Catherine Brahic, environment editor
Like a procession of hooded sinners, these penitentes stand guard on Chile's Chajnantor plain. They get their name from the pointed headgear of the nazarenos, penitents who take part in Holy Week processions across the Christian world.
Charles Darwin described squeezing through a forest of the delicate ice blades in 1835 as he approached the Piuquenes pass on his way from Santiago in Chile to Mendoza in Argentina: they can reach 5 metres high, although most are smaller. He noted the popular local belief that they were formed by winds - a belief that is still held today but turns out to be wrong.
Initially, small dimples in the surface of a snow or ice field trap solar radiation. Temperatures and pressure conspire to make the water crystals sublimate rather than melt: ice and snow turn straight to vapour rather than passing through a liquid phase. As the dimples deepen into troughs, a microclimate sets up inside the troughs where humidity is higher and the snow begins to melt. Pools of water in the troughs bore down into the snow faster still.
It may seem counter-intuitive, but penitentes are thought to slow melting and preserve the remaining snow. The deep troughs create a larger surface area for heat exchange, allowing gentle winds to cool the surface of the glacier more efficiently.
Fields of penitentes are mostly seasonal, but in the Andes, where they form only above 4000 metres, they can last for years. Geologist and meteorologist Javier Corripio of meteoexploration.com reports skiing over fields of 15-centimetre-high blades in the Alps. The ride, he says, was "quite bumpy". His best guess is that those in the photo are between 50 centimetres and 1 metre tall.