Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigor

TL;DR

This study demonstrates that nitrogen ice layers on Pluto are capable of convection, which explains the surface cell patterns and suggests ongoing geological activity driven by internal heat and unique sluggish lid convection.

Contribution

It provides the first numerical evidence that nitrogen ice layers thicker than 1 km can convect under Pluto's current heat conditions, revealing a novel sluggish lid convection regime in the Solar System.

Findings

Nitrogen ice layers >1 km thick can convect on Pluto.

Convection explains the observed surface cell patterns.

Surface renewal times are estimated at ~500,000 years.

Abstract

The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's geological activity[1,2]. Composed of molecular nitrogen, methane, and carbon monoxide ices[3], but dominated by N2-ice, this ice layer is organized into cells or polygons, typically ~10-40 km across, that resemble the surface manifestation of solid state convection[1,2]. Here we report, based on available rheological measurements[4], that solid layers of N2 ice approximately greater than 1 km thick should convect for estimated present-day heat flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-km-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of N2-ice viscosity implies that the SP ice layer convects in the so-called sluggish lid regime[5], a unique convective mode heretofore not definitively observed in the Solar System. Average surface horizontal velocities of a few cm/yr imply surface transport or renewal times of ~500,000 years, well under the 10 Myr upper limit crater retention age for Sputnik Planum[2]. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help explain the high albedos of some of them.