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The Planetary Society Blog
By Emily Lakdawalla
Saturn's north polar hexagon
Mar. 28, 2007 | 10:33 PDT | 17:33 UTC
Yesterday the Cassini VIMS team released a spectacular set of images showing a hexagonal structure encircling Saturn's north pole.
Saturn's north polar hexagon
Credit: NASA / JPL / U. Arizona |
The image was taken at a mid-infrared wavelength of 5 microns. It's winter at Saturn's north pole right now, so you're not seeing anything in this image that's illuminated by the Sun. Instead, you're seeing thermal radiation (that is, heat) welling up from Saturn's gloomy depths. Some of the heat radiation is blocked by clouds floating around in Saturn's atmosphere about 75 kilometers below the cloud tops that we see when we look at Saturn in visible wavelengths of light. The pressure at that level is about three times Earth's atmospheric pressure. So the patterns are created by alternating cloudy and clear areas.
They also released a movie of the same region. The full-size version includes 37 frames captured over a period of about an hour; I cut out 2/3 of the frames in the thumbnail below to make it quicker to download. Most of the motion that you see here is just planetary rotation -- in one hour, Saturn rotates about 30 degrees. But if I stare at this movie long enough, I can see some differential motion -- I think I see the tiny spot at the center rotating faster, for instance. But with the resolution that VIMS achieves it's tough to be certain about relative cloud motions. What I expect the VIMS team will do when they analyze this image is to reproject their data so that you appear to be looking down on the south pole, and take out the planetary rotation component of the motion, so that you have a chance to spot the motions that are actually taking place within the clouds. You can learn more about the VIMS instrument here.
I'll note here that the imaging technique and the interesting structures observed are pretty similar for the bizarre south polar vortex observed at Venus by another infrared instrument measuring the glow of thermal radtiation -- it's even measured at roughly the same wavelength. For these images, though, the contrast is not reversed, so holes in the clouds show up as bright features, while cloudy areas are dark. Venus' south pole at 5.05, 4.65, and 4.08 micronsThe VIRTIS instrument on Venus Express can look to different depths in Venus' atmosphere by employing different wavelengths in the infrared spectrum. These three views, captured on May 29, 2006 from about 64,000 kilometers (40,000 miles), show the three-dimensional structure of Venus' south polar vortex. The left image (taken at 5.05 microns) corresponds to an atmospheric altitude of about 59 kilometers (36.7 miles), just about the Venusian cloud deck. The central image (taken at 4.65 microns) corresponds to an atmospheric altitude of about 60 kilometers (37.2 miles). The right image (taken at 4.08 microns) corresponds to an altitude of about 65 kilometers (40.4 miles), just in the upper clouds. Brighter color indicates that more radiation is leaking out from below, so they identify "holes" in Venus' atmosphere. The vortex is encircled by a faintly visible dark circle of thicker clouds that block the radiation. Credit: ESA / VIRTIS / INAF-IASF / Obs. de Paris-LESIA |
What is Saturn's hexagon, and how is it a stable feature? It's been around since the Voyagers passed by in 1980 and 1981. (Or, more accurately, it was also spotted in 1980 and 1981; there's no way of knowing if it's been there continuously since that time.)
Space enthusiast Richard Hendricks pointed out an interesting paper, published in 2006, titled "Polygons on a Rotating Fluid Surface," (here's the article as a PDF), in which a team of physicists writes about creating stable polygons in rotating fluids in the laboratory. Their abstract begins: "we report a novel and spectacular instability of a fluid surface in a rotating system. In a flow driven by rotating the bottom plate of a partially filled, stationary cylindrical container, the shape of the free surface can spontaneously break the axial symmetry and assume the form of a polygon rotating rigidly with a speed different from that of the plate. With water we have observed polygons with up to 6 corners." Here are three images from their study, showing polygons with 3, 4, and 5 sides. (You can see more images and a movie from their experiment here.)Polygons formed in a rotating fluidThree top views of an experiment in fluid dynamics show stable polygonal shapes formed in a rotating fluid. The experiment consisted of a stationary cylindrical container in which a circular plate was rotated by a motor. The rotation of the plate creates a centrifugal force that presses the fluid outward, forming a circular depression in the center. When the rotation rate is increased sufficiently high, the circular depression spontaneously forms corners, eventually forming stable polygonal shapes that rotate at a different rate from the plate below. From " Polygons on a Rotating Fluid Surface." Credit: Thomas Jansson et al., 2006 |
This study is probably not directly applicable to Saturn. They continue to say, "We speculate that the instability is caused by the strong azimuthal shear due to the stationary walls and that it is triggered by minute wobbling of the rotating plate. The slight asymmetry induces a tendency for mode-locking between the plate and the polygon, where the polygon rotates by one corner for each complete rotation of the plate." In other words, the polygons form as a result of interaction between the rotating fluid and the edges of the container. Saturn has no edges. But the important result of this study is that you can create stable, rotating polygonal shapes in rotating fluids in the laboratory. Saturn has its own way of doing it -- now the atmospheric physicists just have to figure out how!
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