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In the 1960’s Venus’ clouds were first studied by spectroscopy and polarization. On the basis of this, the presence of sulphuric acid was only clearly established in 1973, and confirmed by in-situ analyses throughout many Soviet missions from Venera-12 (1978) and the American mission Pioneer Venus (1979).

Clouds in the higher atmosphere

Particles, in liquid phase, form at very high altitudes, at the level of the top of the upper cloud layer, where the ultraviolet radiation from the sun leads to the photolysis on some atmospheric constituents on Venus.

In particular, SO2 gas forms SO3 in a reaction with O, coming from the photolysis of CO2, and then makes H2SO4 from H2O, which passes to liquid state due to the partial pressure of the sulphuric gases in the surrounding gas.

Clouds in the low atmosphere

In the low atmosphere, we can see the decomposition of fine droplets of sulphuric acid H2SO4 migrating through the stratified cloud structure at the slow speed of about 1 mms-1. They are vapourised when they reach the hotter layers of the base of the clouds, at about 40km altitude.

Venus Low Clouds Infrared
Infrared image of the lower clouds of Venus. This false-colour image is a near infra-red view of the clouds on the night side of Venus, obtained by the instrument NIMS on the Galileo probe as it approached the planet on the 10th February 1990.

The view shows the middle atmosphere (corresponding to altitudes 50-55km above the surface) as turbulent and cloudy. The red colour indicates the heat emitted by the lower atmosphere, shining through the cloud layer, which seem 10 times darker than the brighter holes between the clouds. This cloud layer is at a temperature of -30°C. The clouds seem fluffy but form large compact groups; in the north they form filaments oriented from the east to the west and at the poles they form a complete blanket.

Fascinating UV phenomenon in the atmosphere of Venus

Ultraviolet observations of the cloud layer revealed an intriguing phenomenon. Some zones absorb half of the solar energy received by the planet and reemit it in the form of ultraviolet radiation.

Many explanations of varying levels of fantasy, where put forward: at 50km altitude microbes could be feeding on sulphuric avid and using UV light in an exotic photosynthetic process to extract energy. This is a hypothesis for extreme exobiology but to verify it, one would have to go there with balloon probes.

Venus in UV
Ultraviolet image obtained by the space telescope Hubble (NASA/ESA) 24 January 1995. In ultraviolet light, the structures within the clouds become much more visible. One can see, for example, a horizontal structure in the shape of a Y near the equator. Similar structures were seen by the Mariner 10 and Galileo probes. The structure would indicate the presence of atmospheric waves, similar to cells of high and low pressure on Earth. Clouds at high latitudes seem to follow lines at the same latitude. Darker regions show regions of clouds with higher quantities of sulphuric acid at the top of the clouds.


An image from the Galileo probe dated 4 February 1990. A wide-band spatial filter has been applied to highlight the smaller details. Blue tones were used to show the subtle contrasts in the centre of the clouds, and to remind viewers that the image was taken using a violet filter. The sulphuric acid clouds show a significant dynamo in the equatorial band of the planet, with a movement towards the left (west). Credits: NASA.
False-colour images of the low clouds present on the night side of Venus, obtained by NIMS on the Galileo probe as it approached the planet on the 10th February 1990.
This images shows the heat energy emitted by the lower atmosphere through the clouds. Credits NASA.