Presentation #308.04 in the session Outer Irregular Satellites.
The existence of an absorption feature near 0.7 μm and attributed to aqueous alteration products has been suggested in both grey and reddened broadband photometry of some outer irregular jovian satellites. Dynamical clusters of these satellites are mixed between objects that do and do not show this absorption feature. Observations of some objects suggest surface variegation.
To confirm and define this feature, narrowband spectroscopy 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. The spectra are centered near 0.64 μm covering the 0.7-μm feature (generally ranging from 0.57 to 0.83 μm in C-complex asteroid spectra), with a dispersion/element of ~6 Å. These spectra sample three prograde (i = 28o), four retrograde (i = 149o, 165o) and one independent satellite.
In our analysis, we constrained the mixing modeling to materials that have been spectrally identified and studied in CM2 carbonaceous chondrites. We argue that these materials derived from extraterrestrial rocks provide reasonable direction for narrowing compositional possibilities. Based on the spectral similarities with C-complex asteroids and the presence of Fe-bearing serpentine-group phyllosilicates in CM2 meteorite laboratory analyses, we consider Fe-bearing serpentines in our mixture of materials. Based on our study of carbon in the Solar System, the presence of carbonaceous matter in the CM2 meteorites, the low geometric albedos of both jovian satellites and carbonaceous chondrites, and the presence of the Trojan D-class asteroids, we include carbon products we expect at different heliocentric distances.
Our results suggest that the prograde (i = 28o) cluster satellites represent fragments probing the cluster’s original parent body, with the largest satellite Himalia being the core of the parent body, Elara preserves the geochemical/mineralogical transition between the core to an outer layer of the body, and Lysithea probes farther from the center of the parent body. At twice the distance from Jupiter, the retrograde (i = 149o) cluster anchored by Pasiphae also suggests that Ananke preserves the transition between the core to an outer layer of a parent body. The spectra of Sinope are sufficiently different than Pasiphae that it is difficult to conclude that it originated as part of the i = 149o cluster. Both Sinope and Carme are similar to D-class asteroids.
This research has been supported by SSERVI TREX.