Paris: 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.

The European Space Agency s (ESA) Venus Express, orbiting its prey since April 2006,seeks to explain this astonishing divergence.

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 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 and has a pressure equivalent, on Earth, to being a kilometer under the sea.

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

Source : PTI