Venus' Morphing Vortex Trumps Saturn's Hexagon

New analysis of images taken by ESA's Venus Express orbiter has revealed surprising details about the remarkable, shape-shifting collar of clouds that swirls around the planet's South Pole (left). This fast-moving feature is all the more surprising since its centre of rotation is typically offset from the geographical pole.

Several planets in the Solar System, including Earth, have been found to possess hurricane-like polar vortices, where clouds and winds rotate rapidly around the poles. Some of these take on strange shapes, such as the hexagonal structure on Saturn.

"The longevity of Saturn's hexagon makes this something special, given that weather on Earth lasts on the order of weeks," said Kunio Sayanagi, a Cassini imaging team associate at the California Institute of Technology. "It's a mystery on par with the strange weather conditions that give rise to the long-lived Great Red Spot of Jupiter."

The hexagon was originally discovered in images taken by the Voyager spacecraft in the early 1980s. It encircles Saturn at about 77 degrees north latitude and has been estimated to have a diameter wider than two Earths. The jet stream is believed to whip along the hexagon at around 100 meters per second (220 miles per hour).

'Now that we can see undulations and circular features instead of blobs in the hexagon, we can start trying to solve some of the unanswered questions about one of the most bizarre things we've ever seen in the solar system, said Kevin Baines, Atmospheric scientist at NASA's Jet Propulsion Laboratory after viewing Cassini images in 2009. Solving these unanswered questions about the hexagon and Venus's vortex will help us answer basic questions about weather that we're still asking about our own planet.

Scientists have known about the presence of swirling clouds around the poles of Venus since they were first imaged by Mariner 10 in 1974. At the same time, it was discovered that Venus' upper winds sweep westwards around the planet in only four days, 60 times faster than the rotation of the solid surface of the planet -- a phenomenon known as superrotation.

Thermal infrared imagery from the Pioneer Venus spacecraft subsequently revealed an enormous depression in the cloud blanket at the North Pole. This relatively warm polar 'hole' was thought to be caused by downward movement of gases, rather like water flowing down a drain. However, detailed examination of the thick clouds and dense atmosphere over the South Pole had to wait until the arrival of Venus Express in April 2006.

During its first orbit around the planet, multi-wavelength observations confirmed for the first time the presence of a huge 'double-eye' atmospheric vortex at the planet's South Pole. Some 2000 km across, it was comparable to the structure that had previously been detected at the North Pole.

This animation is a reconstruction of the motion of the polar atmosphere of Venus, created by taking 3.8 micron radiance maps of the polar region, obtained during May 2007, and applying a shift (a rotation and translation) to the image based on the measured wind speeds. The cross marks the South Pole and the white circle marks the centre of rotation of the polar atmospheric vortex. The outer edge of the figure is the latitude circle of 75 degrees.

Since then, high-resolution infrared measurements obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on Venus Express have revealed that the southern vortex is far more complex than previously believed. The VIRTIS images, taken at wavelengths of 3.8 and 5.0 microns, are ideal for tracking polar features on both the day and night sides of the planet, probing the polar cloud layer at an altitude of about 65 km.

The new observations, reported this week in the journal Science on the Science Express website, show that the vortex has a highly variable shape and internal structure. Images show that its morphology is constantly changing on timescales of less than 24 hours, as a result of differential rotation.

"The southern vortex is very dynamic compared with a hurricane on Earth, which remains stable for several days," said Håkan Svedhem, ESA's Venus Express Project Scientist. "It can take almost any shape, so although it often looks like an 'S' or figure 8, it may become completely irregular, even chaotic, in appearance."

The rapid shape changes indicate complex weather patterns, which are strongly influenced by the fact that the centre of the vortex does not coincide with the geographical pole.

The VIRTIS images show that the speeds of the zonal (east-west) winds change rapidly with latitude, revealing that the vortex is continually being pulled and stretched by wind shear. The apparent reversal of the zonal wind close to the pole is a consequence of the non-alignment of the centre of rotation of the vortex with the planet's rotational axis. This causes an apparent inversion of the wind direction between the pole and the centre of rotation.

The new analysis also reveals that the centre of rotation of the vortex drifts right around the pole over a period of 5-10 Earth days. Its average displacement from the geographical South Pole is about three degrees of latitude, or several hundred kilometres. The data show no evidence of any link between the positions of these 'centroids' and local solar times, as might result from a solar tide-related forcing of the superrotation.

The Venus Express mission is currently funded until 2014, offering further opportunities for in-depth studies of the super-rotating atmosphere and the shape-shifting southern polar vortex. However, its highly elliptical orbit means that Venus Express flies too close to the planet's North Pole for detailed imaging studies of its other polar vortex.

"It seems likely that the northern polar vortex has a similar structure and behaves in a similar way," noted Svedhem. "However, the spacecraft flies very close to the North Pole, so it can only see a small region. Confirmation of the behaviour of the northern vortex will have to wait until future missions."

"Results such as these show how interesting Venus is to study and how important it is to study other worlds," Svedhem added. "They enable us to compare the processes that take place on Venus, a planet with a thick atmosphere, with those on our Earth."

The perplexing mysteries of Saturn's hexagon was first spotted as a hexagonal weather pattern when NASA scientists stitched together Voyager images of Saturn's north pole. Cassini has obtained higher-resolution pictures of the hexagon –- which tells scientists it's a remarkably stable wave in one of the jet streams that remains 30 years later -– but scientists are still not sure what forces maintain the object.

After waiting years for the sun to illuminate Saturn's north pole again, cameras aboard NASA's Cassini spacecraft captured detailed images yet of the intriguing hexagon four times the size of Earth crowning the planet.

Because Saturn does not have land masses or oceans on its surface to complicate weather the way Earth does, its conditions should give scientists a more elementary model to study the physics of circulation patterns and atmosphere, said Kevin Baines, an atmospheric scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., who has studied the hexagon with Cassini's visual and infrared mapping spectrometer.

The images of the hexagon, whose shape is the path of a jet stream flowing around the north pole, reveal concentric circles, curlicues, walls and streamers not seen in previous images.
The last visible-light images of the entire hexagon were captured by NASA's Voyager spacecraft nearly 32 years ago, the last time spring began on Saturn.

After the sunlight faded, darkness shrouded the north pole for 15 years. Much to the delight and bafflement of Cassini scientists, the location and shape of the hexagon in the latest images matched up with what they saw in the Voyager pictures.

Early hexagon images from Voyager and ground-based telescopes suffered from poor viewing perspectives. Cassini, which has been orbiting Saturn since 2004, has a better angle for viewing the north pole. But the long darkness of Saturnian winter hid the hexagon from Cassini's visible-light cameras for years. Infrared instruments, however, were able to obtain images by using heat patterns. Those images showed the hexagon is nearly stationary and extends deep into the atmosphere. They also discovered a hotspot and cyclone in the same region.

Scientists are still trying to figure out what causes the hexagon, where it gets and expels its energy and how it has stayed viable for so long. They plan to search the new images for clues, taking an especially close look at the newly identified waves that radiate from the corners of the hexagon -- where the jet takes its hardest turns -- and the multi-walled structure that extends to the top of Saturn's cloud layer in each of the hexagon's six sides. Scientists are also particularly intrigued by a large dark spot that appeared in a different position in a previous infrared image from Cassini. In the latest images, the spot appears in the 2 o'clock position.

The Daily Galaxy via European Space Agency, JPL/NASA and Science Express

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