LITHOSPHERIC STRESS AND FAILURE OF A DESPUN PLUTO: PREDICTIONS FOR FLATTENING AND TECTONICS

Pluto and Charon arrived at their mutual spin-orbit synchronous state (6.39 day period) from much higher spin rates following the (likely) collisional formation of Charon. Despinning of planetary bodies may leave a tectonic signature indicative of the relaxation or partial relaxation of an equatorial bulge. This pattern can be complex depending on the thickness of the lithosphere, and it can be modified by volume changes, other tidal effects, and large impacts.

We model lithospheric stresses for spin-down of Pluto for a range of ice lithospheric thicknesses and initial spin periods (methods similar to Singer and McKinnon, 2011, Icarus 216, 198–211). For a differentiated Pluto, we assume an indurated rock core may retain its original flattening, and thus calculate stresses only due to relaxation of the icy shell. The predicted maximum differential stresses for Pluto from a high early spin period (e.g., 6 hrs), using the rigidity of unfractured ice, are ~160-to-140 MPa for 50-to-75 km thick lithospheres (corresponding to high early heat flows of ~20-to-12 mW m^-2, respectively). At such stresses the entire lithosphere fails brittlely. A slower initial spin period, or equivalently, failure followed by brittle strength support during terminal spin-down, should allow for retention of “modest” equatorial bulge. Modest in this case corresponds to flattenings >1% (radii differences >10 km) even for a strengthless icy lithosphere. And for a fully relaxed core we expect at least a 2-3 km fossil bulge supported by the icy lithosphere, unless impacts have allowed the ice shell to fully relax. These values are notably larger than predicted for Pluto’s present-day hydrostatic figure, and do not depend on the presence or absence of an internal ocean.

Tectonic faults, fractures, and lineaments (and their orientations) should be revealed in Pluto’s water-ice “basement,” and on Charon’s water-ice surface, by New Horizons in July 2015. Lineaments may be expressed as straight segments of complex crater walls, extensional fractures or troughs, strike-slip offsets, ridges, or scarps. With lithospheric thicknesses of 50-75 km on Pluto, despinning fractures are predicted to be strike-slip in the equatorial to mid-latitudes, and normal towards the poles. If the lithosphere was thicker during spindown, equatorial thrust faults may be seen.