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

Abstract

We investigate viscoelastic impact basin relaxation on Pluto for a variety of thermal evolution scenarios encompassing both convective and conductive ice shells. Basins smaller than 200 km in diameter do not relax appreciably, while relaxation fractions 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. Other volatiles, such as CO2 or NH3, if present in the ice shell in sufficient quantities could increase the predicted relaxation fraction of basins. 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. Future observations of the relaxation states and tectonics of impact basins are therefore likely to provide a key constraint on Pluto's thermal history.

Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto

“…For basins with initial depths in the range 0-7 km, positive gravity anomalies only occur with N2 loads > 40 km thick and Te values < 15 km (so that the space required by the N2 can be accommodated). The required N2 thickness is much larger than that inferred 10,11 and the Te value is smaller than predicted 18 . The large negative gravity anomaly generated by the present-day 3.5 km negative topography is hard to overcome with N2 loading alone.…”

“…17 George Mason University, Fairfax, Virginia 22030, USA. 18 Planetary Science Institute, Tucson, Arizona 85719, USA. 19 University of Arizona, Tucson, Arizona 85721, USA.…”

“…the deflection due to a load thickness L is calculated using a thin-spherical-shell approach 30 (see Methods). Shaded region denotes estimated elastic thickness range 18 . The characteristic wavenumber of SP is taken to be (4/3)/D where D is the diameter (=1000 km).…”

“…In this case, we assume that deposition of N2 of thickness L took place at a later epoch by which time the crust had an elastic thickness Te. Thermal evolution models predict that Te always exceeds 40 km, depending mainly on when SP formed 18 . Given d0 and the present-day topography h, the load thickness L and the resulting gravity anomaly g can be calculated (Figure 2a; Methods).…”

Formation of the Sputnik Planum basin and the thickness of Pluto's subsurface ocean

“…We produced simulations with five different preimpact ocean thicknesses of 0, 50, 100, 150, and 200 km, while keeping the combined thickness of the ice shell and ocean constant at 328 km (i.e., ice shell thicknesses of 328, 278, 228, 178, and 128 km, respectively). We assume a surface temperature of 44 K and an initial thermal gradient of 1.36 K/km [Kamata and Nimmo, 2014]. We assume a surface temperature of 44 K and an initial thermal gradient of 1.36 K/km [Kamata and Nimmo, 2014].…”