Cassini Spacecraft Provides Compelling Evidence for Patterns Resembling Spokes on a Pinwheel in Saturn's Outer Ring

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Cassini spacecraft provides compelling evidence for patterns
resembling spokes on a pinwheel in Saturn's outer ring

Nov. 9, 2005

By Lauren Gold



By watching a distant star as it passed behind Saturn's outer rings,
Cornell University astronomers on NASA's Cassini-Huygens mission to
Saturn have found the most direct evidence to date of patterns,
called gravitational wakes, within the planet's outer rings.

The patterns, thin, parallel striations like spokes on a pinwheel,
have been theorized since the 1970s, but their small scale (just 100
meters -- 328 feet -- wide) makes them impossible to see even with
the spacecraft's high-resolution camera. The new evidence of their
existence, says Phil Nicholson, Cornell professor of astronomy, gives
scientists clues about how thick Saturn's rings are and how their
constituent bodies interact.

Nicholson presented his findings in September at the American
Astronomical Society's 37th Division for Planetary Sciences meeting
in Cambridge, England.

Nicholson and postdoctoral researcher Matt Hedman used Cassini's
visual and infrared mapping spectrometer (VIMS) to take spectra of
the star Omicron Ceti (also known as Mira) in quick succession during
four three-hour intervals, or stellar occultations, during which the
star passed behind Saturn's A and B rings. (The A ring is the
planet's outermost visible ring; the B ring is closer to Saturn.)
With the more than 100,000 spectra from each occultation, Nicholson
and Hedman plotted the amount of near-infrared light that filtered
through the rings. They then compared the optical depth -- the amount
of light blocked by the ring material -- at several points throughout
the occultation. In the comparison, they noticed an unexpected
asymmetry: More light filtered through at points on the star's way
out of the occultation than at corresponding points equidistant from
the planet on the star's way in.

At first, Nicholson and Hedman considered whether the asymmetry could
be explained by the spacecraft's slight shift in vantage point over
the occultation. But Nicholson pointed out that the distance between
the spacecraft and the star is virtually infinite. Lines drawn
between the two at either end of the occultation are for all
practical purposes parallel -- so the angle between the ring plane
and the line of sight between Cassini and the star doesn't change
significantly.

More likely, said Nicholson, is that the small chunks of water ice
that comprise the rings are arranged in stripes radiating outward at
a skewed angle, like spokes on a pinwheel. The so-called
gravitational wakes form when the small objects' gravitational
attraction to each other competes with the tendency of tidal forces
from Saturn to pull them apart.

"When the wakes are seen almost end on, the A ring appears at its
most transparent," said Nicholson, "whereas when the wakes are seen
from the side, the ring becomes almost opaque."

Though Cassini, which arrived at the giant planet in June 2004, can't
get close enough to see the wakes directly (the spacecraft was about
1.6 million kilometers away from Saturn when the occultations
occurred), Nicholson has already heard from other Cassini researchers
with independent observations supporting the existence of
gravitational wakes.

"I suspect in the end that many Cassini instruments will be seeing
the same kind of phenomenon," he said. Still, while he expected some
evidence of the wakes, the spacecraft's position during the
occultations -- with only 3.5 degrees between the ring plane and the
line of sight to the star -- made the effect striking.

"We were certainly surprised," he said. "We were not expecting it to
be as obvious. At one level, it's just kind of neat to us who study
rings. For those of us in the business it's nice to get direct
evidence of the wakes."

On a more practical level, though, the finding gives scientists a new
piece of information about the rings' micro-structure and internal
dynamics: specifically, how the ice chunks move as they are pulled
toward each other and collide, and as Saturn's tidal force shears
them apart again.

It also gives researchers a tool for judging the overall thickness of
Saturn's rings. Throughout Cassini's four-year mission, astronomers
will collect data from dozens of similar experiments. Since each will
be made from a different viewing angle, astronomers will then compare
how marked the wakes' effect is across the series. That information
will allow them to estimate the rings' thickness (their north-south
span) -- which is thought to be as little as 10 meters -- 33 feet.

Using the wakes to estimate the rings' thickness is similar to using
sunlight to estimate the widths of slats on a set of vertical blinds.
If the blinds are open and the sun is high in the sky, the blinds
will let in nearly all the light. But as the sun sets to one side,
the blinds -- in the same position -- will begin to block more and
more light, because of their width. Similarly, if the rings are very
thick, the spacecraft will continue to see the wakes' effect from
higher elevations; if they are very thin, evidence of the wakes will
begin to diminish when the angle between Cassini's line of sight and
the ring plane is still quite small.

"This may be the best way of directly learning the thickness of the
rings," Nicholson said.

Cassini's VIMS team is led by Robert Brown at the University of
Arizona's Lunar and Planetary Laboratory. During Cassini's Saturn
tour, the spacecraft will complete 74 orbits of the planet, 45 flybys
of the moon Titan and many flybys of Saturn's other moons.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a division
of the California Institute of Technology in Pasadena, manages the
Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter was designed, developed and assembled
at JPL.

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