Lure of the rings

As the Cassini orbiter enters Saturn's 'city limits,' scientists hope to uncover secrets of the planet and its moon Titan.

It's an expedition Galileo Galilei might have given his beard to join. Nearly 500 years after the Renaissance genius first trained his crude telescope on Saturn, some 300 scientists from 17 countries are set to explore the planet, its spectacular ring system, and its moons with a $3.4 billion orbiter named Cassini and its Huygens probe.

Next Tuesday, Cassini enters Saturn's "city limits" - the gravitational boundary where the ringed planet tugs at the craft with more strength than the sun. The transition marks the final leg of Cassini's seven-year, 3.45 billion-kilometer trip.

On July 1, the craft begins orbiting Saturn. Six months later it is slated to release Huygens. The probe will parachute to the surface of the moon Titan to give scientists an unprecedented look at a body whose atmosphere resembles a young Earth's and whose surface is thought to be oozing with organic compounds held to be among the chemical building blocks of life.

As the most sophisticated, scientifically ambitious civilian space probe ever launched, Cassini "will allow us to study this magnificent system in excruciating detail," says Carolyn Porco, who heads the mission's imaging team. What's more, it may have a universal story to tell.

The Saturnian system resembles a solar-system wannabe frozen in a state of arrested development, she says. Like Jupiter, Saturn is a giant ball of gas that never gained enough mass to heat its interior to the point of igniting. Saturn's rings - chunks of ice and rock - may be relics of the early disk of dust and gas that surrounded the solar system's young sun some 4.6 billion years ago. They also resemble the protoplanetary disks seen around other stars. And Saturn's inner moons are plowing gaps in the rings, similar to the gaps seen in the dusty, gaseous disks detected around other stars. From such gaps astronomers have inferred the presence of planets around these stars.

Thus, an intimate study of processes at Saturn could help researchers interpret what they see in fledgling solar systems around nearby stars.

"As a planetary system, Saturn holds the greatest promise for answering questions that have a far broader scientific reach than Saturn itself," Dr. Porco says.

Since February, Cassini's cameras have been trained on the planet to provide colleagues and the public with visual appetizers. A team led by John Clarke at Boston University has been using the Hubble Space Telescope in tandem with Cassini's instruments to tease out details of the planet's "space weather" - the interaction of its magnetic fields with charged particles coming from the sun, known as solar wind. The results of their work are slated to be published this summer.

Yet the mission is so complex that team members have been spending the lion's share of their time refining the craft's observing program and the software to execute it. This has left little time to do more than cast an appreciative glance at the images coming in. Deep study comes later.

Cassini's 'crown jewel'

Astronomers also have been intensifying their focus on Saturn's moon Titan, which Goddard Space Flight Institute astrophysicist Michael Flasar dubs "the jewel in Cassini's crown." Earlier this month, Cassini beamed back its first images of Titan. They show a surface that appears to vary greatly.

During Cassini's baseline mission of orbiting Saturn, set to last four years, Titan is drawing more fly-bys than any other object save Saturn itself. First discovered by Dutch astronomer Christiaan Huygens in 1655, Titan is the only moon in the solar system with an atmosphere. And it's slightly larger than Mercury, which probably would earn it the title of planet if it were orbiting the sun on its own.

Over the years, research has suggested that "on Titan, you have kind of a deranged version of Earth," says Caitlin Griffith, an astronomer at the University of Arizona's Lunar and Planetary Laboratory in Tucson. It boasts a nitrogen-based atmosphere, as Earth does. The atmosphere's surface pressure is thought to be similar to Earth's. But where the balance of Earth's atmosphere is mostly oxygen, Titan hosts organic compounds, such as methane. It appears that methane, a heat-trapping "greenhouse gas," may be hoarding what faint heat Titan receives from the sun to help drive weather patterns.

Those patterns remained shrouded in a hazy hydrocarbon "smog" at the top of the atmosphere until four years ago. In April 2000, Dr. Griffith and two colleagues published observations of Titan's atmosphere using a near-infrared camera on the eight-meter Gemini Telescope on Hawaii's Mauna Kea. The results suggested that Titan has an active weather system with cloud cover that varies daily. The data also implied that it had convective storms that rained methane and other chemicals capable of condensing under Titan's conditions. Much of that weather seems to concentrate near the moon's south pole.

Ground-based observations that are stretching the limits of earthbound technology also have been teasing out clues regarding the moon's surface. Notions of a body covered with a vast ocean of liquid methane or surface features blanketed with organic "goo" up to 800 meters thick are giving way to a more complex picture.

Last spring, Griffith and colleagues reported additional telescope observations that pointed to evidence of ice on the surface. This suggested that vast expanses of the moon's "bedrock" of ice and rock were exposed at the surface. Then in October, a team led by Cornell University astronomer Donald Campbell reported the discovery of radar returns from Titan's surface that appeared to have been reflected off smooth surfaces. Signals seemed to bounce off broad seas or lakes of liquid hydrocarbons, or perhaps ice. If it's ice, he says, it's so reflective that "it's as if someone has driven over it with a Zamboni."

Exploring the surface

Settling some of these issues will fall to the Huygens probe, built by the European Space Agency. It will carry six instruments designed to analyze the composition of the atmosphere and photograph its features. It also carries instruments to determine the properties of the surface at its landing site.

"We really want to understand the chemistry that's going on," Griffith says.

She says the late Carl Sagan conducted experiments in which he filled a vessel with nitrogen and methane in abundances similar to those found in Titan's atmosphere. "Then he zapped it" with charged particles, she says. "They found that there was orange stuff that collected on the side of the vessel. They analyzed it and found it contained amino acids. So there's some possibility that there's some very interesting building-block molecules for life."

Beyond all the expectations for Cassini at Saturn, it's already proven its mettle in salvaging research at Jupiter. In December 2000, it flew within 9.72 million kilometers of Jupiter. From its vantage point, it snapped some 26,000 images, many of which will be used to analyze Jupiter's weather patterns in ways the Galileo orbiter couldn't. [Editor's note: The original version misstated how close Cassini came to Jupiter.]

One piece of conventional wisdom the images overturned involved regions where Jupiter's atmosphere was rising and subsiding. White bands were held to be the tops of convective storm clouds, while darker bands were thought to be regions of intense downdrafts. In fact, says Porco, it was the opposite. "We found the heftiest storms in the dark bands," she says.

Exactly what that says about the Jovian weather and its energy sources will have to wait for an unlikely break in preparations for Cassini's arrival, she sighs.

Secrets of Saturn

• Saturn is the second-largest planet in our solar system, next to Jupiter. If you could line them up, more than nine Earths would fit across Saturn.

• The average distance from Earth to Saturn is more than 800 million miles. However, the Cassini orbiter used gravity assists from planets and has not flown a direct route to Saturn. It will have traveled about 2 billion miles by the time it reaches Saturn.

• The planet consists mostly of hydrogen and helium. While it has heavier materials in the core, it has no hard surface. It could float in water.

• Saturn's main rings could cover almost the entire distance from Earth to the moon, yet they are less than a half mile thick.

• Scientists have found 31 moons orbiting Saturn. Titan is its largest moon and the second-largest moon in the solar system, bigger than Mercury and Pluto.

Source: NASA website

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