Available Abstracts, Preprints, Articles.
Eileen V. Ryan1 and Clark R. Chapman2,
1Planetary Science Inst. 620 N. 6th Ave., Tucson AZ 85705, USA, 2Southwest Research Inst., 1050 Walnut, Suite 429, Boulder CO 80302, USA.
We have done a detailed spectral examination of Ida's surface using Galileo SSI color images that span the wavelength range 0.4 to 1.0 microns. Spectra calibrated and processed by Helfenstein et al. [1] have been further smoothed, to decrease scatter, for our studies. We find definite color variations on Ida in all parts of the spectrum, but particularly in the depth of the 1 micron pyroxene-olivine band. We interpret the color variations as being largely unrelated to compositional variations. Instead, they indicate a space-weathering trend because of a prominent correlation in reflectance spectra with the apparent relative geological ages of the units. As the surface of Ida is cratered, bright, bluer-than-average materials are excavated. As the fresh surfaces age and degrade with time, they darken slightly and their spectra become redder, showing less curvature in the visible and a weaker absorption band. These changes are similar, but less strong, than space weathering trends well known on the Moon. Much of Ida is fairly homogeneous, perhaps mature, exhibiting the reddish spectra typical of S-type asteroids. Among the younger terrains, our studies indicate a greater range of diversity than is implied by lumping all bluer regions into a single Terrain B [2]. The reflectance spectrum for Dactyl is similar to that for some younger terrains on Ida.
Figure 1 shows spectra of three different areas of Ida: regions containing the large, fresh crater Azzurra and its ejecta; a small, fresh crater (2 degS, 52 degE, see map in [3]) located within the larger crater Sterkfontein; and background, weathered terrain (similar to Terrain A of [2]). Also shown in the figure are laboratory spectra for the common L5 and L6 ordinary chondrites [4]. The spectrum of the small, recent crater has a 1æm absorption band about 2/3rds the depth of the band in L5 ordinary chondrites; thus it appears that L5 chondrites are plausible analogs for the unweathered composition of Ida, which has important implications for the ongoing debate about the possible relationship between S-type asteroids and ordinary chondrites [5].
While Azzurra is comparatively young compared with other large craters on Ida, its weaker absorption band suggests that it is somewhat older and more weathered than the small crater. The background units, typical of much of Ida, are most optically mature. In the Azzurra region, there may be greater reddening of terrain with increasing distance from the central impact, perhaps indicating a degree of vertical or lateral mixing with mature materials near the thinner, distal edges of the ejecta blanket.
Fig. 1 (go to Ida Research from CRC's home page to see this figure). Comparison of SSI 4- and 5-color spectra for geologically young terrains on Ida and for background regions with laboratory spectra of L5 and L6 ordinary chondrites. The appreciable depth of the 1 micron band for the spectra of the younger areas (Azzurra, fresh crater) may reflect more pristine materials on Ida, indicating that the general background areas have undergone weathering.
Available Abstracts, Preprints, Articles.
Clark
R. Chapman's Publications.
Home Page