Astrophile: Two craters that launched 1000 meteorites

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Impact craters on the asteroid Vesta
Size: Up to 500 kilometres wide

Vesta thought its days of being the solar system's punching bag were over. Despite 3.5 billion years of pummelling, the protoplanet had managed to hold itself together. The bullies of the asteroid playground had mostly settled down, and aside from the occasional shove or taunt, things were starting to look up. If only poor Vesta had known of the two bullies that were still to come.

Fresh images of 4.5 billion year old Vesta show the poor protoplanet was pummelled by a 60-kilometre-wide rock not once, but twice, in the past two billion years. And those strikes dug up enough material to create an entire class of meteorites. If Helen of Troy had "the face that launched a thousand ships", then Vesta has Rheasilvia and Veneneia, the craters that launched a thousand meteorites.

Astronomers had suspected for decades that Vesta – the solar system's second-biggest asteroid – was the source of the howardite-eucrite-diogenite, or HED meteorites. This common group of space rocks makes up about 6 per cent of the meteorites seen to fall to Earth: the Meteoritical Bulletin Database lists 1082 that have been found on the ground.

Both the asteroid and HED meteorites give off similar spectral signatures that were different from other classes of meteorites, and both looked like basaltic lava of the sort that is found in Hawaii. Vesta's orbit around the sun is also just right for sending debris to Earth. Meanwhile, observations with the Hubble Space Telescope in 1997 showed a giant crater in the South Pole. That was the "smoking gun", says Paul Schenk of the Lunar and Planetary Institute in Houston, Texas. But a few mysteries remained.

Groovy impact

Could a single hole in Vesta's South Pole really have dug up enough material to account for all HED meteorites? How deep is the hole, and how long ago did it form? And how many impacts were there? A group of smaller asteroids called Vestoids are thought to be chips of Vesta knocked off in a giant impact. But some studies of the way they move had suggested that these are divided into two distinct populations – one about a billion years older than the other.

New images from NASA's Dawn spacecraft are filling in the details. Dawn slipped into orbit on 17 July, 2011 and has mapped nearly 80 per cent of the asteroid's surface. The images show that Vesta's northern hemisphere is pockmarked with craters, a record of billions of years of pummelling. "It's been hammered quite a bit," Schenk says.

But whatever craters had been preserved in the southern hemisphere were obliterated by one huge impact, which Schenk and colleagues estimate came about a billion years ago. The crater this vast asteroid bully left behind, called Rheasilvia, stretches to 500 kilometres, wider than the distance from London to Dublin and spanning most of Vesta itself. It's at least 19 kilometres deep, and has a central peak that rises 20 kilometres high, higher than Mauna Kea on Hawaii.

That means the object that hit Vesta must have been 50 to 60 kilometres wide, bigger than the object that made the Chicxulub impact crater thought to be responsible for killing off the dinosaurs. Debris from the crater can be found 100 kilometres away from the rim. The force of the impact formed deep grooves that circle Vesta's equator.

Meet Veneneia

"It's the largest possible ring you can make due to an impact," says Chris Russell of the University of California, Los Angeles. "We've never seen anything like that before."

The team calculated that the impact scooped a million cubic kilometres worth of material out of Vesta's South Pole, and scattered much of it into space. The HED meteorites plus the Vestoids have a total estimated volume of 100,000 cubic kilometres, so the Rheasilvia impact alone could have been responsible for the lot.

But beneath all the destruction the researchers also found a second impact crater, Veneneia, almost as large as Rheasilvia but half-hidden underneath it. About 400 kilometres wide and 12 kilometres deep, it formed about two billion years ago, Schenk and colleagues determined.

Iron core

"That was a surprise, that in fact the southern part of Vesta had been subject to two very large impacts," Russell says. Vesta probably would have been destroyed by the one-two punch, if not for a characteristic that makes it seem more planet than asteroid: an iron core. In its first million years of existence, Vesta was completely molten, Russell says, and the heavier elements like iron sank to its centre to congeal into a solid metallic core.

"It was anchored by that iron core, and survived," he says. "And fortunately so, because we don't have many ways of getting back that far in history, to get some evidence of what was going on in those very early days."

There's another silver lining to Vesta's tortured playground years: HED meteorites are so numerous, you could own one. "You can go to a meteorite collection store, and they'll sell you a piece of Vesta," says Russell.

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