Evidence of the changing seasons is all around. Leaves aflame with color, frost in the morning, snow pushing in from the west. Many of us feel wistful about seeing summer end but find renewed energy and a fresh point of view as fall sweeps it away.
I wonder what Saturnians think about their changing seasons? Saturn’s axis tilts 27 degrees, a few more than Earth’s, giving that planet a full round of seasons, too. While autumn on Earth blows by in 3 months, Saturn’s 29-year-long orbit means each season lasts for more than 7 years.
As on Earth, autumn manifests itself on the ringed planet as a change in color. Since there’s no solid surface or possibility for trees, we look to Saturn’s atmosphere for signs of fall and find an delightful autumnal blue.
Ultraviolet light from the sun works on the planet’s atmospheric chemistry to increase the amount of haze. During Saturnian summer, the intensity of UV is greater and the air gets hazier. In winter, UV light drops off and Saturn’s atmosphere clears. Air molecules can now better scatter the visible sunlight and tint the planet’s upper atmosphere blue. Our own sky is blue for the very same reason. Methane, which comprises almost half a percent of Saturn’s air and absorbs red light, further enhances the blue in a clearing atmosphere.
Earth’s tipped axis is the cause of the seasons. On one side of our orbit, the northern hemisphere is oriented toward the sun, giving us summer; on the other side the hemisphere faces away from the sun, bringing us winter. Any planet with a fair amount of tilt experiences seasons.
Jupiter’s axis is tipped only 3.1 degrees or nearly straight up and down. Sunlight shines equally across the planet during an entire Jupiter year, which is equal to 12 Earth years. While we have to wait a while yet for Saturn to reappear in the dawn sky, Jupiter’s easy to spot. Tonight and tomorrow night the waning gibbous moon will pass very near it in the northeast. Clear skies!