December 1, 2005
Scientists Find Huygens Probe Landing Site
Arizona -- Cassini/Huygens scientists have discovered exactly where on Saturn's largest moon, Titan, the European Space Agency's (ESA) Huygens probe landed last January. Knowing the landing location will allow them to directly compare data from Huygens with remote sensing data from NASA's Cassini orbiter.
"Based on a truly project-wide collaboration among a number of members of the extended Cassini/Huygens community, we feel we are finally in a position to announce a definitive correlation between a section of radar data taken on the T8 (the Oct. 28, 2005 Titan flyby) and a DISR high-altitude mosaic," Bashar Rizk of the University of Arizona Lunar and Planetary Laboratory (LPL) and Steven Wall of NASA's Jet Propulsion Laboratory told project scientists earlier this month.
Expressed in Titan longitude and latitude, the Hugyens probe landed within about 5 kilometers (1.4 miles) of 192.4 degrees west longitude (or 167.6 degrees east longitude) and minus 10.2 degrees south latitude, Rizk and Wall said. That's a mere 7 kilometers (4 miles) away from where Cassini/Huygens scientists predicted the probe would land, they noted.
Locating the landing site required the joint effort of members of the radar, imaging, visual and infrared mapping spectrometer and DISR teams, as well as the essential participation of Larry Soderblom and Randy Kirk of U.S. Geological Survey astrogeology division in Flagstaff, Ariz., Rizk and Wall said.
The DISR team scientists analyzed landform features and albedo (brightness) patterns in both the radar and optical (DISR) images by making overlays to locate boundaries and match landform orientations and shapes. It took considerable skill, patience and some luck.
"It's important that we know from an orbital perspective what kind of terrain the Huygens probe landed in," Rizk said. "It allows us to connect what Huygens found in detail about a small patch of Titan's surface to what the orbiter is accumulating now. We had a pretty good notion of what the landing site was before, but connecting it with the radar data allowed us to use the magnificent, absolute knowledge of the location transferred through the Cassini orbiter."
Last week Rizk made a minute-long animation, Titan descent movie, from images taken during the Huygens probe's two-and-one-half hour alien-world plunge. The animation is online at the DISR Website, http://www.lpl.arizona.edu/DISR/. Rizk created the animation from Cassini imaging, radar, and visual and infrared mapping spectrometer data as well as DISR data. It traces the actual descent profile that the probe took as it swung and spun east down to Titan's surface.
LPL Professor Jonathan I. Lunine presented the movie earlier today at an ESA press conference in Paris. "The combination of DISR and Cassini remote sensing data provide a tantalizing hint that the action of liquids eroding and shaping the landscape near the landing site is repeated elsewhere in the much larger region covered by the orbiter data," Lunine said.
The new Huygens descent animation starts at an altitude of 300 km (about 186 miles) and moves eastward along the trajectory that the Huygens probe traveled on its journey to Titan's surface. Data from the imaging system, radar and the visual and infrared mapping spectrometer on the Cassini orbiter are displayed in quick succession, followed by DISR mosaics from increasingly lower altitudes. The surface color is about what a human observer riding on the probe would see if it were possible to see the surface through Titan's atmospheric haze. The longitude and latitude grid lines are separated by 2 degrees. Near the end of its descent, the probe reversed direction, setting almost straight down until, finally, it landed, facing south on a dry lakebed strewn with ice cobbles.
Overall, the entire set of DISR observations from 150 kilometers (93 miles) high in Titan's atmosphere through landing outlines the major role methane plays in shaping Titan's surface and controlling its meteorology, said Bruno Bezard of Observatoire de Paris, France, a co-investigator on DISR, at the ESA press conference.
DISR was enveloped in thick haze as soon as it began taking data at 150 kilometers (93 miles) altitude, Bezard said, and the haze reaches undiminished all the way to the surface. The haze was so thick that DISR's three different cameras began discerning surface features only at about 55 kilometers (34 miles) altitude.
DISR scientists used the different camera views to reconstruct the probe's descent trajectory and measure wind velocities. At 50 kilometers high (31 miles), 90 kph (60 mph) winds swept the probe eastward. But at about 7 kilometers altitude (4 miles), windspeed dropped to less than 3kph (less than 2 mph) and the winds changed direction. This may be a convective region where local winds disconnect from Titan's main jet-streams, the scientists said.
At 700 meters altitude ( about 1/2 mile), DISR turned on a landing lamp so spectrometers could analyze light reflected from the near-surface atmosphere and the surface itself. The spectrometers measured five percent methane in Titan's mostly nitrogen atmosphere at 20 meters (66 feet) altitude. That's three times more methane than in Titan's stratosphere and confirms that methane is condensing near Titan's surface, DISR scientists concluded.
The team had planned to measure light reflected from Titan's surface to learn just what that surface is made of. The dark, frigid surface would look reddish to the human eye. It reflected no more than 15 percent to 20 percent at infrared (longer-than-visible) light wavelengths. Light reflected from Titan's surface showed there are organic materials (carbon-and-hydrogen containing compounds) and water ice, but also water ice laced with an unknown constituent. Scientists will have to further analyze DISR data and organic materials manufactured in the laboratory to identify the unknown constituent.
But it's the DISR images of Titan's striking landscape that have thrilled millions of people worldwide. When DISR scientists assembled the descent images into panoramic mosaics, they saw bright, high terrain cut by deep channels and flat, darker, lower terrain that resembled a dried lakebed. It is Earth-like desert topography clearly marked by fluid flow.
"Titan's surface is shaped by winds, liquid and tectonic forces as on Earth, but under exotic conditions and involving organic deposits as well as water ice," Lunine noted at the press conference.
There appear to be two types of channel networks. Steeply sloped main drainage channels from 100 to 200 meters wide (roughly 300 to 650 feet) and 50 to 100 meters deep (roughly 150 to 300 feet) branch through the bright highlands. They are believed to have been cut by rapidly flowing rivers of liquid methane. A second type are the short, stubby channels that often begin - or end - in dark circular areas. They are thought to be spring-fed channels.
One of DISR's most memorable images is the well-known view from Titan's surface taken after landing. Fifteen-centimeter (six-inch) rounded water-ice cobbles lie scattered over a darker, fine-grained "ice gravel." It's more evidence for powerful erosion by flowing liquid.
The British science journal Nature will publish an issue on Huygens probe results, including an article on DISR results, on Dec. 8.
The Cassini-Huygens mission to Saturn and Titan is a joint mission of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI). ESA supplied and manages the Huygens probe that descended to Titan's surface Jan. 14, 2005. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington, D.C. NASA funded the Descent Imager-Spectral Radiometer, which was built by Lockheed Martin.
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