Keck Telescope Images of Uranus Reveal Ring, Atmospheric Fireworks

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Keck Telescope images of Uranus reveal ring, atmospheric fireworks
By Robert Sanders
UC Berkeley Press Release
10 November 2004

LOUISVILLE, Ky. - As summer draws to a close in the southern hemisphere
of Uranus, storm clouds are brewing in the upper atmosphere, northern
hemisphere winds are gusting to 250 miles per hour, and the planet's
rings are getting brighter every day.

This weather report comes from researchers using the Keck II 10-meter
telescope atop Mauna Kea volcano in Hawaii, where recent observations
are proving that Uranus is not the "boring and unchanging" planet people
have assumed, according to Imke de Pater, professor of astronomy at the
University of California, Berkeley. The new images were obtained with
the second-generation Near Infrared Camera (NIRC2) using the adaptive
optics system on the Keck II telescope.

"It's really intriguing, the planet seems to be getting more active as
the equinox approaches," said de Pater, who, with colleague Heidi B.
Hammel of the Space Science Institute in Boulder, Colo., has been
observing Uranus since 2000 with adaptive optics on the Keck II telescope.

"When the Voyager 2 spacecraft flew by Uranus in 1986, it saw almost no
discrete cloud activity - you could literally count the number of
discrete clouds on your fingers: 10! Most astronomers decided that
Uranus was a boring, static planet," Hammel added. "What we are seeing
now is the opposite, that actually there are changes, and they are
visible to Keck and the Hubble Space Telescope."

The team also spotted an inner ring, the 11 th, that was seen only once
before - in a single image taken by Voyager. Really a sheet of rocky
debris, the ring - called 1986U2R - is the innermost of the 11,
calculated by the team to be about 3500 kilometers (2,200 miles) wide
and centered about 39,600 kilometers (24,600 miles) from the planet
core. Though a thousand times dimmer than the brightest ring - the
epsilon ring - the inner ring is now visible because it is nearly
edge-on to the sun and Earth, and so reflects more light back to Earth
observers. The team also found that the nine main rings of Uranus
consist of a single layer of particles, a so-called monolayer, never
before seen in other rings.

De Pater, Hammel and colleague Seran Gibbard of Lawrence Livermore
National Laboratory report their observations this week at the American
Astronomical Society's Division for Planetary Sciences meeting, which is
being held Nov. 8-12 in Louisville, Ky. De Pater and Hammel also will
discuss their findings at a media briefing on Nov. 10 at 12 noon.

Additional members of the atmospheric team are Wes Lockwood of Lowell
Observatory in Flagstaff, Ariz., and Kathy Rages of the SETI Institute
in Mountain View, Calif.

The Voyager 2 spacecraft made the first close-up observations of Uranus
when it flew by the planet 18 years ago. At that time, the planet was at
the peak of its southern summer, with the entire southern hemisphere
bathed in continuous sunlight. The spacecraft observed faint,
low-altitude bands around the planet, a faint echo of the colorful bands
on Jupiter and Saturn, plus just 10 discrete clouds in the sunlit
southern hemisphere compared with Jupiter's thousands and Saturn's
hundreds. The relatively static atmosphere led some astronomers to label
Uranus a dull and uninteresting planet, theorizing that the thick haze
covering the planet's southern hemisphere somehow set up a stratified
atmosphere that inhibited convection.

The bands and clouds are thought to be made of crystals of methane
condensing in the planet's atmosphere, which is mostly hydrogen and
helium. Astronomers predict that the atmosphere surrounds a core of
rock, surrounded by water, ammonia and methane slush . The Voyager
spacecraft also measured the length of the Uranian day - 17 hours, 14
minutes in Earth time - based on the rotation of the planet's magnetic
field.

Starting in 1994, observations by the Hubble Space Telescope and later
by Keck have shown that the banded structure near the southern pole has
been evolving very slowly. Also, in the northern hemisphere, bright
high-altitude clouds have been popping up ever since sunlight started to
reach north of the equator.

