Keck Zooms In On The Weird Weather of Uranus

http://www.news.wisc.edu/10402.html

Keck zooms in on the weird weather of Uranus
Terry Devitt
University of Wisconsin - Madison
November 10, 2004

Capitalizing on the incomparable optical capabilities of the Keck
Telescope, scientists have gained an
unprecedented look at the atmosphere of Uranus, providing new insight
into some of the most enigmatic weather in the solar system.

A pair of images unveiled here at a meeting of the Division for
Planetary Sciences of the American Astronomical Society, reveal
more cloud features - an abundance of atmospheric phenomena that
vary dramatically in size, brightness and longevity - than have
been observed before on Uranus.

"The cloud features range from small to large, from dim and diffuse to
sharp and bright, from rapidly-evolving systems to stable features that
last for years," says Lawrence Sromovsky, a senior scientist at the
Space Science and Engineering Center.

What's more, the new Keck images captured several Uranian weather
oddities, including a big southern hemisphere storm feature that, during
the course of several years, seesaws over 5 degrees of latitude.

"It's weird behavior that hasn't been recognized before on Uranus. It's
similar to what's been seen on Neptune, although there the oscillation
is much more rapid," Sromovsky explains. "It is not surprising to see
cloud features drifting in latitude, but our models don't show these
oscillations. We don't know what makes it keep coming back to its
starting point."

Another unusual Uranian weather feature is a long, narrow complex of
cloud features that is probably the largest group of atmospheric
features ever seen on the planet. Spotted in the northern hemisphere of
Uranus, the 18,000-mile-long complex of clouds dissipated completely
during the span of a month.

"These more dynamic systems seem to develop at northern latitudes where
they rise to even higher latitudes, apparently using up energy and
dissipating relatively rapidly," says Sromovsky, who conducted the new
Keck observations with Patrick Fry, also of the Space Science and
Engineering Center.

Together, the new images of Uranus reveal more than 30 cloud features,
exceeding the total observed in all pictures obtained by the Voyager,
the Hubble Space Telescope and other telescopes up to the year 2000.

The scientific value of the new pictures lies in their ability to help
scientists unravel some of the atmospheric novelties of Uranus, the
seventh planet from the sun. The cloud features they show, for example,
are being used to trace and help define wind patterns and predict the
motions of the large storm systems that sweep across the pale blue planet.

"The large, longer-lived cloud features of Uranus may be underpinned by
giant hurricane-like vortices, as we've seen on Neptune," says
Sromovsky, "but it's unlikely that they are as violent as the hurricanes
that routinely batter Florida, for example."

Earthly hurricanes, he explains, dispense a lot of energy. Uranus, which
is 19 times as far from the sun as the Earth, has far less solar energy
to dissipate.

Uranian storms seem to survive and thrive because the atmosphere is
"slippery," providing less of the atmospheric resistance that help
storms on Earth dispense their energy.

"There is very little temperature contrast and very little energy to
drive the weather in Uranus," says Sromovsky. "Whatever is happening has
to be well lubricated; it has to be a low-friction environment."

The quality of the new images, says Sromovsky, demonstrates the
astonishing optical capabilities of one of the world's premier
ground-based telescopes: "The image quality is better than anything
we've seen before. It is amazing that the amount of detail we can see
from the ground with Keck far exceeds what we could see with Voyager
during its relatively close pass by the planet. And we can get better
resolution than Hubble, especially at the near infrared wavelengths we
use to improve cloud contrast."

Situated at the summit of Hawaii's dormant Mauna Kea volcano, the Keck
Telescope boasts a 10-meter diameter segmented mirror, and a
revolutionary adaptive optics system that detects and corrects most of
the atmospheric effects that blur viewing.