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Take the Venusian vortex tour

— What's behind the vortex on Venus? Astronomers have been studying the atmospheric swirl at the Venusian south pole for more than three decades, and the latest crop of imagery from the European Space Agency's Venus Express orbiter documents quick changes in what appears to be the eye of a 1,200-mile-wide (2,000-kilometer-wide) hurricane. But they still haven't figured out the exact mechanism behind the vortex.

The fresh view of Venus' giant swirlie is just one of the curiosities documented in this week's wave of interplanetary imagery - taking its place alongside fresh close-ups of a mysterious Saturnian moon and dark halos on the planet Mercury.

An animated image from Venus
Express shows the swirl of gases in
the Venusian vortex. Click on the
image for a larger view.

First, about that swirlie: When scientists reviewed the Venus Express imagery, they were surprised to see how quickly the polar vortex could change shape. The core of the vortex shows up brightly in thermal infrared imagery of the storm structure, and that probably indicates that a lot of atmospheric gases are moving downward in the polar region. In this week's Venus Express advisory, ESA scientists say that action creates a depression in Venus' cloud tops, raising the core temperature.

"Simply put, the enormous vortex is similar to what you might see in your bathtub once you have pulled out the plug," said Giuseppe Piccioni, co-principal investigator for the probe's Visible and Infrared Thermal Imaging Spectrometer, or VIRTIS.

The VIRTIS team's findings were published last November in the journal Nature, but this week's pictures serve as a virtual public tour of the vortex. The animated image you see here is a time-lapse compilation of imagery from just five hours, showing how the vortex changes its shape in a matter of days or even less time.

Scientists didn't expect to see the shape-shifting happen so quickly, and they're still trying to work out the atmospheric dynamics behind the phenomenon.

"One explanation is that atmospheric gases, heated by the sun at the equator, rise and then move poleward," the University of Oxford's Colin Wilson said in the advisory. "In the polar regions, they converge and sink again. As the gases move toward the poles, they are deflected sideways because of the planet's rotation."

The Venus Express team plans to keep a close eye on the vortex to gain a better understanding of its inner workings. That puts the Venusian vortex right in there with the Saturnian hexagon and Jupiter's red spots as alien storms worth watching.

Next stops on the tour
Venus isn't the only stop on this week's tour: We've already mentioned the Cassini orbiter's Enceladus flyby and the Messenger observations of Mercury's dark halos.Today, the European Space Agency is highlighting an image from its Mars Express orbiter showing ancient lava flows on a plain called Daedalia Planum. The interesting thing about this landscape is that it shows evidence of multiple epochs of volcanic activity. The most recent eruptions are thought to have come perhaps 100 million years ago, when dinosaurs walked on Earth.

This week also brought a fresh batch of images from NASA's Mars Reconnaissance Orbiter, including overhead views of the proposed landing sites for the Mars Science Laboratory, a Hummer of a rover due for launch in 2009. You'll also find a very weird picture of defrosting dunes in Mars' north polar region. When the ground thaws, dark dust trickles down the dunes, creating bizarre patterns

Coming back to Earth
Closer to home, the space shuttle Endeavour's mission to the international space station is starting to yield some great pictures, ranging from the launch to close-up views of the shuttle as seen from the station. As Endeavour's flight continues, you can check for updates from NASA's shuttle mission Web site and its space station photo gallery.

Meanwhile, Earth-watching satellites are providing fascinating perspectives on the planet below. For example, Europe's Envisat spacecraft has documented this month's breakup of the massive A53A iceberg, just east of South Georgia Island in the southern Atlantic. NASA's Earth Observatory always has something worth seeing, including this unorthodox view showing how Earth's nighttime side looked during different phases of last month's total lunar eclipse. And there's something new every day from NASA's MODIS Web.

Take the grand tour
For the ultimate tour of space imagery, click through the latest installment of our "Space Shots" slide show.  You'll find killer views of the lunar eclipse, northern lights and other space attractions.

Folks always ask where they can find larger versions of the slide show images, for use as desktop wallpaper or for printing on photo stock. Many of the images come from copyrighted sources, but here are pointers to some of our sources: