Wind or Rain or Cold of Titan’s Night?
|This image shows a possible caldera from which liquid water or slush may have once flowed on Ganymede.|
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’.
|Titan in different wavelengths and atmospheric depths. |
Image Credit: NASA/JPL
"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."
|Ultraviolet image of Venus obtained by Pioneer-12.|
Image Credit: BNSC
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.
|Shorelines may be dry but intermittently defined by drainage channels of methane rain. Click image for larger view. Credit: ESA|
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.
Earth-like Dynamic Moon?
Since most of the cloud activity observed on Titan with Cassini has occurred over the south pole, scientists believe this may be the place where the cycle of methane rain, channel carving, runoff, and evaporation is most active, an hypothesis that could explain the presence of the extensive channel-like features seen in this region.
|Titan seen in different filters tuned for atmospheric depths. Click image for larger view. |
Image Credit: Keck
With presently active geologic and erosional processes similar to those shaping the land areas of Earth, Titan offers scientists an intriguing place to explore and study in the years ahead.
"Throughout the Solar System, we find examples of solid bodies that show tremendous geologic variation across their surfaces. One hemisphere often can bear little resemblance to the other," said Dr. Carolyn Porco, Imaging Team leader. "On Titan, it’s very likely to be this and more. Who knows, we may get lucky and have the chance to observe the surface change with time. It’s a good thing we’ll be coming back for more."
Titan is about the same size and density as Jupiter’s largest moon, Ganymede. Unlike Ganymede, though, it probably has not undergone tidal heating – a well-known internal engine for modification of planetary surfaces. For these reasons, Titan was expected to have a surface at least as old as Ganymede’s and pocked with at least as many large craters. Over the past billion years, Titan should have accumulated as many as a hundred craters, 20 kilometers (12 miles) wide and larger, across its entire surface. Yet, that is not what is seen in the images of this world Cassini has obtained so far.
|Icy pebbles on Titan. Click image for larger view. Credit: ESA|
Images collected over the last eight months during a distant flyby of the south polar region and three close encounters of Titan’s equatorial region have covered 30 percent of its surface with spatial resolutions high enough to pick out features as small as 1 to 10 kilometers (0.6 to 6 miles). At this scale, what has been discovered are geologically young terrains with signs of tectonic resurfacing, erosion by liquid hydrocarbons, streaking of the surface materials by winds and only a few large circular features thought to be impact craters formed in the ice ‘bedrock’. (The largest of these – a 300-kilometer (190-mile) wide, double-annulus structure to the northeast of the large region called Xanadu – was recently imaged by Cassini’s Radar instrument, providing independent confirmation of an impact origin.)
Any large craters that were once there – and there should have been hundreds of them if Ganymede is any guide – appear to have been eliminated or obscured by a combination faulting, viscous relaxation (in which features subside over time due to flow of surrounding material), erosion, and burial. Titan’s surface appears to be as complex as planet Earth’s, though the rates at which the various forces modify its surface may be much slower than on our planet.
Tectonism (brittle fracturing and faulting) has clearly played a role in shaping Titan’s surface. Linear boundaries between bright and dark areas are pervasive on Titan at the global and regional scales seen from orbit, as well as the smaller scales seen by Huygens.
Dr. Alfred McEwen, imaging team member from the University of Arizona, said, "The only known planetary process that creates large-scale linear boundaries is tectonism, in which internal processes cause portions of the crust to fracture and sometimes move – either up, down, or sideways. Erosion by fluids may then serve to accentuate the tectonic fabric by depositing dark materials in low areas and enlarging fractures. This interplay between internal forces and fluid erosion is very Earth-like."
|Do surface features suggest some present tectonic activity? Click image for larger view. Credit: ESA|
Cassini images collected from orbit have also shown dark, curvilinear patterns in various regions on Titan, but mostly concentrated near the south pole. Some in the polar region extend up to 1,500 kilometers (930 miles) long. Images collected by the Huygens probe during its descent down to the Titan surface in January showed clear evidence for small channels a few kilometers long, probably cut by liquid methane. Cassini imaging scientists are suggesting that the curvilinear patterns seen in their images of Titan may also be channels, though there is no direct evidence for the presence of fluids. If these features are channels, it would make the ones seen near the south pole the Titanian equivalents of the Snake River.
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):