Images of Uranus reveal for the first time a dark spot in
the planet's northern hemisphere.
During the past decade, many bright spots have been seen on Uranus,
in both red and near-infrared filters. But this is the first dark spot ever
seen on the planet. A team led by Lawrence Sromovsky of the University of
Wisconsin and including Kathy Rages of the SETI
Institute, Heidi Hammel of the Space Science Institute (Boulder, CO), and
Patrick Fry of U. Wisconsin, observed the dark spot on Aug. 23 using the Hubble
Space Telescope (HST).
Other images taken a day later also show the dark spot near
Uranus' limb, a sure sign that the spot had a lifetime of at least several
days. In fact, we can be sure the spot had a lifetime of at least two months. A
team led by Mark Showalter of the SETI Institute also has been observing Uranus
with Hubble in an effort to understand the planet's rings
and satellites. Some of their images taken on June 16 show the dark spot,
two months prior to its discovery.
The recent observations also showed a bright spot just to the
north of the dark spot, which may have some people scratching their heads and
asking "Why do I think I've seen that before?" They probably remember the
Voyager images of Neptune's
Great Dark Spot and its associated Bright Companion. It would be very
exciting to see similar features forming on Uranus as spring comes to the
northern hemisphere after decades of winter darkness. Uranus' bright spot,
however, is located at 30°N latitude, an area where bright
spots have been seen on many previous occasions. In enlarged images of this
region the bright spot doesn't appear to be connected to the dark one, and is probably
just a case of two spots passing by each other.
There is a reason for the recent spate of activity on
Uranus. Since the planet is "tipped on its side," the seasons are more drastic
on Uranus compared with those on Earth." In contrast to Earth, with an axial
tilt of 23°, Uranus is tilted 98°. To put it another way: On June 21 on Earth, the Sun is
directly overhead in Mazatlan, Mexico. If Earth had Uranus' axial tilt, on June
21 the Sun would be directly overhead along the northern coast of Ellesmere
Island, in the Canadian Arctic.
At the conclusion of Voyager's planetary encounters, Uranus
held the undisputed title of "blandest planet in the outer Solar System." In
part this was due to the timing of the Voyager encounter. Uranus reached southern
solstice in October 1985, and Voyager flew past only three months later--a Uranian
"date" about the equivalent of Dec. 23 on Earth. With the Sun almost directly
over the Uranian south pole, only mid-to-high southern latitudes were clearly
visible. This region on Uranus is covered by a substantial, and nearly
featureless, cloud of methane ice crystals, giving the planet the appearance of
a cue ball in the Voyager images. However, latitudes north of around 45°S are much clearer.
Now, Uranus is approaching equinox. Almost a full season has
passed since the Voyager
2 encounter. On Uranus it is now late winter in the northern hemisphere, or
Mar. 16 on Earthly calendars, with equinox due to occur in December 2007. The
clear northern regions of Uranus' atmosphere are easily visible from Earth. And
as the Sun has risen over Uranus' northern hemisphere for the first time in
decades, it has revealed much more activity than Voyager ever saw.
Previously, most of this activity has taken the form of small
bright spots, which may be akin to thunderheads rising high into the
atmosphere. The newly discovered dark spot, on the other hand, may be caused by
a thinning of the underlying methane "stratus" layer.
During the next few years, as Uranus passes through equinox,
Hubble and ground-based telescopes around the globe will turn their lenses
toward the planet. We will find out if the dark spot has a life span of years
or just months and whether other dark spots will develop. We expect to witness
the development of a bright cloud over Uranus' north pole similar to the one
that has been in place over the south pole for the past 20 years. These are
interesting times, indeed, for Uranus watchers.
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