ESA's Venus Express spacecraft has discovered a thin ozone layer high in the planet's atmosphere, a result that will help test ideas for finding suitable atmospheric conditions for life on other worlds outside of our Solar System.

Venus Express' SPICAV instrument made the detection by looking at stars through the edge of the planet's atmosphere, measuring the fingerprints of the atmospheric gases as they absorbed light at different wavelengths. Ozone, which contains three oxygen atoms, revealed its identity when it absorbed some of the ultraviolet radiation from the starlight.

The team, led by Franck Montmessin, found that the ozone layer sits at an altitude of 100 kilometres, about four times higher in the atmosphere than Earth’s and one hundred to a thousand times less dense. Models suggest that the ozone on Venus forms when sunlight breaks up carbon dioxide molecules, releasing oxygen atoms, which are then swept around to the night side of the planet where they combine to form either two-atom oxygen molecules or three-atom ozone molecules.

"The ozone cycle on Venus bears very strong similarities with ozone on the Earth where it is destroyed through catalytic reactions with chlorine (the origin of the ozone hole on Earth)," explains Montmessin. "This view is supported by the fact that the ozone formation mechanism on Venus is due to O2 and O recombining in the nightside after they have been transported from the dayside by the so-called "solar-to-antisolar" circulation of the Venus upper atmosphere."

But there are some oddities, too. "The 100km altitude at which the ozone layer is observed is comparable with the altitude at which O2 molecules form in the nightside...but the locations seem to differ significantly," continues Montmessin. "For instance, O2 emission in the infrared (which traces the recently recombined O2 molecules) is observed essentially around the antisolar point of Venus (midnight at the equator). This is not the case of O3 which exhibits a rather uneven distribution within the nightside; there is no particular region where O3 likes to concentrate."

Montmessin speculates that the ozone is sustaining the action of chlorine radicals which have also been released in the dayside and conveyed to the nightside in the same way as the oxygen atoms. The result is a complex balance between formation and loss processes of the ozone in the nightside, which likely explains the lack of correlation with O2 emission.

On Earth, it is thought that oxygen-excreting microbes began building up the ozone layer around 2.4 billion years ago, and that plant and life-forms continue to replenish it today. There is too little ozone on both Mars and Venus – the only two other planets where it is known to exist – for life to have generated it; indeed, astrobiologists suggest that a planet’s ozone concentration must be 20 percent of Earth’s value before life should be considered as a cause. But the new observations will still help astronomers target potentially habitable planets

"There are some strategies that are currently considered of trying to detect CO2-H2O-O3 simultaneously on exoplanets as a proof of biogenic activity," Montmessin tells Astronomy Now. "The remarkable fact here is that both Venus and Mars atmospheres possess these three species and yet do not harbour any kind of life, as far as we know it. It does not mean that our observations invalidate the viability of the CO2-H2O-O3 criterion, but our observations support the idea that for life to be evidenced on other worlds, it takes a whole lot more ozone (in relative proportions) than what we see on Mars and Venus."

Montmessin adds that any strategy that will be used in the future for the search of biomarkers will have to be validated against all the planetary examples we have in the Solar System, and that understanding the chemistry and climate of Venus will certainly help.