In the sky above eastern Antarctica, airborne ice crystals sculpt the sun’s rays into a ring.
This phenomenon, called a 22-degree halo, is the result of sunlight passing through tiny, six-sided ice cylinders in high-altitude clouds. The crystals act like prisms, bending incoming light 22 degrees off course. Millions of crystals at various orientations can cast a full circle of light around the sun. While dramatic over polar regions, these halos can occur worldwide, even at the equator. Some halos are decorated with bright spots known as sun dogs or mock suns.
Italian photographer Enrico Sacchetti captured this halo in 2013 over Concordia Station, a joint Italian-French research base on the Antarctic Plateau, one of the coldest places on Earth. While ice in the air puts on a light show, the ice surrounding the station provides researchers an opportunity to study the planet’s history: Accumulating layers of snow trap bubbles of atmospheric gases and, over time, build up an archive of past climates. Ice cores from near Concordia provide the oldest records of atmospheric carbon dioxide, dating back at least 800,000 years.