What scientists may learn from Saturn's 'Groundhog Day'

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    Saturn's moon Epimetheus begins to throw its shadow across the planet's A ring, a broad expanse of icy debris that spans some 9,000 miles. The moon shares its orbit with another moon, Janus. The pair orbit a scant 40 miles apart. Every four years they swap positions as the innermost moon of the pair.
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Nope, it's not the tentative rise of Punxsutawney Phil. It's Saturn's moon Epimetheus serving as a suitable stand-in during the planet's rough equivalent of a northern hemisphere's Groundhog Day.

Scientists released this image this week. It was captured by a camera on the Cassini orbiter, currently exploring Saturn and its moons. The researchers also pulled several shots together into a time-lapse video of the event. You can watch the video here.

It's the first time planetary scientists have seen the moon-shadow-on-ring phenomenon. It's a signal that the sunlight is beginning to strike the rings at just the right angle to reveal ring features no one has seen before.

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"It's a good time for ring studies for a couple of reasons," says Larry Esposito during a phone chat. He's a planetary scientist at the University of Colorado at Boulder and leader of the team using Cassini's Ultraviolet Imaging Spectrograph to study the rings.  Dr. Esposito is credited with discovering Saturn's F ring during the Pioneer 11 mission's fly-by in 1979.

First, he explains, as the planet approaches the spring equinox (in its northern hemisphere), the stunning array of rings that orbit around the planet's equator increasingly appear edge-on to the incoming sunlight. Sunlight hits the rings at increasingly shallow angles.

With this lighting, it's easier to spot differences in the vertical structure of a ring, Esposito says.  And the low-angle lighting can illuminate thinly spread portions of rings that scientists can't see at other times. "So very diffuse rings light up at this period," he notes.

"We know there are waves in the rings, where the rings are actually corrugated," he says. "Those should become quite evident during this period because the peaks of those waves will be lit."

Second, as the sunlight hits at increasingly shallow angles, the rings cool: When sunlight strikes the rings edge-on, it doesn't heat them at all. This could affect on the behavior of ring "spokes" that have appeared in previous Cassini images by changing the electrical charges that affect the spokes.

Researchers are particularly interested in the broadest of the rings – the A and B rings. These are only about 33 feet thick. The A ring measures some 9,000 miles across; the B ring extends for nearly 16,000 miles.

The problem? These rings lie inside the F ring, Saturn's outermost ring. While Cassini has taken some edge-on shots of the ring system before, its view of these larger rings has been blocked by the F ring, which is narrower but thicker than the inner rings. Moon shadows and the increasingly shallow-angle light coming from the sun are expected to reveal interesting high and low spots along the main rings' surfaces.

Observing the rings under these changing conditions will help researchers understand the physics governing the rings, Esposito says. "But probably the most exciting things will be things we haven't thought of yet. Whenever we look with Cassini instruments at a different environment, we see things we never expected."

Launched by the National Aeronautics and Space Administration in 1997, Cassini has been orbiting Saturn since 2004. Since last June, the space probe has been on science mission v 2.0 – an extension dubbed Cassini Equinox.

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