Bright hazes that mysteriously appear and then disappear on Venus in a matter of days have revealed a new dynamic feature of the planet?s cloudy atmosphere that is unlike anything on Earth.

The European Space Agency's Venus Monitoring Camera (VMC) captured a series of images showing the development of a bright haze over the southern latitudes of the planet in July 2007. Over a period of days, the high-altitude veil continually brightened and dimmed, moving towards equatorial latitudes and then back towards the south pole.

These transient dark and bright markings indicate regions on the cloud-covered world where solar ultraviolet radiation is being absorbed and reflected by sulfuric acid particles, mission scientists said this week.

Gaseous sulfur dioxide and small amounts of water vapor are usually found below altitudes of about 43 miles (70 kilometers) in Venus' carbon-dioxide rich atmosphere. These molecules are usually shrouded from view by cloud layers above that block our view to the surface at visible wavelengths.

ESA scientists think the sulfuric acid particles that make up the bright haze are created when some atmospheric process lifts the gaseous sulfur dioxide and water vapor high up above the cloud tops where they are exposed to ultraviolet radiation from the sun.

The UV radiation breaks up the molecules, making them highly reactive. The fragments of the molecules eagerly seek each other out and combine to form the sulfuric acid particles.

"The process is a bit similar to what happens with urban smog over cities," said mission team member Dmitri Titov of the Max Planck Institute for Solar System Research in Germany.

Exactly what causes the sulfur dioxide and water vapor to well up is not known, but Titov says it is likely some internal process of Venus' atmosphere.

The transient dark markings on the VMC images are even more of a mystery. They are caused by something that absorbs UV radiation, but scientists don't yet know what the chemical is.

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