Boggling chemistry

Venus has lessons for earthlings, and lessons of an unromantic kind!

It was the detection of a sulphur dioxide layer at 90-110 km by ESA's Venus Express orbiter in 2008 that posed a complete mystery and finally has been solved with some thinking. In the process throwing some darts at geo-engineering.

Venus is blanketed in sulphuric acid clouds that block our view of the surface. The clouds form at altitudes of 50-70 km when sulphur dioxide from volcanoes combines with water vapour to make sulphuric acid droplets. Any remaining sulphur dioxide should be destroyed rapidly by the intense solar radiation above 70 km. So, how did the layer form at 100 kms?

Some sulphuric acid droplets could have evaporated at high altitude, freeing gaseous sulphuric acid that is then broken apart by sunlight, releasing sulphur dioxide gas.

Nobel prize winner Paul Crutzen has recently advocated injecting artificially large quantities of sulphur dioxide into Earth's atmosphere at around 20 km to counteract the global warming resulting from increased greenhouse gases. The proposal stems from observations of powerful volcanic eruptions, in particular the 1991 eruption of Mount Pinatubo in the Philippines that shot sulphur dioxide up into Earth's atmosphere. Reaching 20 km in altitude, the gas formed small droplets of concentrated sulphuric acid, created a haze layer that reflected some of the Sun's rays back into space, cooling the whole planet by about 0.5°C.

But now the Venus story shows that we cannot predict some things yet, like how quickly the initially protective haze will be converted back into gaseous sulphuric acid. The gas unlike the droplets is transparent and so allows all the Sun's rays through.

All the more reason why geo-engineering can only be the very last resort.