The European Space Agency's Venus Express orbiter made the discovery while observing stars seen right at the edge of the planet through its atmosphere. Instruments measured the characteristic fingerprints of gases in the atmosphere as they absorbed starlight at specific wavelengths, an ESA release said Thursday.
The finding may help in the search for life on other planets, astronomers said.
Ozone, a molecule containing three oxygen atoms, has only previously been detected in the atmospheres of Earth and Mars. It is of vital importance on Earth because it absorbs much of the sun's harmful ultraviolet rays.
The build-up of oxygen, and consequently ozone, in Earth's atmosphere began 2.4 billion years ago.
Scientists theorize microbes excreting oxygen as a waste gas must have played an important role in the process. As a result, some astrobiologists say the simultaneous presence of carbon dioxide, oxygen and ozone in an atmosphere could be used to tell whether there could be life on a planet.
Although scientists agree there is no life on Venus, the detection of ozone there brings it a step closer to Earth and Mars, as all three planets have an ozone layer.
"It is yet more evidence of the fundamental similarity between the rocky planets, and shows the importance of studying Venus to understand them all," Hakan Svedhem, ESA Project Scientist for the Venus Express mission, said.
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