The large asteroid Vesta has had volcanic activity for 30 million years

When'astronomer German Heinrich Olbers discovered on March 29, 1807 theasteroid Vesta, he could not have imagined that he would one day be visited by an emissary of Humanity, the probe Dawn of the Nasa who studied it closely, from July 2011 to August 2012, before leaving for the dwarf planet Ceres. Dawn revealed a highly cratered surface for this celestial body which became the largest asteroid in the main belt of the Solar system between March and Jupiter. This title was formerly held by Ceres before we reclassify it as dwarf planet.

Olbers could not know either that with an average size of around 530 kilometers, Vesta would be considered in the XX^e century as a representative fossil planetesimals, these small celestial bodies whose sizes are roughly between 10 and 1,000 kilometers and which were introduced into the cosmogonic theories of the planets, first by Chamberlin and Moulton, and above all by Viktor Safronov. It is by colliding under the effect of their gravitational pulling forces that the planetesimals gave birth to the embryos of planets and then to the planets themselves. We therefore grasp all the interest of the study of Vesta by Dawn to understand more precisely the genesis of the Solar System about 4.56 billion years ago.

Meteorites similar to basalts on Earth

Dawn confirmed by her spectral analyzes of the mineralogical composition of the surface of Vesta what we already knew from similar instruments equipping the telescopes earthly for decades, namely the presence of igneous rocks witnesses of past volcanic and magmatic activity. Better, the ghosts obtained are very similar to those of a group of three types of meteorites achondrites well known on Earth Vesta, whose origin, notably due to the similarity of the spectra, was already attributed to Vesta for a long time. meteorites HED (for Howardites-Eucrites-Diogénites).

The eucrites are very similar to basalts that can be found on Earth, for example in the form lava flows from Hawaiian volcanoes. They are essentially made up of small crystals of pyroxene and plagioclase, which implies rapid cooling while the diogenites, also made up of pyroxene and plagioclase, with a littleolivine, have much larger crystals. This implies a much slower cooling and therefore suggests that the diogenites come from the depths of Vesta and that they were excavated by very violent impacts, the very ones that formed the large craters on the surface of Vesta. Howardites are breccias composed of fragments of eucrites and diogenites, sometimes with some chondrules carbon.

The eucrites were therefore issued by a volcanic activity important since a good part of Vesta's surface is covered with it. Complemented by the study of the field of severity of Vesta by Dawn, all these data indicate that the asteroid is a differentiated body, like the Earth, having a dense nucleus of nickel and of iron whose diameter would be between 214 and 226 kilometers, a coat and an crust. Vesta can therefore be considered to have frozen a glimpse of how rocky planetary bodies, including the Earth, cooled, solidified and formed during the first tens of millions of years in the history of the Solar System. .

Now, you should know that there are good reasons to think that many planetesimals at the beginning of the history of the Solar System were heated strongly and quickly, to the point of developing volcanoes, essentially by the disintegration of a isotope of the'aluminum short duration of life, aluminum 26. It would have been produced by the explosion of a star in supernova, whose shock wave would have caused thecollapse of the nebula protosolar who would become the young Sun surrounded by protoplanetary disc, in which the radioactive isotope of aluminum was found. More content in ²⁶Al of the rock bodies was important, so their size, plus the production of heat was important.

Surviving pockets of an ocean of magma

Cosmochemical planetologists at Curtin University in Western Australia have just brought up the eucrites found in Antarctic by dating them more precisely by mass spectrometry using abundances of isotopes ⁴⁰Ar /³⁹Ar, a so-called radiometric dating method Argon-argon, which refines the potassium-argon dating method. The results of this study were published in the journal Geochimica and Cosmochimica Acta.

Professor Fred Jourdan of the School of Earth and Planetary Sciences at Curtin University explains some of the results in these terms in a statement from the University:

"The data revealed that Vesta was volcanically active for at least 30 million years after its initial formation, which occurred 4,565 million years ago. While it may sound short, it is actually much longer than most others digital models predicted, it was unexpected for such a small asteroid. "

For this reason, Fred Jourdan adds: "Considering that all of the radioactive elements providing heat would have completely disintegrated by that time, our research suggests that pockets of magma must have survived on Vesta, and were potentially linked to an ocean of slowly cooling partial magma located inside the crust of the asteroid. "

Trudi Kennedy, colleague of Fred Jourdan, also adds: " Our data consolidate the hypothesis that the first flows of wash eruptions on Vesta have been buried deep in its crust by more recent lava flows, essentially by overlapping each other. They were then "cooked" by the heat of the mantle of the protoplanet, modifying the rocks … It is very exciting for us, because our measurements bring a lot of new information on the first 50 million years or so. Vesta's ancient history, which all future models must now take into account. It also suggests now that if the volcanism could last longer than imagined on a protoplanet, so perhaps volcanism on primitive Earth itself could have been more energetic than we currently believe. "

The Mystery of Vesta's Missing Olivine

In the region occupied by Vesta's largest crater, Rheasilvia, scientists expected to find rocks rich in olivine from the mantle of this asteroid. Unfortunately, the Dawn probe did not confirm this prediction. The reason is not well understood. It is undoubtedly necessary to review the history of this small celestial body, probably more complicated than expected.

Rheasilvia is a huge crater almost 500 km in diameter centered near the south pole ofasteroid Vesta. It was first discovered in images taken by Hubble in 1997. It occupies almost 90% of the average diameter of Vesta, whose shape is close to that of an oblong ellipsoid measuring almost 580 km in its longest and 460 km in its smallest. It was expected that such an impact would allow direct access to the deep rocks of this small celestial body by bringing them to the surface.

Now, the probe Dawn, which allowed to better know Vesta since it stayed for some time orbit around this asteroid (before leaving for the dwarf planet Ceres, which it will reach in 2015), was equipped with the instrument Visible and Infrared Mapping Spectrometer (VIR). With him it was possible to determine the mineralogical composition of the Vesta surface to a certain point. The researchers therefore went in search of traces of olivine.

To understand the reason, you must know that with its size, which could almost qualify it as a dwarf planet, the asteroid could have evolved like other bodies in the Solar System, such as Earth or Mars. Indeed, there were good reasons to think that heat sources, whether radioactive elements or because ofinduction, had led Vesta to differentiate itself with the formation of a metallic core, a crust and a coat rich in olivine.

Vesta, a partially differentiated asteroid?

But as planetologists explain in an article by Nature, and to their surprise, no trace ofolivine was found in association with Rheasilvia, when they expected natural drilling giving access to the rocks of Vesta's mantle. On the other hand, VIR revealed the presence of olivine where it had not been foreseen, namely in two craters in thenorthern hemisphere, Arruntia and Bellicia.

This enigmatic result forces researchers to consider Vesta evolution scenarios more complicated than they imagined. Perhaps the differentiation of this celestial body occurred only partially, with pockets of rocks containing olivine. Perhaps there are simply layers of material deposited on Rheasilvia during an unknown event, hiding the rocks rich in olivine that we expected to discover. This is in any case a beautiful subject of study for a mission future which would land on Vesta to explore the geology.