By John Matson

Venus is Earth’s closest sibling, in terms of size and proximity, but it remains relatively little explored compared with Earth’s other planetary neighbor, Mars. For instance, NASA and the European Space Agency (ESA) currently have three working Mars orbiters and one active Mars rover between them, whereas at Venus, ESA’s Venus Express spacecraft has the place to itself.

Given how little is known about Venus, the exclusive access Venus Express currently enjoys offers plenty of room to make new discoveries about how the planet works. One new such wrinkle in the Venusian sphere is the finding that Venus’s swirling cloud cover, which obscures the planet’s surface from view in optical light, has a shape-shifting, rapidly migrating vortex over the south pole. A team of scientists from Portugal, Italy, France and the U.K., drawing on observations made by Venus Express, announced the finding in a study published online April 7 in Science.

Venus rotates exceedingly slowly on its axis (and in the opposite direction of the solar system’s other planets)—if you were unlucky enough to live on the scorching surface of the planet, you would experience only about two sunrises and sunsets for every trip around the sun. But its super-rotating atmosphere moves much faster, with clouds zipping along at 60 times the rotational speed of the surface. Explanations for the atmospheric super-rotation have been kicking around since the 1970s, but the source of the phenomenon has not been settled conclusively.

That atmospheric swirling causes vortices at the poles with bright filaments that are visible to infrared eyes such as those of Venus Express’s Visible and Infrared Thermal Imaging Spectrometer. An S-shaped northern polar vortex was discovered in the infrared by NASA’s Pioneer Venus spacecraft in the late 1970s, and Venus Express found a similar-looking feature at the south pole in 2006. But the story got more complex as Venus Express returned to the south polar region time and time again to find the southern vortex had moved or changed shape entirely.

“I think the most striking thing about it is it changes so much from day to day,” says lead study author David Luz, a planetary scientist at the University of Lisbon in Portugal. Venus Express is in a 24-hour polar orbit, so it can take a snapshot of the south pole relatively often but cannot watch gradual changes unfold over timescales of several hours. “When it comes back the next day the feature has changed,” Luz says.

The center of rotation of the vortex is offset from the planet’s south pole by about three degrees of latitude, but it migrates around the pole over the course of several days. “We still haven’t figured out what causes it to move around, but we suspect it is related to what is called the meridional circulation,” Luz says, referring to an atmospheric circulation pattern that moves air at high altitudes from the equator to the poles, where the air sinks for an equatorward return at lower altitudes. “We expect that over the poles it is down-welling like a drain,” Luz says. “If the center of rotation is drifting, then we think it probably means that the point of maximum down-welling is drifting.”

Figuring out why the vortex at the pole moves and changes so quickly might help planetary scientists develop a better understanding of Venus’s extreme atmospheric system. “We would like to know how the meridional circulation relates to the motion of the vortex, how the global circulation is feeding the vortex—that’s the missing link,” Luz says.

For now, Venus Express’s latest finding has at least provided a bit more detail to the somewhat sketchy picture scientists have of our neglected planetary neighbor. “It doesn’t solve the problem of the super-rotation” of Venus’s atmosphere, Luz says. “But it’s a clue.”

http://www.scientificamerican.com/article.cfm?id=venus-polar-vortex