[meteorite-list] Blue Ring Discovered Around Uranus

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Apr 7 00:21:10 2006
Message-ID: <200604070103.k3713DD29790_at_zagami.jpl.nasa.gov>

http://www.berkeley.edu/news/media/releases/2006/04/06_bluering.shtml

UC Berkeley Press Release

Blue ring discovered around Uranus

By Robert Sanders
06 April 2006

BERKELEY - The outermost ring of Uranus, discovered just last year, is
bright blue, making it only the second known blue ring in the solar
system, according to a report this week in the journal Science.

Perhaps not coincidentally, both blue rings are associated with small
moons.

"The outer ring of Saturn is blue and has Enceladus right smack at its
brightest spot, and Uranus is strikingly similar, with its blue ring
right on top of Mab's orbit," said Imke de Pater, professor of astronomy
at the University of California, Berkeley. "The blue color says that
this ring is predominantly submicron-sized material, much smaller than
the material in most other rings, which appear red."

The authors of the paper in the April 7 issue of Science are de Pater,
Mark Showalter of the SETI Institute in Mountain View, Calif.; Heidi B.
Hammel of the Space Science Institute in Boulder, Colo.; and Seran
Gibbard of Lawrence Livermore National Laboratory in California.

The similarity between these outer rings implies a similar explanation
for the blue color, according to the authors. Many scientists now
ascribe Saturn's blue E ring to the small dust, gas and ice particles
spewed into Enceladus' orbit by newly discovered plumes on that moon's
surface. However, this is unlikely to be the case with Mab, a small,
dead, rocky ball, about 15 miles across - one-twentieth the diameter of
Enceladus.

Instead, the astronomers suspect both rings owe their blue color to
subtle forces acting on dust in the rings that allow smaller particles
to survive while larger ones are recaptured by the moon.

"We know now that there is at least one way to make a blue ring that
doesn't involve plumes, because Mab is surely too small to be internally
active," said Showalter. He and astronomer Jack Lissauer of NASA Ames
Research Center in Mountain View, Calif., discovered Mab in Hubble Space
Telescope images in 2003.

The likely scenario to explain Saturn's blue ring was proposed before
plumes were discovered last November as the Cassini spacecraft flew by
Enceladus. As modeled for the E ring, meteoroid impacts on the surface
of Enceladus scatter debris into its orbit, probably in a broad range of
sizes. While the larger pieces remain within the moon's orbit and
eventually are swept up by the moon, smaller particles are subject to
subtle forces that push them toward or away from the planet out of the
moon's orbit. These forces include pressure from sunlight, magnetic
torques acting on charged dust particles, and the influence of slight
variations in gravity due to the equatorial bulge of Saturn.

The net result is a broad ring of smaller particles, most less than a
tenth of a micron across - a thousandth the width of a human hair - that
scatter and reflect predominantly blue light.

"This model can be transferred directly to what we now see in Uranus,
although we still need to understand the details of the process," de
Pater said.

All other rings - those around Jupiter, Saturn, Uranus and Neptune - are
reddish. Though they contain particles of many sizes that reflect many
wavelengths of light, red dominates not only because larger particles -
many microns to meters across - are abundant, but also because the
material itself may be reddish, perhaps from iron.

"Arguing by analogy, the two outermost rings, the two rings that have
satellites embedded in them, are both the blue rings. That can't be
coincidental, there has to be a common thread of dynamics that is
causing both of these phenomena," Showalter said.

The discovery of the blue ring came after combining ground-based
near-infrared observations by the Keck Telescope in Hawaii and
visible-light photos taken by the Hubble Space Telescope. De Pater,
Hammel and Gibbard have observed Uranus since 2000 with the
second-generation NIRC2 infrared camera using the adaptive optics system
on the Keck II telescope, and in August 2005 obtained 30 new images of
the planet in hopes of seeing new features as the ring plane moves
edge-on to Earth.

Showalter and Lissauer, on the other hand, captured numerous
visible-light images of Uranus between 2003 and 2005 with Hubble's
Advanced Camera for Surveys.

Neither team realized it had captured pictures of new rings until an
extensive analysis, basically piling image upon image until faint
features stood out from the background. In December 2005, as Showalter
and Lissauer reported finding two new rings - Uranus's 12th and 13th -
and two new moons, Mab and Cupid, numbers 26 and 27, de Pater, Hammel
and Gibbard reported seeing the red, innermost of the two new rings but
not the outermost. The blue ring peaks in brightness about 97,700
kilometers from the planet's center, exactly at Mab's orbit.

Further analysis proved to both teams that the outer ring seen in
visible light was definitely not observable in the near-infrared, and so
must be blue. The analysis also showed that Mab, which like its ring
could not be seen in the infrared, is probably covered with water ice,
like the other outer moons of Uranus, and is probably Uranus's smallest
moon.

De Pater's research is supported by the National Science Foundation and
the Technology Center for Adaptive Optics at UC Santa Cruz. Hammel is
supported by NASA, while Gibbard is supported by the U.S. Department of
Energy's National Nuclear Security Administration.

Showalter's work is supported by NASA through the Space Telescope
Science Institute.
Received on Thu 06 Apr 2006 09:03:12 PM PDT


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