Penitente (snow formation)
Penitentes, or nieves penitentes (Spanish for "penitent-shaped snows"), are snow formations found at high altitudes. They take the form of elongated, thin blades of hardened snow or ice, closely spaced and pointing towards the general direction of the sun.
The name comes from the resemblance of a field of penitentes to a crowd of kneeling people doing penance. 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 were first described in scientific literature by Charles Darwin in 1839. On March 22, 1835, he had to squeeze his way through snowfields covered in penitentes near the Piuquenes Pass, on the way from Santiago de Chile to the Argentine city of Mendoza, and reported the local belief (continuing to the present day) that they were formed by the strong winds of the Andes.
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. A mathematical model of the process has been developed by Betterton, although the physical processes at the initial stage of penitente growth, from granular snow to micropenitentes, still remain unclear. The effect of penitentes on the energy balance of the snow surface, and therefore their effect on snow melt and water resources has also been studied.
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.
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- "Icy ridges only before seen on Earth are found on Pluto". Mail Online. Retrieved 2017-01-05.
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- Vance Bergeron, Charles Berger, and M. D. Betterton, 2006. "Controlled Irradiative Formation of Penitentes", PRL 96, 098502 (2006), webpage: Arxiv-PDF-601184.
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- Lliboutry, L.: 1998, "Glaciers of the Dry Andes", in R. S. J. Williams and J. G. Ferrigno (eds), Satellite Image Atlas of Glaciers of the World - South America, United States Geological Survey Professional Paper 1386-I, webpage: USGS-p1386i.