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Vesta seems more planet than asteroid

Spacecraft explorations reveal a layered, beat-up body

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9:09pm, March 22, 2012
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THE WOODLANDS, Texas — The enormous asteroid Vesta is more like a small, rocky planet than other space rocks wandering around the asteroid belt between Mars and Jupiter. Among other planetlike characteristics, Vesta’s interior is probably divided into layers like Earth’s — and scientists have detected traces of an ancient magnetic field.

“We have a hard time working on this body and not thinking about it as a planet,” said UCLA’s Christopher Russell, principal investigator of the Dawn spacecraft that has been buzzing around Vesta since July.

Like Earth, Vesta probably has an iron core, a mantle and crust. Scientists don’t know how thick the crust is, but Dawn measurements suggest that the core’s radius is between 107 and 113 kilometers, Carol Raymond of the Jet Propulsion Laboratory said on March 22 at the Lunar and Planetary Science Conference. Vesta is only about 530 kilometers across, meaning that the core occupies almost half its diameter. And new gravity maps from Dawn reveal anomalies in the crust, or areas where there’s “likely mantle material close to the surface,” Raymond reported.

Other presenters at the meeting relied on data gathered from bits of Vesta that had crashed to Earth.

Two chips off the old Vestal block contain traces of a strong magnetic field, said Roger Fu, a graduate student at MIT, who studied meteorites known as Millbillillie (which landed in Australia in 1960) and ALHA 81001 (found in Antarctica in 1981). The field’s strength suggests that an active, liquid metallic core generated the magnetic signature, which became locked into Vesta’s crust as the asteroid cooled. “There was a magnetic field on the surface of Vesta after 3.6 billion years ago, and there probably still is,” Fu said.

Vesta also bears multiple scars from two giant impacts, including mammoth basins in its south pole and equatorial troughs that formed as the rock’s crust reverberated from the impact. The larger impact basin, called Rheasilvia, is well-known: at 505 kilometers across, it consumes essentially the entire south pole and hosts one of the tallest mountains in the solar system, which towers 20 kilometers from the basin’s floor.

But the second impact basin, which sits under and slightly to the side of Rheasilvia, is a newly named structure. Called Veneneia, the basin — described by Paul Schenk from the Lunar and Planetary Institute in Houston — is older than Rheasilvia and about 395 kilometers across. 

The double bull’s-eye at the south pole suggests that Vesta must be incredibly resilient, Schenk said.

Even so, the impacts probably penetrated Vesta’s crust and scattered minerals that normally live deep underground over the surrounding surface. Scientists will continue studying and characterizing these details because even though Vesta might resemble Earth, it embodies one crucial difference: Instead of erasing pages from its history, the asteroid’s biography still contains records of its evolution dating back to the dawn of the solar system.

“By studying the surface,” Russell said, “we will go back in time right to the beginning of the solar system.”

Dawn will hover near Vesta until August, continuing to transcribe the tales it tells. It will then head toward Ceres, the most massive body in the asteroid belt, with a planned arrival in 2015.

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