Scientists hope hostile Venus can answer Earth's secrets of evolution

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Venus may get its name from the goddess of love but it is more like a vision of medieval hell.

It is the solar system's most inhospitable world. Its surface temperature, about 460C, would melt lead and sear a human to death in seconds while his or her remains were crushed to pulp by an atmosphere 92 times denser than Earth's. For good measure, thick clouds of toxic sulphuric acid perpetually cloak the planet.

Nevertheless, astronomers and space engineers - whose robot craft first revealed the nature of the horrors on Venus 50 years ago - have rekindled a desire to return to this blighted, uninhabitable world. They believe it could provide crucial information about the existence of habitable exoplanets orbiting other stars in our galaxy. It is also reckoned Venus could hold clues about the geological evolution of our own planet and the emergence of life here.

As a result, travel to Venus has found its way back on to the interplanetary agenda.

In recent months, a flurry of proposals to send unmanned spacecraft to our closest planetary neighbour have been put forward to the European Space Agency (Esa) and its United States counterpart, Nasa.

Venus, Earth's evil twin, may soon find itself back in the spotlight.

"Venus and Earth are, superficially, the two most similar planets in the solar system," says Colin Wilson, of Oxford University. "They are almost exactly the same size while their orbits both lie in a relatively warm habitable zone round the sun. Yet one of these worlds is balmy and pleasant while the other has turned out to be utterly inhospitable. The question is: why?"

At present, scientists do not have answers. Why good planets go bad, as happened to Venus, remains a mystery and that ignorance has implications for the search to find habitable exoplanets elsewhere in the galaxy.

"We may be able to use powerful space telescopes to detect an exoplanet in a promising orbit round a star but that may not be enough to say it is habitable," says Richard Ghail, of Imperial College London. "It could turn out that the planet that we are looking at is another Venus, a world hostile to life even if it is in a promising location. So we need to know what factors favoured Earth and what ones doomed Venus if we are to have hope of finding other planets that could support life."

Hence the plethora of missions that are now being proposed by scientists. One of these projects, led by Ghail, is known as EnVision and would involve Esa launching a robot probe, at a cost of about 500 million ($722 million), that would put a powerful synthetic aperture radar (SAR) device in orbit around Venus. This would allow scientists to peer through the thick clouds that shroud the planet so they could map its surface in detail.

Two similar missions have been proposed to Nasa by other groups of US scientists. One is known as Raven (Radar at Venus). The other is called Veritas. In each case, engineers would take advantage of improvements in the SAR technology first used by Nasa's Magellan spacecraft to map Venus's surface in the early 1990s.

By peering through Venus's clouds, Magellan revealed a world covered in lava plains but which found little sign of plate tectonic activity as there is on Earth. Crucially, the probe showed that these lava plains were relatively unscathed by meteorite bombardments, suggesting they were formed fairly recently, possibly about only 500 million years ago. "Around that time, complex life was beginning to evolve on Earth," says Ghail.

It remains uncertain if this resurfacing of Venus took place as a single volcanic episode or involved a sequence of smaller eruptions. In 2005, Europe's Venus Express began a nine-year mission that found evidence - in the form of sudden jumps in sulphur dioxide levels - that suggested the volcanoes there may still be active.

So, by returning to the planet with a new generation SAR device, scientists believe that they can resolve that issue.

- Observer