Impact basin relaxation as a probe for the thermal history of Pluto

Abstract:

Viscous relaxation of large impact basins provides a potential method for investigating the thermal evolution of terrestrial planets and of icy satellites of giant planets. Pluto, an icy dwarf planet, is likely to possess large impact basins. In this study, we use a viscoelastic code [1] to investigate relaxation of impact basins on Pluto for a variety of thermal evolution scenarios [2] encompassing both convective and conductive ice shells. We found that impact basins smaller than 200 km in diameter do not relax appreciably, while the relaxation fraction can be up to ~60% for large impact basins. The main control on basin relaxation is the amount of radiogenic heat produced in the rocky core; our results are insensitive to the formation time of the basin, the ice reference viscosity adopted, and the presence/absence of a subsurface ocean. Relaxation causes extensional stresses interior to the basin; the orientation of the resulting tectonic features is controlled by the effective elastic thickness beneath the basin. We also found that the relaxation fraction of impact basins on Charon, the largest satellite of Pluto, should be less than 10% as long as tidal heating is small. Future observations of the relaxation states and tectonics of impact basins by New Horizons are therefore likely to provide a key constraint on Pluto's thermal history and on the evolution of the Plutonian system.

[1] Kamata et al. (2012) JGR, 117, doi:10.1029/2011JE003945. [2] Robuchon and Nimmo (2011) Icarus, 216, 426-439.