Views of Uranus in July 2004 with the Keck 10-meter telescope
"Fourth of July fireworks" in Uranus' southern hemisphere (bright spot
at left of lower left image) indicate that vigorous convection is
starting up, bringing methane clouds to high altitudes for the first
time in decades. The near infrared Keck II images at 1.6 microns
wavelength (top) show low altitude clouds, in particular the polar
collar and a nearby bright cloud. Observations at 2.2 microns, however,
reveal high-altitude features, including the cloud's bright co
presumably material rising up to high altitudes in Uranus' atmosphere.
(Credit: Heidi Hammel and Imke de Pater )

Now, for the first time, de Pater, Hammel and Gibbard have observed
high-altitude cloud activity in the planet's southern hemisphere,
indicating "vigorous convection" in the atmosphere. Convection - the
turbulent upward and downward movement of large masses of air - creates
clouds on Uranus just as it does on Earth.

"One of several large clouds in the southern hemisphere surprised us by
showing a very bright core. This bright core was also visible at a
wavelength of 2.2 microns, which normally shows a very dark planet due
to absorption by methane," said de Pater. "We have never seen such
vigorous convective activity in the southern hemisphere before. We think
this might be a response to the changing geometry, because this region
of Uranus now experiences both day and night, whereas during the Voyager
flyby it was always in sunlight."

Hammel referred to the bright cloud feature as "Fourth of July
fireworks," because they first surprised the team on July 4 and then
faded below detectable levels by the 9 th of July. Other astronomers
could not detect the feature when looking at Uranus a few days later.

"Thus, this is the first time southern features have penetrated upward
high enough to be detected at these wavelengths," Hammel explained.
"Clouds are not unprecedented in this hemisphere, but penetration of
these clouds' activity to higher altitudes is unprecedented."

The beginning of autumn in the planet's southern hemisphere and the
arrival of spring in the north is a sight never before seen with
high-resolution telescopes like the Keck, since it occurs only once
every 84 Earth years as the planet trundles around the Sun. As the 2007
equinox draws closer, churning activity is starting at high altitudes in
the southern hemisphere .

"The amount of sunlight hitting different parts of the planet is
changing as Uranus moves around the Sun and presents different faces to
the Sun," Hammel said. "So, the changing radiation pattern the clouds
are experiencing is changing the heat balance in the atmosphere. This
could be causing convection, or it could be turning off processes that
had been inhibiting it."

Uranus has extreme seasons because its rotational axis is tilted about
90 degrees from the plane of its orbit - that is, its rotational axis
lies nearly in the plane of its orbit.

"In 1986, the southern half of the planet was pointing almost directly
at the Sun, just sitting there baking in the sunlight, and the northern
half of the planet was just radiating heat into the blackness of space,"
Hammel said. "Now, the southern part that had been getting huge amounts
of sunlight is experiencing increasing darkness every Uranian night."

The northern hemisphere of Uranus, on the other hand, is getting more
and more sunlight, and by the year 2028, the planet's north pole will
point directly at the Sun, a complete reversal of the situation when
Voyager flew by.

"We're seeing more of the north as the planet rotates into view, and we
are measuring the northernmost features we've ever seen on Uranus,"
Hammel said. She expects soon to see a bright collar develop around the
north pole, similar to the collar that so far has been a constant
feature around the south pole. "But who knows?" Hammel pointed out.
"Maybe that southern collar will dissipate first. It's fun to speculate,
and even more fun to watch."

Other new observations reported by Hammel, de Pater and Gibbard include
the fastest winds ever recorded on Uranus. The winds, between 107 and
111 meters per second (240 and 260 miles per hour), were measured in
October 2003 on the northernmost parts of the planet visible at that time.

"At least as interesting as Uranus' atmosphere is its ring system," said
de Pater. "The rings are slowly closing up while approaching the 2007
equinox, so we are starting to see faint sheets of dust like the
innermost Uranian ring, called 1986U2R, that had been seen only once
before."

The team has been able to extract much more information on this ring,
including its brightness and radial extent, than had been possible from
the single Voyager image that first revealed it.

Perhaps most surprising is the team's discovery that in Uranus' main
rings, the ring "particles," which were deduced to be large boulders
from Voyager data, are distributed within a single layer.

"We had never seen such an odd configuration in other ring systems,"
said de Pater. "Rings in other systems, like Saturn's rings, are usually
depicted as being many particles thick. Uranus' ring system is unique in
this respect."

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.

The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy, a scientific partnership of the California
Institute of Technology, the University of California and NASA. The
Space Science Institute is a nonprofit organization with the unique
mission to integrate world-class scientific research with education and
public outreach. The institute's research program includes earth
science, space physics, planetary science and astrophysics.