With data from Venus Express, scientists have learnt that the equatorial areas on Venus that appear dark in ultraviolet light are regions of relatively high temperature, where intense convection brings up dark material from below. In contrast, the bright regions at mid-latitudes are areas where the temperature in the atmosphere decreases with depth. The temperature reaches a minimum at the cloud tops suppressing vertical mixing. This annulus of cold air, nicknamed the ‘cold collar’, appears as a bright band in the ultraviolet images.


 
Using Venus Express, it is possible to compare what the planet looks like at different wavelengths, giving scientists a powerful tool with which to study this planet’s turbulent atmosphere.

The lower left shows a differential temperature map (not absolute values) of the venusian cloud tops, derived from the Visible and Infrared Thermal Imaging Spectrometer, VIRTIS, on the planet’s night-side. The darker the region, the colder the cloud tops. To the upper right is an ultraviolet image of the venusian day side, captured by the Venus Monitoring Camera, VMC, simultaneously with the night-side infrared image.

The ultraviolet reveals the structure of the clouds and the dynamical conditions in the atmosphere, whereas the infrared provides information on the temperature and altitude of the cloud tops.


 
A Venus Monitoring Camera ultraviolet image with a superimposed colour mosaic, showing the altitude of the cloud tops. The colour mosaic was derived from simultaneous pressure measurements by the Visible and Infrared Thermal Imaging Spectrometer.


 
In this mosaic, infrared images taken at a wavelength of 5 micrometres (in red) are overlaid on ultraviolet images, taken at 0.365 micrometres.

The bright areas in the infrared images represent the temperatures at the cloud tops (dark regions denote lower temperatures). The oval feature that stands out in these images is the giant eye of a hurricane, or the polar vortex, at the planet’s south pole. Its centre is displaced from the south pole and the structure measures about 2000 km across, rotating around the pole in about 2.5 days. The atmosphere rotates anticlockwise in the figure.


 
The lines indicate regions of constant temperature. They were derived from spectrometry data from the soviet Venera-15 spacecraft, taken in the northern hemisphere of Venus. Bright blue regions show areas that appear bright in the ultraviolet whereas dark blue regions indicate areas that appear dark.



Release date: 14 July 2009