The Cloud
Layer
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.
Here is a
more detailed
discussion
of the infrared imaging from the Galileo spacecraft shown in the right
photograph, including an explanation of the "false color" used in such images.
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.
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