Convection of Volatile Ices on Pluto

The 2.5-km deep, volatile-ice-filled basin named Sputnik Planitia (SP) is central to Pluto's geological, glaciological, and climatological activity. Composed of nitrogen, methane, and carbon monoxide ices, but dominated volumetrically by N _{2}-ice, this ice sheet is organized over much of its extent into cells, typically 10-40 km across, that strongly resemble the surface manifestation of solid state convection. We have previously shown that solid layers of N _{2} ice >1 km or so thick should convect for Pluto's estimated present-day, steady state radiogenic heat flow (even considering a wide range of uncertainty for the latter). We have also shown that convective overturn in a several-km-thick layer of solid N _{2}(-CH _{4}-CO) can explain the great lateral width of the cells. We have argued that the temperature dependence of N _{2}-ice viscosity implies that the SP ice layer convects in the sluggish lid regime, a unique convective mode heretofore not definitively observed in the Solar System. In this talk we will describe ongoing efforts to understand this convection and its implications, focusing on the questions of the aspect ratio and planform of the convective cells and the implications for the thickness of the ice sheet (depth to the water ice basement), Pluto's heat flow, and the potential for N _{2}-ice melting at the base of the ice sheet. We perform CITCOM convection calculations in 2-D and 3-D to constrain the depth of this N _{2} and thus the depth of the Sputnik impact basin and related properties and conditions. For an extended but plausible range of Rayleigh numbers and viscosity contrasts for solid nitrogen, convection can occur in all possible regimes: small viscosity-contrast, sluggish lid, or stagnant lid, or the layer could be purely conducting. Convective dynamics are complicated at the vicinity of regime transitions, thus we conduct a systematic analysis of convection regimes using 2-D convection simulations. Moreover, the conditions at the base of the nitrogen ice sheet are uncertain. It may be directly in touch with the water ice bedrock, or nitrogen-rich melts may be present if the base reaches the melting temperature of nitrogen (63 K). This bottom boundary condition affects the dynamics of the convection system and thus the surface manifestation, which is more prominent in 3-D models. Sputnik Planitia is a deep, ∼1150 km x 900 km elliptical basin likely formed by the impact of an ∼200-km-wide ancestral Kuiper belt object more than 4 billion years ago. Based on depths of smaller, fresher craters formed in Pluto's water-ice bedrock, SP was likely originally no deeper than 10 km, which implies an upper limit of ∼7 km for the N _{2}-ice sheet today. Preliminary topography determined by photoclinometry on the highest resolution New Horizons LORRI transects indicates the cells themselves are broad domes ∼100 m high. These topographic profiles are more consistent with greater as opposed to lesser viscosity contrast across the convecting layer. The vertical scale is also indicative of the integrated thermal anomaly (which provides the buoyancy that drives the convection). For N _{2}-ice thermal expansion (which is large), 100 m of uplift is consistent with a temperature contrast of 10 K over a depth of 5 km, and greater thermal anomalies are required as the viscosity contrast increases. It is hard to reconcile these requirements with solely top-down cooling due to climactic temperature variations (small temperature changes), which has been proposed for SP.