Spectroscopic Links among Giant Planet Irregular Satellites and Trojans

Abstract

We collect near-infrared spectra (∼0.75–2.55 μm) of four Jovian irregular satellites and visible spectra (∼0.32–1.00 μm) of two Jovian irregular satellites, two Uranian irregular satellites, and four Neptune Trojans. We find close similarities between the observed Jovian irregular satellites and previously characterized Jovian Trojans. However, irregular satellites’ unique collisional histories complicate comparisons to other groups. Laboratory study of CM and CI chondrites shows that grain size and regolith packing conditions strongly affect spectra of dark, carbonaceous materials. We hypothesize that different activity histories of these objects, which may have originally contained volatile ices that subsequently sublimated, could cause differences in regolith grain size or packing properties and therefore drive spectral variation. The Uranian satellites Sycorax and Caliban appear similar to TNOs. However, we detect a feature near 0.7 μm on Sycorax, suggesting the presence of hydrated materials. While the sample of Neptune Trojans have more neutral spectra than the Uranian satellites we observe, they remain consistent with the broad color distribution of the Kuiper Belt. We detect a possible feature near 0.65–0.70 μm on Neptune Trojan 2006 RJ103, suggesting that hydrated material may also be present in this population. Characterizing hydrated materials in the outer solar system may provide critical context regarding the origins of hydrated CI and CM chondrite meteorites. We discuss how the hydration state(s) of the irregular satellites constrains the thermal histories of the interiors of their parent bodies, which may have formed among the primordial Kuiper Belt.

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

“…The mixed red and gray broadband photometry of this cluster (Grav et al 2003), with mixed results from testing for the presence of the 0.7 μm feature in this photometry (Paper I), support, however, the potential coexistence of two types of material in the parent body of the cluster. Sharkey et al (2023) show moderate-resolution NIR spectra of Pasiphae and Sinope, where the spectrum of Sinope is redder at lower wavelengths but alters at 1.6 μm to a less reddened slope in agreement with the spectrum of Pasiphae. This could agree with our spectral findings.…”

“…The two spectra for Sinope show a redder slope than Carme but fall within the spectral range of the ECAS values. A recent VNIR spectrum of Jovian irregular satellite JXVIII Themisto (Sharkey et al 2023) shows similar reddening to the spectra of Carme and Sinope, suggesting a similar composition and possibly origin.…”

Revealing Callisto’s Carbon-rich Surface and CO2 Atmosphere with JWST

“…The presence of such organic residues on Callisto, if they formed in an H 2 O-poor environment, supports delivery in dust grains from irregular satellites that may also be H 2 O-poor. Upcoming JWST observations of Jovian irregular satellites (Sharkey et al 2023a) will shed light on the possible compositional ties between these objects and Callisto's surface chemistry (Sharkey et al 2023b), in particular for clarifying the origin of the 4.57 μm feature.…”