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Nowhere in the solar system are impact morphologies observed in greater variety than on the icy Galilean satellites. This is likely a consequence of the structural and thermal state of the crust at the time of impact, and perhaps impact velocity. Palimpsest-type impact features show smooth enclosed central plains surrounded by undulating plains, within which are distributed concentric arcuate ridges, and no recognizable rim. Buto Facula on Ganymede is the best resolved of any palimpsest, having mostly been imaged at 190 m/pixel and optimum lighting, allowing insight into the circumstances that form this type of impact feature. Part of an impact crater on the eastern edge of Buto Facula has been buried by undulating plains material, suggesting that at the time of their emplacement the undulating plains behaved as a low-viscosity flow advancing across the landscape around the impact zone, encroaching on landforms that it encountered. We evaluated hypotheses for the formation of undulating plains using impact and ejecta modeling. We do not attribute the source of Buto's undulating plains to "dry" impact ejecta due to the existence of impact features larger than Buto that are not surrounded by undulating plains deposits. We performed iSALE impact simulations incorporating a subsurface liquid layer (or low strength layer) at various depths. An impact into a surface with a pre-existing, 5 km-deep, 5 km-thick fluid layer results in excavation of fluid material from that layer, producing a nearly flat final surface profile that is consistent with Buto's flat profile and the distribution of its undulating plains material. A liquid layer at a depth of 20-40 km results in impact feature profiles that resemble classic impact craters. We offer tests of this shallow subsurface liquid layer hypothesis for palimpsest formation that could potentially be performed by JUICE and Europa Clipper.

The icy Galilean satellites display impact crater morphologies that are rare in the Solar System. They deviate from the archetypal sequence of crater morphologies as a function of size found on rocky bodies and other icy satellites: they exhibit central pits in place of peaks, followed by central dome craters, anomalous dome craters, penepalimpsests, palimpsests, and multi-ring structures. Understanding the origin of these features will provide insight into the geophysical factors that operate within the icy Galilean satellites. Pit craters above a size threshold feature domes. This trend, and the similarity in morphology between the two classes, suggest a genetic link between pit and dome craters. We propose that dome craters evolve from pit craters through topographic relaxation, facilitated by remnant heat from the impact. Our finite element simulations show that, for the specific crater sizes where we see domes on Ganymede and Callisto, domes form from pit craters within 10 Myr. Topographic relaxation eliminates the stresses induced by crater topography and restores a flat surface: ice flows downwards from the rim and upwards from the crater depression driven by gravity. When the starting topography is a pit crater, the heat left over from the impact is concentrated below the pit. Since warm ice flows more rapidly, the upward flow is enhanced beneath the pit, leading to the emergence of a dome. Given the timescales and the dependence on heat flux, this model could be used to constrain the thermal history and evolution of these moons.

We have performed topographic and geological mapping of 19 large impact features on Ganymede and Callisto in order to gather morphometric and crater age data that allow us to quantify how the diverse morphologies of these features transition with size and age. The impact features are divided into two main morphological groups, craters and penepalimpsests/palimpsests. The morphologies of pit and dome craters are independent of age or geologic context, indicating that the impacts that formed them were small enough to only ever penetrate into a cold, rigid ice layer, with evolution of a pocket of impact melt contributing to the development of their central pits and surrounding raised annuli, while the domes formed mainly via viscous relaxation of the pit topography. The subdued rims of anomalous dome craters indicate the increasing effect of a warm subsurface ice layer on impact feature morphology with increasing size. The low topographic relief of penepalimpsests and palimpsests indicates that these impacts penetrated the ice shell to liberate large volumes of underlying, pre-existing liquid. Penepalimpsests show a higher frequency of concentric ridges within their interiors, indicating a more robust subsurface state that could support the rotation and uplift of solid material during impact. The size and age overlap of anomalous dome craters, penepalimpsests, and palimpsests, with a few impact features appearing to be transitional between anomalous dome craters and penepalimpsests, indicates that impactor size, time of impact, and variation in temperature gradient across the satellite are all factors in determining which of these morphologies emerges.

We have investigated the 2009 July impact event on Jupiter using the ZEUS-MP 2 three-dimensional hydrodynamics code. We studied the impact itself and the following plume development. Eight impactors were considered: 0.5 km and 1 km porous (\rho = 1.760 g cm^{-3}) and non-porous (\rho = 2.700 g cm^{-3}) basalt impactors, and 0.5 km and 1 km porous (\rho = 0.600 g cm^{-3}) and non-porous \rho = 0.917 g cm^{-3}) ice impactors. The simulations consisted of these bolides colliding with Jupiter at an incident angle of \theta = 69 degrees from the vertical and with an impact velocity of v = 61.4 km s^{-1}. Our simulations show the development of relatively larger, faster plumes created after impacts involving 1 km diameter bodies. Comparing simulations of the 2009 event with simulations of the Shoemaker-Levy 9 events reveals a difference in plume development, with the higher incident angle of the 2009 impact leading to a shallower terminal depth and a smaller and slower plume. We also studied the amount of dynamical chaos present in the simulations conducted at the 2009 incident angle. Compared to the chaos of the SL9 simulations, where \theta is approximately 45 degrees, we find no significant difference in chaos at the higher 2009 incident angle., Comment: 8 pages, 7 figures

We have studied plume formation after a Jovian comet impact using the ZEUS-MP 2 hydrodynamics code. The three-dimensional models followed objects with 500, 750, and 1000 meter diameters. Our simulations show the development of a fast, upward-moving component of the plume in the wake of the impacting comet that "pinches off" from the bulk of the cometary material ~50 km below the 1 bar pressure level, ~100 km above the depth of greatest mass and energy deposition. The fast-moving component contains about twice the mass of the initial comet, but consists almost entirely (>99.9%) of Jovian atmosphere rather than cometary material. The ejecta rise mainly along the impact trajectory, but an additional vertical velocity component due to buoyancy establishes itself within seconds of impact, leading to an asymmetry in the ejecta with respect to the entry trajectory. The mass of the upward-moving component follows a velocity distribution M(>v) approximately proportional to v^-1.4 (v^-1.6 for the 750 m and 500 m cases) in the velocity range 0.1 < v < 10 km/s., Comment: 5 pages, 4 figures. Accepted for publication in The Astrophysical Journal