24 October 2012

Snowshoe hares and Saturn’s rings

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The evolving understanding of just what lies within Saturn's giant disk.
Saturn rings NASA

In this simulated image of Saturn's rings, colour is used to present information about ring particle sizes in different regions based on the measured effects of three radio signals. Credit: NASA/JPL

Artists’ renderings of Saturn’s rings tend to show swarms of boulders in wide, thin bands. And to a large extent, that’s probably still a correct vision. But as scientists look closer at the ‘particles’ composing the rings, the picture that emerges is becoming ever more complex.

Instead of simply being a bunch of ice blocks, these particles may be more like mini-planets – or at least mini-asteroids – complete with regolith crusts. And they themselves, may be capable of amalgamating into short-lived clumps many times thicker than the rings as a whole.

This new, still-tentative, picture stems largely from efforts to determine the size of the particles. Thanks to the Cassini spacecraft now orbiting Saturn, there are several ways to do this, none perfect.

One method looks at how the rings interfere with the spacecraft’s radio signal when Cassini is behind the rings. Another uses starlight passing through the rings – which are not dense enough to block the light completely – as a probe for particle density and size. Big particles would make the light flicker randomly as they randomly pass in front of the star. Small ones produce a more consistent dimming of the light.

Other instruments use infrared or ultraviolet light. And one enterprising researcher, Tracy Becker, a graduate student at the University of Central Florida, used the blurring (technically known as a diffusion pattern) from accidentally misaligned instruments to estimate particle size in one part of Saturn’s F ring when the rings were backlit by the Sun.

Each of these methods, however, focuses on particles of a certain size. That’s because, just as radio waves on Earth pass easily through even heavy rainclouds while light does not, electromagnetic radiation interacts poorly with particles smaller than the wavelength of the radiation itself.

Thus, Cassini’s signals are perfect for looking for particles in the centimetre to metre size range, but not much use for looking for smaller ones. Not that this isn’t itself useful. Using this method on Saturn’s C ring, Essam Marouf of San Jose State University reported at a meeting of the American Astronomical Society’s Division for Planetary Sciences, in Nevada, that the largest particles were six to nine metres, depending on which part of the ring he examined. But at the same meeting, Ryuji Morishima of NASA’s Jet Propulsion Laboratory in Pasadena, California, used infrared data to conclude that the maximum ‘grain size’ in the rings is between one and ten centimetres.

“If all the ring particles had one defined size – if they were all just big bowling balls, which is probably on average what they are – it would be pretty simple to figure out how big they are and how densely they are packed,” says Philip Nicholson of Cornell University, Ithaca, New York.

But instead, what’s happening is a lot like the proverbial blind men and the elephant. “When we look over many orders of magnitude in wave length, we tend to see different things,” Nicholson says.

The traditional answer is that the rings are composed of a wide range of particle sizes. But maybe it’s a bit more complex. The emerging model, Nicholson says, is that the particles are composed of icy cores covered with layers of smaller grains, much like the loose regolith found asteroids, the Moon, and many other bodies. It was the size of these grains, in fact, that Morishima’s experiment was designed to detect. In the infrared measurements he used to study them, they would look identical whether they were laying on objects the size of bowling balls or of school buses.

The exact size of these grains is still uncertain, although studies like Morishima’s and Becker’s puts them somewhere between dust motes and marbles. The source of the grains is unclear, but they may simply come from eons of slow-speed collisions between the larger particles.

“These particles are packed like the chairs in this room,” Nicholson said, motioning to the conference hall in which he was speaking. “They’re pretty densely packed.”

The particles mostly orbit at the same speed, but even in the most sedate portions of the rings, they bump into each other every few hours, Nicholson says. Even if the relative speeds of these collisions are only a few millimetres per second, that might be enough, over time, to “crunch up” the surface, he suggests.

Evidence for this comes from Larry Esposito, a planetary scientist at the University of Colorado, Boulder, who has studied interaction between large particles, clumps of large particles, and the grains.

At certain places in the rings, he reported at the planetary sciences meeting, the particles are orbiting in ‘resonance’ with one of Saturn’s inner moons, such as Janus or Mimas – with the ring particles making, for example, six orbits for every five orbits of the moon.

What this means is that with each orbit, particles in these special zones reach places where the moon’s gravity alternately damps down their relative motions or stirs them up.

In the damped-down zones, their collisions are super-gentle, and they tend to stick together, building up into clumps that can be a kilometre or more in diameter – big enough to poke above the plane of the rings and leave highly elongated shadows when the Sun illuminates the rings edge-on. (These shadows have indeed been photographed.)

In the stirred-up regions, however, the clumps are battered enough that particles are knocked away from the clump. “They hit each other pretty hard,” Esposito said. In addition, regolith grains are knocked off the colliding particles, producing bright clouds known as halos that show up in Cassini’s highest-resolution images.

Esposito compares the cyclical growth and decline of the clumps to a boom/bust economic cycle or the famous predator/prey interactions of organisms like snowshoe hares and arctic foxes. But the amazing thing, he says is that it happens quickly – with a full cycle taking about 10 hours. In the process of that short time, he says, the mass of the clumps can change by a factor of ten – something that can be verified with instrument sweeps of the clump-forming parts of the rings.

When you think about it, it’s rather stunning. “The rings are billions of years old,” Esposito says, “but nonetheless, some of these things are changing before our eyes, in hours.”

Richard A. Lovett is a science writer based in Portland, Oregon, USA.
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