This false-colour ultraviolet view taken April 12, 2006, is one of the first images of the southern hemisphere of Venus, and shows the day and night sides over the South pole. (ESA/MPS, Katlenburg-Lindau, Germany)

Named by the Romans for the goddess of love but known since ancient times, Venus has fascinated astronomers, mariners and observers of the heavens. Depending on the time of year, Venus is either the first and brightest planet or star seen at night in the western sky, or the last seen in the morning in the eastern sky.

It owes its brightness to its proximity to Earth and its dense, impenetrable atmosphere, a thick soup of carbon dioxide that reflects much of the sun's light back into space while at the same time trapping much of its heat down on the surface of the planet.

Though many of the landscape features and conditions of the planet are not comparable to Earth, Venus's environment can be viewed as an extreme case of global warming, said David Grinspoon, a scientist and curator of astrobiology with Venus Express, the European Space Agency (ESA) probe that is currently the only spacecraft currently studying the planet.

"Venus is the worse-case scenario of what can happen to a planet when an extreme runaway greenhouse effect happens," he said. For that reason, he says, the planet bears watching.

A year after the probe arrived in orbit around the planet, the data accumulated by the probe backs up Grinspoon's assertion, suggesting the planet's "huge inventory" of greenhouse gases is responsible for the planet's unique weather patterns and high temperatures.

In an overview of a series of Nature articles published online on Nov. 28 in advance of print, European astronomers also suggest Venus's lack of a magnetic field and closer proximity to the sun caused the planet to vaporize what water it did have.

"The differences between Venus and Earth are much less mysterious than previously thought," wrote lead author Hakan Svedhem of the ESA. "They are consistent with theoretical ideas and interpretations suggesting that the two planets had similar surface environments and they evolved differently, with Earth's oceans converting most of its atmospheric CO2 to carbonate rocks, and Venus losing most of its water to space."

Venus 101

Venus is closer to the sun than any other planet save for Mercury. It travels in a relatively circular orbit around the sun at a mean distance of about 108 million kilometres, about 30 per cent closer than Earth.

The volcano Maat Mons is displayed in this computer generated three-dimensional perspective of the surface of Venus created in 1992. It is not known if Venus is still geologically active. Maat Mons is approximately 0.9 degrees north latitude, 194.5 degrees east longitude with a peak that ascends to eight kilometres above the mean surface. (NASA/JPL)

It is often referred to as Earth's twin because of similarities in size, mass, density and volume. But viewed up close, the planet's behaviour is much different.

Venus rotates slowly on its axis, with a day on the planet lasting 243 Earth days, or more time than the 224.7 Earth days it takes the planet to complete a rotation of the sun. Venus also rotates in the opposite direction than it orbits the sun, making the sun rise in the West and set in the East.

Its temperature and atmosphere also make the planet inhospitable to life. The surface temperature of the planet is 477 C, the hottest in the solar system, and its atmosphere is toxic, made up of 96 per cent carbon dioxide with clouds of sulphuric acid.

Earth's atmosphere, on the other hand, is made up of mostly nitrogen (77 per cent) and oxygen (21 per cent). At the surface, the atmospheric pressure is over 90 times that of Earth at sea level.

Venus's thick sulphuric acid clouds have made viewing the planet's surface difficult, and probes travelling down to the planet haven't been able to handle the planet's extreme conditions for long.

But we do know that Venus's surface is hot and dry, with mountain ranges and ring-like structures called coronae rising up from the landscape. There are also impact craters, but few enough that geologists speculate the present surface of the planet is less than one billion years old.

Missions to Venus

Venus was the first planet outside our own to be observed by a spacecraft. The unmanned U.S. spacecraft Mariner passed within 34,760 kilometres in 1962 and took the first reading from the planet. The Soviet Union sent two more probes (Veneras 2 and 3) to explore Venus, with Venera 2 passing by the planet and Venera 3 crashing on the planet's surface.

