Stratigraphy, Chronology, Tectonics, and Subsurface Properties of Ganymede: Deciphering Light Terrain Formation Using Impact Craters

Abstract

Ganymede's geologic evolution has intrigued scientists since the first images were sent back from spacecraft visiting the Jovian system. In particular its unique surface features such as the complex network of light terrain that crisscross the ancient heavily cratered surface has been of major interest, since it has been formed during a period of geologic activity. Its formation is tightly related to Ganymede's internal evolution. In order to solve this relationship, it is needed to gain more insight into the stratigraphic position and the details of the formation process of the light terrain. Previous studies concentrated on the structural analysis of linear features in the dark and light terrain. In this work the focus lies on Ganymede's numerous impact craters and their potential to reveal the surface age and the stratigraphic position of the light terrain with respect to Ganymede's geologic evolution, how craters have been affected by the tectonic activity and which details can be derived about the light terrain formation processes due to the crater's potential to investigate the stratigraphy of Ganymede's subsurface. The study was performed based on the data set collected by the Voyager, Galileo and Juno spacecrafts, which provide the only available data set with sufficient image resolution and coverage of Ganymede's surface so far. To address the research questions various methodologies were employed. Next to photogeological mapping techniques for defining and mapping geologic relevant surface units such as Ganymede's different terrain types as well as impact craters, measurements of the crater size-frequency distribution of dark and light terrain units were performed in order to derive their geologic ages and relative stratigraphy. Structural analysis as used in previous works for several dark and light terrains are applied to a special crater type, i.e. polygonal craters. The structural analysis of these craters in comparison to adjacent dark and light terrains were used for identifying which tectonic processes affected these craters and are responsible for their peculiar shape. The obtained results were then applied to evaluate the currently available crater counting models and their use to estimate the time and duration of the light terrain formation and thus to constrain the tectonic activity in Ganymede's geologic history. Furthermore, based on the recent knowledge about the impact process, the stratigraphy of the subsurface have been reconstructed for special crater types. The nature and distribution of their ejecta and depending on their position in either the dark or light terrain or at the border between them are used to investigate possible variations in the subsurface properties of Ganymede's dark and light terrain. This investigation explores any implications for the type of processes forming the light terrain in particular the mode of extension, whether it involves rifting or spreading. The duration of this formation process of Ganymede's light terrain strongly depends on the cratering models used, specifically the Lunar-Derived chronology Model (LDM) and Jupiter family Comet derived chronology Model (JCM) employed for deriving absolute crater ages. Using the LDM, the light terrains generally exhibit older ages and a shorter formation period compared to the JCM. However, both models support that Ganymede's light terrain began to form early in its history, with a short time gap of ~ 0.2 Ga between the end of dark terrain formation and the beginning of light terrain formation. The onset of tectonism in the formation of light terrains has been previously discussed to be attributed to various factors, including internal differentiation leading to global expansion, tidal heating due to Laplace resonance with Europa and Io, orbital recession, nonsynchronous rotation, and large impact events. Given the early start of the tectonic activity, internal differentiation and global expansion probably played a major role in the formation processes. It is possible that these factors or a combination of them contributed to the process and thus significantly affected the duration of the tectonic activity. The presence of polygonal-shaped impact craters equally distributed across the surface, even in dark cratered terrains, furthermore support that Ganymede's tectonic activity not only concentrated on the light terrain, but that the dark cratered terrain also experienced normal faulting or spreading tectonism, leading to the gradual development of light terrain units. Polygonal craters suggest hidden fractures, which are indicators of the initiation of light terrain formation. During the modification stage of these craters, their rims aligned with tectonic linear structures through slumping or faulting along preexisting fracture and fault planes of weakness. These craters also support the idea that grooves and other fractures are indeed traces of faults. Ganymede's unique dark ray and halo craters as well as bright ray craters emplaced onto the border between dark and light terrain support that the dark material of Ganymede's ancient dark terrain forms a thin layer of less than 1 km on top of an icy crust with no significant differences in the subsurface properties of both terrains at greater depth. The in-depth analysis of dark halo craters located in Ganymede's light terrain reveals significant heterogeneity in the uppermost portions of the icy crust at various locations. The presence of multiple layers of subsurface dark material is needed to explain the existence of small dark halo craters in the light terrain and their distribution of the dark ejecta deposits with maximum excavation depth of ~ 3 km. Such occurrences of near-surface dark terrain material imply a tectonic rifting mode as responsible for these portions of light terrain, indicating downfaulting of the dark terrain material in these locations.