Keck telescope captures faint new ring around Uranus (Forwarded)

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University of California-Berkeley

Media Contacts:
Robert Sanders
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Imke de Pater, UC Berkeley
(510) 642-1947

Heidi Hammel
(203) 438-3506

FOR IMMEDIATE RELEASE: Thursday, December 22, 2005

Keck telescope captures faint new ring around Uranus

Astronomers have made the first ground-based observations of one of two
new rings discovered recently around the planet Uranus by the Hubble Space
Telescope and announced today.

The ground-based detection was conducted with the Near Infrared Camera
(NIRC2) using the adaptive optics system on the Keck II telescope atop the
Mauna Kea volcano in Hawaii.

The new discoveries within the Uranus ring system are being made thanks to
the superb instrumentation that researchers are now using on both the Keck
and Hubble telescopes, and because of the current angle of Uranus' rings
as seen from Earth, according to Imke de Pater, a professor of astronomy
at the University of California, Berkeley, and leader of the ring science
Keck observing team.

De Pater explained that our perspective on the planet's rings changes as
Uranus and Earth orbit the sun. The rings are now "closing up" and in 2007
will appear edge-on from Earth, providing astronomers with an opportunity
to see faint rings. The path lengths through the dusty rings increase as
the rings close up, so that any faint rings will become progressively more
visible.

De Pater, Heidi B. Hammel of the Space Science Institute in Boulder,
Colo., and Seran Gibbard of Lawrence Livermore National Laboratory,
announced their observations of the innermost new ring today in a circular
issued by the International Astronomical Union (IAU). The initial
discovery of Uranus' two new rings was announced in the same IAU circular,
issued Dec. 22, and will appear in the Dec. 23 issue of the journal
Science. Authors of that report are Mark R. Showalter of the SETI
Institute and Jack J. Lissauer of the NASA Ames Research Center, both
located in Mountain View, Calif.

"We have been puzzled by the fact that we cannot detect the outer ring,
even though it is more than twice as bright as the inner ring in the
Hubble data," de Pater said. "The difference is probably caused by the
fact that we are working in the infrared, at a wavelength four times
longer than that used by the Hubble telescope in the rings' discoveries.
This seems to suggest a color difference between the two rings, which
would indicate that their constituent particles are very different."

Specifically, the team concludes, the outer ring is much less red, which
would suggest it is made of tinier particles. This would be consistent
with the hypothesis by Showalter and Lissauer that the inner ring may have
a large population of unseen source bodies, whereas the outer ring may be
composed entirely of dust ejected from Uranus' tiny moon, Mab.

"A normal dusty ring is red," said de Pater, noting that Jupiter's rings
and Saturn's G ring are distinctly red. Only Saturn's E ring, nearest to
the moon Enceladus, is blue.

"If a ring has a lot of tiny particles, it will be bluer, because smaller
particles do not scatter efficiently in the red," added Gibbard.

"Because our team was observing in the near infrared -- an invisible part
of the spectrum with wavelengths longer than that of visible red light --
we concluded that the outermost ring must be more neutral in color than
the innermost ring," Hammel explained.

The new discoveries by Showalter and Lissauer, made in visible light with
Hubble's Advanced Camera for Surveys, are faint, dusty rings orbiting well
beyond Uranus's previously known system of 11 distinct rings. This brings
the total number of known rings up to 13. Uranus also has 27 known moons,
one of which, Mab, seems associated with the outermost new ring.

When the Keck team was first informed of the discovery a few months ago,
de Pater carefully processed data they had obtained in August with the
infrared camera on the 10-meter Keck II telescope. After combining 30
images (equal to a full hour of integration time), she spotted the
innermost of the two new faint rings. In subsequent observations in
October, the team specifically looked farther from the planet, but failed
to see the outermost of the two new rings.

The innermost new ring is about 67,700 kilometers from Uranus, just
outside the previously known outermost and brightest ring, called the
epsilon ring. This is in accord with the NASA team's results -- that the
peak brightness of the innermost new ring is 67,300 kilometers from the
center of the planet. The outermost ring, which de Pater and her team did
not see, was reported to have a peak brightness at 97,700 kilometers from
the planet's center.

De Pater, Hammel and Gibbard have been observing the planet since 2000
with the second-generation NIRC2 using the adaptive optics system on the
Keck II telescope. Last year, they spotted an inner ring, the 11th, that
was seen only once before by the Voyager spacecraft.

The team has uncovered various other interesting aspects of the Uranian
rings. They saw, for example, that faint dust sheets in between the main
rings do not have the same spatial brightness distribution as the dust
seen by the Voyager spacecraft almost 20 years earlier in forward
scattered visible light. These results have just been published in the
Jan. 2006 issue of Icarus.

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 the National Aeronautics and Space Administration (NASA),
while Gibbard is supported by the U.S. Department of Energy's National
Nuclear Security Administration.

Web Links to additional Information:

Images of Uranus and the innermost of its two new rings can be viewed and
downloaded after the embargo lifts at
http://www.berkeley.edu/news/media/download/
or from

http://astron.berkeley.edu/~imke/Infrared/UranusAo/Uranus_outerring_2005.htm


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