Saturn’s blue autumn

Northern Minnesota reached peak fall color this past weekend. The view was sumptuous from an overlook on the Superior Hiking Trail near Little Marais, Minn. Photo: Bob King

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.

Saturn’s largest moon Titan passes in front of the planet in a picture taken by the Cassini spacecraft last May. The rings (thin yellow line) casts broad shadows on Saturn’s cloudtops. The arrival of autumn in the southern hemisphere creates the blue cast. Click to enlarge. Credit: NASA/JPL-Caltech/Space Science Institute

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.

It’s all in the tip. As Earth – and other planets – orbit, first one hemisphere and then the other is oriented toward the sun. This changes the height of the sun in sky which changes the length of the day and drives the seasons.

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.

Watch for a nice pairing of the moon and Jupiter in the east tonight and tomorrow night. The two will come up around 10 p.m. This map shows the sky around 11 o’clock. Created with Stellarium

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!

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