March 9, 2005
New Titan Images Reveal an Active, Earth-like World
NASA -- Saturn's largest and hazy moon, Titan, has a surface shaped largely by Earth-like processes of tectonics, erosion, winds, and perhaps volcanism. The findings are published in this week's issue of the journal Nature.
Titan, long held to be a frozen analog of early Earth, has liquid methane on its cold surface, unlike the water found on our home planet. Among the new discoveries is what may be a long river, roughly 1,500 kilometers long (930 miles). Scientists have also concluded that winds on Titan blow a lot faster than the moon rotates, a fact long predicted but never confirmed until now.
Tectonism (brittle fracturing and faulting) has clearly played a role in shaping Titan's surface."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," said Dr. Alfred McEwen, Cassini imaging team member from the University of Arizona, Tucson.
"Erosion by fluids may 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."
Cassini images collected during close flybys of the moon show dark, curving and linear patterns in various regions on Titan, but mostly concentrated near the south pole. Some extend up to 1,500 kilometers (930 miles) long. Images from the European Space Agency's Huygens probe show clear evidence for small channels a few kilometers long, probably cut by liquid methane.
Cassini imaging scientists suggest that the dark, curved and linear patterns seen in the Cassini orbiter 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 near the south pole nearly as long as the Snake River, which originates in Wyoming and flows across four states.
Since most of the cloud activity observed on Titan by Cassini has occurred over the south pole, scientists believe this may be where the cycle of methane rain, channel carving, runoff, and evaporation is most active, a hypothesis that could explain the presence of the extensive channel-like features seen in this region.
In analyzing clouds of Titan's lower atmosphere, scientists have concluded that the winds on Titan blow faster than the moon rotates, a phenomenon called super-rotation. In contrast, the jet streams of Earth blow slower than the rotation rate of our planet.
"Models of Titan's atmosphere have indicated that it should super- rotate just like the atmosphere of Venus, but until now there have been no direct wind measurements to test the prediction," said Cassini imaging team member Dr. Tony Del Genio of NASA's Goddard Institute for Space Studies, in New York. DelGenio made the first computer simulation predicting Titan super-rotation a decade ago.
Titan's winds are measured by watching its clouds move. Clouds are rare on Titan, and those that can be tracked are often too small and faint to be seen from Earth.
Ten clouds have been tracked by Cassini, giving wind speeds as high as 34 meters per second (about 75 miles per hour) to the east -- hurricane strength -- in Titan's lower atmosphere. "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.
"We've only just begun exploring the surface of Titan, but what's struck me the most so far is the variety of the surface patterns that we're seeing. The surface is very complex, and shows evidence for so many different modification processes," said Dr. Elizabeth Turtle, Cassini imaging team associate in the Lunar and Planetary Laboratory at the University of Arizona, Tucson and co- author of one of the papers in Nature.
"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, Cassini imaging team leader, Space Science Institute, Boulder, Colo. "On Titan, it's very likely to be this and more."
These results are based on Cassini orbiter images of Titan collected over the last eight months during a distant flyby of the south pole and three close encounters of Titan's equatorial region. Cassini cameras have covered 30 percent of Titan's surface, imaging features as small as 1 to 10 kilometers (0.6 to 6 miles). Cassini is scheduled to make 41 additional close Titan flybys in the next three years.
Image 1: Titan's Variety -- This map of Titan's surface brightness was assembled from images taken by the Cassini spacecraft over the past year, both as it approached the Saturn system and during three closer flybys in July, October and December 2004.
Due to Titan's thick, hazy atmosphere, the size of surface features that can be resolved is a few to five times larger than the actual pixel scale. The pixel scales of the individual images in the map range from 88 to 2 kilometers (55 miles to 1 mile), so the scales of the surface features that can be resolved range from 180 to 10 kilometers (112 to 6 miles).
The images were acquired using a near-infrared filter (centered at 938 nanometers) that has been proven effective at peering through Titan's haze to its troposphere and surface. Similar to a cloudy day on Earth, these images indicate only brightness variations; there are no shadows or topographic shading effects.
The map reveals complex patterns of bright and dark material on Titan's surface. The large scale features, including Xanadu Regio - the large, bright feature that extends from approximately 80 degrees to 130 degrees west near the equator - have been observed from Earth over the past several years.
The patterns seem to vary with latitude. Close to the equator there is more contrast in the large-scale bright and dark features, with some strikingly linear boundaries that are suggestive of geologic processes at work within Titan's crust. The southern-middle latitudes are more uniformly bright, whereas there is more dark material near the south pole. The very bright features near the south pole are clouds. High northern latitudes are not illuminated during the current season on Titan, which is southern summer.
