The Shape-Shifting Southern Vortex of Venus (Venus Express)

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid8596

The shape-shifting southern vortex of Venus
European Space Agency
07 Apr 2011

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. This fast-moving
feature is all the more surprising since its centre of rotation is
typically offset from the geographical pole. The results of this study
are published online in Science Express today.

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, but none of them are as variable or
unstable as the southern polar vortex on Venus.

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.

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 Hߥkan 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. Although the mean zonal wind
is retrograde (blowing from east to west) and approximately uniform
equatorward of 84°S, its speed decreases toward the pole and becomes
prograde (blowing from west to east) close to 87°S. A slightly slower
wind is detected at 75°S, coinciding with the poleward edge of the cloud
collar which coincides with a region of colder air surrounding the
centre of the vortex.

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.

"Our results highlight the importance of the polar region in the global
dynamics of the Venus atmosphere," said David Luz of the Centre for
Astronomy and Astrophysics at the University of Lisbon, Portugal, the
lead author of the Science paper. "/They provide new insights into the
complex processes that shape the polar vortices and their role in the
atmosphere's 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."

Notes for Editors

The results of this study are published online in Science Express on 7
April 2011. Other authors of the paper were: by D.L. Berry of the
University of ߉vora, Portugal; G. Piccioni and R. Politi of INAF-IASF
Rome, Italy; P. Drossart and S. Erard from Observatoire de Paris,
France; C.F. Wilson of Oxford University, United Kingdom; and F.
Nuccilli from INAF-IFSI Rome, Italy.

Reference publication
Luz, D., et al., "Venus's Southern Polar Vortex Reveals Precessing
Circulation", published online on Science Express, 7 April 2011.
DOI:10.1126/science.1201629

Contacts

David Luz
Centre for Astronomy and Astrophysics
University of Lisbon, Portugal
Phone: +351 938553711
Email: dluzoal.ul.pt

Colin F. Wilson
Oxford University, UK
Phone: +44 7939 202 151
Email: wilsonatm.ox.ac.uk

Giuseppe Piccion
VIRTIS-Venus Express PI
Istituto Nazionale di Astrofisica-IASF, Italy, Rome
Phone: +39 06 45488 445
Email: giuseppe.piccioniiasf-roma.inaf.it

Pierre Drossart
VIRTIS-Venus Express PI
LESIA, Observatoire de Paris, France
Phone: +33 145077664
Email: pierre.drossartobspm.fr

Hߥkan Svedhem
Venus Express Project Scientist
Directorate of Science & Robotic Exploration
ESA, The Netherlands
Phone: +31 71 565 3370
Email: hsvedhemrssd.esa.int