Vesta sheds light on planet formation

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The crust of the asteroid Vesta is almost three times thicker than expected, according to a new study that questions contemporary models of rocky planet formation, including that of Earth.

Asteroid Vesta

Asteroid Vesta is one of the largest and oldest ‘planet embryos’ in the Solar System, residing in the main asteroid belt between Mars and Jupiter. Until recently, it has remained an unexplored world, only known through modest observations from the Hubble Space Telescope.

Thanks to thousands of new images and data gathered by the NASA spacecraft Dawn, which orbited Vesta from 2011 to 2012, a team of European and US researchers is edging closer to understanding the evolution of Vesta and its internal structure, as reported in Nature.

In its infancy, Vesta suffered two massive meteorite collisions that excavated its southern pole and catapulted a lot of material to the surface.

“What we found from numerical simulation of the impacts and analysis of the data is that the first layer of Vesta, the crust, is much thicker than what we thought [from predictions based on classical models of Vesta’s formation],” says Harold Clenet from École Polytechnique Fédérale de Lausanne (EPFL) in a video accompanying a press release.

Mapping Vesta’s mineral makeup

To map the mineral composition of Vesta’s surface, the researchers used data collected by a visible and infrared spectrometer aboard the Dawn spacecraft. Measuring the electromagnetic radiation reflected off the surface, the instrument records the ‘spectral signatures’ of materials in rocks and soils. Many materials absorb radiation at specific wavelengths, which reveal the nature of the chemical bonds in the materials. Scientists can use these characteristics as ‘fingerprints’ to identify minerals such as olivine and pyroxenes.

Clenet and colleagues found plenty of pyroxenes all over Vesta’s surface, but olivine was conspicuously missing from the southern hemisphere, even in the impact craters where it was expected to be. As a main component of planetary mantles, olivine should have been found if the two impacts had punctured the crust and exposed the mantle – the layer beneath the crust.

Explaining the ‘missing’ olivineThe layers of asteroid Vesta

“This means that the two giant impacts were not important enough to excavate deep into the mantle of Vesta,” says Clenet, indicating that the crust of the asteroid is not 30 km thick, as suggested by previous models, but more than 80 km.

“A thicker crust and a thinner mantle mean that the composition of the material that aggregated to form Vesta is probably different from what we previously thought. And this is probably also the case for the other terrestrial planets in the Solar System.”

Challenging the models that describe the formation of not only Vesta but other rocky planets including Earth, Mars, Venus and Mercury, the researchers argue that a different model of planet formation needs to be considered.

“Far from the last word”

There is much that cannot be discovered about a parent body from meteorites alone, according to the principal investigator of NASA’s Dawn mission, Chris Russell from the University of California, Los Angeles, who was not involved in the study published in Nature.

“…and that is why we flew the Dawn mission to Vesta. Going beyond the zeroth order we find that the models that had served us so well are simplistic,” Russell writes via email.

One of the predictions of these simplistic models, he explains, is that planetary differentiation – the physical and chemical separation of materials into distinct layers – would make a deep magma ocean in which olivine was the major, if not the sole, constituent of the mantle.

“A pure olivine mantle clearly does not exist. But why? Is there a different chemistry? Did the body not heat up as much as was needed to make an olivine mantle? Do we not understand how the radionuclides are sequestered in the differentiating protoplanet? Do they percolate through the overlying material slowly, in conduits, plutons, or some other way? The community is now sorting out the possibilities,” Russell continues.

He regards the new study as “an early attempt at an explanation of the ‘missing’ olivine but far from the last word on the topic.”

by Lillian Sando



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