Moon Casts Shadow on Saturn Ring

June 12, 2009 -- Tiny moons embedded in Saturn's ring system create gravitational speed bumps that redirect the flow of particles in the otherwise flat rings, new findings from the Cassini spacecraft show.

Taking advantage of a very low sun angle relative to the planet's rings, Cassini scientists found long shadows in a ring gap created by the five-mile-wide Saturn moon Dephnis, which orbits in Saturn's outer A ring.

Extrapolating from the results, the team determined the moon was creating a vertical structure about one mile in height -- about 150 times as high as the rings are thick.

The planet's three main rings are about 30 feet thick.

Scientists had predicted that gravitational forces associated with embedded moons would create vertical structures as they dip above and below the ring plane, but they've never before observed it.

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"It's one thing to get data back that shows you're right about something but to get pictures that are this stunning is really wonderful," said John Weiss, a Cassini researcher with the Space Science Institute in Boulder, Colo., and the lead author of a paper appearing in this week's Astronomical Journal.

The images were taken over the past several weeks as part of ongoing studies of Saturn's ring system. The Cassini spacecraft was about a million kilometers away and nearly directly above the rings for the shoot, which was made possible by a sun angle that occurs once every 15 years.

In addition to unraveling some of the inner workings of the Saturn system, the studies are expected to help scientists refine measurements of moon's embedded in the planet's rings. The research also could help scientists understand the physics of protostellar systems and other disk systems, like galaxies.

"There is a great deal of commonality in the underlying physics," Carolyn Porco, head of the Cassini imaging team wrote in an email to Discovery News. "To know one is to go a long way toward knowing the other."

Scientists will continue looking for structures in Saturn's rings while the sun is in favorable alignment throughout the next several months. They also are trying to find tiny moonlets that may be creating 10 unaccounted for gaps in the planet's rings.

Cassini arrived at Saturn in 2004 and completed its primary four-year mission last year. The team won funding for a two-year extension to study the planet during equinox, when the sun will be directly overhead at noon on the planet's equator.

The alignment, which occurs every half-Saturn year, or about 15 Earth years, lowers the sun angle relative to the plane of the rings, causing any out-of-plane structures to cast long shadows across the rings' surface.

Related Links:

Cassini Imaging: Central Laboratory for Operations

The Cassini Project

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