Near-Infrared Observations of the Outer Jovian Satellites

On September 21, 1998, the outer jovian satellites Himalia (J6), Elara (J7), Carme (J11), Pasiphae (J8), and Sinope (J9) were detected in the J, H, and K_s bandpasses in the course of the Two-Micron All Sky Survey. Similarities in near-infrared colors are consistent with the hypothesis that the outer satellites in prograde orbits are fragments of a captured C-type asteroid. The retrograde satellites in general exhibit considerable diversity. The nondetection of retrograde Ananke is significant in that it requires visible to near-infrared reddening close to that of prograde Himalia and very different from the other retrograde satellites. It is suggested that the retrograde jovian satellites are fragments of a captured parent body more heterogeneous than the prograde parent. Alternatively, it is suggested that the retrograde satellites arose from more than one retrograde capture event and possible subsequent fragmentation.