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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