Insights about Stratigraphy and Composition From Ray and Halo Craters on Ganymede

Abstract

Ganymede, the largest moon in the solar system, captivates with its complex geology and potential habitability. ESA's JUICE mission currently focuses on exploring Ganymede [1, 2]. We investigate ray and halo impact craters on its surface, which exhibit diverse morphologies and ejecta materials [3], including bright icy and dark non-ice materials [4, 5] found in various locations on Ganymede. In order to understand stratigraphy of Ganymede’s crust, we investigate formation of ray and halo impact craters using the Z-model [8] and the iSALE 2D, which is a multirheology and multimaterial Hydrocode code [e.g. 9] for numerical simulations.

We mapped ray and halo impact craters using global mosaic created by [10]. Additionally, we incorporate NIMS-derived data on varying water ice abundance, dark non-ice material distribution, and water ice grain sizes as presented in [11], wherever available. For iSALE, the projectile resolution used was 10 cells per projectile radius, corresponding to an impactor size of 1 km. Approximately 120-160 zones were used in the extension zone, with a 5% increase in cell size from one neighboring cell to the next. For Antum, an impactor velocity of 15 km/s was employed.

Excavation depth measurements for different crater types were collected based on [7] and [8] (Z = 3, Z = 4). These measurements reveal that dark ray craters such as Antum and Mir suggest the dark terrain at Marius Regio is relatively thin, not exceeding 2.3 km. In contrast, dark halo craters like Nergal and Khensu on light terrain indicate that excavated dark material originates from depths of ~1.4 km and 2.5 km, respectively, suggesting heterogeneity in the crust and the presence of subsurface dark material. Dark ray craters in light terrain, like Kittu, indicate that dark terrain material originates from a depth of around 2.3 km. From iSALE, for Antum, dark material is ejected furthest followed by bright material, where the dark material is estimated to ~1.3 km thick. These findings support the possibility of rifting contributing to the formation of light terrain wherever dark halo and dark ray craters are present, indicating subsidence of dark material into the subsurface. While bright ray craters imply light terrain formation via tectonic spreading. Our preliminary findings support iSALE modeling results for Antum are consistent with results from Z-model [8].