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Venus Express, which was launched by the European Space Agency in 2005 and has been orbiting Venus since April 2006. (European Space Agency)

Like the Earth and Mars, Venus has an ozone layer, European scientists have discovered.

An ozone layer a hundred to a thousand times less dense than Earth's sits at about an altitude of 100 kilometres above the surface of Venus and is five to 10 kilometres thick, the European Space Agency reported in a news release Thursday.

Venus's ozone layer, located about four times higher up than Earth's, was detected by the agency's umanned spacecraft Venus Express. Launched in 2005, it has been orbiting the Earth's nearest planetary neighbour, the second-nearest planet to the sun, since April 2006.

Instruments aboard Venus Express detected the chemical signature of ozone from the specific colours of starlight that it absorbs.

Franck Montmessin, an atmospheric scientist at the Centre National de la Recherche Scientifique in France, said in a statement that researchers were actually looking for other molecules in Venus's atmosphere when they noticed the strong ozone signature in some of their plots. That prompted them to go back and search for it in other scans.

"This ozone detection tells us a lot about the circulation and the chemistry of Venus' atmosphere," says Håkan Svedhem, ESA project scientist for the Venus Express mission in a statement.

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The red dots represent areas where Venus Express detected ozone on the night side of Venus's atmosphere. (European Space Agency)

Based on computer models, the scientists believe the ozone molecules, which are made up of three oxygen atoms, are formed in a multi-step process that begins when sunlight breaks up carbon dioxide in Venus's atmosphere, releasing oxygen atoms. Those oxygen atoms get blown by winds to the night side of Venus, facing away from the sun, where they recombine into oxygen molecules, which have two oxygen atoms, and sometimes ozone.

The ozone layer on Mars is also thought to have been generated this way.

Astrobiologists predict that if the ozone concentration in a planet's atmosphere is 20 per cent of the ozone concentration in the Earth's atmosphere, living things were likely responsible for generating it.

The concentration on Venus, where there is no known life, is consistent with that prediction, the ESA reported.

On the other hand, on Earth, most of the ozone and oxygen in the atmosphere is thought to have been produced by living things, and is at much higher levels than on Venus or Mars.

Interestingly, the researchers did not find ozone in areas of Venus's atmosphere where there was  the most oxygen. They believe that is because the highest concentrations of ozone-destroying chlorine compounds — the kind that generate ozone holes above Earth's poles — are also located there.

The presence of an ozone layer on Venus is "yet more evidence of the fundamental similarity between the rocky planets," Svedhem said, "and shows the importance of studying Venus to understand them all."