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 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. 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.
- "Planetary Analogue". Retrieved 8 June 2015.
- Lliboutry, L. (1954a). "Le Massif du Nevado Juncal ses penitentes et ses glaciers". Revue de Géographie Alpine. 42: 465–495. doi:10.3406/rga.1954.1142.
- Lliboutry, L. (1954b). "The origin of penitentes". Journal of Glaciology. 2: 331–338.
- Lliboutry, L. (1965). Traité de Glaciologie, Vol. I & II (in French). Paris, France: Masson.
- Naruse, R.; Lieva, J.C. (1997). "Preliminary study on the shape of snow penitents at Piloto Glacier, the Central Andes". Bulletin of Glacier Research. 15: 99–104.
- Darwin, C. (1839). Journal of researches into the geology and natural history of the various countries visited by H. M. S. Beagle, under the command of Captain Fitz Roy, R.N., 1832 to 1836. London, UK: H. Colburn.
- Betterton, M.D. (2001). "Theory of structure formation in snowfields motivated by penitentes, suncups, and dirt cones". Physical Review E. 63: 12. doi:10.1103/physreve.63.056129.
- Corripio, J.G. (2003). Modelling the energy balance of high altitude glacierised basins in the Central Andes (PDF) (PhD. thesis). Edinburgh, UK: University of Edinburgh. p. 151. Retrieved 7 September 2013.
- Corripio, J.G.; Purves, R.S. (2005). "Surface Energy Balance of High Altitude Glaciers in the Central Andes: the Effect of Snow Penitentes". In de Jong, C.; Collins, D.; Ranzi, R. Climate and Hydrology in Mountain Areas (PDF). London, UK: Wiley & Sons. p. 18. Retrieved 7 September 2013.
- "Icy ridges only before seen on Earth are found on Pluto". Mail Online. Retrieved 2017-01-05.
- Talbert, Tricia (2017-01-04). "Scientists Offer Sharper Insight into Pluto’s Bladed Terrain". NASA. Retrieved 2017-01-05.
- Moores, John E.; Smith, Christina L.; Toigo, Anthony D.; Guzewich, Scott D. (2017-01-12). "Penitentes as the origin of the bladed terrain of Tartarus Dorsa on Pluto". Nature. 541 (7636): 188–190. doi:10.1038/nature20779. Retrieved 13 January 2017.
- Vance Bergeron, Charles Berger, and M. D. Betterton, 2006. "Controlled Irradiative Formation of Penitentes", PRL 96, 098502 (2006), webpage: Arxiv-PDF-601184.
- Kotlyakov, V. M. and Lebedeva, I. M.: 1974, "Nieve and ice penitentes, their way of formation and indicative significance", Zeitschrift für Gletscherkunde und Glazialgeologie, Vol. X, 111-127.
- 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.