A Ringside Seat Reveals Unexpected Phenomena
Safely in orbit around Saturn, NASA’s Cassini spacecraft sent back its first close-up images of the massive planet’s rings Thursday, revealing an unexpectedly varied terrain featuring surprisingly sharp edges, braids and delicate ridges.
“We’re seeing phenomena today that have never been imaged before,” said Ed Weiler, NASA’s associate administrator. After seven years of traveling to reach Saturn, “now the science is going to start.”
Voyager flew part of the same route through the rings in 1981, but the cameras on Cassini have five times the resolution of Voyager’s, said imaging team leader Carolyn Porco of the Space Science Institute in Boulder, Colo.
“The beauty and clarity of these images are so shocking that I thought at first that my team was playing tricks on me, showing me simulations” rather than real pictures, she said.
The school bus-sized spacecraft flashed through the gap between two of Saturn’s rings, known as F and G, Wednesday evening, fired its rocket motor for 95 minutes to slow the craft just enough for it to be captured by Saturn’s gravity, then flew once more through the gap in the rings, taking black-and-white pictures all along the way.
Cassini will never be as close to the rings again, so the 61 images beamed 900 million miles back to Earth by the craft after its crossing offer a treasure trove of information about the origin, structure and dynamics of the rings. “Scientists will have a field day with this,” Porco said.
Astronomers are eager to study Saturn’s ring system because it is a nearly perfect model of the processes that produced our solar system and can be seen producing similar systems around distant stars.
Cosmologists think that our sun was once surrounded by a gigantic ball of dust that collapsed first into a series of rings and eventually into planets.
Understanding how Saturn’s rings formed and are maintained will thus provide insight into how the solar system developed. If we want to understand our own celestial history, “this is the place to be,” Weiler said.
Cassini, which was launched in 1997, is scheduled to spend at least four years in the Saturnian system studying the planet itself, its rings and its 31 known moons, especially the largest, Titan. Researchers hope that data from the mission will lend new insights into the formation of the solar system and perhaps even the origins of life on Earth.
About half of the pictures were taken from the sunlit side of the rings and half from the back side. The latter are like photographic negatives of the former. In the images taken from the sunlit side, the rings -- ice and rock ranging from house-sized boulders to specks of dust -- are bright, looking much like they do when seen from Earth, albeit in much greater detail.
But in the images taken from the opposite side of the ring plane, dark bands in the images occur where the rings block sunlight and bright bands occur where there are just enough particles to scatter light.
The images were more detailed than scientists expected. “They can be read like a book telling us what kinds of properties the particles have and how densely they are packed,” Porco said.
The photos reveal two kinds of waves in the rings, both produced by interaction with Saturn’s tiny moons. Density waves, which are usually concentric circles spreading farther apart and fading as they recede from the moons, are produced by clumping of the particles of ice and rock in small, thin bands, leaving minute gaps between them.
The so-called bending waves are spiral in structure and extend outward perpendicular to the plane of the rings, giving the rings the appearance of corrugated cardboard. Bending waves are similar to the arms of spiral galaxies, Porco said.
Other pictures showed delicately braided rings and scalloped edges caused by a tiny moon orbiting in the ring gap. Repeated passage of the moon “forces eccentricities in the orbits of particles on the edge of the ring ... building up beautiful, classic sinusoidal patterns,” Porco said. “This is textbook ring physics.... I can’t describe how exciting this is for us.”
Some images left astronomers scratching their heads.
One photo of the thin band known as the A ring shows an unexpected structure that Porco said looked a bit like straw. “We’re seeing something here and we literally don’t have a clue about what it is,” she said. “Maybe it is something no one has ever predicted before.”
Researchers hope that close examination will reveal how old Saturn’s rings are and how long they will persist. Most research suggests that they are no older than a few hundred million years, Porco said.
Cassini itself behaved perfectly during the orbital insertion, said project manager Robert Mitchell. “There is not a single red alarm, not a single indication of any faults,” he said.
The craft entered a broad, looping orbit that will take 116 days to complete, almost exactly what the team had planned, said navigation team chief Jeremy Jones.
On its second orbit, the craft will fly by Titan and is expected to get a gravitational tug that will tighten the orbit and reduce its length to 60 days. A second passage will reduce the orbital period to 32 days, eventually putting the craft into an orbit in which it will repeatedly fly by Titan.
That orbit is required for Cassini to release the European-built Huygens probe on Christmas Eve.
Three weeks later, Huygens will plummet to the surface of Titan, thought to be the only moon in the solar system with a dense atmosphere. Titan is rich in organic compounds, and researchers think its conditions are similar to Earth’s before life evolved.
Over the next four years, Cassini will take an estimated 300,000 photos of the Saturnian system. With its nuclear power plant, the craft could continue its mission for as long as 15 years.
The $3.3-billion mission is a joint project of NASA, the European Space Agency and the Italian Space Agency.
