Pluto's surface is surprisingly young and geologically active. One of its youngest terrains is the near-equatorial region informally named Sputnik Planum, which is a topographic basin filled by nitrogen (N2) ice mixed with minor amounts of CH4 and CO ices. Nearly the entire surface of the region is divided into irregular polygons about 20-30 kilometres in diameter, whose centres rise tens of metres above their sides. The edges of this region exhibit bulk flow features without polygons. Both thermal contraction and convection have been proposed to explain this terrain, but polygons formed from thermal contraction (analogous to ice-wedges or mud-crack networks) of N2 are inconsistent with the observations on Pluto of non-brittle deformation within the N2-ice sheet. Here we report a parameterized convection model to compute the Rayleigh number of the N2 ice and show that it is vigorously convecting, making Rayleigh-Bénard convection the most likely explanation for these polygons. The diameter of Sputnik Planum's polygons and the dimensions of the 'floating mountains' (the hills of of water ice along the edges of the polygons) suggest that its N2 ice is about ten kilometres thick. The estimated convection velocity of 1.5 centimetres a year indicates a surface age of only around a million years.

We simulate the formation of the large elliptical impact basin associated with Pluto's Sputnik Planum (SP; informal name). The location of SP suggests that it represents a large positive mass anomaly. To find the conditions necessary for SP to have a positive mass anomaly, we consider impacts into targets with a range of thermal states and ocean thicknesses. Assuming the basin evolves to its current‐day configuration, we calculate the mass and gravity anomalies associated with SP. We find that SP can only achieve a large positive mass anomaly if Pluto has a more than 100 km thick salty ocean. This conclusion may help us better understand the composition and thermal evolution of Pluto. Furthermore, our work supports the hypothesis that SP basin has an impact origin.

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2014 MU69 (named Arrokoth), targeted by New Horizons, has a unique bilobate shape. Research suggested that there is a large circular depression feature with a diameter of ∼7 km on the smaller lobe of this object. This feature, called Maryland, is surrounded by topographically high regions and faces perpendicular to the shortest axis of this object. Here, following the interpretation by earlier work that Maryland is formed by an impact, we investigate how the Maryland impact affects the structure of a neck of this object. We find that to avoid a structural breakup driven by this impact, MU69 needs high cohesive strength, at least tens of kilopascals depending on the bulk density. The cohesive strength at this level is much higher than that of other small bodies observed at high resolution, which is usually reported to be a few hundred pascals. It may be possible that MU69 actually has such a high cohesive strength, which may challenge the current knowledge about the cohesive strength of small bodies. Alternatively, we hypothesize a scenario that the Maryland impact actually broke the neck structure and made the shape settle into the current configuration. Considering this scenario, we obtain that the bulk density of MU69 should be between 300 and 500 kg m−3.

The dwarf planet Ceres exhibits a collection of craters that possess concentric fractures beyond the crater rim. These fractures typically range from a few hundred meters to a few kilometers in length and are less than 300 m wide. They occur preferentially on elevated regions around the crater and are located less than a crater radius beyond the rim. In total there are 17 craters exhibiting concentric fracturing beyond the rim. They are located in the midlatitudes. The craters' diameters range between 20 and 270 km. We investigate the concentric fractures of three craters (Azacca, Ikapati, and Occator) in detail and suggest that the formation of such concentric fractures can be explained by a shallow (<10‐km) low‐viscosity (~1020‐Pa·s) subsurface layer extending underneath the crater and its surroundings. Finite element modeling of such a scenario applied to a typical concentrically fractured crater of 50‐km diameter implies that the depth of the low‐viscosity layer is comparable to the crater depth and the layer does not extend to the surface. Given that not every crater of comparable size on Ceres exhibits concentric fractures, it is also suggested that these conditions are only met locally and may be related to the surface temperature. Correlations of concentrically fractured craters with other volatile related features, such as pitted terrains and floor fracturing, suggest that the low‐viscosity subsurface layer may be enriched in ice.