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tokens: ). Thus, a subsurface strengthened layer has to be very large and thick to avoid lifting by gas released beneath it. If we consider a disc of strengthened material with a 50-m radius and a 10-m thickness, then for a density of 531 kg m −3 and g = 2 × 10 −4 m s −2 , the gravity force exerted on the material underneath (e.g. gas cavity, pristine material) is F grav ∼ 8340 N. If the pressure is enhanced under a small fraction of the disc (i.e. 1 per cent), then the condition F gas > F grav can be met when p ∼ 0.1 kPa. When the edges of the strengthened layer are under elevated surrounding terrain, then the critical gas pressure is higher. In any case, the tensile strength of the strengthened material is important. 3.2 Tensile strength Fig. 3 plots the profiles of σ T versus depth, calculated when the initial tensile strength σ T0 = 1 kPa. The curves are drawn for selected locations for the times when the gas pressure in given locations satisfies conditions: p 0.1 kPa (upper panel), p > p crit,1 = 1 kPa (middle panel) and p > p crit,2 = 10 kPa (lower panel). Profiles shown in the middle panel are taken 113 h (nine rotation periods) after the profiles presented in the upper panel. During this period, the thickness of the strengthened layer increased by at least 0.5 m. In one location, a secondary strengthened layer formed above the crystallization front of the amorphous water ice. The thickness of the main strengthened layer is ∼1 m when the gas pressure approaches 0.1 kPa, 0.8–1.8 m when p > p crit,1 and 0.8–3.3 m when p > p crit,2 . A strengthened layer about 1 m thick will probably be broken or lifted. Thus, in locations marked with white triangles, the presence of uncovered ice-rich material should be expected. Increased emission of more volatile components (such as carbon monoxide) is also likely. Figure 3. Tensile strength versus depth, when the CO pressure p beneath the strengthened layer is p 0.1 kPa (upper panel), p > p crit,1 = 1 kPa (middle panel) and p > p crit,2 = 10 kPa (lower panel). The initial tensile strength is 1 kPa. The remaining parameters of the model parameters are given in the legends. It is interesting that when σ T0 = 1 kPa the ejection of material is possible in all considered locations, while the uncovered ice was found only in the pit. This difference can be explained by the non-uniform concentration of highly volatile CO, but other options should also be considered, such as the non-uniform granulation or porosity of the material. Fig. 4 is similar to the upper and lower panels of Fig. 3 . The profiles shown in Fig. 4 are calculated for σ T0 = 0.1 kPa instead of σ T0 = 1 kPa. It can be seen that the strengthening of the loosely bonded mixture of grains is slower than with moderate initial strength. The thickness of the strengthened layer at the moment when the CO gas pressure reaches the threshold value p crit,1 is ∼1 m for σ T0 = 0.1 kPa and 1.1–1.9 m for σ T0 = 1 kPa. The next threshold p crit,2 is exceeded when the