craters produced by impacts on its surface in the last 4 billion years seem to have erased the traces of the much older ones formed in the circumsolar disk. On the other hand, the possible greater thickness of its crust with respect to what was suggested by the HED meteorites (which the Dawn mission confirmed being Vesta's crust fragments that landed on Earth) makes the information provided by the crust survival very vague.

Are we back to square one, then? The results of an international team of researchers led by Diego Turrini of INAF-IAPS, published in the journal Icarus, suggest differently. "It is all about changing our perspective and, instead of focusing only on the destructive effects of impacts as we have done so far, consider the constructive ones too," Diego Turrini explains.

"Each impact removes material from the surface of Vesta, but at the same time brings new material to it," continues Vladimir Svetsov, member of the team and co-author of the study. "By balancing the two effects it is possible to extract more and more precise information from the data of Dawn and HEDs."

The results of laboratory studies of HED meteorites had revealed in recent years how some of these meteorites present superabundances of water and of siderophilic elements (i.e., elements showing affinity to metals and therefore expected to be mainly segregated in Vesta's metallic core) and provided upper limits to their global presence in Vesta's crust.

Depending on its composition (for example a rocky asteroid or a comet), each impact can bring one or both of these materials, modifying the composition of the crust: the team then wondered if it was possible to use the global limits on the presence of these materials set by the HEDs in synergy with the data provided by Dawn to constrain the primordial flow of impacts on Vesta and, consequently, the ancient history of the solar system.

"Instead of focusing on the direct search for the true evolutionary path of the primordial solar system, in our study we use a 'Sherlock Holmes' approach showing how the destructive and constructive effects of impacts can be used to exclude all impossible scenarios (as incompatible with observational and laboratory data), leaving only the realistic ones," explains Guy Consolmagno, also member of the team and co-author of the study.

"Sherlock Holmes said to eliminate the impossible, but sometimes it's not so easy to admit what is impossible," adds Consolmagno. "The Vesta we found when Dawn arrived was different from what we had expected, but we have to deal with the Vesta that is, not the Vesta that we thought."

Given the large number of parameters that can be varied in this type of investigation, the team focused on a case study based on the formation and migration of Jupiter. Scientists now believe that the giant planet formed in a different region of the solar system than the one it inhabits nowadays and then progressively migrated to its current orbit.

The results obtained by the team indicate how this approach not only allows to establish whether Jupiter has migrated or not, but also allows to constrain how much did it migrate.

"While we could consider only a subset of all proposed migration scenarios, balancing the crust erosion by impacts with water accretion and mass accretion allows us to reject three scenarios out of the four we simulated," explains Svetsov. "While a modest migration of Jupiter by about 0.25 au is consistent with the data, larger migrations (up to 1 au) or no migration at all are to be excluded."

"The most innovative aspect of our results is possibly the fact that the joint use of the information provided by Dawn and the HEDs allows to perform, for the first time, quantitative comparisons between different models," Turrini concludes.

"Our main goal in this study was to provide the planetary science community with a new tool of investigation. The different groups involved all around the world in the study of the origins of the solar system can now use it to verify their models and evolutionary scenarios."

Research Report: "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," D. Turrini, V. Svetsov, G. Consolmagno, S. Sirono and M. Jutzi, 2018 Sep. 1, Icarus

Related Links
Italian National Institute Of Astrophysics
Asteroid and Comet Mission News, Science and Technology

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