Spectroscopic Characterization of Icy Moon Surfaces: Compositions, Origins, and Implications from Jupiter toNeptune

The surfaces of icy satellites preserve a complex record of the processes that shape their formation and evolution, reflecting a combination of endogenic emplacement, exogenic delivery, and irradiation-driven modification. In this thesis, I use multi-wavelength spectroscopy to disentangle these competing processes and assess the extent to which surface compositions can be used to infer interior chemistry and system history across the outer solar system, from Jupiter to Neptune. Chapters 2 and 3 focus on Europa, whose surface may provide a window to the subsurface ocean composition, but is strongly altered by irradiation. Using VLT/SINFONI observations, I show that a 2.07 μm absorption feature on Europa’s trailing hemisphere correlates with irradiation patterns but not with large-scale geology, indicating a predominantly exogenic origin and challenging models that invoke radiolytically altered ocean-derived salts to explain this band. I then use Galileo/NIMS observations of the young Pwyll and Manann’an craters to constrain the equilibrium timescale of Europa’s radiolytic sulfur cycle, finding that hydrated sulfuric acid is depleted in these regions and that equilibrium is reached on timescales ≳2–4 Myr—orders of magnitude longer than laboratory estimates. In Chapter 4, I present near-global ultraviolet–visible spectral mapping of Callisto with HST/STIS. These data reveal absorptions plausibly associated with iron-bearing silicates, organics, and potentially NaCl, and show little evidence for SO2 or other sulfur-bearing species. The spatial distribution of these spectral features reveals significant compositional heterogeneity, including localized features associated with major impact basins, challenging suggestions that Callisto’s dark material is dominated by a thick (∼100 m) blanket of irregular satellite dust. Instead, these observations indicate that the dark material reflects a complex mixture of endogenic material excavated by impacts, delivered exogenic material, and radiolytic alteration. In Chapter 5, I use JWST/NIRSpec observations to show that Neptune’s ring-moons exhibit a 2.72 μm absorption diagnostic of Mg-phyllosilicates, indicating extensive aqueous alteration within larger precursor bodies. These results suggest that Neptune’s present-day moons are reaccreted fragments of differentiated satellites disrupted during Triton’s capture, providing direct access to deep interior material from icy outer solar system bodies.