March 17, 2008 -- With its choking carbon dioxide atmosphere, thick clouds of sulfuric acid and lead-melting heat, Venus has been a living laboratory for meteorologists fascinated by wild weather. Among the most provocative storms are massive hurricanes over the planet's north and south poles.
The systems are not only long-lived -- they were first spotted more than 30 years ago by NASA's Mariner and Pioneer spacecraft -- but fickle. Scientists have discovered that the eye of the south pole storm, which is centered in a 2,000-kilometer (1,243-mile) wide vortex, can change its shape within days.
It is not yet known if its sister storm in the north is doing the same.
The finding, announced this week by the European Space Agency, is based on infrared images taken by Europe's Venus Express spacecraft, which has been studying Earth's sister planet for almost two years.
One picture, taken Feb. 26., shows a classic hourglass-shaped structure in the south pole vortex, nearly identical to previous findings. Another image taken just 24 hours earlier however, shows the structure at the center of the vortex, which is similar to the eye of a hurricane, to be nearly circular.
Scientists aren't sure what is causing the phenomenon. Colin Wilson, with the University of Oxford, theorizes that atmospheric gases are being heated by the sun at Venus' equator rise and shift toward the poles, where they then regroup and sink back down again.
"As the gases moves toward the poles, they are deflected sideways because of the planet's rotation," Wilson said.
Viewed in the infrared -- Venus' atmosphere is too dense for imaging details in visible light -- the core of the vortex appears very bright, indicating a region hotter than its surrounding environment.
The downward shift of gases would create a depression at the cloud tops, making the region hotter, scientists said.
"The vortex is similar to what you might see in your bathtub once you have pulled out the plug," said Giuseppe Piccioni, a lead scientist on the Venus Express program.
The spacecraft already has surpassed its design life of 500 Earth days, or two Venus years, with studies continuing. Scientists are trying to figure out why Venus, Earth and Mars, which are about the same size and composition, evolved into such different worlds and whether life ever took hold anyplace else besides Earth.
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