Tuesday, April 2, 2013

Vortices on Venus

A new study of Venus’ South Polar Vortex has shown that its cyclone is much more chaotic and unpredictable than previously thought. The study shows that the centre of rotation of the polar vortex wanders around the pole differently and at different altitude levels within the Venusian clouds.

The study used infrared images from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), aboard the European Space Agency's Venus Express spacecraft. VIRTIS combines three observing channels in one instrument: two are devoted to spectral mapping and the third is devoted to spectroscopy. This allows spectral images to be obtained at all levels of the atmosphere including the upper and lower Venusian cloud decks. More on VIRTIS: http://bit.ly/XSQeN1.

While Venus is similar to Earth in size, it is on average 108 million km from the Sun, about 30% closer than the Earth. Venus rotates much more slowly around its axis so that one day on Venus is equivalent to 243 Earth-days (a Venusian year is 224.7 Earth days). Venus rotates in a retrograde direction from east to west, the opposite direction of Earth’s spin. Venus has a dense atmosphere of carbon dioxide and a surface pressure 90 times that found on Earth; these cause a runaway greenhouse effect that raises surface temperatures up to 750 K (477 °C). A dense layer of sulphuric acid clouds 45-70km above the surface covers the planet. These clouds move up to 360km/h and take four Earth days to spin around the planet. This phenomenon is known as superrotation, where the atmosphere of a planet rotates faster than the surface. The only other planetary body in the Solar System where atmospheric superrotation is common is on Titan.

At the poles of Venus, permanent vortices are produced by the atmospheric circulation. These vortices can change shape and size every twenty four hours. The new analysis shows that the winds in this vortex also change from day-to-day. It is the unpredictable nature of the Venus polar vortices that sets them apart from the more stable and predictable polar vortices found on planets like Earth and Saturn.

The cyclone on Venus extends vertically through the atmosphere over more than 20 kilometres. The centres of rotation at two different altitude levels (42 and 62 km above the surface) are not aligned however, and both wander around the Venusian south pole with no pattern at velocities of up to 55km/h. the new research also found that even when the cross-winds were about the same at both altitudes, there was still a strong vertical gradient; the winds increased by as much as 3km/h for every kilometre of height, which may cause atmospheric instabilities.

The atmospheric superrotation feds the vortices, which are trapped in the polar regions of Venus by a wide, shallow collar of cold air in subpolar latitudes. The eye at the centre of the vortex is about 2200 kilometres by 1400 kilometres in area. Though scientists have observed the vortex for many years, how the vortex is able to alter its shape in one day or remain stable for weeks is still unknown.

The upper panels of the image show the upper clouds at 63km above the surface and the lower panels present the vortex as observed in the lower clouds at 42km altitude level.

More on Venus: http://on.fb.me/14A5NQL

I. Garate-Lopez, R. Hueso, A. Sánchez-Lavega, J. Peralta, G. Piccioni, P. Drossart. A chaotic long-lived vortex at the southern pole of Venus. Nature Geoscience, 2013; DOI: 10.1038/NGEO1764 (http://www.nature.com/ngeo/journal/v6/n4/full/ngeo1764.html)

Image: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Universidad del País Vasco (I. Garate-Lopez)

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