Sublimation pit distribution indicates convection cell surface velocities of ∼10 cm per year in Sputnik Planitia, Pluto

Abstract

The ∼10^6 km^2 Sputnik Planitia, Pluto is the upper surface of a vast basin of nitrogen ice. Cellular landforms in Sputnik Planitia with areas in the range of a few × 10^2–10^3 km^2 are likely the surface manifestation of convective overturn in the nitrogen ice. The cells have sublimation pits on them, with smaller pits near their centers and larger pits near their edges. We map pits on seven cells and find that the pit radii increase by between 2.1 ± 0.4 × 10^(−3) and 5.9 ± 0.8 × 10^(−3) m m^(−1) away from the cell center, depending on the cell. This is a lower bound on the size increase because of the finite resolution of the data. Accounting for resolution yields upper bounds on the size vs. distance distribution of between 4.2 ± 0.2 × 10^(−3) and 23.4 ± 1.5 × 10^(−3)m m^(−1). We then use an analytic model to calculate that pit radii grow via sublimation at a rate of 3.6_(−0.6)^(+2.1)×10^(−4) m yr^(−1), which allows us to convert the pit size vs. distance distribution into a pit age vs. distance distribution. This yields surface velocities between 1.5_(−0.2)^(+1.0) and 6.2_(−1.4)^(+3.4) cm yr^(−1) for the slowest cell and surface velocities between 8.1_(−1.0)^(+5.5) and 17.9_(−5.1)^(+8.9) cm yr^(−1) for the fastest cell. These convection rates imply that the surface ages at the edge of cells reach ∼4.2–8.9 × 10^5 yr. The rates are comparable to rates of ∼6 cm yr^(−1) that were previously obtained from modeling of the convective overturn in Sputnik Planitia (McKinnon et al., 2016). Finally, we investigate the surface rheology of the convection cells and estimate that the minimum ice viscosity necessary to support the geometry of the observed pits is of order 10^(16)–10^(17) Pa s, based on the argument that pits would relax away before growing to their observed radii of several hundred meters if the viscosity were lower than this value.

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