The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's geological activity 1,2 . Composed of molecular nitrogen, methane, and carbon monoxide ices 3 , but dominated by N2-ice, this ice layer is organized into cells or polygons, typically ~10-40 km across, that resemble the surface manifestation of solid state convection 1,2 . Here we report, based on available rheological measurements 4 , that solid layers of N2 ice ≳1 km thick should convect for estimated present-day heat flow conditions on Pluto. More importantly, we show numerically

The strongly temperature-dependent viscosity of rocks leads to the formation of nearly rigid lithospheric plates. Previous studies showed that a very low yield stress might be necessary to weaken and mobilize the plates, for example, due to water. However, the magnitude of the yield stress remains poorly understood. While the convective stresses below the lithosphere are relatively small, sublithospheric convection can induce large stresses in the lithosphere indirectly, through thermal thinning of the lithosphere. The magnitude of the thermal thinning, the stresses associated with it, and the critical yield stress to initiate subduction depend on several factors including the viscosity law, the Rayleigh number, and the aspect ratio of the convective cells. We conduct a systematic numerical analysis of lithospheric stresses and other convective parameters for single steady-state convection cells. Such cells can be considered as part of a multi-cell, time-dependent convective system. This allows us a better control of convective solutions and a relatively simple scaling analysis. We find that subduction initiation depends much stronger on the aspect ratio than in previous studies and speculate that plate tectonics initiation may not necessarily require significant weakening and can, at least in principle, start if a sufficiently long cell develops during planetary evolution.

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We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature‐dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh‐Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable.

The prospect of subsurface oceans in icy satellites presents an exciting area of research to understand their diverse processes and astrobiological potential. Induced magnetic fields were detected by Galileo on Europa, Ganymede, and Callisto (Khurana et al., 2009;Kivelson et al., 2000;Zimmer et al., 2000), which implies a subsurface ocean. Gravity measurements and surface features indicate that the icy shells are decoupled from the interior (cf. Hussmann et al., 2015). Direct imaging of erupting plumes on Enceladus from Cassini and potentially on Europa as well, from Hubble Space Telescope and magnetic field and plasma wave observations from Galileo (Jia et al., 2018) point to subsurface water sources. The surface observations offer clues about the composition of the subsurface ocean. Measurements of Saturn's E-ring indicate a sodium salt-rich source derive from Enceladus' interior (e.g., Postberg et al., 2009). Spectroscopic data from Galileo's NIMS suggests the presence of irradiated salts on the surface of Europa that may reflect the composition of the subsurface ocean (McCord et al., 1998;Trumbo et al., 2019). This non-water ice material is prominent in linear and chaos features on the surface. What are the sources of these salty materials? Can their presence on the surface reflect the composition in greater depths, for example, from the subsurface ocean or even from the silicate interior? How are they transported to the surface, and is the dynamics in the subsurface ocean expressed on the surface? Current understanding of icy subsurface oceans is drawn from studies of the Earth's ocean, fundamental knowledge of geophysical fluid dynamics, numerical simulations, and laboratory experiments. Topics of heat and material transport by hydrothermal systems and the circulation in the subsurface ocean have been explored with various assumptions about its conditions and physical properties (e.g., Amit et al., 2020;Goodman et al., 2004;Kvorka