Lord of the Rings

At least for astronomical audiences, the ‘Lord of the Rings’ trilogy currently includes three missions: Pioneer, Voyager and an ambitious finale, the mission to land on Saturn’s moon, Titan, which premiers in 2005. This time next year, ESA’s Huygens spaceprobe will be descending through the atmosphere of Saturn’s largest moon, becoming the first spacecraft to land on a body in the outer Solar System.

Earlier this month, the giant ringed planet Saturn was closer to Earth than it will be for the next thirty years. All the planets orbit the Sun as if on a giant racetrack, travelling in the same direction but in different lanes.

The Huygens probe descends through Titan’s murky, brownish-orange atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter. The probe is equipped with a variety of scientific sensors to measure the physical properties of the moon’s atmosphere; it also carries an imaging device to return pictures of Titan’s possibly hydrocarbon-lake-dotted surface.
Credit: ESA

Those in the outer lanes have further to travel than those on the inside lanes. So, Earth regularly ‘laps’ the further planets. On New Year’s Eve 2003, Earth overtook Saturn, drawing closer than at any time in the next three decades.

Through a small telescope, Saturn is normally visible as a creamy yellow ‘star’. You may be able to see the ring system that the planet is famous for, and its largest moon Titan will show up as a tiny dot of light.

That tiny dot is the destination for ESA’s Huygens probe and may hold vital clues about how life began on Earth. Titan is the only moon with a thick atmosphere in the Solar System.

Astronomers think this atmosphere might closely match the one Earth possessed millions of years ago, before life began. Certainly Titan’s atmosphere is rich in carbon, the chemical necessary for life on Earth. What is more, this is all stored in ‘deep freeze’, ten times further from the Sun than the Earth.

Titan’s atmosphere, ten times as massive as Earth’s, is primarily nitrogen laced with such poisonous substances as methane and ethane. Titan is thickly veiled by a dense hydrocarbon haze that forms in the high stratosphere as atmospheric methane is destroyed by sunlight. The haze is much thicker than Earth’s worst city smog. It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s.

Scientists would like to know the origin of the atmospheric patches imaged on Saturn’s moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith

The big mystery is Titan’s surface, which is hidden by a cloud layer. This is why ESA built Huygens, to probe through this layer which is impenetrable by Earth-based observations.

In January 2005, Huygens will parachute below the clouds to see what is really going on. Its battery of instruments will return over 1000 images as it floats down and samples the chemistry of this exotic place.

Titan’s haze
was first breached by the Hubble telescope, scanning in the near infrared wavelengths, in 1994. Surface features included a bright spot the size of Australia.

In October, radar observations from the world’s largest radio telescope at Arecibo [Radar Evidence for Liquid Surfaces on Titan], detected specular — or mirrorlike — glints from Titan with properties that are consistent with liquid hydrocarbon surfaces.

Cornell astronomer Donald Campbell, who led the Arecibo radar observation team, does not rule out that the reflections could be from very smooth solid surfaces. “The surface of Titan is one of the last unstudied parcels of real estate in the solar system, and we really know very little about it,” he said. Titan, with a diameter about two-fifths that of Earth, ranks second largest of all the solar system’s moons. Only Jupiter’s moon Ganymede is larger. The atmospheric pressure near its surface is 60 percent greater than on Earth at sea level.

The Titan probe was named Huygens in honor of the Dutch astronomer who discovered Titan in 1655. Launched in October 1997, Huygens is currently in space, hitching a ride on NASA’s Cassini spacecraft.

What’s Next

European missions planned in 2004 include the launch of Rosetta to the comet Churyumov-Gerasimenko in February, the mission of ESA astronaut Andre Kuipers to the International Space Station in April, and the European Space Agency’s (ESA) first Earth Explorer mission Cryosat, planned for the autumn. ESA’s ongoing Aurora Programme, launched two years ago, is studying the technologies needed for robotic and human missions to the Moon and to Mars. Launched in June last year, Mars Express has now been placed into its final orbit around Mars and everything so far is going according to plan. The first high-resolution images will be released next week.

The Cassini spacecraft itself is slated to arrive at Saturn in July 2004, and it will look closely at the competing processes of destruction and re-capture in Saturn’s rings to confirm and quantify whether small moons sweep clean portions of the outer ring (or F ring) using their own gravity. Some explanations of ring formation propose that they are debris piles of rubble left over and shaped by tiny moons. Some of the fragments that make up the rings may be re-accreted instead of being ground to dust. New evidence shows some debris has accumulated into moons or moonlets rather than disappearing through collisional erosion. The F Ring in particular was discovered using data from NASA’s Voyager 2 mission to the outer planets launched in 1978.

The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life’s door if liquid water makes an occasional appearance on Titan– which some scientists believe it may well do.

Studies of Titan so far have indicated enough evidence for both temporal and spatial variability, two signatures required for the presence of organic molecules. The by-products of methane molecules destroyed in the sun’s ultraviolet light react with other molecules in Titan’s atmosphere, forming organic droplets and particulates that fall onto the moon’s surface, blanketing the icy bedrock and forming lakes and oceans.

So look forward to seeing more of Saturn and a tiny European spacecraft called Huygens, that in one year’s time will make an historic landing in the quest to uncover the origins of life.

Related Web Pages

Titan’s Icy Bedrock
Saturn– JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame

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