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Planetary Radio Q and A: Saturn's Hexagon

Posted by Emily Lakdawalla

04-11-2009 16:14 CST

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Before I get to this week's Q and A, I should mention that the Carnival of Space can be found over at the Next Big Future.

This week's Planetary Radio features Lou Friedman on the Augustine Human Spaceflight report. On "Questions and Answers" I answered this question:"How do they explain the hexagon near Saturn's north pole?"

Actually, the reader said more than that; they seemed to be under the bizarre impression that the Cassini team was hiding something about this odd feature of Saturn's atmosphere. Far from it; there's been quite a lot of published research on the topic. My answer was drawn from a few abstracts, including this one

Saturn's north polar hexagon was discovered in Voyager images in 1980, and has been observed ever since. It's a long-lived, stable feature of Saturn's atmosphere, akin to Jupiter's Great Red Spot.

From the side, it looks like a wavy boundary between a light band of clouds and a dark band of clouds, located at seventy-six degrees north latitude. But if you compose a lot of images into a synthetic view looking down on Saturn's north pole, you'll notice there are exactly six up-and-down waves, so that the band of clouds appears like a hexagon centered on the pole.

Saturn's north polar hexagon

NASA / JPL / U. Arizona

Saturn's north polar hexagon
This movie of Saturn's north pole was taken by Cassini's VIMS spectrometer at a mid-infrared wavelength of 5 microns. It was winter at Saturn's north pole; all illumination is thermal radiation (heat) welling up from Saturn's depths. Some of the heat radiation is blocked by clouds floating in Saturn's atmosphere at about 75 kilometers below the cloud tops that can be seen in visible wavelengths. The pressure at that level is about three times Earth's atmospheric pressure. The patterns in the image are created by alternating cloudy and clear areas. The images have been contrast-reversed so that the glouds show up as bright spots, while open areas appear dark. The 37 frames in the animation (only 13 of which are shown in the thumbnail version) were taken over a period of about an hour, as Saturn rotated about 30 degrees.

Observations of Saturn in infrared wavelengths have shown that the hexagon is a deep feature, too, extending down to pressures of several bars. While it may be deep, it's not mysterious. Scientists have developed computer simulations of Saturn's atmosphere that contain a jet of wind at that latitude. With just a tiny bit of turbulence introduced into the simulation, the jet quickly develops a wavy motion. How long the waves are depends upon various characteristics of the atmosphere -- density, pressure, and so on -- but because Saturn is a globe, the waves meet themselves after journeying 360 degrees around the planet.

Under these circumstances, the waves naturally organize themselves into a standing wave, with an integer number of ups and downs. There's nothing special about the number six; if the jet were at a different latitude, Saturn might have wound up with a pentagon or a septagon. Cassini wasn't able to observe it in visible light until recently, because the north pole was in winter darkness when Cassini arrived. Now that spring has come to Saturn's north, we can look forward to terrific new images of the hexagon.

[The time constraints of radio did not allow me to clarify the last paragraph by noting that Cassini's infrared instruments can see Saturn by the thermal radiation emitted from its interior, so have been getting fantastic views of the hexagon and other atmospheric structures since Cassini arrived, including the animation I show above.]

 
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