Uncovering Ganymede's past: Tectonics at Nippur/Philus Sulci

Abstract

Observations of Jupiter's largest moon Ganymede by Voyager and Galileo revealed fractures and evidence of strike-slip faulting, leading to studies for the mechanisms of these structures focused on extensional tectonism and gravitational tidal stresses. In this analysis, we investigate the geologic history of Ganymede in the area of Nippur/Philus Sulci (175°E, 36.9°N) by examining Galileo high-resolution data available for this region (∼100 m/pixel) and conducting a tidal stress investigation of Ganymede's past. Several crosscutting bands of light terrain in the Nippur/Philus Sulci site show varying degrees of tectonic deformation, ranging from smooth and less distorted bands to highly grooved and deformed terrain. The chronology of tectonic activity implied by mapped crosscutting relationships reveals three eras of distinct geologic activity: 1. Ancient, 2. Intermediate and 3. Youngest. Indicators of shear deformation are found throughout all ages of terrain, but mapped early-stage indicators of strike-slip features are exclusively located in intermediate and youngest terrain. An investigated offset feature in intermediate-aged terrain corresponds in slip direction to the predictions from modeling stresses of a higher past eccentricity. However, the en echelon structures found in a younger geologic unit do not align in slip direction with typical first-order R Riedel shears. This suggests that these features might have formed through another process and the observed en echelon folds could be passive structures, which can be created through relatively rapid large displacement on a fault. Through modeling of Ganymede's past tidal stresses, we can conclude that a past higher eccentricity (e > 0.02) could have distorted the Nippur/Philus Sulci region in its second, intermediate phase of main deformation, yielding a stress field conducive to shear failure of the icy upper crust. New data from NASA's Juno mission, as well as future observations by ESA's Jupiter Icy Moons Explorer (JUICE) mission and NASA's Europa Clipper mission, will further aid our understanding of the tectonic history of Ganymede and improve the resolution of tectonic feature analysis.