Have they got the wrong asteroid?

Image of Vesta’s south polar region showing large central peak that rises 22km above the surrounding terrain (vertical scale 1.5 x horizontal). (image: NASA)

As the NASA Dawn spacecraft continues its study of the asteroid Vesta the images being released have become increasingly detailed and breathtaking. It is true that the huge southern crater, with its large central peak, had been known about from earlier observations. However, the complexity of this feature, as revealed by the new data from Dawn, has been completely unexpected. Recent analysis suggests this feature consists of two impact basins: an earlier one with a diameter of about 375km, and a later basin, named Rheasilvia by researchers, with a diameter of 475km.

Coloured and shaded relief map showing the outline of an older 375 km diameter impact basin beneath the more recent Rheasilvia impact structure. (image:NASA)

The scale of the central peak within Rheasilvia is truly astonishing, rising 22km above the surrounding terrain. To put it into context, that’s more than twice as high as Mount Everest, on an asteroid which has a diameter 24 times smaller than the Earth (530km compared to 12,700km). In fact the impact that made the Rheasilvia basin appears to have got close to totally shattering the asteroid. A series of dramatic troughs and ridges that circle the equatorial region of Vesta are thought to have been created by this impact.

View of Vesta taken by the Dawn spacecraft showing groove-like features that run around the equatorial region of the asteroid. (image NASA)

However, one major consequence of the impact that created the Rheasilvia basin cannot be studied directly by the Dawn spacecraft. This cataclysmic event would have ejected an enormous amount of debris into space. This material produced a string of smaller asteroids known as the vestoids. These are located between Vesta and a gap in the asteroid belt known as the 3:1 resonance, where material can be thrown into an Earth-crossing orbit.

It had been known for a while that a group of meteorites known as the HEDs (Howardites, Eucrites and Diogenites) had a composition similar to the surface of Vesta. But the giant asteroid lay in an unfavourable position within the asteroid belt in terms of supplying meteorites to Earth. The vestoids provided the missing link between the HEDs and Vesta.

So that’s sorted then! HEDs, which represent about 6% of the meteorites that arrive on Earth, come from Vesta. That’s many, many more than come from the Moon and Mars combined, which are much closer to us than Vesta. In many ways the reason Dawn got funded to visit Vesta was because most scientists agree that it was the ultimate source of the HED meteorites.

But scientists are a sceptical bunch.

Now, recent data on the magnesium isotopic composition of HED meteorites has cast doubt on their link to Vesta. These new results, published by a team led by Martin Schiller from the Natural History Museum of Denmark, suggest that Vesta might be too large a body to be the source of the HED meteorites. The HED meteorites are generally viewed as being formed when their parent body underwent almost total melting to produce a “magma ocean”. Schiller and colleagues interpret their new data as indicating that the body which produced the HED meteorites melted and then crystallized very early in solar system history. They suggest it must have been a very small asteroid, less than 100km diameter. If correct, then Vesta cannot be the source of the HED meteorites after all.

So has NASA gone to the wrong asteroid?

Well it is, as they say, early days. Dawn will be studying Vesta in ever greater detail for another nine months before heading off to rendezvous with Ceres in 2015. The new interpretation presented by Schiller and colleagues is interesting and provocative, but will require further scrutiny. One thing’s for sure, it’s going to make for some interesting discussions at the Lunar and Planetary Science Conference in Houston next March.


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