Abstract DGP2026-81

Ganymede, Jupiter's largest moon and a primary target of the European Space Agency's JUICE mission [1], exhibits a dichotomous surface comprising ancient low-albedo (dark ice) and younger high-albedo (light ice) terrains [2]. Impact craters on these terrains display distinct morphologies and ejecta patterns that encode information about subsurface stratigraphy and material properties. Here, we utilized numerical simulations using the iSALE2D shock physics code [3, 4, 5] to constrain the subsurface layering and strength parameters responsible for three representative crater types [6]: Antum (dark ejecta on dark terrain), Kittu (dark ejecta on light terrain), and Nergal (dark halo surrounded by bright ejecta). We used a global photomosaic of Ganymede [7] compiled from Voyager 1 and 2, Galileo, and Juno imaging data to characterize crater dimensions and ejecta distributions. Numerical simulations assumed vertical impacts with projectile velocities of 20 km/s, Ganymede surface gravity (1.43 m/s²), surface temperature of 100 K [8], and a subsurface thermal gradient of 10 K/km, with spatial resolution of 50 m per cell. Layer thicknesses were systematically varied until simulated ejecta distributions matched observed patterns. Our simulations consider an elasto-plastic constitutive model, fragmentation, strength, porosity & compaction, dilatancy, and multiple material properties [3, 4, 5, 9, 10, 11].

Our models reveal that dark ice exhibits significantly higher cohesive strength than light ice, consistent with its greater age and thermal processing. For Antum crater, a 1.0 km thick dark ice layer overlying light ice substrate reproduces the observed ejecta pattern. Kittu crater formation requires a more complex stratigraphy: 0.8 km of light ice above 0.4 km of dark ice, both overlying dark ice basement. Nergal crater's distinctive morphology demands the most elaborate layering: alternating layers of light ice (0.1 km), dark ice (0.3 km), light ice (0.3 km), and dark ice (1.05 km) above a light ice substrate. These stratigraphies suggest multiple episodes of surface modification through tectonic resurfacing and possible cryovolcanism. Comparative analysis of equation of state (EOS) implementations reveals significant differences between ANEOS [12] and Tillotson [13] formulations. ANEOS simulations produce shallower craters with high-density (potentially liquefied) material on crater floors, reflecting explicit treatment of shock-induced phase transitions from solid to vapor states. This vaporization generates bimodal ejecta distributions absent in Tillotson simulations, where ejecta travels farther as a continuous blanket. The ANEOS treatment provides more realistic representation of shock processing in ice, though computational expense limits resolution.

While iSALE2D simulations incorporate simplifying assumptions (homogeneous material properties within layers, vertical impacts, absence of strain localization), our results provide quantitative constraints on Ganymede's near-surface stratigraphy and ice rheology. The derived strength contrasts and layer thicknesses offer testable predictions for JUICE mission observations and inform geophysical models of Ganymede's thermal and geological evolution. Future three-dimensional simulations incorporating oblique impacts and heterogeneous target properties will refine these interpretations and enhance our understanding of impact processes on icy satellites.

References: [1] Grasset et al., Planet. Space Sci., 78, 1-21, 2013; [2] Pappalardo et al., in Jupiter, 363-396, Cambridge Univ. Press, 2004; [3] Amsden et al., Los Alamos Nat. Lab. Rep., LA-8095, 1980; [4] Collins et al., Meteorit. Planet. Sci., 39, 217-231, 2004; [5] Wünnemann et al., Icarus, 180, 514-527, 2006; [6] Baby et al., Earth Space Sci., 11, e2024EA003541, 2024; [7] Kersten et al., EPSC2022-450, 2022; [8] Bray et al., Icarus, 231, 394-406, 2014; [9] Melosh, et al., JGR, 97(E9), 14735–14759, 1992; [10] Ivanov, et al., International Journal of Impact Engineering, 20, 411–430, 1997; [11] ​Collins, G. S. JGR: Planets, 119(12), 2600–2619, 2014; [12] Thompson & Lauson, Sandia Nat. Lab. Tech. Rep., 1972; [13] Tillotson, Gen. Atomic Rep. GA-3216, 1962