Carbonaceous meteorites and asteroids.

Carbonaceous meteorites and asteroids 

Scientists have long sought to identify similarities between certain types of meteorites, chunks of space rock that fall on Earth, and asteroids, large blocks of rock orbiting the sun, which astronomers study largely by spectra measured through telescopes. A kind of meteorite called eucrites, for example, has been tentatively associated with the large asteroid Vesta, though this link is typical of such studies in that the limited data available fall far short of proving the eucrites actually came from Vesta.

A particularly intriguing possible parallel exists between a class of meteorites known as carbonaceous chondrites (primarily types C1 and C2) and the so-called C-type asteroids. Studies of their compositions suggest that water was involved in the formation of both. Such an association interests astronomers, in part because some believe both types of objects represent examples of some of the solar system's more primitive material. Now, new results tighten the link between them.

Two overlapping lines of evidence are involved. Larry A. Lebofsky of the University of Arizona in Tucson and Thomas D. Jones, now with the Central Intelligence Agency in McLean, Va., report that the infrared spectra of some C-type asteroids include absorption bands with a 3.0-micron wavelength characteristic of water-of-hydration--a sign that water was present when the asteroids formed--and indicating the presence of clay minerals called phyllosilicates. At the same meeting, Faith Vilas of NASA's Johnson Space Center in Houston, and Michael J. Gaffey of Rensselaer Polytechnic Institute in Troy, N.Y., report high-resolution, visible-light spectral absorption features that they say are due to iron oxide in such clays. Spectral measurements of the asteroids indicate the oxidation state of the iron, which researchers can compare to the iron oxide in carbonaceous chondrites.

The spectral features and phyllosilicates have both been found in carbonaceous chondrites, but Lebofsky and Vilas agree that the linkage with specific asteroids will require collecting samples from the asteroids themselves.