Impact-driven Resurfacing at Antum Crater (Ganymede)

The icy crust of Ganymede, Jupiter’s largest moon, preserves evidence of complex interactions among impact, tectonic, and chemical processes. Antum Crater, a 25-km dark-rayed feature located in Marius Regio, offers a valuable case study to investigate these dynamics. By integrating Galileo and Voyager geological mapping, spectral analysis from the Near Infrared Mapping Spectrometer (NIMS), and iSALE2D impact simulations, this study reconstructs Antum’s formation and explores its implications for Ganymede’s subsurface volatile inventory.

Geological mapping based on a 359 m/pixel global mosaic identifies three distinct albedo-defined facies within the Antum crater region. The oldest terrain is densely fractured, while the crater’s ejecta display progressively darker albedo with distance, indicating stratigraphic differences or compositional gradients. A revised crater rim diameter of 25 km—significantly larger than previous estimates of 15 km based on the inner dark rim—and an asymmetric ejecta pattern suggest a south-southeast to north-northwest impact trajectory. This directional asymmetry is consistent with the redistribution of dark-rayed material enriched in non-ice components.

Spectral analysis of NIMS data (~2 km/pixel) reveals distinct compositional regimes. The crater floor shows strong 1.5–2.0 µm water ice absorption bands and coarser grain sizes (1–3 mm), indicative of relatively pristine subsurface ice, likely shielded from surface irradiation by Ganymede’s magnetic field. In contrast, the dark ejecta exhibit weaker ice bands, redder spectral slopes, and finer grains, along with diagnostic features of hydrated salts—hydrohalite, bloedite —as well as complexed CO₂  and hydrated sulfuric acid. Nonlinear spectral unmixing using a Hapke model confirms that CO₂ is embedded in a water ice matrix and resolves residual features consistent with up to 20% carbonaceous chondrite material. Hydrated phases dominate the spectral signal, suggesting excavation of volatile-rich subsurface strata.

Impact modeling with iSALE2D constrains Antum’s formation to a collision with a 600–750 m ice projectile (ρ = 910 kg/m³) at a velocity of 20–10 km/s. The resulting 25-km crater and its shallow depth-to-diameter ratio (~0.10) are consistent with formation in a low-cohesion ice shell (~10 kPa). Simulations indicate that steeper thermal gradients (5–15 K/km) can inhibit central peak formation, in agreement with Antum’s observed morphology. Ejecta trajectories from the model align with the spatial distribution of spectrally distinct materials, indicating excavation from compositionally heterogeneous subsurface layers—likely a mix of ancient hydrated non-ice material and impact-redistributed salts.

Antum’s ejecta rays overprint older, concentric structural rings and interact with the adjacent grooved terrain of Tiamat Sulcus, suggesting that the impact may have modified local stress fields. Bright, patchy deposits within the ejecta blanket could represent transient frost from volatile sublimation. The detection of hydrated sulfuric acid in distal regions further supports the presence of pre-impact radiolytic processing of surface or near-surface sulfur compounds by Jovian magnetospheric ions.

Together, these findings point to Antum Crater as a key site where impact processes have mobilized and exposed subsurface brines and radiation-shielded volatiles. The crater serves as a natural excavation into Ganymede’s interior, revealing the interplay between surface and subsurface geochemistry. This interpretation will be directly testable with data from ESA’s JUICE mission: JANUS imaging will resolve fine-scale ejecta stratigraphy and monitor frost dynamics; MAJIS hyperspectral imaging (0.5–5.1 µm) will map hydrated salts and organics at sub-100 m resolution; and RIME radar soundings, guided by JANUS- and GALA-derived topography, will probe for subsurface liquid or frozen brine reservoirs.

The Antum case highlights the value of impact craters as probes of icy moon interiors and demonstrates how impact-driven resurfacing connects surface chemistry with subsurface processes. The approach developed here is broadly applicable to other ocean worlds where similar processes may govern volatile cycling and habitability.

 

Acknowledgements: The authors acknowledge support from the ASI-INAF grant n. 2023-6-HH.0 (CUP F83C23000070005) “Missione JUICE - Attività dei team scientifici dei Payload per Lancio, commissioning, operazioni e analisi dati” (2023-ongoing), and from the INAF Mainstream project “Ganimede dal 2D al 3D: Un approccio multidisciplinare in preparazione a JUICE” (2019-2022).