Cassini-Huygens scientists are investigating what causes the latitudinal variation in brightness. One possibility is that, similar to Earth, some parts of the surface receive higher amounts of precipitation than others over Titan's long year (29.5 Earth years), resulting in different amounts of erosion across the surface.
Image 2: Scrutinizing Titan's Surface -- The six close-up views of Titan's surface shown here are composed of images acquired by the Cassini spacecraft during flybys in October (see http://photojournal.jpl.nasa.gov/catalog/PIA06158) and December (see http://photojournal.jpl.nasa.gov/catalog/PIA06159) of 2004. These close-up views illustrate that a variety of processes have shaped the surface of Titan, just as diverse geologic processes are responsible for what we see on Earth's surface.
Image (a) shows a prominent bright-dark boundary near the western edge of the Xanadu region which exhibits a sharp, angular edge between the materials. Three bright, discontinuous circles can be seen (two near the top of the image and another near the lower left). These may be large impact craters; the upper two are approximately 30 kilometers (30 miles) in diameter and the lower one is approximately 50 kilometers (20 miles) in diameter. Titan's thick atmosphere will screen out small projectiles, but if the surface were as old as Titan itself, it should have many more craters of these sizes. Therefore, Cassini scientists think that, like Earth's surface, Titan's surface has been modified more recently by other geologic processes. However, such processes on Titan may take much longer than on Earth, acting over hundreds of millions of years.
Image (b) shows bright features that appear to be streamlined as if were they formed by winds in Titan's atmosphere moving from west to east. The landing site of the Huygens probe is in the upper left corner of this image (see http://www.nasa.gov/mission_pages/cassini/multimedia/pia07239.html).
Image (c) shows a bright feature surrounded by dark material. Several long, dark and narrow lines running through the bright area may be larger examples of the dark channels seen by the Huygens probe (see http://www.nasa.gov/mission_pages/cassini/multimedia/pia07236.html). These lines are on the order of 2 kilometers (1 mile) wide, and tens of kilometers long.
Image (d) shows dark material within the bright area to the west of Xanadu. The linear nature of these features suggests that they may have formed by faulting. They may be dark due to modification by other surface processes occurring on Titan, in the same way that on Earth, fault-lines can be enhanced by erosion and/or deposition of material by water and wind.
Image (e) shows brightness variations in the region southeast of the Huygens landing site. The features indicated by arrows exhibit shapes that are similar to drainage patterns seen on Earth and Mars, where the source of the liquid is underground springs rather than rainfall.
Image (f) shows a region near the northwestern edge of Xanadu where the boundary between the bright and dark materials is quite complicated. Here some of the bright patches appear as if they represent thin surface plates that have been broken apart and spread apart over underlying dark material.
The white bars above each image are 200 kilometers (124 miles) long. Imaging Titan through its thick atmosphere is a challenge, and the narrow, straight lines within the images are seams between individual images that have not been completely removed. North is to the top of each frame.
Image 3: Tracing Surface Features on Titan - Mosaic -- This mosaic of Titan's south polar region was acquired during Cassini's first and distant encounter with the smog-enshrouded moon on July 2, 2004. The spacecraft approached Titan at a distance of about 340,000 kilometers (211,000 miles) during this flyby.
For images and information on the Cassini mission visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini and http://ciclops.org.
This is a contrast-enhanced version of a previously released image (see http://www.nasa.gov/mission_pages/cassini/multimedia/pia06109.html), which allows surface details to be seen more easily. The very bright features near the south pole are clouds.
Due to Titan's thick, hazy atmosphere, the sizes of surface features that can be resolved are a few to five times larger than the actual pixel scale. At this distance, pixel scale is 2 kilometers (about 1 mile), so features larger than several kilometers across are resolved in the images.
Image 4: Tracing Surface Features on Titan - Close-Ups -- These images of Titan's south polar region were acquired during Cassini's first distant encounter with the smog-enshrouded moon on July 2, 2004. The spacecraft approached Titan at a distance of about 340,000 kilometers (211,000 miles) during this flyby.
This montage contains pairs of close-up images, with the original images (at left) and also versions in which some of the narrow, dark, curvilinear and rectilinear surface features have been traced by red lines (at right). These dark features may be examples of surface channels and deeper crustal structures such as faults. The longest features (in the third and fourth pairs from the top) extend for as much as 1,500 kilometers (930 miles) across the surface and are as narrow as 10 kilometers (6 miles) across. At the bottom left, a single frame shows a small, dark, circular feature, which could be an impact crater. For reference, the white bar at the bottom right is a 1,000-kilometers-long (620 mile) scale bar.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.
On the Net: