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Space Topics: Venus Express

The Year in Pictures: 2007

Venus Express: Observing the Swirling Currents in Venus' Clouds

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Venus' south polar vortex
Credit: ESA / VIRTIS-VenusX / INAF-IASF / Obs. de Paris-LESIA (A.Cardesin Moinelo, IASF-INAF)

This animation is cropped from a much larger one (GIF, 3.5 MB) captured by Venus Express, focused on Venus' south polar dipole, a complex atmospheric vortex-like feature situated over the south pole of the planet. The spacecraft is in a highly elliptical orbit with a one-day period, so for several hours on each orbit it travels slowly near its apoapsis about 65,000 kilometers below the southern hemisphere. The slow drift of the spacecraft near apoapsis allows it to observe the swirling motions of the south polar dipole for almost 20 hours a day. This animation contains observations from two different days (August 7 and 10, 2007); for each day, the individual frames are separated by half an hour.

The video was obtained by the VIRTIS imaging spectrometer at an infrared wavelength of 3.8 microns, allowing the instrument to see thermal emission from the tops of Venus' clouds at an altitude of 60 to 65 kilometers above the surface.

How does that compare with cloud patterns on Earth? A convenient way to compare the two planets is to compare what happens at similar pressure levels.  The air pressure is at a maximum at the surface -- 1 bar for Earth's surface and a crushing 90 bars for Venus' -- and decreases exponentially with altitude.  Clouds on both planets form at approximately the same pressure level, between the 1- and 0.1-bar levels.  For Earth, this is between 0 and 15 kilometers above the ground, at the same elevations as Earth's mountains, so the swirls of Earth's clouds are intimately connected with the topography, temperature, and moisture content of the land and sea below them.  On Venus, however, the 0- to 0.1- bar level lies at around 50 to 60 kilometers above the ground; in other words, there are fully 50 kilometers more atmosphere below Venus' cloud deck. Venus' surface is thus drowned under a global "sky ocean," with currents and motions unconnected to the ground. Venus Express is now elucidating the dynamics of Venus' atmosphere, helping scientists understand the currents in this airy ocean.