New NASA images of the 4.5 billion year old pock-marked surface of asteroid Vesta, located in the asteroid belt between Mars and Jupiter, show the protoplanet was hit by a 60-kilometer-wide rock not once, but twice, in the past two billion years, creating enough material to create an entire new class of meteorites. Ever since the Hubble Space Telescope first spied a huge crater in the asteroid's south pole in 1997, scientists surmised it was carved by a collision with a celestial object, most likely a smaller asteroid.
"Vesta got whacked twice with large impacts," said Christopher Russell of the University of California, Los Angeles, who heads a team of scientists exploring the asteroid. NASA scientists concluded the rim belonged to a smaller, older crater gouged by an impact 2 billion years ago, obscured by the larger crater, created by an impact a billion years later.
Astronomers had long suspected that Vesta – the solar system's second-biggest asteroid – was the source of the howardite-eucrite-diogenite, or HED meteorites, which makes up about 6 per cent of the meteorites seen to fall to Earth. The Meteoritical Bulletin Database shows that 1082 HED's have been found aound the world. About 1 out of every 20 meteorites found on our planet came from Vesta.
New images from NASA's Dawn spacecraft which went into Vesta orbit on 17 July, 2011 and has mapped nearly 80 per cent of the asteroid's surface show that Vesta's northern hemisphere is pockmarked with craters, a record of billions of years of being hammered.
But craters in the southern hemisphere were obliterated by one huge impact, which Schenk and colleagues estimate came about a billion years ago called Rheasilvia, that stretches to 500 kilometers, at least 19 kilometres deep, and has a central peak that rises 20 kilometers, higher than Mauna Kea on Hawaii. 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.
The NASA Dawn team has concluded object that hit Vesta must have been 50 to 60 kilometers wide, bigger than the object that made the Chicxulub impact crater thought to be responsible for the mass extinction of the dinosaurs.
"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 released a million cubic kilometers 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; 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.
Scientists concluded that Vesta probably would have been destroyed by the one-two impact, if not for its iron core. In its first million years of existence, Vesta was completely molten, Russell said, 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," Russell added. "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."
Dawn will move out of its Vesta orbit in late summer, firing its ion propulsion engines to cruise on to a bigger target — an asteroid named Ceres where it will arrive in 2015.
The Daily Galaxy via NASA Dawn Spacecraft/JPL
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