The Enduring Enigma of Ganymede’s Muted Apex-Antapex Cratering Asymmetry

Theoretical calculations going back to the pioneering work of E.M. Shoemaker predict a strong apex-antapex gradient in crater formation rate on Ganymede (and other satellites), but while both Ganymede’s bright and dark terrain (younger and older, respectively) exhibit such asymmetries, they are not nearly as pronounced as predicted, by more than an order of magnitude. Several explanations have been offered: 1) crater saturation (plausible for dark terrain–and Callisto–but not bright terrain); 2) planetocentric impactors (these would have to come from beyond Callisto, as sesquinaries are ruled out given Gilgamesh ejecta fragments are too small); 3) mega-impact temporary unlocking of synchronous rotation (though the necessary young basins are unknown); 4) nonsynchronous rotation (a perennial possibility); and 5) true polar wander (TPW) due to insolation-driven shell thickness variations. We have previously addressed the latter possibility (McKinnon et al., Fall AGU 2023), calculating the shell thickness as a function latitude and deriving the degree-2 gravitational response as function of compensation state. For highly compensated shells overlying an internal ocean, excess polar surface topography drives inertial-interchange TPW in which the poles rotate about the tidal a-axis to align with the leading-trailing direction of motion. Earlier in Ganymede’s history, when its global heat flow was high, repeated 90° episodes of polar wander, or continual drift of the icy shell about its tidal axis, could have markedly reduced the satellite’s ultimate (or cumulative) apex-antapex cratering asymmetry. Only at later times, when Ganymede’s ice shell was sufficiently thick for solid state convection, would the latitudinal shell thickness variation be muted if not eliminated and the potential for TPW curtailed.

The attitudinal instability of the ice shell depends on the mechanism of shell compensation. For static models of isostasy (material boundaries), the classic model of pressure balance is the least stable, whereas equal masses above and below (neutral buoyancy) is the most stable. The former prescription leads to unbalanced body forces, however, whereas equal masses leads to unbalanced pressures at depth. We adopt the principle of total force balance: body forces (buoyancy) + basal traction (pressure difference) = 0, which is an intermediate case.

Finally, there is the issue of Ganymede’s enigmatic subjovian dome. As fully revealed by Juno stereo, it is ~700 × 450 km across and ~3 km high. There is no obvious surface construction (cryovolcanic or otherwise) and a 3-km thick water laccolith also seems implausible. Could the dome be a remnant of a thickened polar shell, frozen into a now thickened ice lithosphere and strength supported (and ultimately rotated to a-axis)? Implied stresses are ~1.5 MPa (supportable), but its ~3-km height would imply a >~30-km former isostatic root, difficult to accept given basal ice flow and oceanic heat transport. Future JUICE observations of both Ganymede’s sub- and antijovian region (where a similar dome should exist if this hypothesis is correct) should be telling.