By Paul Rincon BBC News science reporter

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Nasa's Magellan spacecraft mapped 98% of Venus' surface with radar Enlarge Image

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Some ancient astronomers believed it was two separate objects: a "morning star" and an "evening star".

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Venus is the brightest object in the sky except for the Sun and Moon. As such, the second planet from the Sun has fascinated humankind for millennia.

Europe's Venus Express spacecraft made its rendezvous with our nearest planetary neighbour in April to study it from orbit.

The mission aims to shed light on an enduring mystery about this world: how a planet so similar to our own in size, mass, and composition has evolved so differently over the last 4.6 billion years.

Earth can certainly take a very uncomfortable step towards Venus, though it's not likely to go all the way Prof Fred Taylor, Oxford University

A dense atmosphere, composed chiefly of carbon dioxide (CO2), generates a surface pressure 90 times greater than that on Earth.

Exotic vision

Through Venus Express, scientists hope to understand better the mechanics of climate change on our own planet.

"Earth can certainly take a very uncomfortable step towards Venus, though it's not likely to go all the way," comments Fred Taylor, Halley Professor of Physics at the University of Oxford, UK.

"The point is, we're moving in that direction and many of the constituents and mechanisms involved are the same. Even a few degrees change in temperature can be a disaster on Earth."

Greenhouse hell: How the surface of Venus might look

The prevailing view of Venus used to be very different. "Even though Venus is closer to the Sun, its cloud cover is very shiny and reflective," Fred Taylor explains.

"If you work out the energy balance, Venus is actually absorbing less heat from the Sun than the Earth is. By-and-large, you might expect surface conditions to be the same."

Probe train

With a 25km-thick layer of opaque cloud concealing the planet's surface from view, free rein has been given to science fiction writers, who have depicted Venus with lush jungle, swamp and oceans.

It was only when unmanned space probes were sent to explore the planet at close range that its romantic image was finally shattered.

Between 1961 and 1989, the US and USSR launched more than 30 spacecraft towards Venus. Some reached their destination, others did not.

Venera 13 took this colour photo of Venus' surface

Later missions sent back information about the thickness and chemical composition of the cloud layers as well as surface conditions.

The USSR's Venera 7 probe was first to land, surviving for 20 minutes before succumbing to the crushing pressure and burning heat. Its successor, Venera 9, sent back the first images from the surface.

Orbiting spacecraft have carried out radar surveys, peering through the planet's opaque clouds to map its surface. Principal among these was Nasa's Magellan probe which circled Venus eight times a day from 1990 to 1994.

Metallic 'snow'

Magellan's images show a youthful landscape where volcanism has covered up most of the ancient past. Volcanoes of every size and type are scattered across vast plains formed by lava coursing out from beneath the planet's surface.

The question is: did it once also have a landscape touched by running water, even large seas?

"Venus was born with the same sorts of constituents as Earth, which means there was some water in the environment. Some more probably came in from comets," Andrew Coates explains.

Reflective clouds ensure Venus absorbs less sunlight than Earth

Magellan also revealed that the Venusian highlands are unusually bright, reflecting radar beams much better than lower elevations. Several explanations have been put forward, ranging from the presence of loose soil to a covering of metal "snow".

At Venus's hot lower layers, many metals would be vaporised and exist as a metallic mist. But at higher elevations, where it is a little cooler, they could condense to form a thin, highly reflective layer on the ground.

As for what the snow is made from, the rare metal tellurium and iron pyrites, or "Fool's Gold", have both been proposed as candidates.

Recent work by a team at the Washington University at St Louis suggests the metallic frost is more likely to be sulphides of lead or bismuth.

Acid rain

Fred Taylor thinks the greenhouse effect on Venus has been driven all along by volcanism. Active volcanoes would have pumped copious quantities of the greenhouse gas carbon dioxide (CO2) into the atmosphere.

Water evaporating from the surface would then have acted in concert with CO2 to trap solar radiation and heat up the planet (water vapour is a stronger greenhouse gas per molecule than CO2).

Sunlight then split the water into its constituents, hydrogen and oxygen.

Unlike Earth, Venus is not protected by a magnetic field, so particles can be siphoned off from the top of its atmosphere. This allowed "light" hydrogen to escape from Venus, leaving more of its heavier form - deuterium - in place.

"On Earth, the amount of carbon dioxide is controlled because it dissolves in the surface water, that's why you get deposition on the ocean floor of carbonates," says Dr Coates.

"On Venus, there was no surface water and no natural 'sink' for CO2."

Today, Venus' atmosphere consists almost entirely (97%) of carbon dioxide, with clouds containing droplets of sulphuric acid along with compounds of chlorine and fluorine.

These precipitate an acid rain called virga, which evaporates before it has the chance to reach the surface.

In the upper part of the atmosphere, clouds swirl by at a rate of 300km/h, driven by fierce winds.

Venus itself sluggishly rotates in an East to West (retrograde) direction, turning on its axis once every 243 Earth days.

This is strange - especially since its orbital path around the Sun lasts 225 Earth days. If one were to stand on the surface, one would get the impression that the day was longer than the year.

A 'relaxed' Venus

Scientists have no way of telling how long present-day conditions have existed on Venus.

Mission guide: Venus Express

If this idea is discarded for a moment, and one assumes that the greenhouse warming is driven by volcanism, then conditions could be fluctuating on a relatively short timescale.

"At some point the volcanism will run out. It ran out on Mars a long time ago, and it has partially run out on Earth," says Fred Taylor.

"The volcanism will eventually subside to the point where it can't maintain these high temperatures and pressures. It's interesting to ask yourself what will happen then."

Recent atmospheric models predict that the harsh conditions on Venus will eventually relax, and the planet will become more Earth-like.

"You might get the oceans back, if there's enough water vapour to condense. And once you get the oceans back, they hasten the removal of CO2 from the atmosphere because they dissolve it and turn it into rocks," said Professor Taylor.

"You might end up with a Venus that's quite a lot like people thought it was going to be in the 1950s."

PLANETS COMPARED
	VENUS	EARTH
Distance from Sun	108,200,000km	149,600,000km
Diameter	12,103.6km	12,756.3km
Mass	0.82 Earth masses	1.0 Earth masses
Axial rotation period (sidereal day)	243 Earth days (retrograde)	23 hours 56 minutes
Year length	224.7 Earth days	365 days
Atmosphere	96% carbon dioxide 3% nitrogen	77% nitrogen 21% oxygen
Mean temperature	464C	15C
Surface pressure	92 bar	1 bar
Moons	0	1
Global magnetic field	No	Yes