Longon: Venus has a special place in the sun’s family of planets. It is the solar system’s most inhospitable world. Its surface temperature, around 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.

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

Nevertheless, astronomers and space engineers — whose robot craft first revealed the nature of the horrors on Venus 50 years ago — have recently rekindled a desire to return to this blighted, uninhabitable world. Ironically for such an inhospitable place, they believe it could provide crucial information about the existence of habitable exoplanets orbiting other stars in our galaxy. For good measure, it is reckoned Venus could hold important 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 new proposals to send unmanned spacecraft to our closest planetary neighbour have been put forward to the European Space Agency (Esa) and to its US 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 around 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 around 500m (Dh2 billion), that would put a powerful synthetic aperture radar (SAR) device — a form of radar that can create images of an object or a landscape — 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 also 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 that was first used by Nasa’s Magellan spacecraft to map Venus’s surface in the early 90s. By peering through Venus’s clouds, Magellan revealed a world that was 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. “Yet on Venus, it is clear something very different happened.” It remains uncertain if this resurfacing of Venus took place as a single volcanic episode or involved a gradual sequence of smaller eruptions. Magellan found evidence of ancient calderas on Venus but it provided no evidence that any were still active.

However, in 2005, Europe’s Venus Express, the last major probe to orbit the planet, began a nine-year mission that found tantalising evidence — in the form of sudden jumps in sulphur dioxide levels in the planet’s atmosphere — that did suggest the volcanoes there may still be active. So, by returning to the planet with a new generation SAR device, one that could measure features with a resolution of around a few metres compared with the 100-metre resolution of Magellan, scientists believe they can resolve that issue. “You would be able to see the ground swell and sink as it does on Earth when a volcano gets set to erupt,” says Ghail. “If we saw that we would have evidence for active volcanism on Venus.”

And understanding the nature of the volcanism on the second rock from the Sun could be crucial in understanding the creation of the hellish conditions that now dominate the planet.

“The clouds on Venus are made of sulphuric acid and they keep the planet’s vast store of carbon dioxide, which produces its intense greenhouse heating, trapped underneath,” says Wilson. “Those clouds should disappear over time but they have endured. That suggests they are being replenished by sulphur dioxide, most probably from volcanoes. Hence our interest in Venusian volcanology.”

Astronomers also point to another key factor that may have affected the development of Venus’s searing atmosphere: a complete lack of water. Although it is well within the solar system’s Goldilocks zone, that band of space round the sun where conditions for planets were thought to be “not too cold and not too hot” to prevent a world from becoming habitable,

Venus is nevertheless utterly lacking in water. This aridity may not be surprising given the extraordinary temperatures on the planet’s surface. However, even the upper levels of Venus’s atmosphere, which are relatively cool, lack water vapour. Some event in the planet’s past must have triggered its evaporation.

“The atmosphere on early Earth was made of water vapour and carbon dioxide,” says Wilson. “Various processes, including the appearances of living organisms, led to a decrease in carbon dioxide and an increase in oxygen. That never happened on Venus though we suspect its early atmosphere was also made of water vapour and carbon dioxide. Its proximity to the sun made it that little bit hotter and its water may have then been driven off into space leaving only carbon dioxide which, of course, is a potent greenhouse gas. Hence the heat there.” Thus only a very slight difference in initial conditions doomed Venus.

On Earth water endured and the primitive marine lifeforms that evolved there eventually made a major difference to carbon dioxide levels here. They absorbed carbon dioxide and turned it into calcium carbonate for their shells. Those creatures then died and their shells formed thick layers on the seabed. The end result has been the vast limestone deposits that now cover much of the planet. Thus carbon dioxide was taken out of the atmosphere and dumped on the sea floor before being turned into sedimentary rock. Scientists estimate that if these limestone layers were cooked, they would release levels of the carbon dioxide that would match those on Venus and which would turn our world into a hothouse.

Thus conditions on early Earth, which were very slightly cooler than Venus, prevented our planet from losing its water. This in turn played a critical role in permitting the formation of lifeforms that then reduced levels of greenhouse gases in our atmosphere and kept our planet habitable.

In short, the parameters that make Earth so attractive may be very tight indeed. Venus orbits the Sun at a mean distance of 67 million miles (108 million km), Earth at 93 million (150 million km) and Mars at 155 million (250 million km). (Worlds in the outer solar system have vastly bigger orbits, for example like Saturn which is a billion miles from the sun.) “All three were thought to lie with the sun’s habitable zone,” says Wilson. “But we now know Venus is uninhabitable and that Mars looks lifeless. So it may be we will have seriously to tighten up our idea of what is a habitable zone around a star.”

—Guardian News & Media Ltd