Sublimation dunes on Pluto

The dwarf planet Pluto was flown over by NASA’s New Horizons spacecraft in July 2015, providing a unique opportunity to study some of its geomorphological features. Photos taken during the flyby revealed kilometer-scale regular bedforms on the nitrogen ice surface of Sputnik Planum. Contrary to their interpretation as sedimentary dunes [1] or ice penitentes, we demonstrate that their formation is due to a hydrodynamic instability associated with the coupling of nitrogen ice sublimation and turbulent heat mixing.

The modulation of the temperature field controls the sublimation rate of the ice surface. In turn, the bed elevation profile influences the modulation of the turbulent flow [2], generated by the thermodynamic imbalance of Sputnik Planum, and thus the advection-diffusion of heat. We show that the pattern wavelength is selected by a transitional value of the Reynolds number, similar to dissolution patterns [2]. The pattern observed on Pluto, with a wavelength of a fraction of a kilometer, is therefore analogous to meter-scale sublimation waves on the Martian north polar cap [3]. Estimates of atmospheric parameters (wind shear velocity, viscosity, temperature, and heat flux from the atmosphere to the surface) accurately predict the observed wavelength. This sublimation instability contrasts with that of penitentes, which is due to the self-illumination of the surface.

[1] Telfer et al., Dunes on Pluto, Science 360, 992-997 (2018).

[2] Claudin, Durán & Andreotti, Dissolution instability and roughening transition, J. Fluid Mech. 832, R2 (2017).

[3] Bordiec et al., Sublimation waves: Geomorphic markers of interactions between icy planetary surfaces and winds, Earth-Science Reviews 211, 103350 (2020).