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

Burkhard, Liliane M. L.; Costello, Emily S.; Smith-Konter, Bridget R.; Cameron, Marissa E.; Collins, Geoffrey C.; Pappalardo, Robert T. (2024). Uncovering Ganymede's past: Tectonics at Nippur/Philus Sulci. Icarus, 408, p. 115823. Elsevier 10.1016/j.icarus.2023.115823

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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.

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