A compositional study of asteroid 243 Ida and Dactyl from Galileo NIMS and SSI observations

[1] On 28 August 1993 the Galileo spacecraft encountered the asteroid 243 Ida and its moon Dactyl. A variety of observations of this asteroid system were collected, including visible wavelength (0.4–1.0 μm) imagery with the Solid State Imager (SSI) instrument and infrared wavelength (0.7–5.2 μm) with the Near Infrared Mapping Spectrometer (NIMS) for both objects. These data indicate that 243 Ida has an orthopyroxene/(orthopyroxene + olivine) ratio of 0.35, a value consistent with that of LL chondrites. The ratio does not vary significantly for the portions of 243 Ida observed by the Galileo NIMS instrument. Asteroid 243 Ida is a SIV subtype of the S type asteroid population. Two very similar spectral units were identified in a combined SSI and NIMS spectral data set. The primary discriminator between the spectral units is a small difference in the amount of “red” slope. A larger “red” slope value corresponds to regions of 243 Ida where ejecta from the crater Azurra are present. Dactyl has a relatively deep absorption centered around 0.97 μm. This suggests that Dactyl has larger mineral grains on its surface in comparison to 243 Ida and thus was formed by a younger collision than that which formed 243 Ida.