The dynamic atmosphere of Saturn's haze-enshrouded moon Titan is revealed in the first Cassini Imaging Team report on Titan, to appear in the March 10 issue of Nature.
Imaging scientists, analyzing images of Titan designed to allow views of the surface and lower atmosphere, have discovered that the winds on Titan blow a lot faster than the moon rotates. In contrast, the jet stream of Earth blows a lot slower than the surface of our planet moves.
Titan is a particularly slow rotator, taking 16 Earth days to make one full rotation. Yet, despite its slow period, model simulations made a decade ago predicted that winds in its atmosphere should blow faster than its surface rotates, making it, like its slowly rotating cousin Venus, one of the solar system's 'super-rotators'.
"It has long been known that winds in Venus's atmosphere blow many times faster than the solid planet itself rotates," said imaging team member Dr. Tony DelGenio of NASA's Goddard Institute for Space Studies, or GISS, in New York, who made the first computer simulation predicting Titan super-rotation a decade ago. "Models of Titan's atmosphere have indicated that it too should super-rotate just like Venus, but until now there have been no direct wind measurements to test the prediction," he said.
Titan's winds are measured by watching its clouds move. Clouds are a rare occurrence on Titan, and those whose motions can be tracked are often small (about 100 kilometers or 60 miles across) and faint; in other words, the clouds are too inconspicuous to be seen from Earth. The discovery of moving clouds required careful manipulation of Cassini images in which cloud features are hard to distinguish through the moon's ubiquitous haze and against the backdrop of Titan's complex bright and dark surface. DelGenio and his associate John Barbara, also of GISS, used Cassini images that had been taken through special filters designed to see through the haze to detect surface features as well as clouds. "To discriminate clouds from surface features, I took images of the same region at different times and subtracted them from each other," said Barbara. "When I did this, time-variable clouds stood out as regions of changing brightness."
Ten such clouds have been tracked, giving wind speeds as high as 34 meters per second (about 75 miles per hour) to the east – hurricane strength – at an altitude somewhere in Titan's middle and lower troposphere. "This result is consistent with the predictions of Titan weather models, and it suggests that we now understand the basic features of how meteorology works on slowly rotating planets," said Del Genio.
Cassini images also reveal much larger cloud streaks – 1,000 kilometers (620 miles) long – elongated generally east-west. These clouds occur at preferred locations and move at only a few meters per second. Apparently these streak clouds originate closer to Titan's surface, perhaps from places where methane is released to the atmosphere from below Titan's surface, or places where wind blows over topography.
In Titan's hazy stratosphere, it looks as though modelers may have to go back to the drawing board. Voyager images of Titan detected a faint detached haze layer above Titan's main stratospheric haze, at altitudes of 300-350 kilometers (190 to 220 miles). Cassini ultraviolet images, which are sensitive to scattering of sunlight by small particles, detect a similar detached haze layer, but at an altitude of 500 kilometers (310 miles) instead.
"The change we see in the detached haze over the 25 years since Voyager suggests that either the photochemical process that produce the hydrocarbon haze particles, or the atmospheric circulation that distributes them around the planet, may change with the seasons," said imaging team member Dr. Bob West of the Jet Propulsion Laboratory, who designed all the Titan atmosphere imaging sequences for the Cassini mission. "It will be a challenge for models to be able to predict how and where these detached hazes occur," he said.
Images of Titan's night side, in which high haze layers are backlit by the Sun, surprised scientists by showing evidence of an entire series of haze layers. These may be evidence of gravity waves, the atmospheric equivalent of ripples on a pond, propagating up to Titan's upper stratosphere by disturbances that originate at lower levels. If so, then analysis of the properties of these waves may yield insights into the temperature and wind profiles of Titan's stratosphere and how they change over the course of the mission.
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