MAUNA KEA, Hawaii (Nov. 10, 2004) Observations of Uranus conducted at the
W. M. Keck Observatory in Hawaii are surprising astronomers with the level
of detail they can see from the ground. Two separate teams of
astronomers, one from Berkeley/SSI and one from Wisconsin, used advances
in Keck adaptive optics (AO) to help make major scientific discoveries
regarding the planet’s atmosphere and ring system. The results are a
powerful example of how ground-based telescopes are helping astronomers
study planets in the outer solar system that once could only be studied
from space. Early results were announced today at the 36th meeting of the
American Astronomical Society’s Division for Planetary Sciences.

Images: http://www2.keck.hawaii.edu/news/science/uranus/index.html

“We are stunned by the quality and detail of these images,” said Dr.
Frederic Chaffee, director of the W. M. Keck Observatory in Hawaii.
“These are the best pictures of Uranus that have ever been produced by a
telescope, and they are opening new windows of understanding for this
unique and special world.”

The most recent observations of Uranus show the planet as it approaches
its southern autumnal equinox, which takes place in 2007 (the length of
the year on Uranus is 84 Earth years). The two teams used narrow filters
at infrared wavelengths to study features in the atmosphere and ring sets,
both of which are enormously enhanced by the Keck adaptive optics system.
Ground-based telescopes are helping astronomers track climatic changes in
the planet’s atmosphere.

“People may think that Uranus is relatively inactive, but these images
show that Uranus is definitely changing, and perhaps quite dramatically,”
said Imke de Pater, professor of astronomy at University of California,
Berkeley, lead investigator for the team responsible for the Berkeley
observations. “What is causing it, no one knows for sure. Only time will
tell.”

The new images are the result of many general improvements to the Keck
adaptive optics system. A new calibration technique removes artifacts
previously present in the images when measuring the atmospheric distortion
with a planet instead of a point source of light. Another major
improvement is a new wavefront reconstructor to improve the data
processing within the AO system. This dramatically reduces the effect that
“noise” or errors in measuring the atmospheric distortion have on the
image quality.

A dramatic visualization of the power of adaptive optics (Figure 1,
attached link) was made by Dr. Heidi Hammel of the Space Science Institute
in Boulder, Colorado and Dr. Imke de Pater of UC Berkeley, California.
They took images of Uranus and its rings with the second-generation Near
Infrared Camera (NIRC2) behind the AO system on the Keck II telescope,
first with the AO system off, and then with the AO system on. In this
figure, the ring system is more readily visible through the 2.2-micron
filter because methane absorption at this wavelength renders the planet
extremely dark except for a few high altitude clouds. In contrast, the
1.6-micron image shows deeper atmospheric cloud structure, including many
discrete features peppering the planet’s northern hemisphere. At 1.6
microns, the rings are just barely visible as a faint streak across the
planet’s northern hemisphere.

“The differences are stunning,” said Hammel. “The detail provided by
Keck’s AO system for the atmosphere and the rings of Uranus fundamentally
changes the science we can achieve.”

Later observations conducted by the team at University of
Wisconsin-Madison, also with the Keck II AO system, were formed into a
composite image in which the highest clouds appear white, the middle level
clouds appear bright green, and the lower clouds appear darker blue
(Figure 2). The color balance used to reveal the cloud structure in these
infrared exposures, which are not normally visible to human eyes, makes
the ring system appear red in these images and is an artifact of the
process. The higher clouds are most abundant in the planet’s northern
hemisphere.

Dr. Lawrence Sromovsky, principal investigator for the Wisconsin
observations said, “Twenty years ago we simply couldn’t see the types of
details in the outer solar system the way we can today with large,
ground-based telescopes like Keck. These images actually reveal many more
cloud features than the Voyager spacecraft found after traveling all the
way to Uranus.”

Until recently, little was known about the oddball planet, which gets its
name from the Greek word “Ouranos,” a mythological god who personifies the
heavens. Uranus lies tipped on its side, probably the result of an ancient
cosmic collision, and its magnetic field lays strangely off-set from and
tilted with respect to the planet’s rotational pole. In 1986, the Voyager
2 spacecraft sent pictures to earth of what appeared to be a non-descript
ball suspended in space. At that time, Uranus’ South Pole was pointed
almost directly at the sun, and the North Pole pointed away. Now, more
than 18 years later, the planet is drawing near the point in its orbit
where the planet’s equator will be pointing toward the Sun, and both poles
will get about 17 hours of sunlight a day.

On Earth, the massive storms captured in the new pictures of Uranus would
engulf nations as large as the continental United States, about 3 million
square miles. But at a distance of more than 1.6Â billion miles, even
such large storms are barely detectable and require the use of the world’s
most powerful telescopes.

Funding for the telescopes and the Keck II adaptive optics system is
provided by the W.M. Keck Foundation. Optimization for the Keck
adaptive optics system is provided by the National Science Foundation and
the Technology Center for Adaptive Optics, managed by UCSC under
cooperative agreement No. AST-9876783 (de Pater). Additional funding was
provided by NASA grants NAG5-11961, NAG5-10451 (Hammel) and NAG5-12206
(Sromovsky).

The W. M. Keck Observatory is operated by the California Association for
Research in Astronomy (CARA), a non-profit 501 (c) (3) corporation whose
board of directors includes representatives from the California Institute
of Technology, the University of California, and the National Aeronautics
and Space Administration. For more information, please visit
www.keckobservatory.org.

EDITOR’S NOTE: Additional information is available on the Web from both UC
Berkeley and the University of Wisconsin-Madison:

UC Berkeley:
http://astron.berkeley.edu/~newstar/Infrared/UranusAo/pressrelease2004.htm

University of Wisconsin-Madison: http://www.news.wisc.edu/10402.html