Asteroid Vesta rocked by mighty impacts

By Jonathan Amos
Science correspondent, BBC News

image captionImages from Dawn reveal grooves parallel to Vesta's equator. They cover 240 degrees of longitude

The scale of the pummelling Asteroid Vesta has taken through its history is starting to become clear.

Analysis of data returned by the orbiting Dawn spacecraft shows this giant rock took a mighty double beating in its southern polar region.

One impact had long been recognised from images of the asteroid acquired by the Hubble telescope.

But Dawn's measurements indicate that Vesta was also hit much earlier in time in almost exactly the same location.

These two major impacts gouged out depressions hundreds of kilometres in diameter, and sent shockwaves rippling around the body.

Scientists are fairly confident that the deep system of troughs extending around Vesta's equator and northern hemisphere are fractures that opened up in the surface as a direct consequence of the southern bombardments.

"We can see some things in the visual imagery of Vesta, but it's only when you get in close enough to map the topography that other features come to light," said Debra Buczkowski from the Applied Physics Laboratory at Johns Hopkins University in Baltimore, US.

"And when we did this it was like 'woah! - there's another impact basin there'.

"We simply didn't know of its existence until we had the topography," she told BBC News.

'Impressive' peak

Craters on the asteroid are being named after the vestal virgins - the priestesses of the goddess Vesta - and famous Roman women.

image captionBefore Dawn, our best image of Vesta was supplied by the Hubble Space Telescope

In that vein, the biggest crater at the south pole has been called Rhea Silvia, the mother of Romulus and Remus.

The impact depression measures some 475km in diameter, and gives Vesta the look of a punctured football in Hubble pictures.

Since Nasa's Dawn spacecraft arrived at the asteroid in July, scientists have been able to examine the basin's key features in detail.

It has a central mountain that would have formed when rock that had been depressed in the impact rebounded upwards.

The peak's height is impressive - it rises about 22km (13 miles) above the surrounding terrain, making it more than twice as high as Mount Everest here on Earth.

This much has been clear now for several weeks, but as Dawn has circled closer in to the asteroid it has been able to get better and better information on the surface shape of Vesta.

An analysis of this data has shown that Rhea Silvia actually overlies another, older impact basin that is only marginally narrower in extent - about 375km across.

image captionBetter information on surface shape (topography) reveals that Rhea Silvia sits on top of an older impact crater. Rhea Silvia itself may be 2.5 billion years old. The equatorial trough system is also visible in this image.

This story of multiple large impacts is one of many surprises scientists have encountered at Vesta since Dawn's arrival in July.

It may also help to explain a puzzle about the Vesta meteorites found on Earth.

Something like 5% of all space rocks found on our planet have their origins on this asteroid. The assumption is that many were part of the huge mass of rocky material knocked off Vesta's southern pole.

"We actually see several other large basins to the north [of Rhea Silvia and this other basin] that are very old as well," explained Paul Schenk from the Nasa-funded Lunar and Planetary Institute in Houston, Texas.

"There's one that's 275km across; there's one that's 150km across.

"The reason these basins are interesting is because we believe Vesta is the source for a large number of meteorites that fall to Earth.

"These meteorites, called the HED meteorites, have a spread of ages; and having multiple large impacts gives us potential sources for those meteorites."

image captionA computer generated view of Rhea Silvia's central mountain generated from Dawn data. The mountain is more than twice as high as Mt Everest, rising some 22km above the surrounding terrain

Dr Buczkowski's main interest has been the striking trough systems that circle the asteroid at its equator and further to the north.

The equatorial system is the younger of the two sets. Its grooves are sharply defined with steep walls and flat bottoms.

The biggest trough is 380km long and 15km wide.

The set of troughs to the north has an older, degraded appearance. The topography is more muted, and in places the troughs are showings signs of infilling.

Dr Buczkowski's structural analysis ties the formation of these features to the southern polar impacts.

"As I drew the sets, I was able to analyse them, and could see they had common points of formation," she told BBC News.

"And when I look at those points of formation, the set of equatorial troughs is incredibly close to where Paul Schenk has mapped the centre of the Rhea Silvia basin.

"And then the centre of formation for those northern troughs is remarkably close to the centre of that older, underlying basin that we've only just discovered."

The Johns Hopkins researcher said that extensive ring fractures were often associated with impacts. On a 570km-wide asteroid like Vesta, those rings encompass nearly the whole girth of the rocky body.

No planet

Vesta was discovered in 1807, the fourth asteroid to be identified in the great belt of rocky debris orbiting between Mars and Jupiter.

At the time, its great scale meant it was designated as another planet but it later lost this status as researchers learnt more about the diversity of objects in the Solar System.

The only other object in the asteroid belt that is larger than Vesta is the dwarf planet Ceres (950km in diameter).

Dawn will visit this object after completing its studies at Vesta.

Drs Buczkowski and Schenk presented their analyses of the latest Dawn data at the 2011 Annual Meeting and Exhibition of the Geological Society of America in in Minneapolis, Minnesota.

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