PARIS, Nov 28, 2007 (AFP) - Once styled as Earth's twin, Venus was transformed from a haven for water to a fiery hell by an unstoppable greenhouse effect, according to an investigation by the first space probe to visit our closest neighbour in more than a decade.

Like peas in a cosmic pod, the second and third rocks from the Sun came into being 4.5 billion years ago with nearly the same radius, mass, density and chemical composition.

But only one, Earth, developed an atmosphere conducive to life. The other, named with unwitting irony after the Roman goddess of love, is an inferno of carbon dioxide (CO2), its surface hot enough to melt steel.

Preliminary data from the probe reveal a Venus that is more Earth-like than once thought - but not in ways that are reassuring.

At first blush, the two worlds, 42 million kilometres (26 million miles) apart at their closest points, could hardly be more different.

Earth's temperature range has remained largely stable and its atmosphere has maintained a balance of gases - and this, with the precious water covering two-thirds of its surface, has allowed riotous biodiversity to flourish.

Venus' atmosphere, though, overwhelming comprises suffocating CO2 and a permanent blanket of clouds laced with sulphuric acid. Oxygen is nowhere to be found, nor is any water except in atmospheric traces.

Its surface hovers at 457 degrees Celsius (855 degrees Fahrenheit) and has a pressure equivalent, on Earth, to being a kilometer (3,250 feet) under the sea.

But this was not always so, says Hakan Svedhem, an ESA scientist and lead author of one of eight studies published on Wednesday in the British journal Nature.

Venus, he believes, may have been partially covered with water before it became doomed by global warming.

"Probably because Venus was closer to the Sun, the atmosphere was a little bit warmer and you got more water very high up," he told AFP.

As water vapour is a greenhouse gas, this further trapped solar heat, causing the planet to heat up even more. So more surface water evaporated, and eventually dissipated into space.

It was a "positive feedback" - a vicious circle of self-reinforcing warming which eventually caused the planet to become bone dry.

Even today, Earth and Venus have roughly the same amount of CO2. But whereas most of Earth's store remains locked up in the soil, rocks and oceans, on Venus the extreme heat pushed the gas into the air.

"You wound up with what we call a runaway greenhouse effect," Svedhem told AFP in an interview. "(It) reminds us of pressing problems caused by similar physics on Earth."

Venus Express, the first dedicated mission since the US Magellan Orbiter mapped the planet's surface in the early 1990s, is equipped with an arsenal of sensors to peer through the dense clouds across the entire light spectrum.

One surprise already turned up by the 600-kilo (1,320-pound) probe is a 30-40 C (55-70 F) variation between daytime and nighttime temperatures at an altitude of 60 kilometres (40 miles).

At this height, violent winds three times stronger than hurricanes on Earth should even out differences, or so it had been thought.

There are many questions yet to be answered during the mission, which is scheduled to last through 2013.

One is whether there is lightning on Venus. Given the kind of clouds covering the planet, there simply should not be any, Andrew Ingersoll, a professor at Caltech University in Pasadena, California, said in a commentary, also published in Nature.

But Venus Express has detected "whistlers," low-frequency electromagnetic waves that last a fraction of a second and are normally a sure sign of electrical discharges.

Another enigma: sometime within the last 700 to 900 million years, the planet seems to have lost its skin, its topography resculpted by some giant force.

"Venus has quite recently completely changed its surface," said Svedhem. "Some event completely changed everything - this is a strange process we do not completely understand."