Ganymede, then and now: How past eccentricity may have altered tidally driven Coulomb failure

Laplace-like resonances among Ganymede, Europa, and Io may have once led Ganymede to have an eccentricity (presently e = 0.0013) as high as similar to 0.07 (Showman & Malhotra, 1997, ). While diurnal stresses at Ganymede today are small (less than 10 kPa), a previous period of higher eccentricity may have allowed for an order of magnitude increase in the diurnal tidal stresses that could drive fault initiation and result in a past period of active tectonism. To investigate the role of tidal stresses on faulting, we use the numerical model SatStress (Wahr et al., 2009, ) to calculate diurnal tidal stresses on Ganymede's surface assuming e = 0.05, representative of a more eccentric orbit in Ganymede's past. We resolve normal and shear stresses onto discrete mapped fault segments and assess Coulomb failure criteria along three inferred shear zones on Ganymede's surface: Dardanus Sulcus, Tiamat Sulcus, and Nun Sulci. While Coulomb failure is not expected from diurnal stressing at any of the three shear zones for a diurnal model of present-day (low) eccentricity, we do predict Coulomb failure for a past, high eccentricity case, in isolated diurnal slip windows and limited to very shallow depths (similar to 100 to 250 m). In these cases, this model may provide an alternative to invoking additional stresses such as nonsynchronous rotation or true polar wander, as in previous studies (Cameron et al., 2019, ). Additionally, this model is in general agreement with the sense of inferred shear from imagery and structural mapping. Plain Language Summary As the Galilean satellites (Io, Europa, Ganymede, and Callisto) orbit Jupiter, the gravity of each body influences each other, causing tidal stresses, much like how the Moon causes ocean tides on Earth. The orbits of Jupiter's moons are elliptical rather than circular, meaning the tidal stresses vary as the moons change distance from Jupiter. The orbits of Io, Europa, and Ganymede are currently in a configuration, or orbital resonance, where for every one orbit around Jupiter that Ganymede completes, Europa completes two, and Io completes four. This orbital resonance also influences tidal stresses. High enough tidal stresses may have driven past geologic activity, such as the strike-slip faults we observe on Ganymede's surface. Strike-slip faulting occurs when fault walls move past one another laterally, like the San Andreas fault in California. This study examines images of Ganymede and models the amount of tidal stress necessary to have moved these strike-slip faults based on diurnal stresses alone, without the need to invoke more complicated additional stress configurations. We find that a past orbital resonance may have increased Ganymede's tidal stresses enough to allow for shallow movement along a strike-slip fault. We also find that the direction of the movement of the fault predicted in our models generally agrees with the direction of motion inferred from the image.