The penitentes are a curious natural phenomenon found in high altitude regions, typically more than 4000 metres above sea level where it’s very cold and very dry.
The formation evokes the tall, pointed habits and hoods worn by brothers of religious orders in the Processions of Penance during Spanish Holy Week. In particular the brothers' hats are tall, narrow, and white, with a pointed top.
These spires of snow and ice grow over all glaciated and snow-covered areas in the Dry Andes above 4,000 metres or 13,120 feet. They range in size from a few centimetres to over 5 metres or 16 feet.
|Penitentes near the summit of the Agua Negra Pass on the border between Chile and Argentina.|
Louis Lliboutry noted that the key climatic condition for the differential ablation that leads to the formation of penitentes is that the dew point is always below freezing. Thus, snow will 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. In this way peaks, where mass loss is due only to sublimation, will remain, as well as the steep walls, which intercept only a minimum of solar radiation. In the troughs, the ablation is enhanced, leading to a downward growth of penitentes.
Penitentes may be present on Europa, a satellite of Jupiter. According to recent study, NASA's New Horizons has discovered penitentes on Pluto, in a region informally named Tartarus Dorsa.