(Every day until December 25, I’m posting a science-related image and description.)

Day 8

Humans love symmetry, so much that we sometimes see it where it doesn’t actually exist. Sometimes, however, symmetry surprises us when we don’t expect it, as with the hexagonal storm at Saturn’s north pole.

Saturn is very unlike Earth in many respects; in particular, it has no surface. The atmosphere gradually becomes more dense until it becomes liquid, but there’s no sharp boundary between air and ocean like on Earth. Saturn certainly has no land masses, since it has no rocky crust. Earth’s oceans are water over rock, so even where our surface is liquid, it’s a skim over a much more significant solid surface below.

That difference leads to a striking difference in weather. On Earth, continents and oceans shape the patterns of air above; on Saturn, air currents can flow unimpeded. Storms can span the entire planet and (as with Jupiter’s Great Red Spot) last for centuries or more. Such currents also provide the explanation for Saturn’s Hexagon.

Like the jet stream, Saturn has stable currents depending on the latitude, but far more consistent than on Earth. The wind speed varies strongly near the poles, changing direction over relatively short latitudes. The result is a set of atmospheric waves perfectly arranged to make a hexagonal vortex. The currents make points of intense swirling, and others of relative stillness, akin to nodes and antinodes on a vibrating string. In Saturn’s case, the combination of wind currents makes for six points; a different combination could make for five or seven, or (as more often the case) an incoherent vortex.

Emily Lakdawalla described the atmospheric dynamics that produced the Hexagon, and included a description of a lab experiment — with video! — that showed exactly how hexagonal vortexes come to be. What seemed mysterious (and produced some rather hilarious speculation) is completely understandable in the light of science.