The Sciences

Mother of Some Meteorites


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Asteroid 4 Vesta, at 330 miles in diameter, is the third largest asteroid and the only one visible to the naked eye. In the early 1970s researchers discovered that Vesta’s composition is unlike that of any other asteroid: it is made of basalt. This led astronomers to suspect that Vesta was the source of so-called HED meteorites found on Earth, which are also made of basalt. (HED stands for howardite, eucrite, and diogenite--three types of basaltic rock.) Using the Hubble Space Telescope, astronomers recently found a gigantic crater the size of Missouri at the asteroid’s south pole. In the false-color images, south is at the top and blue represents the crater. Higher elevations are colored red and yellow. The black-and-white image has south at the bottom. Peter Thomas, a planetary geologist at Cornell, got a good look at Vesta when it came within 110 million miles of Earth last May. We could see fairly clearly that there was one humongous crater, he says. The crater is 285 miles across and 8 miles deep and no doubt was created when another asteroid slammed into Vesta. Some of the material blasted out of the crater sloshed back toward the middle, creating an 8-mile-high peak in the center (shown in red here). The collision dug out only 1 percent of Vesta’s volume, but that’s over 250,000 cubic miles of rock--more than enough to have created the HED meteorites.

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