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False-color image hints at Gaspra's origin.

Black-and-white is nice, but color really makes a difference.

Last month, NASA released the first close-up image of an asteroid: a black-and-white photo of a body called 951 Gaspra. The Jupiter-bound Galileo spacecraft took that photo through a green filter (SN: 11/23/91, p.326). This week, the space agency released two color portraits of Gaspra, created by combining Galileo images taken through violet, green and near-infrared filters. These color images offer several new clues about the surface composition and origin of the body, says Galileo researcher Joseph Veverka of Cornell University.

Viewed in a mix of hues that approximates natural light, the yellowish, nearly uniformly colored asteroid appears covered with rocks somewhat less gray than rocks on the moon. A second, false-color portrait enhances the subtle variations in color across the surface of Gaspra.

Veverka notes that the infrared-dark patches on Gaspra coincide with ridges and the freshest craters. He and other researchers say this finding bolsters speculation that a type of rocky covering, called regolith, blankets most of Gaspra's surface to a depth of perhaps 1 meter. The dark patches near craters, Veverka says, may represent areas where an impact has scraped away the regolith, exposing material - possibly the the mineral olivine - that absorbs near-infrared light. Dark patches on ridges may indicate regions where regolith has rolled downhill and exposed the underlying rock.

While both moons of Mars and the Jovian moon Amalthea display a similar regolith pattern, the tentative finding on Gaspra puzzles some astronomers, Veverka notes. They had theorized that the asteroid's rocks were too hard to fragment into regolith. Moreover, Gaspra should lack the gravity to hold this soil on its surface. Veverka argues, however, that such models may need revision.

Gaspra's muted color variations, most of which can be accounted for by regolith, hint that the asteroid represents an intact chunck from a primitive, chemically uniform parent - some object whose composition has remained unaltered since the solar system formed, Veverka says. But Michael J. Gaffey of the Rensselaer Polytechnic Institute in Troy, N.Y., cautions that Gaspra's small size suggests an alternative lineage: It may represent a chip off some uniform layer within a chemically diverse parent.

Higher-resolution images from Galileo - not expected until next spring because of continuing problems in transmitting data to Earth - could settle such speculation, Veverka says.
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Title Annotation:asteroid 951 Gaspra
Author:Cowen, Ron
Publication:Science News
Date:Dec 14, 1991
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