Why the Earth's twin became a wasteland

It is a world stripped of water and scarred by searing temperatures hot enough to melt lead. Yet Venus may once have been a planet much like Earth, where vast oceans of water could have supported life.

The first detailed analysis of data gathered by a European space probe has revealed tantalising evidence that Venus often considered Earth's twin planet became so inhospitable for life because of a series of chance events.

Scientists have confirmed that the similarities between Venus and Earth were overshadowed by a shift in the former's history that led to the loss of the Venusian oceans, an atmosphere clogged with carbon dioxide and a runaway greenhouse effect that gave rise to severe global warming.

The Venus Express, launched two years ago and orbiting the planet for a year taking measurements, has helped to shed light on why the climate of Earth's twin is so severe, said Professor Fred Taylor, of Oxford University. "It is now becoming clear why the climate on Venus is so different from Earth, when the planets themselves are otherwise quite similar," he said.

Venus is the closest planet to Earth in terms of its distance from the Sun, its mass, radius, density and chemical composition. Venus differs in terms of its slow rate of rotation once every 243 Earth days and yet it is hard to imagine the two climates being more different.

"These differences are not just down to Venus being closer to the Sun," Professor Taylor said. "We now know that the lack of a protective magnetic field and the differing planetary rotation rates also play a role in ensuring 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 evolved to the state we see now."

Venus Express has confirmed that the lack of a magnetic field made Venus vulnerable to the water-stripping properties of the solar wind, a high-speed stream of charged particles that split water molecules in two and allow the vital hydrogen in water to escape into space.

The oceans on Earth have been critical in trapping carbon dioxide as carbonate rocks; on Venus, the carbon dioxide has been released into its atmosphere to cause a runaway greenhouse effect, with surface temperatures averaging about 450C.

The latest results from Venus Express, in the journal Nature, show the atmosphere is a turbulent, three-dimensional structure which can be divided into four major components from the Venusian equator to the poles, Professor Taylor said. They also reveal that Venus experiences lightning strikes, and that they may even be more common than on Earth, which has important implications for the chemistry of the planet's atmosphere.

More than 30 spacecraft have visited Venus since the first probe, Mariner 2, was sent by the United States in 1962. Venus Express is loaded with scientific instruments designed to monitor a suite of phenomena, such as the amount of heavy water vapour in the atmosphere and the strength of any magnetic field. One of the biggest mysteries is how Venus lost its water, although the lack of a protective magnetic field suggests it was something to do with the solar wind.

Andrew Ingersoll, of the California Institute of Technology in Pasadena, said: "Because Venus is close to Earth in so many ways, it seems likely that the two planets started out similarly. Venus must once have had an ocean's worth of water, but lost it somehow. The escape mechanism probably involves the solar wind the stream of charged particles emanating from the Sun which strips atoms and ions out of the atmosphere. But the details are contradictory."

Venus by the numbers

457C surface temperature 96% of atmosphere made up of carbon dioxide, leading to surface pressure 92 times greater than that of Earth. The sun rises and sets every 117 earth days and the planet spins once relative to the stars every 243 days. As night falls, the temperature can drop by up to 240C. At cloud-level, wind speeds reach 360 kph three times hurricane force. At its poles, instead of cold cores, Venus has cold collars, inside of which air temperature rises by 263C.

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