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).
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.
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 acid 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.
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.