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By Emily Lakdawalla

Saturn's Atmospheric Pearls and Weather on Titan

Oct. 12, 2006 | 08:47 PDT | 15:47 UTC

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by Brad Thomson

I tried to shed my solid-phase boundary bias and soak in some of the interesting new atmospheric results presented at the Wednesday morning press briefing. There were two presentations, one on Saturn's atmospheric dynamics, and another on weather on Titan.

Saturn's Atmospheric String of Pearls
Kevin Baines talked about new results from imaging Saturn's atmosphere in the infrared (~5 microns). In visible wavelengths, Saturn itself looks sort of bland (not considering the rings and crazy moons spewing geysers all over the place, of course). This is in contrast to Jupiter, which shows these wonderful latitudinally banded clouds and huge storms like the Great Red Spot. The reason for Saturn's apparent blandness is a layer of high-altitude haze.

But when seen in the infrared, one sees beneath this haze to the active churning clouds beneath. In particular, Kevin was reporting on the discovery of a bizarre string of pearl-like clouds at 40 degrees N latitude. These clouds are confined to a thin band, and exhibit a periodic spacing [I was too busy looking at the pretty pictures to remember to ask about their dimensions]. Nothing like this is seen on any other planet. It appears to be some sort of wave phenomena and may stretch all the way around the planet.

Click to enlarge >

String of Pearls
In this image, Saturn's fascinating meteorology manifests itself in a "string of pearls" formation, spanning over 60,000 kilometers (37,000 miles). Seen in new images acquired by Cassini's visual and infrared mapping spectrometer and lit from below by Saturn's internal thermal glow, the bright "pearls" are actually clearings in Saturn's deep cloud system. More than two dozen occur at 40 degrees north latitude. Each clearing follows another at a regular spacing of some 3.5 degrees in longitude. Credit: NASA / JPL / University of Arizona

Titan Weather: Local on the 8s
Caitlin Griffith talked next about our new and growing understanding of weather systems on the Saturnian moon, Titan. Titan is a large moon with a significant atmosphere, and was the target of the recent Huygens probe on the Cassini mission. One of the scientific goals of atmospheric folks is understanding how similar weather is on Titan to weather on Earth. Here on Earth, our weather is tied up with the cycling of water between the surface and the atmosphere. Titan appears to have a methane cycle rather than a water cycle due to its much colder temperatures.

Clouds on Titan are concentrated in the south polar region and in a thin strip near 40 degrees south latitude. These are inferred to be mostly convective clouds and generally form over regions of upwelling. A recent GCM (general circulation model) of Titan's atmospheric flow predicts upwelling in southern hemisphere and downwelling in northern hemisphere, which is consistent with the observed cloud distribution. So far, so good.

But not everything can be explained by the model. The clouds at 40 degrees S latitude cluster at a particular longitude, the cause of which is not known. Is volcanism driving these longitude distribution? Or is liquid at the surface concentrating the clouds? If so, what is sustaining the liquid? Recent radar images appear to indicate the existence of lakes in the polar regions.

How deep are these lakes? 10 m perhaps? 20 m? Something like 10 percent of the surface appears covered in the north polar region. If we assume a mean depth of the lakes of about 15 m, this is equivalent to a precipitation layer north of 70 degrees latitude ~1.5 m deep over the entire surface. By making such informed speculative guesses, we can try and tease out the relative amounts of methane sorted in the atmosphere versus on the surface. On Earth, most of the water is held on the surface rather than the atmosphere (2.7 km global average for the surface inventory versus 2.5 cm for the atmosphere). On Titan, the reverse is true. Titan's atmosphere holds a globally-averaged layer of methane about 3 m deep, while the surface has the equivalent of only about 0.7 m.

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