JWST Reveals Varied Origins Between Jupiter's Irregular Satellites

The gist We report JWST NIRSpec (0.7-5.1 µm) observations of eight Jovian irregular satellites across five orbital groups and detect variation in the phyllosilicate content of the three largest members of the Himalia collisional family

JUPITER’S IRREGULAR SATELLITES: POSSIBLE REMNANTS OF OUTER SOLAR SYSTEM MIXING

The origins of Jupiter’s irregular satellites are uncertain, as their highly inclined and eccentric orbits suggest they were captured from asteroid- or comet-like parent reservoirs The (a,i) orbital elements of the 95 known Jovian irregular satellites display major family clusters, including a prograde group dominated by the Himalia family and three major retrograde groups Various processes have been proposed to explain the capture of these satellites, such as pull-down capture and gas-drag Models of planet migration reproduce the overall number and orbital distributions of irregular satellites via three-body interactions between a small body and two closely interacting planets Through the lens of migratory capture, the most likely source reservoir for irregular satellites and Trojans would be the primordial Kuiper belt

Previous Observational and Laboratory Context

Numerous spacecraft missions have explored the Jovian system, but only Himalia has been characterized by spacecraft imaging and spectroscopy Telescopic observations have suggested kinship between irregular satellites and various groups of dark primitive asteroids, with Jarvis et al. (2000) reporting a 0.7 µm feature on Himalia suggesting it contains phyllosilicates Brown & Rhoden (2014) detected and characterized Himalia’s 3µm band, reporting this feature to also match Europa but not match any known phyllosilicates Coarser color photometry surveys have found Jupiter’s irregular satellites to have colors similar to those of Saturn, Uranus, and Neptune The lack of high-quality spectral datasets for Jovian irregular satellites limits the scope of hypothesis testing both for their compositions and their connections to the Galilean moons

SPECTRAL GROUPINGS AND BAND IDENTIFICATIONS

The processed, thermally-corrected reflectance spectrum of each target is shown in Figure 3, which uses a “global” continuum defined by a low order polynomial to identify major features The eight irregular satellites can be roughly described by four groups based on their spectral slopes and 3µm absorption features

** Group 1 consists of Sinope, Carme, and Themisto, which are all red-Trojan-like objects They display steep red slopes from 0.7-2.5 microns and shallow absorption features near 3.0 and 3.4 µm Carme has a deeper band at 3.0 microns than Sinope and Themisto, but they all have similar 3.4 µm depths The Himalia and Elara family display a unique band complex with a sharp 2.7 micron band transitioning into the broad feature from 2.8-3.5 microns Ananke comprises the second group, displaying a deeper, rounded 3.0 micron band and relatively neutral spectral slopes The third group includes Pasiphae and Lysithea, which have 3.0 µm features centered similarly to Ananke but with redder spectral slopes Himalia and Elara show a clear 2.7 micron feature, while Pasiphae and Lysithea display deep 3µm absorption features centered on 3.0 microns similar to less-red Jovian Trojans Himalia and Elara display a feature near 3.9 µm, while Himalia and Lysithea display 2.63 µm features The presence of Trojan-like 3.0 and 3.4 µm absorption bands on small Jovian irregular satellites provides new context for the chemical pathways that could have produced them The less-red irregular satellites defy simple classification, with bands similar to the less-red Trojans but at measurably shorter wavelengths The red group of satellites displays one notable outlier, Carme, which has a deeper 3.4 µm organics band than seen in either the redder or less-red Trojan groups The prograde satellite Themisto and the retrograde satellite Sinope have 3.0 and 3.4 µm bands similar to the red Trojan Leucus The Himalia family contains at least two distinct types of OH-bearing materials that do not correlate with optical or near-infrared color The Himalia family parent body was similar to Ceres in terms of containing water, organics, and NH-bearing materials The observed spectral variation in the Himalia group suggests a stratified mutual parent body The Himalia family members present a complex compositional picture whose only clear spectral trend is the correlation between the presence of ammoniated materials and satellite size The observed spectral variation in the Himalia group, including Elara’s blend of Himalia-like and Lysithea-like material, could relate to a stratified mutual parent body This suggests that the complex hydration states observed within the Himalia family are more consistent with a heterogenous parent body The Himalia family demonstrates that surfaces with similar colors can nonetheless have different OH-bearing materials The red 3.0 and 3.4 µm absorbers must either form within the circumjovian environment or represent material that survives major resurfacing events The less-red irregular satellites defy simple classification, with bands similar to the less-red Trojans but at measurably shorter wavelengths The retrograde satellite Ananke is similar to some hydrated chondrites, suggesting that aqueous alteration is present within the retrograde satellite population Pasiphae has a 3 µm absorption feature similar to Lysithea, complicating a clear association with a single surface type The origins for Himalia and Ananke could represent separate compositional lineages that were not captured into the Jovian Trojans Alternatively, Himalia and Ananke could represent captures from an earlier epoch of Jupiter’s history Alternatively, for Himalia and Ananke to be related to Trojan compositions, they could represent the unseen cores of Trojan-like objects The possible presence of Trojan-like material (Lysithea) within the Himalia family suggests some relationship between Himalia and Trojan-like materials may exist Future tests for 3 µm variability among collisionally evolved Trojans will be useful to explore compositional stratification The origins for Himalia and Ananke could be assessed by understanding if Trojans demonstrate surface- variable 3 µm bands The Himalia family demonstrates that surfaces with similar colors can nonetheless have different OH-bearing materials The red group of satellites displays one notable outlier, Carme, which has a similar 3µm band to the less-red Trojan Patroclus, but a deeper 3.4 µm organics band more similar to those seen amongst the red Trojans The red 3.0 and 3.

