Studying the winds on Venus

The different atmospheric layers and respective wind speeds between the equator and 50-55˚ latitude on the southern hemisphere of Venus, as measured by the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) on board ESA’s Venus Express between April 2006 and June 2007.

Using three different wavelengths as windows to three different atmospheric layers, VIRTIS tracked the movement of clouds and determined the speed of the winds moving them. The instrument has provided the first-ever 3-D picture of the venusian winds for an entire planetary hemisphere.

Observing at the infrared wavelength of 1.74 micrometres on the night-side of the planet, VIRTIS can see down to 45-47 km altitude; the clouds at this altitude can be seen because they absorb infrared light coming from the surface and the lower atmospheric layers, and this light is then re-emitted by the clouds themselves. At this height, the average wind speed is about 210 km/h.

Observing in the near-infrared (about 980 nanometres) and in the blue ultraviolet (about 350 nanometres) on the day-side of the planet, VIRTIS can look down to about 61 and 66 km altitude (the higher part of the cloud layer), respectively. Sunlight reflected by the clouds at the two different layers in the infrared and in the ultraviolet makes it possible to track them. The measured wind speeds are 220 and 370 km/h, respectively.