Uranus Has a Dark Spot

Just as we near the end of the hurricane season in the Atlantic Ocean, winds whirl and clouds churn 2 billion miles away in the atmosphere of Uranus, forming a dark vortex large enough to engulf two-thirds of the United States. CREDIT: NASA, ESA, L. Sromovsky and P. Fry / University of Wisconsin

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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