The first dark spot on Uranus, imaged by Hubble’s Advanced Camera for Surveys in August 2006, is possibly a glimmer of more interesting features to come.

NASA/ESA/L. Sromovsky and P. Fry (University of Wisconsin)/H. Hammel (Space Science Institute)/K. Rages (SETI Institute)

The bland face of Uranus has never been a favorite of schoolchildren, earning it a reputation as the “least interesting planet in the solar system.” In 1986 NASA’s Voyager probe revealed Uranus to be a featureless blue-green ball with narrow rings — no match for the splendor of Saturn or the intrigue of Mars. Ground-based observations in subsequent years have found subtle white bands and bright spots on Uranus, but stormy dark spots like those observed on the other giant planets have been elusive.

Now astronomers have captured the first definitive images of a dark cloud on Uranus using the Hubble Space Telescope. A team led by Lawrence Sromovsky (University of Wisconsin) detected the spot at 27° latitude “north.” Measuring 1,700 by 3,000 kilometers (1,100 by 1,900 miles), the spot is a vortex two-thirds the size of the US.

Why some spots are darker than their surroundings has never been definitively determined, says Sromovsky. They might be darker because the cloud particles comprising these features absorb more sunlight, or because the spots have thinner clouds that permit more absorption of light below the clouds. Upwelling of deeper, more absorbent material is also a possibility, say the researchers.

Coinvestigator Kathy Rages (SETI Institute) adds, “A partial clearing or thinning of the ubiquitous methane haze at a pressure of 1 bar would cause the observed darkening at red and near-infrared wavelengths. This is the kind of thing expected from convective downwelling — just the opposite of thunderheads.”

This 5,000-mile (8,000-kilometer)-wide box shows the dark spot in detail.

NASA/ESA/L. Sromovsky and P. Fry (University of Wisconsin)/H. Hammel (Space Science Institute)/K. Rages (SETI Institute)

The lack of features previously indicated that Uranus had a less active atmosphere than Neptune, despite their similar size and atmospheric composition. Uranus seems to lack the significant internal heat source that creates more vertical convection on Neptune. But things might be heating up for the gas-giant planet. Because Uranus rotates on an axis nearly parallel to its 84-year orbit, its seasons are extreme. The region containing the spot is experiencing an extended springtime after many years of shadow, which may explain the appearance of the dark feature.

Uranus fans hope that with its vernal equinox approaching in December 2007, the continuing seasonal warm-up may introduce a bevy of new details that would once and for all lengthen its entry in planetary textbooks.
Team member Heidi Hammel (Space Science Institute) comments, “We’ve been saying for years how active the atmosphere of Uranus is. It’s going to be a really cool few years for Uranus science.”

To learn more, read the press release at Hubblesite.org.


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