The Puzzling 'Eye Of
A Hurricane' On Venus
(13 March 2008) Venus Express has
constantly been observing the south pole of Venus and has found it to be
surprisingly fickle.
An enormous structure with a central part
that looks like the eye of a hurricane, morphs and changes shape within a
matter of days, leaving scientists puzzled.
The dynamic nature of the South polar vortex can be seen in this video sequence, composed of images obtained on 7 April 2007. The video is composed of a series of ten images taken over a period of five hours at half-hourly intervals, at a wavelength of 3.9 micrometres. The vortex is rotating with a period of about 44 hours. In video, the point of view of the observer has been rotated at the same rate so that the vortex appears stationary in the centre of the image. These images were obtained as part of the ‘VIRTIS movie’ sequence, previously reported on 7 May 2007. This movie shows that the vortex is very complex, with atmospheric gases flowing in different directions at different altitudes. The bright region at the top-centre appears to be the most active region and its brightness suggests that it is where atmospheric gases are flowing downward. Extending leftward from this point is an ‘S’-shaped feature which is seen frequently in the polar vortex. A very similar feature was observed at the northern polar vortex in 1979 by Pioneer Venus. (courtesy: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
The eye of the hurricane is at the centre of
a 2000 km-wide vortex. It was discovered in 1974 by the Mariner 10 spacecraft.
There is a similar structure on the planet's north pole, which was observed by
the Pioneer Venus mission in 1979.
Venus Express scientists have been
studying the structure in the thermal infrared, the wavelength range that
reveals the temperature at the cloud-tops. Seen in this wavelength, the core of
the vortex appears very bright, probably indicating that a lot of atmospheric
gases are moving downward in the region, which creates a depression at the
cloud-tops, making the region hotter.
This image, of the ‘eye of the hurricane’ on Venus was taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. This picture shows a region in the venusian atmosphere about 60 km from the surface, at a wavelength of about 5 micrometres. In this figure, the dipole assumes an eye-like shape and from here until the last image, it is possible to see how its shape evolves rapidly in a span of only 24 hours. The yellow dot in the image indicates the location of the south pole. (courtesy: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
"Simply put, the enormous vortex is similar to what you might see in your bathtub once you have pulled out the plug" says Giuseppe Piccioni, co-Principal Investigator for the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express, at IASF-INAF, Rome, Italy.
This image, of the ‘eye of the hurricane’ on Venus was taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. This picture shows a region in the venusian atmosphere about 60 km from the surface, at a wavelength of about 5 micrometres. This image is taken only 4 hours after the previous one. In this short time, the vortex has already evolved into a different shape. Using two images taken at different times, it is possible to measure the wind speeds and study the dynamics of the region. The yellow dot in the image indicates the location of the south pole. (courtesy: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
The fickle puzzle
In June
2006, the vortex appeared hourglass-shaped, closely matching observations in
the north polar region by Pioneer Venus. Now we know that it changes its shape
within a matter of days, from orbit to orbit. The image taken on 26 February
2007 shows the 'classic' dipole shape at the centre of the vortex, similar to
that which has been observed previously. But an image taken a mere 24 hours
earlier shows the centre of the vortex to be almost circular, indicating that
the shape of this feature can change very fast. At other times, it is typically
oval.
The dynamic nature of the vortex can be seen clearly in the video.
It shows that the vortex is very complex, with atmospheric gases flowing in
different directions at different altitudes.
This image, of the ‘eye of the hurricane’ on Venus was taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. This picture shows a region in the venusian atmosphere about 60 km from the surface, at a wavelength of about 5 micrometres. This image is taken 24 hours after the first of the series, or after one complete Venus Express orbit. Here the vortex has become more circular and less elongated. The yellow dot in the image indicates the location of the south pole. (courtesy: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
What creates the
puzzle?
Scientists are not sure what actually creates the vortex.
Colin Wilson, at the University of Oxford, says, "One explanation is that
atmospheric gases heated by the Sun at the equator, rise and then move
poleward. In the polar regions, they converge and sink again. As the gases move
towards the poles, they are deflected sideways because of the planet's
rotation."
The dynamic nature of this vortex is similar to behaviour
observed in other vortices on Earth, including those observed at the centre of
hurricanes.
This image, of the ‘eye of the hurricane’ on Venus was taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. This picture shows a region in the venusian atmosphere about 60 km from the surface, at a wavelength of about 5 micrometres. This image was acquired a day later (after the previous image). Here the shape of the vortex is stretched out in two opposite directions and is almost hourglass-shaped - this shape is called a dipole. The yellow dot in the image indicates the location of the south pole. (courtesy: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
Investigators will keep a close watch on the polar region and its variability, in order to gain a better understanding of how it works.
This is a picture of Venus’s atmosphere, taken by the Venus Monitoring Camera (VMC) during Venus Express orbit number 458 on 23 July 2007. The view shows the southern hemisphere of the planet. It clearly shows enormous, spiral cloud features that form at mid-latitudes. One explanation for why the polar vortex forms is that atmospheric gases heated by the Sun at the equator, rise and then move poleward. In the polar regions, they converge and sinks again. As the gases moves towards the poles, they are deflected sideways because of the planet’s rotation. At the top of the cloud layer, this gives rise to cloud features seen in the image. (courtesy: ESA/ MPS/DLR/IDA)
The discovery was reported in 'South-polar
features on Venus similar to those near the north pole' by G. Piccioni et al.
published in Nature on 29 November 2007.
(source: ESA)
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