RINGS AROUND CASSINI
By Mike Rogers

Astronomer Carolyn Porco, PhD ’83, who did her PhD research at Caltech on Saturn and its rings, now heads the imaging team for Cassini, which went into orbit around the ringed planet in late June.

After traveling nearly 2.2 billion miles since its launch on October 15, 1997, the Cassini spacecraft slipped through the F and G rings of Saturn on June 30, successfully entered orbit around the sixth planet from the sun, and began sending back pictures that astounded and surprised an international team of investigators.

“The images are mind-blowing,” said Carolyn Porco, PhD ’83, who leads the imaging team from NASA, the European Space Agency, and the Italian Space Agency, which joined forces to send the bus-sized spacecraft to the sun’s second-largest planet at a cost of approximately $3.3 billion. At a press conference at the Jet Propulsion Laboratory, which is managing the mission, Porco said that the level of detail and clarity in the images of the rings was better than she expected.

Above, the image of Saturn’s ring system, taken with the spacecraft’s Ultraviolet Imaging Spectrograph, suggests that there is more ice toward the outer part of the rings than in the inner part, offering possible clues to the rings’ origins and evolution.

“Even though we had a long time to plan, I’m surprised at how surprised I am at the images,” said Porco, who analyzed images of Saturn and its rings taken by Voyager 1 in 1980 while she was a Caltech graduate student. The Cassini images were so shockingly clear, Porco said, “that I thought the team was showing me simulations of the rings and not the actual rings themselves.”

Porco said that she was particularly surprised by a close-up that showed an unexpected straw-like pattern in one region and another image that revealed startlingly detailed structure on the edge of a gap. “We’re seeing structures that have never been imaged before,” said Porco, an adjunct professor at the University of Arizona in Tucson and the University of Colorado in Boulder, and director of the Cassini Imaging Central Laboratory for Operations (CICLOPS). “These images will help us determine the properties of the rings and the particles in them, and will vastly improve our theories of how they behave.”

The last time that spacecraft took pictures of Saturn was more than 20 years ago during the Voyager 1 and Voyager 2 flybys in 1980 and 1981, respectively. Porco said that Cassini’s imaging instrument provides “inherently 30 percent greater resolution,” can photograph larger areas from the same distance, and can discriminate finer contrasts than the Voyager cameras. “And because Cassini is very much closer to the rings than spacecraft have ever been before, overall Cassini ring images are five times more detailed than those returned by Voyager.”

The beauty of Saturn’s rings—which are actually particles of ice contaminated by bits of rock—has captivated the human imagination since the 17th-century Dutch astronomer Christiaan Huygens first saw them, along with Saturn’s enigmatic moon, Titan, through his telescope in the mid-1650s. (Galileo had actually glimpsed the rings roughly 50 years earlier when he was the first to look at Saturn through a telescope, but because of inferior optics he had been unable to determine exactly what he was seeing.) Today, astronomers and planetary scientists believe that the dynamics of Saturn and its ring system may serve as a model for the pre-planetary nebula—the disc of gas and dust around the young sun that ultimately formed the planets.

By studying the new pictures of the rings and comparing these with the images taken by the Voyagers, scientists “hope to study the processes that cause all the observable structure, and to see how the rings have changed, if at all, with an eye towards figuring out their rate of evolution and their age,” Porco said. “The mission may help us understand the processes that occurred in the solar nebula before planets were formed, which may have aided in the formation of planets.”

During its four-year mission, Cassini will circle Saturn approximately once every one to four months, in all snapping about 300,000 pictures of the planet, its moons, and rings. The mission is named after the French-Italian astronomer Jean-Dominique Cassini (1625–1712), who discovered four moons around Saturn, as well as a gap in the planet’s ring system.

Although Saturn’s rings have always fascinated amateur and professional sky watchers alike, many consider the real prize of the Cassini mission to be Titan—the largest of Saturn’s 31 known moons—which has an atmosphere rich in organic material. Cassini is carrying along a probe, named for Huygens, which will be released in December before heading down to Titan in January.

Titan appears to be the only moon in the solar system with a significant atmosphere, but its surface remains largely shrouded in orange haze. As Huygens parachutes in, it will analyze the moon’s weather and its clouds, and will look for lightning. Special instruments will examine Titan’s surface after Huygens lands, assuming it survives the landing.

One scientist anxiously awaiting the Titan rendezvous is JPL director Charles Elachi, PhD ’71, an imaging radar expert who heads the Cassini Titan radar team.

“I’m really interested in the surface of Titan,” said Elachi. “Smaller than Mars but bigger than Mercury, it’s a world in itself. This mission will give us a look through the haze for the first time. Some people think we’ll see oceans of methane and methane rain.” Elachi, who has been in charge of the Cassini Titan radar team since 1990, said, “I’ve moved on to other things since then, but with Cassini, I kept the fun part.”