An MIT astronomer and his colleagues have used the Hubble Space Telescope (HST) to discover a giant impact crater on the asteroid 4 Vesta. The crater, 285 miles across and nearly equal to Vesta's 330-mile diameter, is a link in the chain of events thought to be responsible for an entire class of meteorites that have reached the Earth.
Richard Binzel, associate professor of planetary sciences, was a co-investigator in the HST research, described in the September 5 issue of Science, with the Hubble image of Vesta appearing on the cover. In 1993, he predicted that a substantial impact must have occurred on Vesta. His prediction resulted from ground-based observations, made together with Dr. Shui Xu (then an MIT graduate student), of the visible spectral colors of numerous small asteroids orbiting the sun in the vicinity of Vesta.
The MIT researchers found 20 small asteroids, each about 5 miles across, which had identical spectral colors uniquely matching that of nearby Vesta. Professor Binzel and Mr. Xu reasoned that these must be "chips" blasted off of Vesta by a giant impact. Any impact catastrophic enough to knock off such big chunks should have left behind a substantial wound.
"Although we predicted that Vesta must have suffered a huge impact, the enormous size of this crater still came as a complete surprise," said Professor Binzel.
The impact which formed the crater gouged out 1 percent of the asteroid's volume, blasting more than half a million cubic miles of rock into space. This tore out an 8-mile-deep hole that may go almost all the way through the crust to expose the asteroid's mantle. (Vesta is large enough to be geologically differentiated like Earth; it has a volcanic crust, core and mantle, making it a sort of "mini-planet".)
Because of the asteroid's small diameter and low gravity, the crater resembles smaller craters on the Moon that have a distinctive central peak of material that has "sloshed" back to the bull's-eye center after the impact.
The first visible clue for such a giant crater came in 1994, when Hubble pictures showed that one side of Vesta's football shape appeared flattened. The astronomers then had to wait two years for a better view from Hubble; they got it in May 1996, when the asteroid was 110 million miles away.
Vesta's immense crater lies near the asteroid's south pole. This is probably more than coincidental, say the researchers. The excavation of so much material from one side of the asteroid would have shifted its rotation axis so that it settled with the crater near one pole.
Unlike some other large asteroids that have jumbled surfaces due to the asteroid's breakup and recollapse, the rest of Vesta's surface is largely intact, despite the cataclysm.
VISITORS FROM VESTA
Approximately 6 percent of the meteorites that fall to Earth are similar to Vesta's mineralogical signature, as indicated by their spectral characteristics. Vesta's spectrum is unique among all the larger asteroids. The crater may be the ultimate source of many of these meteorites.
This distinctive mineralogical makeup indicates that Vesta is the only world other than the Moon and Mars for which scientists have a sample of known origin. Most meteorites come from other asteroids, but specific objects of origin cannot be determined.
The mystery of how meteorites could have traveled from Vesta to Earth has also been unraveled. Vesta's "daughter" asteroids -- literally, "chips off the block" which have color characteristics similar to Vesta -- are near a "chaotic zone" in the asteroid belt. Once in the "chaotic zone," Jupiter's gravitational tug can redirect fragments into orbits which intersect Earth's orbit.
The Vesta research team was led by principal investigator Ben Zellner of Georgia Southern University. Co-investigators were Professor Binzel, Dr. Michael Gaffey of Rensselaer Polytechnic Institute, Dr. Alex Storrs of the Space Telescope Science Institute, Dr. Peter Thomas of Cornell University and Dr. Eddie Wells of Computer Sciences Corp. Funding was provided by NASA.
A version of this article appeared in MIT Tech Talk on September 10, 1997.