Venus 'was once Earth's twin and could have supported life'

Venus may once have been a pleasant place to live, scientists said today.Data from an orbiting European space craft show that the planet is even more like a wayward twin of Earth than was previously realised.Venus is the Earth's closest planetary neighbour. The two worlds are about the same size and occupy similar orbits, although Venus is slightly closer to the Sun.

Billions of years ago, Venus would have had conditions very similar to those on Earth and may even have been habitable, scientists now believe.But a runaway carbon dioxide greenhouse effect, atmospheric erosion by solar radiation, and a slower rotation rate sent Venus down a very different development path.Today the planet is stripped of water with a surface temperature hot enough to melt lead, and ravaged by hurricane-force winds and lightning storms.

The findings from the European Space Agency's Venus Express probe, which has been studying the planet for a year, offer a sobering view of how Earth might evolve if there were catastrophic changes to its climate.Professor Fred Taylor, from Oxford University's Department of Physics, one of the Venus Express mission leaders, said:"It is now becoming clear why the climate on Venus is so different from the Earth, when the planets themselves are otherwise quite similar."These differences are not just down to Venus being closer to the sun, we now know that the lack of a protective magnetic field and the differing planetary rotation rates also play a role in ensuring that many of the atmospheric processes we observe on Earth occur at a much faster rate on Venus."Our new data make it possible to construct a scenario in which Venus started out like the Earth - possibly including a habitable environment, billions of years ago - and then evolved to the state we see now."

Scientists used an infrared imaging spectrometer on Venus Express called VIRTIS to take 3D "soundings" of the planet's atmosphere.The measurements showed a turbulent region near the equator with much smoother air flows at higher latitudes.Both poles were marked by swirling vortices several thousand kilometres across and unusual "double eye" cloud formations. These features were similar to winter atmospheric circulation on Earth.The vortex-like structures extended down to where clouds began to form 50-55 kilometres above the surface of the planet.Nine papers from the first year of Venus Express observations appear today in a special edition of the journal Nature.Key findings help to explain how Venus lost its water through the action of atmospheric molecules being torn apart by ultraviolet radiation and blown into space.A water molecule is composed of two atoms of hydrogen and one of oxygen. The new data show for the first time that large amounts of oxygen have been stripped from the Venusian atmosphere along with lighter atoms such as helium and hydrogen."In fact oxygen is lost at about half the rate for hydrogen, consistent with the source being water," said Prof Taylor."This happens at a faster rate on Venus than on the Earth, not just because Venus is closer to the Sun, but also because it lacks the magnetic field that protects the Earth from the solar wind and rotates more slowly than the Earth."These loss processes must have stripped away large amounts of water from Venus during the first billion years or so after the formation of the Solar System."Scientists have also found the first reliable evidence of lightning on Venus.Lightning strikes appear to be more common on Venus than they are on Earth.