Asteroid Surfaces are of Organic Compound

The dark materials covering the surfaces of many asteroids, and which are mixed with ices in the surfaces of the other planet satellites and rings, are probably chemically similar mixtures of clay-like silicates and carbon- rich organic (but not biological) compounds, Dr. Torrence V. Johnson of Jet Propulsion Laboratory, and Dr. Dale P. Cruikshank of University of Hawaii, reported Tuesday, Dec. 9.

The two scientists presented papers before the fall meeting of the American Geophysical Union in San Francisco at session devoted to primitive solar system materials.

They reviewed results from recent analyses of Voyager spacecraft data, telescopic observations and laboratory studies.

Two general classes of dark material are evident in the asteroid belt, Cruikshank said. They are: Type "C," or neutral in the visible color spectrum and similar to C-class asteroids, and believed to be similar to material in primitive carbonaceous chondrites; and Type "D," which are spectrally red, and similar to D-class asteroids, which are enriched with reddish organic compounds comparable to extracts from carbonaceous chondrites.

Johnson reported that the optical properties of the Uranian satellites and rings suggested that they contain Type- material, while the dark side of Iapetus is Type and Phoebe is C. Iapetus and Phoebe are the outermost moons of Saturn.

The existence of the dark material is not in itself surprising, Johnson said, since material similar to primitive meteorites is the expected form of silicate material in most chemical models of outer solar system condensation.

But still unresolved, he said, is how much of the dark material in those systems is original and how much may have been modified from carbon-rich ices by irradiation.

Voyager observations of the Uranian satellites suggest that original dark material may be the most important component in that system, he said.

Both studies were done under grant from NASA. Other papers in the session detailed current work on the study of such materials in meteorites and the effects of irradiation on carbon-bearing materials.