Nov. 10, 2004

Long-Lived Bright Cloud Tracked On Uranus

Planetary Surface, Once Thought as Interesting as a Cue Ball, Reveals Intriguing Distinctive Feature

Mountain View, CA, November 10, 2004 – Images of Uranus obtained by the Hubble Space Telescope and Keck Observatory have revealed the previously unknown presence of a bright cloud that has lasted within the planet’s atmosphere for more than a year. The massive cloud appears as a bright white spot and has been tracked in the planet’s southern hemisphere. The research effort was led by Kathy Rages, a Principal Investigator with the SETI Institute, Mountain View, CA.

“Uranian weather appears to be similar to that of the American Midwest – it becomes more ‘interesting’ as the equinox approaches,” said Rages, who, with colleagues Mark Showalter of Stanford University and Heidi B. Hammel of the Space Science Institute in Boulder, CO, has been studying Uranus with the Hubble Space Telescope (HST).

“When the Voyager 2 spacecraft flew by Uranus in 1986, the planet looked like a cue ball. With a lot of image processing, just ten clouds were finally discerned. Most astronomers decided that the atmosphere of Uranus was dull and uninteresting,” said Hammel, who has been observing Uranus with the 10-meter telescope of the W. M. Keck observatory on top of Mauna Kea, Hawaii. “We are seeing now that that isn’t true, or at least it isn’t always true.”

The cloud, referred to as “the Great Spot at 37° S”, or GS-37S, extends about 2000 km in the north-south direction and 2-3 times as far east-west. For the past 18 months, GS-37S has repeatedly changed in brightness and in the rate at which it drifts westward within Uranus’s atmosphere. The changes in brightness can occur within a few days, while the drift rate varies on time scales of a month or two.

Uranus is the seventh planet from the sun and is one of our solar system’s four gaseous giant planets, comprised predominantly of hydrogen and helium. Its orbit around the sun takes 84 earth years, though its day lasts only 17.24 hours. Uranus is unusual in that its axis is nearly horizontal, rather than largely vertical, with respect to the plane of its orbit. This tilt can make its summer days last for more than 40 Earth years.

HST observed Uranus in 1994, 1997, 2000-2002, and 2003-2004.  In terms of seasons on Earth, this ten-year span corresponds roughly to the month of February and early March. In 2003 and 2004, Uranus was also observed with the 10-meter Keck telescope, using adaptive optics to compensate for the effects of Earth’s atmosphere and obtain Hubble-class resolution.

Throughout the period of HST observation, the most prominent feature at wavelengths longer than 600 nanometers – where methane absorbslight significantly – has been a bright polar cap south of 45 degrees south latitude. Beginning in 1997, observations of discrete bright spots – clouds – became more common. But the brightest spots were in the northern hemisphere. In the southern hemisphere, diffuse clouds were sometimes seen just north of the polar cap. But because observations of the planet always occurred a year or more apart, there was no way to estimate how rapidly these clouds formed or how long they survived.

“Now that GS-37S has been observed on seven different occasions within a year’s time, we can be more confident in concluding that it is a single long-lived feature,” said Rages.

As Uranus moves toward its equinox in 2007, Rages, Hammel and Showalter will be watching the windy weather on the planet and wondering what to expect in the future.  “Based on historical images of Uranus and more recent results from HST and the Keck telescope in Hawaii, we are beginning to suspect that our equinox view of the planet may differ from what we expected just a year ago,” said Hammel, “but the details are Uranus’ secret – for now.”

Contacts:

Images available