Ray and Halo impact craters on Ganymede: Insights into the stratification of icy crust

Ganymede, the solar system's largest moon, is captivating due to its complex geology and potential habitability. ESA's JUICE mission now focuses on exploring Ganymede [1, 2]. We investigate ray and halo impact craters on its surface, which display diverse morphologies and ejecta materials [3], including bright icy and dark non-icy materials [4,5] located at various locations on Ganymede. To better understand the vertical stratification of Ganymede's icy crust and its implications for tectonic processes like rifting or spreading, we use the knowledge of impact cratering processes [6, 7], particularly the excavation flow field of hypervelocity impacts. We utilize a global mosaic created by [8], which combines high-resolution images from the Voyager 1, Voyager 2, Galileo, and Juno spacecraft. Additionally, we incorporate NIMS-derived data on the varying abundance of water ice and dark non-ice material, as well as water ice grain sizes, as presented in [9] wherever available. We included 36 craters in our study. Excavation depths provide clues into the vertical stratigraphy at these locations. Our findings reveal that dark ray and dark halo craters, characterized by dark ejecta, are generally small, with excavation depths not exceeding 3 km. Craters with a final diameter larger than 75 km exhibit bright rays, as exemplified by Melkart and Enkidu craters. Craters displaying both dark and bright ejecta, like Tammuz (diameter ~51 km), likely impacted at the boundary between light and dark terrains. Dark ray craters, such as Antum and Mir, suggest that the dark terrain at Marius Regio is relatively thin, not exceeding 1.2 km. On the other hand, dark halo craters like Nergal and Khensu on light terrain indicate that the excavated dark material originates from depths of ~ 1.36 km, suggesting heterogeneity in the crust there and the presence of subsurface dark material. Dark ray craters in light terrain, like Kittu, indicate that the dark terrain material originates from a depth of around 1.2 km. These findings support the possibility of rifting as a contributing factor to light terrain formation. Additionally, the presence of a very thin uppermost ice layer atop a dark material in the light terrain suggests the potential for flooding of older dark terrain by an icy substrate, contributing to Ganymede's light terrain formation. REFERENCES: [1] Grasset et al. (2013), PSS, v. 78, p. 1-21. [2] Stephan et al. (2021), v. 208, p. 105324. [3] Schenk et al., (2004), Cambridge University Press, p. 427 - 456. [4] Hibbitts (2023), Icarus, v. 394, p. 115400. [5] Schenk and McKinnon (1991), Space Science Reviews, v. 60, no. 1, p. 413-455. [6] Kenkmann  et al (2014), Journal of Structural Geology, v. 62, p. 156-182. [7] Melosh (1989), Oxford University Press. [8] Kersten et al. (2022), pp. EPSC2022-450. [9] Stephan et al. (2020), Icarus, v. 337, p. 113440.