Cassini spies the brightest infrared spot on Titan

When the Cassini spacecraft flew by Titan on March 31 and again on April 16, its Visual and Infrared Mapping Spectrometer (VIMS) saw a spectacular, 300-mile-wide (500 kilometer) bright spot at long infrared wavelengths. The spot was just southeast of the continent-sized region called Xanadu.

At 5-micron wavelengths -- the longest, reddest wavelengths that VIMS sees -- the red spot is the brightest area yet seen on Titan. The feature is the size and shape of West Virginia and 50 percent brighter than bright Xanadu.
The bright spot appears to be at the same location where Cassini's Imaging Science Subsystem (ISS) saw a bright, 345-mile (550-kilometer) wide semi-circle at visible wavelengths in December 2004. ISS saw the arc-shaped feature again at lower resolution during the February 2005 flyby.

VIMS and ISS scientists combined their results for a complementary look at the infrared-bright feature on Saturn's moon, Titan.

"At first glance, I thought the feature looked strange, almost out of place," said University of Arizona Professor Robert H. Brown of the Lunar and Planetary Laboratory (LPL), head of the VIMS team. "After thinking a bit, I speculated that it was a hot spot. In retrospect, that might not be the best hypothesis, but the spot is no less intriguing."

"The spot is maybe only 10 percent brighter at shorter, or bluer, infrared wavelengths," said Jason Barnes, a UA postdoctoral researcher who works with Brown. "But at these longer infrared wavelengths, this is a whopping difference in spectrum, or color. It's just the kind of thing we've been looking for. Unfortunately, we don't know yet what it is."

The VIMS team suggests that the bright red spot is either a surface coloration -- the strange reflection from an unusual patch of Titan's surface -- or mountains, a cloud or a hot spot, Barnes said.

"It's possible that VIMS is seeing a cloud that is topographically controlled by something on the surface, and that this weird, semi-circular feature is causing this cloud," Turtle said.

Image Credit: NASA/JPL/University of Arizona/Space Science Institute

The false-color image on the left was created using images taken at 1.7 microns (represented by blue), 2.0 microns (green), and 5.0 microns (red). The images that comprise this view were taken by the visual and infrared mapping spectrometer instrument on the April 16, 2005, Titan flyby. Several views were stitched together to make a mosaic. The result was then reprojected to simulate the view from the imaging camera so that the two could be directly compared.

The center image was taken by the narrow-angle camera on December 10, 2004, using a spectral filter centered at 0.938 microns (938 nanometers). The image was taken at a distance of 1.5 million kilometers from Titan and has a pixel scale of 9 kilometers (6 miles) per pixel (see PIA06154 for original image). The image is centered on 8 degrees south latitude, 112 degrees west longitude. This image has been contrast enhanced and sharpened to improve surface feature visibility.

"It seems clear that ISS and VIMS are detecting the same basic feature on or controlled by Titan's surface," said ISS team scientist Alfred S. McEwen, who directs LPL's Planetary Imaging Research Laboratory. "This bright patch may be due to an impact event, landslide, cryovolcanism or atmospheric processes. Its distinct color and brightness suggests that it may have formed relatively recently."

"The feature seen by ISS looks like it's on the surface," said LPL's Elizabeth (Zibi) Turtle. Turtle is an associate on the Cassini imaging team, which is headed by UA adjunct Professor Carolyn Porco of the Space Science Institute in Boulder, Colo.

Other bright spots have been seen on Titan, but all have been transient features that move or disappear within hours, and they have different spectral (color) properties than this feature has.

If the feature is a cloud, it reflects light in a way that is uncharacteristic of clouds, Brown said.

Barnes checked VIMS images from previous Titan passes, in July and October 2004, and found a bright spot consistent with the size and shape with the spot first identified by VIMS images from the March 2005 flyby. "If the spot is a cloud, then its longevity and stability imply that it is controlled by the surface," he said. "Such a cloud might result from airflow across low mountains or outgassing caused by geologic activity."

Image Credit: NASA/JPL/University of Arizona/Space Science Institute

This dramatic color (but not true color) image was taken during the April 16, 2005, encounter with Titan. North is to the right. In the center it shows the dark lanes of the "H"-shaped feature discovered from Earth and first seen by Cassini last July shortly after it arrived in the Saturn system. At the southwestern edge of the "H" feature, near Titan's limb (edge), is an area roughly 500 kilometers (300 miles) across. That area is 50 percent brighter, when viewed using light with a wavelength of 5 microns, than the bright continent-sized area known as Xanadu.

Xanadu extends to the northwest of the bright spot, beyond the limb (edge) of Titan in this image. Near the terminator (the line between day and night) at the bottom of this image is the 80 kilometer (50 mile) crater that has been previously seen by the Cassini radar, imaging cameras, and the visual and infrared spectrometer.

At wavelengths shorter than 5 microns, the spot is not unusually bright. The strange spectral character of this enigmatic feature has left the team with four possibilities for its source: the spot could be a surface coloration, a mountain range, a cloud, or a hot spot.

This color image was created from separate images in the 1.7 micron (blue), 2.0 micron (green), and 5.0 micron (red) spectral windows through which it is possible to see Titan's surface. The yellow that humans see has a wavelength of about 0.5 microns, so the colors shown are between 3 and 10 times more red than the human eye can detect.

The bright red spot could be reflections from a patch of terrain made up of some exotic surface material. "Titan's surface seems to be mostly dirty ice. The bright spot might be a region with different surface composition, or maybe a thin surface deposit of non-icy material," Barnes said.

Scientists have also considered that the spot might be mountains. If so, they'd have to be much higher than the 100-meter-high (300-foot) hills Cassini's radar altimeter has so far seen. Scientists doubt that Titan's crust could support such high mountains.

Another possibility is that the bright region is a "hot" spot, an area warmed by a recent asteroid impact or warmed by a mixture of water ice and ammonia oozing out of an ice volcano over the colder surrounding terrain.

The VIMS team will be able to test the hot spot hypothesis on the July 2, 2006 Titan flyby, when VIMS will take nighttime images of the area. If the spot glows at night, researchers will know it's hot.

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, Calif., manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo. The Visual and Infrared Mapping Spectrometer team is based at The University of Arizona in Tucson.

University of Arizona News Release