Fig. 3. Atmospheric CH₄ abundance and surface temperatures drive CH₄ condensation and accumulation on mountain summits.

a Near-surface CH₄ atmospheric mixing ratio in Cthulhu as simulated by our model (filled contours, in %) with arrows indicating winds at 5 m above the surface. b Cross-section of Cthulhu at 5°S showing the diurnal mean CH₄ mass mixing ratio (kg kg⁻¹) above the local surface (red solid line) and the diurnal mean CH₄ mass mixing ratio at saturation (red dotted line), as obtained in our simulation for July 2015; the topography; and the thickness of the surface layer of CH₄ ice (black solid line); and the thickness of the surface layer of CH₄ ice (about ∼40 µm on top of Pigafetta Montes, blue solid line). The shaded areas indicate where there is saturation of CH₄ above the surface (in diurnal mean) that leads to condensation onto the surface. c Same cross-section showing the diurnal mean surface temperature (solid line) as obtained in our simulation. The diurnal minimal and maximal surface temperatures are also indicated (dotted lines). Colder daytime temperatures are obtained where CH₄ ice is present, due to the brighter surface (albedo positive feedback). Maps are shown in the Supplementary Figs. 1 and 2.