Evidence for a hot start and early ocean formation on Pluto

Abstract

Pluto is thought to possess a present-day ocean beneath a thick ice shell. It has generally been assumed that Pluto accreted from cold material and then later developed its ocean due to warming from radioactive decay; in this ‘cold start’ scenario, the ice shell would have experienced early compression and more recent extension. Here we compare thermal model simulations with geological observations from the New Horizons mission to suggest that Pluto was instead relatively hot when it formed, with an early subsurface ocean. Such a ‘hot start’ Pluto produces an early, rapid phase of extension, followed by a more prolonged extensional phase, which totals ~0.5% linear strain over the last 3.5 Gyr. The amount of second-phase extension is consistent with that inferred from extensional faults on Pluto; we suggest that an enigmatic ridge–trough system recently identified on Pluto is indicative of early extensional tectonics. A hot initial start can be achieved with the gravitational energy released during accretion if the final stage of Pluto’s accretion is rapid (<30 kyr). A fast final stage of growth is in agreement with models of the formation of Kuiper belt objects via gravitational collapse followed by pebble accretion, and implies that early oceans may have been common in the interiors of large Kuiper belt objects. Pluto’s subsurface ocean may have formed early due to accretionary heating, a comparison of thermal evolution modelling with observed tectonic structures suggests.