Paris, Feb 22 : Observations by a spacecraft of the European Space Agency has revealed that Venus has a bright haze layer made up of sulphuric acid, which indicates that the planet has a changeable and extremely large-scale weather.
Images from a monitoring camera aboard the Venus Express, indicated that bright hazes appear in a matter of days, reaching from the South Pole to the low southern latitudes and disappearing just as quickly.
Earlier, in July 2007, VMC (Venus Monitoring Camera) captured a series of images showing the development of the bright southern haze. Within days, the high-altitude veil continually brightened and dimmed, moving towards equatorial latitudes and back towards the pole again.
During these episodes, the brightness of the southern polar latitudes increased by about a third and faded just as quickly, as sulphuric acid particles coagulate.
"This bright haze layer is made of sulphuric acid," said Dmitri Titov, VMC Co-Investigator and Venus Express Science Coordinator.
That composition suggests the existence of a formation process to the VMC team.
Though Venus is covered with clouds, and is seen as a featureless, unchangeable globe at visible wavelengths of light, ultraviolet light reveals a truly dynamic nature of the planet.
New images also detect transient dark and bright markings striping the planet, indicating regions where solar ultraviolet radiation is absorbed or reflected, respectively.
Such global weather suggests that fast dynamical, chemical and microphysical processes are at work on the planet.
At an altitude of about 70 km and below, Venus' carbon dioxide-rich atmosphere contains small amounts of water vapour and gaseous sulphur dioxide. These are usually buried in the cloud layer that blocks our view of the surface at visible wavelengths.
However, if some atmospheric process lifts these molecules high up above the cloud tops, they are exposed to solar ultraviolet radiation. This breaks the molecules, making them highly reactive. The fragments find each other and combine quickly to form sulphuric acid particles, creating the haze.
"The process is a bit similar to what happens with urban smog over cities," said Titov.
As to what causes the water vapour and sulphur dioxide to well up in the first place, Titov said that it is probably an internal dynamical process in the planet's atmosphere. Also, the influence of solar activity on haze formation has not been completely ruled out.
With over 600 orbits completed, the VMC team now plans to look for repeating patterns of behaviour in the build-up and decrease of the haze layer.