"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.

"DISR," or the Descent Imager/Spectral Radiometer, was the eyes for the Huygens probe on its journey to Titan's surface on Jan. 14, 2005. The Huygens landing was the most distant touchdown ever made by a human-built science probe. DISR took photographs during the probe's descent, and those photos show that Titan is more like the Earth than any other world seen yet. UA's Martin Tomasko, an LPL research professor, leads the international DISR team.

Expressed in Titan longitude and latitude, the Huygens 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 touse 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. 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 (about 300 to 650 feet) and 50 to 100 meters deep (about 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.

earlier related report

Highlights of ESA's Huygens mission
Paris (ESA) Dec 01 - After a seven-year journey on board the NASA/ESA/ASI Cassini spacecraft, ESA's Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January 2005 and landed on the surface after a parachute descent of 2 hours and 28 minutes.

Clear images of the surface were obtained below 40 km altitude - revealing an extraordinary world, resembling Earth in many respects, especially in meteorology, geomorphology and fluvial activity. The images show strong evidence for erosion due to liquid flows, possibly methane, on Titan.

The probe descended over the boundary between a bright icy terrain eroded by fluvial activity, and a darker area that looked like a dry river- or lakebed. Huygens landed in the dark area. Water-ice pebbles up to a few centimetres in diameter were scattered near the landing site, and the surface here was found to have the consistency of loose wet sand.

Winds were found to blow predominantly in the direction of Titan's rotation, west to east winds, with speeds up to 450 km/h above an altitude of 120 km. The winds decreased with decreasing altitude and then and changed direction close to the surface. An unexpected layer of high wind-shear was encountered between altitudes of 100 and 60 km.

Huygens also surprised the scientists by finding a second lower ionospheric layer, between 140 km and 40 km, with electrical conductivity peaking near 60 km, and its instruments may also have recorded the signature of lightning.

'Haze' was detected all the way down to the surface, contrary to the predictions of pre-Huygens models. It was predicted that the atmosphere would be clear of 'haze' in the lower stratosphere, below around 60 km. Fortunately, the haze was transparent enough for good images of the surface to be obtained below 40 km.

Huygens enabled studies of the atmosphere and surface, including the first in-situ sampling of the organic chemistry and the aerosols below 150 km. These confirmed the presence of a complex organic chemistry in both the gas and the solid phase, which reinforces the idea that Titan is a promising place to observe chemical pathways involving molecules that may have been the building blocks of life on Earth.

Argon 40 was also detected at the surface and its presence indicates that Titan has experienced in the past, and is most likely still experiencing today, internal geological activity.

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