Scientists marvel at photos
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 1, 2004
Making gravity visible, close-up images of Saturn's rings shot by NASA's newly arrived Cassini probe revealed an intricate, never-before-seen tapestry of icy particles herded into spiralling density waves by the effects of nearby moons.
"I don't think you have to be a ring scientist to imagine what last night was like to us," she said of the spacecraft's arrival in Saturn orbit and the initial batch of ring pictures beamed back to Earth early today. "It was beyond description, it was mind blowing, it was every adjective you could think of.
"Even though we've had a long time to think about our images ... I'm surprised at how surprised I am at the beauty and the clarity of these images. They are shocking to me. You are going to see some images now, they were so shocking I thought my team here was playing tricks on me and showing me a simulation of the rings and not the rings themselves. It's just utterly remarkable."
Cassini snapped 61 black-and-white pictures of Saturn's rings early today after completing a 96-minute rocket firing to brake into orbit around the ringed planet. Program manager Bob Mitchell reported this afternoon that engineering data radioed back from Cassini shows the spacecraft survived two ring plane crossings without incident and that all of its myriad subsystems were in good health and operating normally.
Cassini skimmed over the top of the rings as it braked into orbit and shortly after main engine shut down, the spacecraft began carrying out commands to photograph the rings, first from the upper backlit side and then from below, where the thin disk of icy particles was bathed in direct sunlight.
Because of Cassini's enormous velocity - 60,000 mph or so at engine cutoff - its cameras were programmed with shutter speeds of five thousandths of a second to prevent blurring. In the minute required to snap a picture, record the data and be ready for another shot, Cassini moved hundreds of miles, preventing researchers from taking overlapping photos or the multiple images required for color.
But no one was complaining.
Cassini will never again fly so close to the rings and the level of detail the craft's cameras captured was stunning. If there was a central theme to the pictures it was the ubiquitous presence of density waves, regions of alternating brightness and darkness that look like ripples in fine sand. The spacing of the ripples, caused by gravitational interactions with nearby moons, decreases as one moves outward from the planet.
"This is a telltale sign of a density wave, the wavelengths decreases as you go outward and also the amplitude of the wave damps so you see it disappear," Porco said, describing one picture. "These are characteristics ring scientists read like a book to discern what kind of properties the particles have, how densely they're packed and so on. As I said, this is unprecedented resolution for the imaging experiment."
One image showed a density wave thinning out to the right and a so-called bending wave moving to the left across the field of view.
In a bending wave, "it's not the number density of particles that is varying, it is literally the height of the ring plane," Porco said. "You can think of the feature on the right as being like corrugated cardboard where the ring is literally warped and its warped because the moons which are exciting that particular wave excite inclination (tilt) in the particle orbits and the particle orbits get phased in such a way that it forms this pattern, which in fact is a spiral pattern.
"If you followed it around the rings, it would take the spiral form," she said. "These are similar to the spiral arms of spiral galaxies."
Describing a blow up of a density wave image, Porco pointed out strange looking structures that "almost looks like straw. I don't know what this is. We think it's real, we see it in other images. ... So it's not some noise pattern in the image.
"There may be processes going on that make the particles clump on scales that you're seeing here. ... Nonetheless we're seeing something here and I literally don't have a clue. This may be brand new, something no one's ever predicted before."
The picture Porco initially thought was a joke was focused on a gap in the outer A ring known as the Encke division, a narrow void swept out by the tiny moon Pan. Along with showing ultra clear views of spiralling density waves on both sides of the gap, the ring material forming the inner edge had a sharply scalloped appearance. Even to the layman, the picture appeared unusual.
"This is like textbook physics, textbook ring physics right there in one image," she said.
At a news conference Porco was asked why the study of Saturn's rings was important.
"This is standard ring lore, that Saturn's rings especially are our closest analogue of the celestial disk system," she began. "Frank Shu, an astrophysicist, said this many years ago: there are two types of bodies in the universe. There are spheres and there are disks. And under certain circumstances, a sphere can collapse down into a disk and that's what will happen if you have a spherical cloud of debris and the particles are colliding, they lose energy but they preserve angular momentum and they all end up in a plane. That's a very common process and its given rise to lots of disk system.
"One is Saturn's rings, one was the solar nebula out of which our solar system and the planets formed. Astronomers now see lots of disks around other stars and even reaching way far out in size to the spiral galaxies, they are another disk system. Common physics applies to all of them.
"So in studying rings, we hope to study processes that go on in disks in general," Porco said. "And so we think we're seeing in Saturn's rings some of the processes that went on in the solar nebula before the planets formed. In fact, we may be seeing some of the processes that actually aided the development of the planets."
If one is interested in "understanding where the solar system came from or how it got here, how the planets were formed, then this is the place to go."
Ed Weiler, an astronomer by training who serves as NASA's associate administrator for space flight, offered another reason to study Saturn and its rings.
"When I was growing up, this kind of stuff was science fiction," he said. "We compete with a lot of things: Game Boys, X-Boxes and Play Stations. This isn't science fiction, we actually did this. We're in orbit around another planet taking these kinds of pictures with an incredible machine. We did this. This isn't animation, this isn't PowerPoint, this is real. I like data, and this is real data.
"So I hope we can excite at least a few more kids in this country to become scientists and engineers. If we can do that, it was worth every penny we spent on it."