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Saturn's ring found to be biggest in solar system

This graphic shows the intricate relationships between Saturn's immense ring, its source satellite Phoebe, and the embedded satellite Iapetus. The dark face of Iapetus always leads its motion and, like a car's windshield, collects all of the debris in its path. NASA/JPL/Space Science Institute.

When it was discovered in 2009, scientists knew the newest ring around Saturn was big.

Now, they can say just how large. It's the biggest in the solar system.

In a study Wednesday in Nature, scientists concluded that the faint ring known as the Phoebe ring is 270 times the size of Saturn and over 10 times larger that Saturn's E ring, which is the second largest ring in the solar system.

"For the first time ever, we have the ring image in its entirety," said University of Maryland's Douglas Hamilton, who along with University of Virginia's Michael Skrutskie and CalTech's Anne Verbiscer discovered the ring. CalTech's Frank Masci contributed to the latest Nature paper.

"We know how big this ring is now and we didn't before," he said. "The outer edge is starting to give us interesting information about what is happening inside the ring, the forces and processes that make it function."

Here Saturn's Phoebe ring is seen in its entirety for the first time. The bright white and black central regions of the image are due to scattered light from Saturn. The ring itself is the faint horizontal rectangular structure extending outward past the embedded source moon Phoebe (bright white dot at the 9 o'clock position) and on to nearly 300 time Saturn's radius, a distance of over 10 million miles. NASA/U. Virginia

Analyzing new images from NASA's WISE spacecraft, Hamilton and his colleagues found that the planet's outermost ring is made mostly of small dust particles of around 10 microns in size or one-fifth the width of a human hair. In contrast, rocks that are soccer-ball-sized or larger (at least 10 cm) only make up at most 10 percent of this ring.

"I constructed theoretical models for how the dust particles of different sizes would move and then I constructed artificial rings. What would the ring look like if it were made only of large particles? What would it look like with only small particles? What about a mixture?" he said. "The data is good enough to constrain those models so we rule out the possibility of the ring being composed entirely of large particles. The smallest ones seem to be responsible for something like 90 percent of the light or more."

This new ring lies at the far reaches of the Saturnian system, with an orbit tilted 27 degrees from the main ring plane. The bulk of its material starts about 3.7 million miles away from the planet and extends outward roughly 7.4 million miles. It would take about one billion Earths stacked together to fill the voluminous ring.

Hamilton and his colleagues believe the material for the ring is coming from one of Saturn's farthest moons, Phoebe - hence the ring's name.

And with more details on the particles, the researchers have further proof that this new ring is the reason Saturn's moon's Iapetus is dark on one side and bright on the other. The astronomer Giovanni Cassini first spotted the moon in 1671, and years later figured out it has a dark side, now named Cassini Regio in his honor.

An artists' conception of how Saturn's immense Phoebe ring might appear to eyes sensitive to the infrared wavelengths at which the ring was discovered. A vast and diffuse annulus of dusty material surrounds tiny Saturn and is set against a background starfield lit by faint nebular luminosity. NASA/JPL/Space Science Institute

"If the particles are small, that means they drift inwards toward the moon Iapetus rapidly. That tells us that today the material is being created, drifting inward and striking the face of the Iapetus," he said.

"We believe the black face is being painted by these particles coming from Phoebe," he continued. "By discovering that they are so small, we know this process is happening today. It is not something that happened billions of years ago and shut off and we are seeing the remnant of that."

The Phoebe ring brings to eight the number of named rings encircling Saturn. The main rings nearest to Saturn are comprised of icy particles - some the sizes of houses - that exist in a croweded space where collisions happen almost on an hourly basis. The particles thus are short lived.

Further out is the E ring, which is composed of tiny dust grains made up of ice and only 1 micron in size. The ring is supplied by geysers from the moon Enceladus and the grains hang around for hundreds of years.

And then you have the Phoebe ring.

"The Phoebe ring is absolutely immense and so it's huge wide open spaces and very few particles," Hamilton said. "What this means is the collision times are really, really long so the lifetime of these particles can be millions of years or billions of years depending on the sizes."

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