Clues to the Origin of Jovian Outer Irregular Satellites from Reflectance Spectra

Visible/near-infrared narrowband spectroscopy (dispersion per element ∼ 6 Å) was obtained of the Jovian irregular satellites JVI Himalia, JVII Elara, JVIII Pasiphae, JIX Sinope, JX Lysithea, JXI Carme, JXII Ananke, and JXVII Callirrhoe in 2006, 2009, and 2010 using the MMT Observatory Red Channel spectrograph. These spectra sample three prograde ( i = 28°), four retrograde ( i = 149° and 165°), and one independent satellite. Our results suggest that the prograde cluster satellites represent fragments probing the cluster’s original parent body, with the largest satellite, Himalia, being the core of the parent body, while Elara preserves the geochemical/mineralogical transition between the core and an outer layer of the body, and Lysithea formed farther from the center of the parent body. The spectral signatures suggest that the prograde parent body fragmented in the early stages of aqueous alteration. This supports the change from more organic-rich material at Lysithea to more carbonized material at Himalia, consistent with weathering/processing of a carbon-bearing material at Himalia. At twice the distance from Jupiter, the retrograde cluster anchored by Pasiphae also suggests that Ananke preserves the transition between the core and an outer layer of a parent body. Both Sinope and Carme are similar to D-class asteroids. Bluing/flattening near 0.4–0.5 μ m in Carme’s spectrum suggests a carbonized component to Carme’s surface material, consistent with greater levels of weathering/processing. Sinope’s red spectrum is consistent with broadband photometry and does not confirm or negate the proposal that it had a common parent body with the Pasiphae cluster.