Composite images of Saturn's rings, taken by NASA's Hubble Space Telescope, have revealed mysterious color variations that hint that the rings could be made of materials from the outer solar system. These new findings are important because scientists have long questioned whether the rings originated around Saturn, like the planet's retinue of icy moons, or elsewhere.
The Hubble images, captured by a team of scientists between 1996 and 2000, show Saturn's rings from beneath in a wide-open or "tilted" viewpoint from Earth, as the planet's Northern Hemisphere swings from autumn toward winter. When seen edge-on, Saturn's rings, which are only some tens of meters thick, nearly disappear from view.
"The color of the ring material can help tell us what the rings are made of and will help decipher their origin," said Dr. Jeff Cuzzi, of NASA's Ames Research Center and a member of the Hubble team. The color variations indicate that different materials make up the rings. The distribution of the materials provides information about the processes that shaped the rings, he explained.
"Most people don't know that Saturn's rings aren't white but have a faint salmon color, which hints that a few percent of complex organic molecules are mixed in with the water ice the rings are mostly made of," Cuzzi said.
Saturn's seven small icy moons don't have such a reddish color, but many icy objects in the frozen reaches of the outer solar system do, he explained. This leads scientists to suspect that, unlike the moons, the rings were formed from an outer solar system object.
This object, they think, careened too close to Saturn and -- like comet Shoemaker-Levy 9 in 1994 -- was torn apart by the massive planet's gravity, leaving a trail of debris.
Over 100 Hubble images were analyzed in eight different colors that cover, and go beyond, the range of human vision. They include violet, blue, green and red in the visible range and ultra-violet and infrared in the non-visible range.
Cuzzi also has shown that there appear to be at least two unknown materials mixed with the rings' water ice, and that the way these materials are distributed in the rings is unlike anything seen on the surfaces of nearby planets or satellites.
For example, in some rings the color gets redder closer to Saturn, and in others, the color trend reverses in the middle of the ring. In some places, the color gets redder where the concentration of particles increases, and in other places it gets redder where the concentration of particles decreases.
Scientists hope to explain these variations in terms of how the composition of the ring material was initially distributed, and how it has evolved with time.
The Hubble data also show that the ring color changes with viewing angle. The best explanation, said Dr. Francois Poulet of NASA Ames, who is working with Cuzzi, is that the ring particles are actually lumpy aggregates of particles, with many more deep shadows than the relatively smooth surface of a moon or asteroid.
The lit parts of the "lumpy" surface partly illuminate the shadows with their reddish color, so the rings appear redder as more shadows are seen.
Cuzzi believes that observations like these, while not currently understood, eventually will provide insights into the processes by which Saturn's ring structure was formed and continues to evolve.
He noted that the Cassini spacecraft, which recently passed Jupiter en route to a four-year tour of the Saturn system starting in July 2004, carries several instruments that will provide much finer detail and greatly improve the ability to identify these non-icy constituents in the rings and the structure of the particles.
The Cassini mission also includes an atmospheric entry probe into the organic smog-shrouded moon Titan, which might have lakes or seas of liquid ethane on its frigid surface.
Cuzzi's collaborators include Drs. R. French of Wellesley College, L. Dones of Southwest Research Institute, Mark Showalter of Stanford University, and Paul Estrada of Cornell University.
HST Snaps of Saturn: 1996-2000
Space Telescope Science Institute
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RINGWORLDCassini's Tour de Saturn
Cameron Park - May 25, 2001
In my last article on the Cassini Saturn probe, I described the first of the four "phases" of its four-year orbital tour of Saturn and its rings and moons, during which it will use a constant sequence of gravity-assist flybys of Titan - 44 flybys during its 74 orbits around Saturn during those four years - to keep radically modifying its orbit to study different parts of the Saturn system. Each phase represents a period in which Cassini will carry out orbital maneuvers of a different general type to make a particular kind of scientific observation.
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