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Alien Olivine on Vesta?

The olivine found on the surface of the Main Belt's asteroid studied by the Dawn mission may have been deposited in the last billion years by other celestial bodies, as a consequence of their impact. These are the results of a series of simulations conducted by a team of researchers led by Diego Turrini (INAF)
Alien Olivine on Vesta?

The image shows the geographic distribution of the olivine discovered on Vesta by Dawn. Credits: image created by D. Turrini projecting the locations of the olivine deposits over the map of Vesta produced by Dawn’s Framing Camera and Stereo Analysis team

The search for olivine was one of the main tasks of the Dawn mission during its exploration of Vesta. Olivine is the main component of the mantle of rocky planetary bodies as the terrestrial planets and differentiated asteroids like Vesta and, as such, should be buried below their crusts. In the case of Vesta, a planetary body whose geologic evolution ended more than 4 billion years ago, the presence of that mineral on the surface was always assumed to be a result of the mantle excavation caused by impacts. In this scenario, the amount of olivine on the surface of the asteroid would then be directly linked to the number of major collisions Vesta underwent across its life.

The Dawn mission did find olivine on the surface of Vesta, but both its quantity and its distribution did not match with what was expected if the mineral were excavated from the mantle by impacts. A way out from the "missing olivine" problem was proposed a few months ago by the Dawn mission itself (here and here) whose higher resolution data raised the possibility that the origin of the vestan olivine was to be found outside Vesta. Specifically, they suggested that the olivine seen on Vesta was brought in by some of the very impacts that shaped the surface of the asteroid.

The main issue with this alternative scenario was that, up to now, it was not easy to assess how likely it could be. An international team of researchers therefore decided to tag this problem by calculating the probability for such an hypothesis to work. They describe their results in a paper recently published on the journal Icarus.

"In our investigation we could take advantage of the contamination model we developed and tested to study the delivery by impacts of the dark material discovered by Dawn on Vesta" explains Diego Turrini, researcher at the Italian National Institute for Astrophysics and first author of the paper. "Our contamination model allowed us to evaluate how many asteroids (divided by compositional classes and sizes) could be statistically expected to hit Vesta over a given period of time and, from this, to estimate the flux of the main suspects for the delivery of olivine: the rare A-type asteroids (mainly composed by olivine) and more common S-type asteroids (whose olivine content ranges between 30% and 50%). Then we needed "only" to perform impact simulations of said projectiles on Vesta and input the results in our model to obtain an estimate of how much "alien" olivine they would deliver over time".

"These simulations allow us for estimating the amount and the distribution of the olivine left on Vesta by the impacts. Our results, aside for their interest for the specific case of the vestan olivine, suggest that the contamination of planetary bodies by exogenous materials is likely a common process in the Solar system" explains co-author Vladimir Svetsov, expert of impacts at the Russian Academy of Sciences. “Moreover, the velocity range for the impacts on Vesta and in the main asteroid belt is more similar to the one during planetary formation than that of impacts on Mars, the Earth and the Moon. Thanks to Dawn's data, Vesta is therefore proving a natural laboratory for investigating the role of impacts during planetary formation”.

Olivine, a common material in the mantle rocks of the inner planets of the Solar System, has been detected on Vesta with the analysis of hyperspectral data collected by VIR instrument aboard the Dawn probe. In the image, it is indicated in green outcrops of olivine within the Bellicia crater. Credits: Image generated by Alessandro Frigeri and Eleonora Ammannito (INAF) combining data of VIR spectrometer and images of Framing Camera on board Dawn spacecraft

According to the results of the study, most likely the delivery of the olivine identified to date on Vesta occurred only in the last billion years: basically "just yesterday" from the point of view of the long life of the asteroid. "Our results revealed not only that the exogenous hypothesis is indeed plausible, but also that the scenario for the delivery of olivine by impacts fits well the picture depicted by Dawn's data" said Turrini "We can reproduce what Dawn sees, namely the number and sizes of the small craters where olivine has been discovered, and also what we don't see. Specifically, our results can explain why we don't see olivine inside the giant basin Rheasilvia and why we don't see a more diffuse presence of olivine over the vestan surface".

In the best tradition of scientific research, the answers found by the team opened up new questions. If confirmed, an exogenous origin for the vestan olivine will only strengthen the mismatch between the Vesta we thought we knew before the arrival of the Dawn mission and that revealed by Dawn itself, casting more doubts on the real past history of the asteroid. "It’s possible that Vesta is actually not an intact differentiated asteroid at all" suggested co-author Guy Consolmagno, a meteoriticist at the Vatican Observatory. "Maybe instead it was the reaccretion of shards of other differentiated crusts piled on top of a relict iron core, four and a half billion years ago. That would explain why we don’t see any mantle olivine on its surface today, but only contamination from later impacts".

On the other hand, however, the team pointed out that, should the exogenous hypothesis be disproved, their results would suggest that the number of olivine-rich A-type asteroids, thought to be fragments from the mantles of ancient differentiated asteroids destroyed by collisions, should be even lower than what the available data suggests. That would mean that whatever process removed them from the main belt should have been even more intense and efficient than previously thought.

"This would mean that the primordial evolution of the Solar System was more violent than what was assumed until a few years ago and would support the recent scenarios hypothesizing that the Solar System is the result of events associated to one or more episodes of extreme migration of the giant planets" commented Turrini "The only thing we can be sure of is that the data provided by the Dawn mission are providing a unique window on the most ancient past of our planetary system".

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