1. Bold header: Improved Compositional Discrimination for Small Bodies

This system can now discriminate between different formation scenarios by analyzing spectral variations within collisional families, as noted in The Himalia family’s band properties do not just vary in depth: Himalia has a rounded 3.0µm band that is centered ∼ 0.05µm longer than Lysithea’s sharper feature. This allows for testing hypotheses about whether all three objects were primarily formed from the same parent body, with the three objects representing distinct aqueous alteration states.

2. Bold header: Enhanced Origin Hypothesis Testing

The AI can now evaluate competing capture scenarios by correlating spectral features with orbital dynamics, as suggested by the conclusion that The first possibility is that the Himalia and/or Ananke families represent separate compositional lineages that were not captured into the Jovian Trojans. This capability allows for a quantitative assessment of whether Himalia and Ananke could represent captures from an earlier epoch of Jupiter’s history versus being related to Trojan compositions.

3. Bold header: Predictive Modeling of Nitrogen Delivery

The system can now model the potential delivery pathways for nitrogen to the Galilean moons by linking irregular satellite chemistry to moon surface alteration, specifically by assessing Delivery and subsequent irradiation of NH4-bearing material could contribute to the subtle absorption features detected between 3.2 and 3.7 µm and Callisto’s broad 4.57 µm band. This enables predictive modeling of exogenic nitrogen sources for Europa's subsurface ocean based on the presence of NH4-bearing compounds delivered in dust grains.

We report observations of eight Jovian irregular satellites with JWST's NIRSpec instrument: Himalia, Elara, Pasiphae, Sinope, Lysithea, Carme, Ananke, and Themisto. Irregular satellite families, which are presumed to have formed via collisions, contain various Trojan-like and C-type-asteroid-like surfaces. We sample the three largest members of the Himalia satellite family, detecting the presence of complexed CO 2 and a unique absorption band from about2.7-3.6 μm whose character correlates with satellite size. The two largest irregular satellites, Himalia family members Himalia and Elara, contain ammoniated phyllosilicates that are not seen in the meteorite inventory. We propose that the Himalia parent body was heterogeneous and formed with materials similar to Ceres-like ammonium-bearing asteroids. Several small (D about 10km) irregular satellites closely track the colors and absorption bands of ``red'' Jovian Trojans, demonstrating that these compositions are retained amongst the products of collisions that occurred after Jovian capture. We report the first detection of aqueous alteration products in the retrograde satellite swarm, finding Ananke's 3 micron band to closely match phyllosilicates seen in C2 chondrites. Notably, objects with OH absorption features similar to the Trojan asteroid Eurybates are found in both the retrograde Pasiphae family and the prograde Himalia family, confounding a simple link between such materials and a single surface type. The irregular satellites appear consistent with some materials that experienced alteration from liquid water and others that did not. Consequently, Jupiter may have captured bodies that formed from different initial compositions, or bodies that experienced different levels of heating, driving differential alteration processes.