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Scientists find a protoplanet's guts

Georgia Southern U. / Cornell / NASA

The asteroid Vesta has a huge crater on its southern side. This picture shows the asteroid in an image captured by the Hubble Space Telescope (top left), as a reconstruction based on calculations (top right) and as a topographical map (bottom).

Astronomers have identified a space rock that they say came from the interior of Vesta, an asteroid that may be the only mini-world of its type left in the solar system. The finding is just a foretaste of what scientists will learn when a NASA probe begins orbiting Vesta later this year.

Vesta ranks No. 2 on the list of asteroids in terms of mass (after Ceres) and No. 3 in terms of size (after Ceres and Pallas). It's significantly denser and brighter than those other two asteroids, which suggests that it has an iron-nickel core and a rocky mantle layered beneath a cool lava crust. Scientists consider Vesta as well as Ceres and Pallas to be protoplanets — cosmic leftovers that were stuck in an intermediate phase of planet formation. But Vesta's density puts it in a class by itself.

Eons ago, the 325-mile-wide (525-kilometer-wide) Vesta was blasted in a cosmic collision that left a 16-mile-deep (25-kilometer-deep) crater at its south pole. That blast scattered fragments known as Vestoids out into space. Some of those fragments were flung all the way to Earth, ending up as a class of meteorites called the HED group.

Long-lost piece of Vesta
The HED meteorites have given astronomers a good baseline for identifying Vestoids that are still in outer space. One of those Vestoids is the subject of the newly reported research. The roughly kilometer-wide (0.6-mile-wide) asteroid known as 1999 TA10 was first spotted more than a decade ago, but it's only been recently that astronomers used NASA's Infrared Telescope Facility in Hawaii to analyze the rock's composition.

Astronomers from the University of North Dakota and Germany's Max Planck Institute for Solar System Research reported their results this week in a paper published online by the journal Icarus. They found the signatures of calcium-rich wollastonite as well as iron-rich ferrosillite in their spectral observations of 1999 TA10.

"These materials can be found in Vesta's mantle and crust," Andreas Nathues of the Max Planck Institute said in a news release. "However, the ratio is decisive."

The researchers reported that 1999 AT10's concentration of iron is lower than that of any other known Vestoid. "This all points to 1999 TA10 having originated from the interior of Vesta," Nathues said.

If 1999 TA10 was blasted out of Vesta's mantle rather than its crust, that suggests that the crust can't be thicker than the mini-world's south pole crater. The crust would have to be 16 miles thick at most. That kind of information could be useful for determining how the different layers were built up on Vesta — and provide even deeper insights about the processes at work during the solar system's formation.

Here comes Dawn
The big reveal will come starting in August, when NASA's Dawn spacecraft arrives at Vesta and begins its year-long mapping mission. Dawn's observations would confirm whether Nathues and his colleagues are correct about the origins of 1999 TA10 and the composition of Vesta itself.

Christopher Russell, a geophysicist at the University of California at Los Angeles who serves as principal investigator for the $357 million Dawn mission, said the Icarus research could well be on the right track. But he's not ready to draw any conclusions about Vesta's composition quite yet.

"I'm basically withholding judgment until we get there and see the images," Russell told me. "I'm not being skeptical as much as I'm being cautious."

Even though Dawn was launched more than three years ago, the mission's real adventure is just about to begin. After studying Vesta, the spacecraft is due to move on to the dwarf planet Ceres, which could reveal even juicier secrets. Dawn should begin orbiting Ceres in 2015 — just as NASA's New Horizons probe closes in on Pluto, my personal favorite among the dwarf planets. And who knows? Maybe the data from Vesta will lead astronomers to regard that world as a dwarf planet too. So brace yourselves: This could be the decade of dwarf domination. 

More about dwarfs and other planets:

Correction for 1:40 p.m. ET Jan. 8: My conversion of miles to kilometers for Vesta's diameter was mistakenly listed as miles. That's been fixed — sorry about the error.

In addition to Nathues, co-authors of "First Fragment of Asteroid 4 Vesta's Mantle Detected" include Vishnu Reddy and Michael Gaffey of the University of North Dakota.

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