Penitentes

Water ice penitentes.

penitentesThis forest of snow and ice penitentes reflects moonlight shining across the Chajnantor plateau. The region lies in the Chilean Andes at an altitude of 5,000 meters ...

Up to several meters high, the flattened, sharp-edged shapes, and orientation of the penitentes tend to minimize their shadows at local noon. In the dry, cold, thin atmosphere, sublimation driven by sunlight is important for their formation. A direct transition from a solid to a gaseous state, sublimation shapes other solar system terrains too, like icy surfaces of comets and the polar caps of Mars.
Night in the Andes Ice Forest | NASA

Ice Penitentes

Louis Lliboutry noted that the key climatic condition behind the differential ablation that leads to the formation of penitentes is a dew point that remains below freezing. This combined with dry air will cause snow to sublimate. Once the process of differential ablation starts, the surface geometry of the evolving penitente produces a positive feedback mechanism, and radiation is trapped by multiple reflections between the walls. The hollows become almost a black body for radiation, while decreased wind leads to air saturation, increasing dew point temperature and the onset of melting.
Penitente (snow formation) | Wikipedia