VIRTIS Shows Sides of Venus

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The south pole of Venus and its gigantic double vortex have been pictured as never before in a series of videos from the European Space Agency's Venus Express, which has been capturing atmospheric details of Venus's two sides -- day and night -- simultaneously, at different altitudes.

The planet's south pole, the site of a huge double-vortex atmospheric system that the Venus Express mission discovered about a year ago, are key to understanding the global atmospheric dynamics on Venus and will contribute to a better comprehension of the global meteorology of the planet.
A frame from a composite video sequence obtained by the Ultraviolet, Visible and Near-Infrared Mapping Spectrometer (VIRTIS) on board ESA’s Venus Express. The intersection between the polar atmospheric structures seen at different wavelengths is visible in good detail due to the optical properties of the clouds. (Images: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA)

In their search for clues on how to solve the global atmospheric puzzle, the team of scientists behind the ultraviolet, visible and near-infrared mapping spectrometer (VIRTIS) on board Venus Express tried something new by focusing on the south pole from the advantageous position of the orbit apoapsis, or apocentre -- the furthest distance of the spacecraft from the planet. This allows the instrument to keep the target in the field of view longer than in other portions of the orbit, where the spacecraft travels faster. In this position, they made efficient use of the multiwavelength capability of VIRTIS.

By using wavelengths longer than 3 µms in the thermal infrared range, VIRTIS can obtain a combined view of the day and night sides simultaneously. At shorter wavelengths, the difference between the thermal radiation emitted on the day and night sides is too high to observe both regions simultaneously without "blinding" some channels of the camera.

"It is comparable to looking at bright, sun-illuminated snow and at a dark sky without having to change your glasses," said Giuseppe Piccioni, VIRTIS co-principal Investigator. "In addition, within this observation process, not only can we look at the dark and lit sides of the south pole at the same time, but we can also look into the atmosphere at different depths. What we are building is the most complete three-dimensional data set of the Venusian atmosphere to date."

THe VIRTIS videos of the south polar vortex are the result of combined observations at two different wavelengths (3.8 and 1.7 µms, respectively) used at the same time (view videos). The various images were taken over five orbits, over a timespan of about eight hours per orbit. The 3.8-µm channel was chosen because of its compatibility (in exposure time) with the 1.7-µm observations, and for its capability to provide information about the cloud deck at an approximately 65-km altitude over the planet. The 1.7-µm wavelength was chosen to probe the atmosphere below the clouds when looking at the night side.

It is clearly possible to see that the morphology of the vortex changes a lot during the eight-hour observation session and from one orbit to the next (one Venus Express orbit is 24 hours long), Piccioni said. "It is interesting to note that due to "bad weather conditions," by the time of the observations, the videos do not show the maximum achievable image contrast. In fact, the visibility of the polar structure was somewhat reduced by the local increase of the upper atmospheric haze."

If weather permits, by extending the timespan of future observations, it may be possible to obtain even clearer and more detailed views of the polar vortex, he added.

Pierre Drossart, the other co-principal investigator for VIRTIS, said, "With video sequences of this kind, combining all the pieces of information together, we can study the dynamics and the evolution of the vortex both in the short and the long term. "What we want to understand is the overall 3-D thermal structure of the vortex, especially the vertical variation of the horizontal winds."

The next step will be to correlate this data, and data collected in the next sessions, with fluid-dynamics computer models. This will eventually help the scientists create the best possible atmospheric model of Venus to date.

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Published: May 2007
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