by Harvey Leifert Monday, March 5, 2012
Since last July, NASA’s Dawn spacecraft has been orbiting the asteroid Vesta and sending back images and other data to the delight and amazement of researchers, who presented their latest results at the annual meeting of the American Geophysical Union (AGU) in San Francisco, Calif., last December.
Most asteroids look like potatoes, says Vishnu Reddy of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, who has studied thousands of asteroids over the years. But “Vesta is not a potato; it’s an avocado,” he says, referring to the asteroid’s core, one of the newly discovered characteristics that make Vesta unique among known asteroids in the belt between Mars and Jupiter. Another characteristic: Dawn has revealed that “Vesta is one of the most colorful objects in the solar system that we have visited so far,” Reddy said. It is covered with a lot of regolith, or loose surface material, which affects the colors we see.
“Colorful” is relative, of course, in an asteroid belt where almost everything is a dull gray. Dawn’s framing camera, developed by the Max Planck Institute, uses pairs of filters to detect differences between one rock formation and another, resulting in bright Technicolor-like images. The naked-eye impression of color variation, if one were to visit Vesta, would be far more subtle — probably limited to dull green, Reddy noted at the AGU meeting.
The unique characteristics of Vesta, which is about the length of Arizona, extend well beyond shape and color. Unlike most asteroids, it rotates on its axis. It also has an iron core, as revealed by gravitational analysis, says Carol Raymond of NASA’s Jet Propulsion Laboratory at Caltech. Vesta’s gravity, though weak, is not consistent with a homogeneous material throughout, says Raymond, who is Dawn’s deputy principal investigator. In fact, Vesta’s structure resembles Earth’s, comprising a core, mantle and crust.
“We have found a wide variety of surface features on Vesta: dark and bright ray craters, extensive systems of grooves and troughs, scarps, slumps, partially degraded impact craters that have been modified by mass movements, and even possible evidence of impact melts,” said David Williams of Arizona State University, a participating scientist on the Dawn mission, at the meeting. Ray craters are impact craters with dark or bright lines radiating from them, which are probably reflections from loose material ejected from the craters. The quantity and scale of many of these features surprised the researchers, Williams said.
One such feature is the Rheasilvia formation, a 23-kilometer-tall formation in the south polar region that is dominated by an impressive impact crater about 460 kilometers across. The impact that formed Rheasilvia may have produced some of the meteorites found on Earth, Reddy said. Dawn’s Visible and Infrared Mapping Spectrometer “firmly confirms the link between Vesta and three classes of meteorite found on Earth” — the HED (howardite-eucrite-diogenite) meteorites. “This increases our confidence in using laboratory studies of those meteorites to further understand the formation and evolution of Vesta,” Reddy said.
Other than Rheasilvia, the southern region is relatively crater-free, whereas the northern part of the asteroid is heavily cratered. Vesta’s equatorial region is moderately cratered, Williams said. As of December, Vesta had been mapped only to 45 degrees north.
One interesting note, Williams said, is that many of Vesta’s impact craters have been disturbed by deposits of mass movement downslope. “The widespread, very steep slopes across Vesta, combined with the extensive regolith of loose material from impact cratering, make Vesta’s surface very conducive to flow by gravity-driven mass movements,” Williams noted.
Together, Dawn’s findings indicate that Vesta is a transitional body between small asteroids and larger planets, Williams said. Vesta is estimated to have formed about 4.5 billion years ago, soon after the formation of the solar system, Raymond noted.
The Dawn spacecraft launched from Cape Canaveral, Fla., in 2007 and reached Vesta last July. After one year of orbiting Vesta, Dawn will move on to the dwarf planet Ceres, 60 million kilometers away in the asteroid belt, arriving there in 2015. The largest-known object in the belt and nearly twice the diameter of Vesta, Ceres is quite different in character. It has always been cold and represents an earlier phase of planetary formation. Ceres is nearly spherical and lacks a metallic core, although it may contain frozen water.
Before departing for Ceres, however, Dawn still has much to learn about Vesta. It is slowly descending to an eventual altitude of less than 200 kilometers above the asteroid’s surface. At both destinations, Dawn’s goal is to help reveal the processes and conditions that marked the first few million years — the dawn — of the solar system.
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