Fields of frozen blades
On the high-altitude slopes of Isluga Volcano, within the arid landscape of northern Chile, strange fields of ice blades point towards the sun. These formations, known as nieves penitentes ("penitent snows"), are spikes of hardened snow and ice that can grow from a few centimeters to over 5 meters in height. They appear in dense fields above 4,000 meters, where specific atmospheric conditions prevail: intense solar radiation, cold air, and extremely low humidity where the dew point is always below freezing.
The formation of penitentes results from a physical process called sublimation, where ice turns directly into water vapor without first melting into a liquid. The process begins on a smooth snowfield, where small, random depressions form. Sunlight entering these hollows gets trapped, reflecting between the walls and concentrating the solar energy. This focused energy accelerates sublimation at the bottom of the depressions, deepening them. Meanwhile, the peaks of the forming blades receive less concentrated radiation and are cooled by the dry air, allowing them to remain frozen and lose mass much more slowly. This feedback loop carves the snowpack into a forest of sharp, sun-aligned spires.
From the Andes to outer space
The first scientific account of these ice formations comes from Charles Darwin. On March 22, 1835, while crossing the Piuquenes Pass in the Andes en route from Santiago to Mendoza, he navigated through a field of them. In his journal, he noted the local belief that the strong Andean winds were responsible for sculpting the ice. While this theory was incorrect, his description introduced penitentes to the scientific world.
Today, the unique conditions that create penitentes on Earth are an important analog for studying icy surfaces on other worlds. Scientists theorize that similar structures may exist on Jupiter's moon Europa and have identified massive bladed terrain on Pluto. NASA's New Horizons mission discovered features on Pluto, in a region named Tartarus Dorsa, that resemble penitentes. These Plutonian structures are composed of methane ice and are immense, with ridges up to 500 meters high. By studying the formation of penitentes in the high Andes, planetary scientists can model and understand the surface dynamics and atmospheric conditions on these distant, frozen bodies. Researchers have even discovered that the microclimates created within penitente fields can shelter life, providing an "oasis" for snow algae and bacteria in an extreme environment.