Shape and albedo variations of Asteroid 15 Eunomia

Near-infrared reflected and thermal emitted lightcurves were measured concurrently for Asteroid 15 Eunomia and were used to calculate relative projected area and relative albedo variations over its surface. The technique for calculating relative variations in projected area and albedo from simultaneously measured reflected and emitted lightcurves can be used to detect changes in the shape and surface brightness of asteroids for which such data exist. Rotational spectral reflectance data obtained in 1981 were used to correlate variations in shape and relative surface albedo with changes in surface composition. Using these data combined with this relative projected area/relative albedo technique, it is confirmed that 15 Eunomia is an elongate asteroid with a generally ovoid (''egg'') shape. Its surface composition varies from metalliferous with major olivine and lesser pyroxene in subequal metal/silicate ratios (much like stony-iron meteorites) for the more ''pointed'' end to a higher concentration of high-Fe pyroxene and a higher general concentration of silicate (a basaltic composition) for the ''blunt'' end (Gaffey and Ostro, 1987, Lunar Planet Sci. XVIII, 310-311). Asteroid 15 Eunomia may be a cross section through a completely or partially differentiated S-type asteroid parent body. The basaltic blunt end of 15 Eunomia may show a sample of the surface of the parent body with the metalliferous end showing a portion of the core/mantle boundary, If true, this would indicate a mantle thickness on the parent body of approximately 300 km. (C) 1997 Academic Press.