Asteroid Vesta is 'last of a kind' rock

Rhea Silvia basin A computer generated perspective of Rhea Silvia basin, which was dug out by an impact a billion years ago

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Vesta is the only remaining example of the original objects that came together to form the rocky planets, like Earth and Mars, some 4.6 billion years ago.

This assessment is based on data from the Dawn probe which has been orbiting the second largest body in the asteroid belt for the past 10 months.

The findings from the Nasa mission are reported in Science magazine.

They confirm that Vesta has a layered interior with a metal-rich core, just as Earth, Mars, Venus and Mercury do.

Using information about the shape of the asteroid and its gravity field, scientists can even say something about the likely size of this core.

The Dawn team calculates it to be about 220km (135 miles) across, representing about 40% of the radius of Vesta, or roughly 18% of its total mass.

Cutaway model of Vesta Cutaway model: Data indicates Vesta has an iron core that is about 220km across

"This mission at Vesta has been a spectacular success," said Carol Raymond, Dawn's deputy principal investigator from Nasa's Jet Propulsion Laboratory in Pasadena, California.

"It's transformed Vesta from a fuzzy orb into a planetary body, which has exceeded our expectations in many ways."

Dawn has studied in detail the pattern of minerals exposed in Vesta's surface by innumerable impacts through the eons. It has also mapped the diverse geological features that shape its surface.

These observations have enabled scientists to elucidate a history for the colossal rock, which is second in size only to Ceres in the band of material moving between Mars and Jupiter.

The researchers believe Vesta formed within two million years of the first solids coming together in the Solar System, before the planets we know today were assembled.

Short-lived radioactive materials would have generated enough heat to melt Vesta's insides, producing a subsurface ocean of magma.

This melting would have led to differentiation, to denser materials like iron falling to the asteroid's centre.

Other such bodies in the infant Solar System with magma oceans ended up becoming parts of Earth and the other planets. Somehow, Vesta did not; somehow, it survived obliteration in the cascade of impacts that would have marked those early times. Nothing telescopes see today in the asteroid belt quite matches what Dawn has seen at Vesta, suggesting the mighty rock is a unique hangover.

"Vesta is special because it has survived the intense collisional environment of the asteroid belt for billions of years, allowing us to interrogate a key witness to the events at the very beginning of the Solar System," said Dr Raymond.

Crystal structure in HED meteorites The crystalline structure in HED meteorites: These common meteorites can now be tied definitively to Vesta

Another in a series of stand-out discoveries about Vesta is the definitive association that can now be made between it and the howardite-eucrite-diogenite, or HED, class of meteorites.

Based on telescopic observations, researchers had always suspected these common meteorites came from Vesta. But the signatures of pyroxene - a mineral rich in iron and magnesium - in those meteorites have now been matched precisely with the mineral signatures spied in Vesta's surface by Dawn's instruments.

The HED meteorites account for about 6% of all the meteorites seen falling to Earth.

Common they may be, but their value to science is greatly increased following the Dawn mission because researchers now know they offer some remarkable insights into the earliest epoch of planetary formation.

Much of the HED material is likely to have come from two huge impact basins at Vesta's southern pole.

The Rhea Silvia crater measures some 475km in diameter, and gives Vesta the look of a punctured football in global images.

It overlies the Veneneia basin, which is a little less extensive at 375km across.

"Our estimate [is that] about 250,000 cubic miles [were] excavated from Rhea Silvia," said Dr David O'Brien, from the Planetary Science Institute in Tucson, Arizona.

"To put this into another perspective - 250,000 cubic miles is enough to fill the Grand Canyon a thousand times over."

Some of that excavated material lies just outside Rhea Silvia crater, but a lot of it would have gone into space to form the group of much smaller Vesta-like objects seen in the asteroid belt and, of course, those HED meteorites that have fallen to Earth.

The pattern of craters on Vesta has allowed the Dawn team to age Rhea Silvia and Veneneia. The former was created about a billion years ago; the latter was dug out by an impact around two billion years ago.

The ages are much younger than had previously been supposed.

The other great impact basins in the inner Solar System, such as the ones that pock Earth's Moon, are considerably older, dating to a period about 3.8 billion years ago.

Scientists suspect this period to have been a very dynamic phase in the early evolution of the Solar System and even have a name for it - the "late heavy bombardment".

Comparison in size of asteroids visited The size of Vesta compared with the other asteroids visited by spacecraft

The ages of Vesta's big impact basins put some new constraints on the timing of that bombardment and and the region within the inner Solar System affected by it.

Dawn continues to study Vesta. It has an extension on its mission that will allow it to stay at the asteroid until at least 26 August, when it is due to leave on a three-year cruise to Ceres.

The Dawn team is hoping to be able get images of the north pole, which, as a result of the seasonal angle to the Sun, is currently in darkness.

Dr Jim Green, the head of planetary science at Nasa, indicated that the team would be given sufficient time to complete its work.

There is considerable excitement about what might be found at the north pole, with the suspicion being that the terrain may reflect in some way the seismic repercussions from the Rhea Silvia impact at the opposite pole.

"With Dawn's ion engines, which are running much better than predicted, it really enables us to stay at Vesta longer," Dr Green told BBC News.

"We're very excited about that because it will enable us to map the unknown region. That region is very important to look at now that we know about the Rhea Silvia impact basin. How that impact might affect the northern hemisphere will now be revealed."

Craters on Vesta Crater counting is used to date the surface of asteroid Vesta. Rhea Silvia and Veneneia appear as undulating lines. Note how the terrain of the north pole has yet to be revealed

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter

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