The ESA said hydroxyl, an important but difficult-to-detect molecule, consists of one hydrogen and one oxygen atom. It was detected in the upper reaches of the Venusian atmosphere, approximately 60 miles above the surface, by Venus Express's Visible and Infrared Thermal Imaging Spectrometer, or VIRTIS.

Scientists said the molecule was discovered by turning the spacecraft away from the planet and looking along the faintly visible layer of atmosphere surrounding the planet's disc. The instrument detected the hydroxyl molecules by measuring the amount of infrared light they produce.

Venus Express showed the amount of hydroxyl at Venus is highly variable. It can change by 50 percent from one orbit to the next, possibly caused by differing amounts of ozone in the atmosphere.

"Venus Express has already shown us that Venus is much more Earth-like than once thought. The detection of hydroxyl brings it a step closer," said one of the principal investigators of the VIRTIS experiment, Giuseppe Piccioni of The Institute of Astrophysics in Rome.

The discovery is detailed in the journal Astronomy & Astrophysics Letters.