JWST Reveals Varied Origins between Jupiter’s Irregular Satellites

We report JWST NIRSpec (0.7–5.1 μm) observations of eight Jovian irregular satellites across five orbital groups. We detect variation in the phyllosilicate content of the three largest members of the Himalia collisional family (Himalia, D ∼ 140 km, Elara, D ∼ 80 km, and Lysithea, D ∼ 40 km). Himalia contains complexed CO₂ and overlapping absorption features from ∼2.7 to 3.6 μm that match laboratory samples of ammoniated phyllosilicates. Lysithea displays a simpler, single-minimum 3 μm feature caused by an unidentified absorber. Elara presents a 3 μm band that matches a simple average of Himalia and Lysithea. We argue that the Himalia parent body was heterogeneous and formed with materials similar to Ceres-like ammonium-bearing asteroids, consistent with suggestions from previous visible-wavelength observations. The satellites Carme, Sinope, and Themisto have colors and absorption features similar to “red” Jovian Trojans. The satellites Ananke, Pasiphae, and Lysithea each have absorption bands centered from 2.93 to 2.97 μm, intermediate between the longer-wavelength bands observed on Trojan asteroids and the shorter-wavelength bands commonly seen on phyllosilicate-rich C2 chondrites. These intermediate-wavelength absorptions are present in both Trojan-like families and the hydrated Himalia family, confounding a link to a single compositiont. The observed differences between Jupiter’s irregular satellites and Trojans require one of two possibilities: (1) some Trojan parent bodies contained hydrated materials in their cores, or (2) some Jovian irregular satellites were not captured from the Trojan parent reservoir. Future searches for NH-bearing materials across the solar system and 3 μm studies of small-body collisional families may provide means to discriminate between these two hypotheses.