The peculiar polygonal shapes observed on the icy surface of dwarf planet Pluto are likely to have formed as a result of convection within layers of solid nitrogen several kilometres thick, report two papers published in this week’s Nature.
The vast, oval-shaped basin informally named Sputnik Planum in Pluto’s equatorial region is filled with ice (made up mainly of nitrogen) that is organized into polygons about 10 to 40 kilometres across, with centres rising tens of metres above their sides. Both thermal contraction and convection have been proposed as mechanisms to explain the geometry of this terrain.
William McKinnon and colleagues use measurements from the New Horizons spacecraft to show that layers of nitrogen ice more than one kilometre thick must convect to account for the estimated present-day heat flow conditions on Pluto. They find, through numerical modelling, that convective overturn can explain the great lateral width of the polygons, and they estimate a renewal time of about half a million years, based upon the calculated horizontal velocities of the surface ice of Sputnik Planum. The authors suggest that similar convective processes, which allow for continual rejuvenation of the ice layers, may also be at work on other dwarf planets in the Kuiper belt - the disk-shaped region beyond the orbit of Neptune believed to contain comets, asteroids and other small, icy bodies.
In a related paper, Alexander Trowbridge and colleagues use a numerical model of the nitrogen ice on Sputnik Planum to also conclude that vigorous convection is occurring on Pluto. In addition, the lack of brittle deformation within the nitrogen ice layer is inconsistent with thermal contraction, they note.
In an associated commentary published in Nature Geoscience, Paul Schenk and Francis Nimmo write: “Pluto and Charon show us that planetary processes work in similar ways regardless of where in the Solar System you are… The delight is in seeing such variety and complexity, and yet so much familiarity so far from home.”