Lure of the rings

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."

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.

• 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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