Venus Express observed stars seen close to the planet, and thus through its atmosphere. Its SPICAV instrument analysed the starlight, looking for the characteristic fingerprint of ozone.
According to computer models, the ozone on Venus is formed when sunlight breaks up carbon dioxide molecules, releasing oxygen atoms. These atoms are then swept around by winds to the nightside of the planet. Here, they can then combine to form two-atom oxygen molecules - but also sometimes three-atom ozone molecules.
The discovery may help astronomers in their hunt for life on other worlds. Ozone - found previously only in the atmospheres of Earth and Mars - is of fundamental importance to life because it absorbs much of the sun’s harmful ultraviolet rays. Not only that, on Earth, it is thought to have been generated by life in the first place.
The build-up of oxygen, and consequently ozone, in Earth’s atmosphere began 2.4 billion years ago. Although the exact reasons aren't entirely understood, microbes excreting oxygen as a waste gas are assumed to have played an important role. Along with plant life, they still do.
As a result, some astrobiologists have suggested that if carbon dioxide, oxygen and ozone were all to be found in another planet's atmosphere, it could indicate there was life there.
However, the amount of ozone is crucial. The small amount in Mars' atmosphere, for example, is the result of sunlight breaking up carbon dioxide molecules - and the same appears to be true of Venus.
Theoretical work by astrobiologists suggests that a planet’s ozone concentration must be 20 percent of Earth’s value before life should be considered as a cause. The new results support that conclusion, with Venus clearly below this threshold.
“We can use these new observations to test and refine the scenarios for the detection of life on other worlds,” says Franck Montmessin, who led the research.