Thermal convection as a possible mechanism for the origin of polygonal structures on Pluto's surface

Abstract

High‐resolution pictures of Pluto's surface obtained by the New Horizons spacecraft revealed, among other surface features, a large nitrogen ice glacier informally named Sputnik Planitia. The surface of this glacier is separated into a network of polygonal cells with a wavelength of ∼20–40 km. This network is similar to the convective patterns obtained under certain conditions by laboratory experiments, suggesting that it is the surface expression of thermal convection. Here we investigate the surface planform obtained for different convective systems in 3‐D Cartesian geometry with different modes of heating and rheologies. We find that bottom heated systems, as assumed by previous studies, do not produce surface planforms consistent with the observed pattern. Alternatively, for a certain range of Rayleigh‐Roberts number, RaH, a volumetrically heated system produces a surface planform similar to this pattern. We then combine scaling laws with values of RaH within its possible range to establish relationships between the critical parameters of Sputnik Planitia. In particular, our calculations indicate that the glacier thickness and the surface heat flux are in the ranges 2–10 km and 0.1–10 mW m−2, respectively. However, a difficulty is to identify a proper source of internal heating. We propose that the long‐term variations of surface temperature caused by variations in Pluto's orbit over millions of years produces secular cooling equivalent to internal heating. We find that this source of heating is sufficient to trigger thermal convection, but additional investigations are needed to determine under which conditions it can produce surface patterns similar to those of Sputnik Planitia.

Gravity, Topography, and Melt Generation Rates From Simple 3‐D Models of Mantle Convection

“…It is now possible to carry out three‐dimensional time‐dependent fully resolved numerical experiments at Rayleigh numbers of 10 5 –10 7 , and the results of many such experiments have been reported (Christensen & Harder, ; Galsa & Lenkey, ; Houseman, ; Larsen et al, ; Tackley, , ; Sotin & Labrosse, ; Zhong, ; Vilella & Deschamps, , ; Weinstein & Christensen, ). Such experiments have an important advantage over tank experiments, because it is straightforward to calculate the geophysical observables from the numerical solutions.…”

Numerical Simulations of Tidal Deformation and Resulting Light Curves of Small Bodies: Material Constraints of 99942 Apophis and 1I/‘Oumuamua

“…While water ice has a known dynamic viscosity of μ ; 10 13 g cm −1 s −1 (over long-term high-latitude terrestrial temperatures; Fowler 1997), the viscosities of exotic solids such as CO, H 2 , and N 2 ice are far more difficult to measure. Vilella & Deschamps (2017) analyzed the dynamics of N 2 ice glaciers in the Sputnik Planitia of Pluto, and found that the observed polygonal structure is consistent with viscosities of 10 15 -10 17 g cm −1 s −1 , although these values are not well constrained and are highly uncertain. Due to experimental barriers, however, no measurements are available for the viscosity of H 2 or CO ice.…”

Sublimation-driven convection in Sputnik Planitia on Pluto

“…As a comparison, fig. 1(b) shows an equivalent map obtained for convection driven by an imposed temperature difference, which produces a polygonal pattern with topographic highs as sides to the polygons, at odds with the observations 6 . Convection driven by sublimation (fig.…”