Sulphuric acid clouds blanket Venus at an altitude of between 30 and 45 miles, formed by sulfur dioxide from volcanoes combining with water to form sulfuric acid droplets.
Intense solar radiation above 45 miles should destroy any sulfur dioxide above that height, so scientists were puzzled when a European Space Agency probe found a layer of sulfur dioxide at about 55 miles, an ESA release said.
Scientists say computer simulations show some sulphuric acid droplets may evaporate at high altitude, freeing gaseous sulphuric acid that rises to greater altitude then is broken apart by sunlight, releasing sulphur dioxide gas.
"We had not expected the high-altitude sulphur layer, but now we can explain our measurements," Hekan Svedhem of ESA's Venus Express Project said.
This new understanding may mean a proposed method of mitigating climate change on Earth may not be as effective as originally thought.
Scientists have advocated injecting artificially large quantities of sulphur dioxide into Earth's atmosphere to counteract global warming from increased greenhouse gases.
The proposal stems from observations of powerful volcanic eruptions like the 1991 eruption of Mount Pinatubo in the Philippines that shot sulphur dioxide up into Earth's atmosphere, spreading around Earth in a haze layer that reflected some of the sun's rays back into space, cooling the whole planet by about 0.9 degrees Fahrenheit.
The Venus findings suggests such attempts at cooling our planet may not be successful, because it is not known how quickly the initial protective haze would be converted back into gaseous sulphuric acid, which is transparent and would allow all the Sun's rays through.
"We must study in great detail the potential consequences of such an artificial sulphur layer in the atmosphere of Earth," Venus Express scientist Jean-Loup Bertaux says. "Venus has an enormous layer of such droplets, so anything that we learn about those clouds is likely to be relevant to any geo-engineering of our own planet."
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