Missions to the surface of Venus (Russian Venera spacecraft) indicate
that the cloud deck begins about 50 km above the surface. Spectral analysis
of the thick cloud layer surrounding Venus suggests that the clouds are
largly composed of sulphuric acid droplets. The cloud layer is rather
featureless in visible light, but shows structure in other wavelengths.
The adjacent image shows the clouds
photographed by Pioneer 10 at UV wavelengths and computer enhanced to
accentuate differences.
Different Wavelengths
Different wavelengths of light penetrate the atmosphere to varying degrees and therefore may be used to study different layers of the clouds and atmosphere. For example, the following images show the clouds of Venus imaged in visible light (with a violet filter), ultraviolet light, and infrared light respectively.
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| Visible Light | Ultraviolet | Infrared |
High Velocity Winds
A comparison of surface and upper cloud velocities
indicates that there are winds in the upper part of the Venusian atmosphere
with velocities as large as 300 km/hour. These winds are comparable in speed
to jetstreams in the Earth's atmosphere, but extend over much larger regions
in the case of Venus. It is not fully understood why these winds have such
high velocities.
Absence of Water Vapor
The clouds contain little water vapor, and there is little evidence for water in any form on Venus. It is speculated that the absence of water is because most water that may have initially been on Venus made its way to the upper atmosphere, where it was broken down by sunlight and interactions with cosmic rays and the solar wind into oxygen and hydrogen, which was then lost to interplanetary space.
Information supplied by: http://csep10.phys.utk.edu