A swirling atmospheric sink on Venus
17 March 2008
Imagine a planet with two drains, one in the north and the other in the south. Each drain features a whirlpool of atmospheric gases which are swirling around at high speed as they plunge down towards the surface. This seems to be what is happening on Earth’s sister planet, Venus, according to ESA’s Venus Express spacecraft.
Europe’s robotic explorer has been in orbit around the second planet from the Sun since 11 April 2006. Some of its earliest observations showed a dark, hurricane-like feature (known as a vortex) over the south pole. A similar cloud structure had already been found over the north pole.
Since then, Venus Express has been observing the south pole of Venus constantly and the centre of the “whirlpool” has turned out to be surprisingly changeable. On 26 February 2007, it was hourglass-shaped. 24 hours earlier, it was almost circular. At other times, it appears oval (egg-shaped). Clearly, this feature is quite a rapid shape-shifter.
What causes the 2000 km-wide vortex? Venus Express has shown that its centre is warmer than its surroundings. Like the eye of a hurricane, this hotter region marks where atmospheric gases are moving downward, creating a depression at the cloud tops.
“The enormous vortex is similar to what you might see in your bathtub once you have pulled out the plug,” says Venus Express scientist Giuseppe Piccioni.
Scientists believe the carbon dioxide atmosphere is heated by the Sun at the equator. The gas rises and then moves poleward. In the polar regions it sinks, forming the atmospheric “drain”.
