By Lori Stiles

First-ever images of Titan's landscape thrilled millions of people around the world in January, and discoveries from the Huygens Probe landing began immediately and will continue for years, researchers say.

The images were the remarkable achievement of University of Arizona Lunar and Planetary Laboratory (LPL) scientists and their colleagues in Germany and France.

LPL Research Professor Martin Tomasko's Descent Imager-Spectral Radiometer (DISR) rode to the surface of Saturn's giant moon Titan aboard the Huygens probe on Jan. 14. The probe ­ part of the four-year international Cassini-Huygens mission to the Saturn system ­ is hailed as perhaps the European Space Agency's greatest space exploration success to date.

On Jan. 14, Titan became the farthest place in the solar system where humans have landed a spacecraft. Not only did Huygens collect data for two hours and 27 minutes during descent, it took data for more than 82 minutes after landing.

Researchers on Huygens' six experiments will spend years finding new discoveries in all their information. Their data are the critical "ground-truthing" needed to interpret much more information about Titan that 12 experiments on NASA's Cassini orbiter will obtain through at least 2008.

But for many, it's going to be hard to top the first intense days after Jan. 14, which Tomasko called "the most exciting week of my life."

Even for a space mission, this one seemed unusually risky. The six-ton, 22-foot Cassini spacecraft had to precisely change speed on Christmas Day to accurately aim the 9-foot, 700-pound probe on its 22-day cruise toward Titan. NASA Jet Propulsion Lab engineers achieved that speed change to within a third of an inch per second ­ an amazing feat.

Also, Huygens' mylar-covered heat shield had to protect electronics from burning up in temperatures as high as 1,800 degrees Fahrenheit when the disk-shaped probe slammed into Titan's atmosphere at around 13,000 mph. A sequence of parachutes had to open and slow the probe for its two-hour-plus plunge to the moon's surface, instruments all the while taking images and measurements relayed by radio signal back to the Cassini orbiter which, in turn, would beam the signal back to Earth for arrival 67 minutes later.

"We knew that one of the riskiest parts was Huygens' complicated entry," Tomasko said. "Everything had to go like clockwork. Quite a few things could have gone wrong. And if things went wrong, the mission was over."

Tomasko had invested 17 years of his working career in the DISR, and most others on his team had invested a decade or more. They gambled big for high scientific stakes. Their instrument, funded by NASA and built by Lockheed Martin, was designed to study aerosols in Titan's atmosphere as well as photograph Titan's surface.

Huygens scientists assembled at the European Space Agency Operations Center (ESOC) in Darmstadt, Germany, during the probe's descent.

Early that Friday, Jan. 14, mission teams were euphoric. The Robert C. Byrd Telescope at the National Radio Astronomy Observatory in Green Bank, W. Va., had directly picked up the probe's 'carrier' signal shortly after the probe began its descent. That signal meant that Huygens had successfully ejected its back cover, deployed its main parachute, and was transmitting data when it began its plunge.

Hours later, euphoria turned to trepidation when researchers in mission control realized that power to one of the Cassini orbiter's two receiver channels, Channel A, was off. For six, panic-stricken minutes, scientists feared their mission might be lost.

"But then Channel B data starts coming, and coming, and coming," Tomasko said.

"I couldn't wait to get back to our team's portakabin, the trailer where we had our lab, to see the data. It's like it's Christmas, and all the data is wrapped up under the Christmas tree and you just want to tear the package wrappings off and see what you got after 17 years. And everybody on the team did that except ­ first I had to talk to the relatives."

Tomasko was waylaid by crews of media, pressing him for interviews, before he could join his team for a first look at results. More than 320 media from around the world stationed crews in Darmstadt during Huygens mission, and Tomasko was among those most in demand.

When Tomasko did get a first look at results later that night, he and his team realized that the probe had bounced around during a very rough descent ­ and that Titan's haze was much thicker and deeper than expected.

The world couldn't wait to see pictures of Titan, so the team worked around the clock, under enormous pressure, to process photos for the next morning's press conference.

"Putting the first pictures together was like putting together a jigsaw puzzle with half the pieces missing (those not recorded on the Channel A receiver)," Tomasko said. "And imagine that every time the probe tips or tilts during its rough ride, the shape of the picture changes."

"That first night experience was a phenomenon that you can't really control, one that you just have to live through the best you can," said co-investigator Bashar Rizk, head of the imaging team.

"And, the truth is, in the end it all came out," Rizk said. "It was wonderful. People got to see a whole new world. To see that look of genuine surprise on people's faces is something you don't often see, and something you don't soon forget."

Titan is about 746 million miles, or 1.2 billion kilometers, farther from the sun than is Earth. At 10 times the Earth-sun distance, only one-hundredth as much sunlight reaches Titan as Earth. Also, Titan's thick, orange, hydrocarbon smog atmosphere filters and scatters what feeble sunlight it does get.

The DISR's 20-watt landing lamp turned on 2,300 feet above touchdown, exactly as planned, replacing sunlight colors filtered by Titan's haze. The team wanted to see true colors reflected up from the ground to learn what the ground is made of.

The scientists expected their last frame before landing would look a bit like a high-altitude spotlight on the surface. Instead, they saw bright, fuzzy, diffuse light. "We think the landing lamp illuminated such a big area because its light shined off aerosols in the atmosphere even near ground level," co-investigator Lyn Doose said. That is, they discovered Titan's haze reaches all the way down to the ground.

Haze down to the ground explains why every DISR image was flat, so that team members had to heighten contrast to see Titan's surface features.

"That we saw anything at all is an indication of how well the camera worked," Tomasko said.

DISR's down-looking camera managed to photograph asphalt-black surface in about a thousandth of the light on Earth at noon, through haze that reached all the way to the ground. "The downward-looking camera is really busting its tail to pull anything out of that brightness distribution," Tomasko said.

One of DISR's exceptional features was its great dynamic range. The cameras were sensitive to increments of light on a scale between zero and 4,000, where zero is no light and 4,000 is fully saturated. Topographical features on Titan that would be invisible to other digital cameras popped into view. Commercial digital cameras, which commonly have a dynamic range from between zero and about 250, would have photographed nothing but haze. Your own eyes would have seen nothing but haze.

What's under the haze stunned everyone. It looked both familiar and peculiar.

"These images are other-worldly," DISR co-investigator Peter Smith said. "These look like something Chesley Bonestell would draw. You put a probe in, and it's like his drawings."

Tomasko said, "Rather than being an alien world, the images show Titan to be a member of the family of planets with solid surfaces and atmospheres, such as Earth and Mars. Many of the same geological processes that form Earth's surface are shaping Titan's surface. We see evidence for precipitation, erosion, mechanical abrasion and other fluvial activity."

With Titan temperatures hovering at minus 290 degrees Fahrenheit, water on Titan's surface is frozen rock hard. But methane ­ the gas that LPL founder Gerard Kuiper detected when he discovered Titan's atmosphere in 1944 ­ can remain liquid.

Huygens heated Titan's surface where it landed, and its science instruments detected outgassing methane. The evidence supports Tomasko's view that liquid methane rain forms a complex network of narrow drainage channels running from brighter highlands to lower, flatter dark areas. The channels merge into river systems running into lake beds resembling the Earth's.

DISR's surface picture also shows small, rounded pebbles in a dry riverbed. Spectra (color) measurements say the pebbles are water ice, not silicate rock. "The sand in the river beds is really ice chips," Tomasko said, "and the pebbles seen in the surface image are 'stones' of rounded ice cubes."

The dark material on Titan consists of particles of organic smog that have flaked like snow from the atmosphere. These "tholins," as they're called, blanket the land below, except where washed away by methane rain.

LPL scientist Ralph Lorenz and other members of Huygens' Surface Science Package team discovered that when the probe landed, it broke through a few inches of crust and sunk into underlying material with a consistency of wet sand. That's evidence that although Titan's rivers and lakes seem to be dry at the moment, rain must have fallen not long ago.

"Titan is the first place where we have evidence of liquid eroding the surface in both images and direct chemical analysis of the liquid itself," UA planetary sciences professor Jonathan Lunine, one of three 'interdisciplinary' scientists for Huygens, said.

"We have a lot to be thankful for," Tomasko said. "Titan is a beautiful world."