Venus has an ozone layer, too

Long ago, Venus was thought to be Earth's twin – until measurements of its atmosphere revealed it to be a sweltering hellhole stifled by a runaway greenhouse effect. Now Europe's Venus Express spacecraft has found a new trait that both Earth and our sister planet share: an ozone layer.

The finding could help astronomers home in on life on other planets.

Venus Express found ozone's spectral signature in a layer 100 kilometres up in the planet's atmosphere, at concentrations of no more than 1 per cent those found in Earth's atmosphere.

Computer models suggest that Venus's ozone is formed when sunlight breaks up carbon dioxide molecules. The oxygen atoms freed in this reaction meet up on the planet's cooler night side to form molecular pairs (O₂) and triplets (ozone, or O₃).

"The key chemical reactions operating in Earth's upper stratosphere may also operate on Venus," write Franck Montmessin of the LATMOS atmospheric research centre in France and his colleagues in a paper describing the results.

Ozone is important for life on Earth because it blocks damaging ultraviolet radiation from the sun. On Earth its abundance suggests the breakup of CO₂ by sunlight was not its only source. Instead, ozone, along with molecular oxygen, O₂, also originated from oxygen atoms generated by CO₂-eating photosynthetic microbes at least 2.4 billion years ago.

Some astrobiologists have suggested that an atmosphere containing carbon dioxide, molecular oxygen and ozone all at once could be a sign of life.

But the new result, along with the fact that Mars had previously been found to have ozone at a concentration of 0.3 per cent that of Earth, suggests the mere presence of these molecules is not enough to prove the existence of life below.

Instead, it bolsters the idea that a planet must have at least 20 per cent as much ozone as Earth to suggest life. "On Venus as on Mars, the positive identification of this triplet is not associated with biological activity on this planet," the researchers write.