Newly released images of a double hurricane-like feature over Venus's south pole show that it is even more complex than first thought. Identifying the forces that shape it could help unlock the secrets of the planet's thick atmosphere, which keeps Venus scaldingly hot via a runaway greenhouse effect.
Scientists first noticed the structure, which looks like a double-eyed hurricane, in images taken by the European Space Agency's Venus Express spacecraft in April 2006, just after it entered orbit around the planet (see Venus's double vortex raises new mystery (http://www.newscientistspace.com/article/dn9380.html)). Other spacecraft had previously seen a similar structure over the north pole.
The pattern is thought to be a product of the super-strong winds that tear westward around the planet. But scientists do not understand why the winds produce a double vortex rather than a single one.
The newly released images show the double vortex over the south pole is even more complex than first thought. Its structure varies with height, with the double vortex more prominent and intricate at lower altitudes.
These new images were taken on 29 May 2006, from a distance of 64,000 kilometres – about three times closer than the April images. Watch an animated gif of the double vortex on Venus's south pole (http://esamultimedia.esa.int/images/Science/VenusExpress/South_polar_vortex_close_up_1_top_down_b_anim_H.gif).
"We are building a true 3D view of the vortex," says Venus Express scientist Giuseppe Piccioni at the Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF) in Rome, Italy. "Then we hope to be able to better understand what are the driving forces that shape it."
Because the temperature of the atmosphere changes with altitude, scientists can cut through the cloud layers to probe different atmospheric depths by studying different wavelengths of infrared light. The new images measure the double vortex between 59 km and 70 km above the planet's surface.
"It’s a really important part of the picture of how Venus's atmosphere works," says Mark Bullock of the Southwest Research Institute in Boulder, Colorado, US, who is not part of the spacecraft team.
Understanding the origins of the double vortices could help scientists figure out how Venus's super-strong winds themselves are generated, he says. And because the vortices influence how the atmosphere circulates, they "probably play a fairly important role in establishing the temperature of the atmosphere", Bullock told New Scientist.
Scientists have also been scrutinising some mysterious dark stripes at the top of Venus's cloud layer, which had been spotted by previous spacecraft. They are called "UV absorbers" or "blue absorbers", because they absorb light strongly at ultraviolet and blue wavelengths.
Scientists are eager to determine their composition because they account for almost half of the solar energy Venus absorbs at all wavelengths, so they play a big role in maintaining Venus's hellish climate. The average temperature on the surface is about 460° Celsius.
The spacecraft has also detected what may be traces of long-vanished seas on Venus, by identifying "heavy" water in the planet's atmosphere. One or both of the hydrogen atoms in a heavy water molecule contains a neutron, and its extra weight makes it harder for heavy water molecules to be lost to space.
Observations suggest all of the planets began with the same ratio of normal water to heavy water. But decades ago, NASA's Pioneer Venus mission discovered that ratio has changed on Venus, suggesting it has lost significant amounts of water over its 4.5 billion year history.
More precise measurements from Venus Express should allow scientists to pin down just how much water Venus lost, which conceivably might have been enough to fill entire oceans.
The spacecraft is also making measurements of how quickly Venus's atmosphere is escaping into space, which should tell them the rate at which it lost its ancient water. "The prevailing theory is that it took about 600 million years, which is pretty quick," says Bullock. "But it is possible that Venus kept its oceans for much longer than that."
But the spacecraft is not making as many observations as astronomers had originally hoped. The Planetary Fourier Spectrometer (PFS), the spacecraft's highest-resolution instrument for measuring the spectrum of light coming from Venus, has been stuck in a position that does not allow it to view Venus (see Mirror jams on Venus Express spacecraft (http://www.newscientistspace.com/article/dn9123)).
Mission scientists still hope to get it working, but if they fail, they say they should be able to make up for most of the loss with other instruments.