Abstract
The existence of subsurface oceans on the satellites of the giant planets and Trans-Neptunian objects has been predicted for some time. Liquid oceans on icy worlds, if present, exert a considerable influence on the dynamics of the ice–ocean system and, because of the astrobiological potential, represent an important objective for future missions to the outer solar system. The Pluto–Charon system is representative of an icy moon orbiting a dwarf planet that is believed to have formed from the remnants of a giant impact. The evolution of icy moons is primarily controlled by the mode and efficiency of heat transfer through the outer ice shell, which is influenced by the presence of impurities, by tidal dissipation in the ice shell, and by the radioactive element budget in the silicate core. Previous studies on the evolution of the Pluto–Charon system generally considered either only the thermal or the tidal evolution, and in the cases where both were considered, the important effect of the presence of impurities in the liquid oceans was not addressed. Here, we consider the joint tidal–thermal evolution of the Pluto–Charon system by combining a comprehensive tidal model that incorporates a viscoelastic description of the tidal response with a parameterized thermal convection model developed for icy worlds. This approach enables an extensive analysis of the conditions required for the formation and maintenance of subsurface liquid oceans up to the present. Our results show that because of relatively fast circularization and synchronization of the orbits of Pluto and Charon, tidal heating is only important during the early stages of evolution (<1 Myr). As part of our study, we test the sensitivity of our results to a number of parameters that pertain to the orbital and thermal history. In all the studied cases, oceans on Pluto are always predicted to remain liquid to the present, ranging in thickness from 40 km to 150-km, whereas oceans on Charon, while in-place for approximately 4 Gyr, have solidified. This is supported by New Horizons observations of primarily extensional faults on Pluto and both extensional and compressional faults on Charon.
Highlights
The Kuiper belt harbours numerous planetary objects of diverse internal structure and surface features, including the dwarf planet Pluto
The main heat sources affecting the thermal evolution of Pluto and Charon are those associated with radiogenic heating in the silicate parts and tidal dissipation in the ice shell
The initial orbital eccentricity, semi-major axis, and spin rates employed are ∼0.4, ∼0.65, and ∼5 times the initial mean motion, corresponding to rotation period around 10 hr, respectively, where is the present-day observed semi-major axis. These values are based on the Pluto-Charon impact simulations of Canup (2005) and reflect the fact that in the postcollisional state, Charon’s orbital eccentricity is high and both bodies commenced with spin rates that are higher than their initial orbital mean motion, indicative of a closer-in satellite
Summary
The Kuiper belt harbours numerous planetary objects of diverse internal structure and surface features, including the dwarf planet Pluto. The Pluto system is, because of Pluto’s size and relative brightness, presently the beststudied of all of the Trans-Neptunian objects (TNOs) (Hussmann et al, 2006). This is a consequence of a protracted history of Earth-based remote sensing (Malhotra and Williams, 1997; Dobrovolskis et al, 1997; Olkin et al, 2003) and not least the flyby of the New Horizons spacecraft in 2015 (Spencer et al, 2020). In analogy with the Earth-Moon system, is believed to have formed as a result of a collision between Pluto and a Kuiper belt object (Canup, 2010; Sekine et al, 2017; Arakawa et al, 2019). Like Earth’s Moon, Charon would initially have been closer to Pluto, but because of tidal dissipation within the two bodies, Charon would have been driven further away until it reached its current synchronous state (Dobrovolskis et al, 1997)
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