IDA AND DACTYL - SPECTRAL REFLECTANCE AND COLOR VARIATIONS

Galileo SSI color data between 0.4 and 1.0 mu m demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra , Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1- mu m pyroxene-olivine band. Ida's surface can be classified into two c olor terrains: Terrain A has a shadower 1-mu m absorption and a steepe r visible red slope than does Terrain B. Qualitatively, the color-albe do systematics of these two terrains follow those noted for color unit s on Gaspra and the variations in 1-mu m band depth with weathering de scribed by Gaffey et al. (Gaffey, M. J., J. F. Bell, R. H. Brown, T, H . Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993. Icarus 106, 57 3-602). Terrain A, with its slightly lower albedo, its shallower 1-mu m band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the ''more processed,'' ''more mature,'' or the ''more weathered'' of the two terrains. Consistent with this inte rpretation is that Terrain A appears to be the ubiquitous background o n most of Ida, while Terrain B is correlated with some small craters a s well as with possible ejecta from the 10-km Azzurra impact structure . Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fail within the range known to occur within the Koronis family. The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does n ot conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-mu m band is deeper than that o f Terrain B, but its albedo is lower, rather than higher, By itself, t he deeper band depth could be interpreted, following Gaffey et at, to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the difference s between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Th omas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granah an, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996 Icarus 120, 48-65), thus ruling out any dramatic texture differences between the t wo surfaces, the most likely explanation is that the satellite has a s lightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzel et al. (Binzel, R. P ., S. Xu, and S. J. Bus 1993. Icarus 106, 608-611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith proces ses. (C) 1996 Academic Press, Inc.