Convection in a volatile nitrogen-ice-rich layer drives Pluto’s geological vigour

The volatile-ice-filled basin informally named Sputnik Planum is central to Pluto’s geological activity; this ice layer is organized into cells or polygons, and it is now shown that convective overturn in a several-kilometre-thick layer of solid nitrogen can explain both the presence of the cells and their great width. NASA's New Horizons spacecraft has revealed fascinating details of the surface of Pluto, including a vast ice-filled basin known as Sputnik Planum, which is central to Pluto's geological activity. Much of the surface of Sputnik Planum, consisting mostly of nitrogen ice, is divided into irregular polygons that are tens of kilometres in diameter and whose centres rise tens of metres above their sides. Two papers in this issue of Nature analyse New Horizons images of this polygonal terrain. Both conclude that it is continually being resurfaced by convection, but arrive at contrasting models for the process. Alexander Trowbridge et al. report a parameterized convection model in which the nitrogen ice is vigorously convecting, ten or more kilometres thick and about a million years old. William McKinnon et al. — from the New Horizons team — show that 'sluggish lid' convective overturn in a several-kilometre-thick layer of solid nitrogen can explain both the presence of the cells and their great width. The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto’s vigorous geological activity1,2. Composed of molecular nitrogen, methane, and carbon monoxide ices3, but dominated by nitrogen ice, this layer is organized into cells or polygons, typically about 10 to 40 kilometres across, that resemble the surface manifestation of solid-state convection1,2. Here we report, on the basis of available rheological measurements4, that solid layers of nitrogen ice with a thickness in excess of about one kilometre should undergo convection for estimated present-day heat-flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-kilometre-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of nitrogen-ice viscosity implies that the ice layer convects in the so-called sluggish lid regime5, a unique convective mode not previously definitively observed in the Solar System. Average surface horizontal velocities of a few centimetres a year imply surface transport or renewal times of about 500,000 years, well under the ten-million-year upper-limit crater retention age for Sputnik Planum2. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help to explain the high albedos shown by some of these bodies.