Sublimation driven convection in Sputnik Planitia on Pluto

The flyby of Pluto by the New Horizons spacecraft in 2015 revealed stricking features in a nitogen ice filled basin named Sputnik Planitia (SP). On the large (few 10 km) scale, irregular polygons [1] have been interpreted as resulting from solid-state convection [2, 3], with down-welling currents at the polygons’ edges [4]. It was first proposed that convection was driven by heating from below, associated with radioactivity in Pluto’s core but it produces a flow that is opposite to the observations with upwelling currents at polygons’ edges [5] (fig. 1a). At the small scale, we can observe pits that have been explained as produced by sublimation of nitrogen ice. The development of this pattern when getting toward the polygons’ sides has been used to estimate the flow velocity on several polygons, of the order 10 cm yr−1 [4]. We proposed [6] that sublimation is also at the origin of convection by cooling the surface: sublimation consumes latent heat which results in the surface of SP being ∼ 5 K colder than the surroundings [7].

We developed a theory to take into account sublimation in a convection model and showed that it leads to a mixed-type (Robin) thermal boundary condition which, in the case of SP is close to that of a fixed temperature. Starting from a uniform temperature in the ice layer, imposing a colder surface value leads to cooling and convection. With the best estimates of the various parameters for SP, a Rayleigh number of order 1 × 105 can be obtained, justifying the possibility of convection. We used a mantle convection model to explore systematically the effect of surface cooling on convection in a temperature-dependent viscosity solid. Depending on the Rayleigh number and the viscosity contrast, we obtain different regimes (fig. 1b), including one with polygons that match the observations of SP (fig. 1c). The five cases that we obtained in that regime can be used to explore the relationships between observed quantities, like the polygonal sizes, and unconstrained ones, like the thickness of the ice layer or the amplitude of topography [6]. Such large-scale dynamics of a solid state layer driven by climate is unprecedented in the knowledge of planetary bodies but might also operate on other volatile rich moons and TNOs.

Figure 1: Convection pattern for bottom heated convection (a), convection driven by sublima- tion at the surface (c) and regime diagram as function of the Rayleigh number and viscosity contrast (b).

References: 1. White, O. L. et al. Icarus 287 (2017). 2. Trowbridge, A. J., Melosh, H. J., Steckloff, J. K. & Freed, A. M. Nature 534 (2016). 3. McKinnon, W. B. et al. Nature 537 (2016). 4. Buhler, P. B. & Ingersoll, A. P. Icarus 300 (2018). 5. Vilella, K. & Deschamps, F. J. Geophys. Res. 122 (2017). 6. Morison, A., Labrosse, S. & Choblet, G. Sublimation-driven convection in Sputnik Planitia on Pluto. Nature 600 (2021). 7. Gladstone, G. R. et al. Science 351 (2016).