Both the U.S. and the Soviet Union continued to send probes in the 1960s and 1970s, with the Soviet Venera 7 the first to land (safely) on the planet's surface and Venera 9 the first to return photos of the surface. In all, 15 Soviet and six U.S. spacecraft have either flown by or soft-landed on Venus, with the last mission finishing in 1994 when the U.S. Magellan space probe plunged into the planet after four years of radar mapping the planet's surface from orbit.

The ESA's Venus Express, which entered orbit around the planet a little over a year ago, marks the first spacecraft to visit the planet in 12 years. Part of the reason for the time between missions has been a change in focus from Venus to Mars, a planet thought to be more hospitable to our technology.

"Venus has been less on the radar because it's a much more difficult environment to explore," said Grinspoon. "It's more of a sure bet to go to Mars."

Mars also holds the possibility of discovering liquid water, a compound said to be a likely precondition to sustaining life. Although Venus's atmosphere contains small amounts of water vapour, any liquid water on the planet's surface would quickly boil away because of its high temperatures.

Mysterious climate

It wasn't always so hot on Venus, said Grinspoon. Theories of the birth and growth rate of our sun suggest that about four billion years ago, it was only 70 per cent as bright, raising the possibility that temperatures on Venus might have once been cool enough to have liquid water, he said.

Such conditions could have led to a similar process as on Earth: carbon dioxide in the atmosphere would have dissolved in water and then come down to the planet's surface, where it would be absorbed by the planet's rocky surface.

But as the sun grew in strength, temperatures began to rise and this liquid water would have evaporated, leaving the carbon dioxide to stay in the atmosphere and trap heat in the planet, which in turn would have caused more water to boil away. This runaway greenhouse effect is often used to explain the planet's present conditions. The planet's lack of a magnetic field also left the water vapour in the atmosphere exposed to the solar wind, the high-energy particles emanating from the sun.

Preliminary data from one of the Venus Express instruments backs this theory, according to the ESA report in Nature. The Analyser of Space Plasmas and Energetic Atoms 4 instrument — or ASPERA 4 — was able to, for the first time, track the composition of the escaping planetary atoms that had lost electrons after colliding with the solar wind, and found that the two most common departees were the positive ions of hydrogen (H+) and oxygen (O+), which together with their electrons can combine to form H2O, or water.

As the authors in the Nature overview write, "these loss processes must have removed large amounts of water from Venus during the first billion years or so after the formation of the Solar System."

Venus's weather has also puzzled scientists for years; powerful winds circumnavigate the planet in just four Earth days in the upper atmosphere, while the planet itself takes 243 days to perform a full rotation, but it was not known why this happens. A host of regional atmospheric processes, particularly at the poles, also appear to be at work on both the planet's sunny side and dark side, and have yet to be explained with an accurate model.

The Venus Express scientists suggest the planet's unique weather patterns are less mysterious than originally thought, and could arise from the combination of the planet's slow rotation and the dense atmosphere of greenhouse gases. But many of the processes, including the atmospheric circulation at the north and south poles and close to the equator, bear a resemblance to similar processes on Earth, though Venus's unique conditions mean they are larger and more energetic.

The planet's geological activity, however, is mostly unknown and Venus's lack of a magnetic field raises questions about the composition and activity in its core.

The Venus Express mission, slated to run until 2010, hopes to provide a detailed analysis of the driving forces behind the super-rotation of the atmosphere.

Grinspoon would like to see further missions for a planet he feels has been neglected of late but is essential to our understanding of Earth.

"Climate modelling is more art than science here on Earth," he said. "What Venus provides is an almost controlled experiment, a way to test what we know about the effect of runaway carbon dioxide on a planet's temperature and atmosphere.

"But we have this big gap in knowledge on how the climate of Venus works because of the gap between missions to the planet," he said.

Separated at Birth?

Venus and Earth are both thought to have formed around 4.5 billion years ago and share many similarities. Here's a brief comparison of some key characteristics of the planets that are similar.

Planet	Venus	Earth
Orbital distance from sun	108 million km	150 million km
Diameter	12,104 km	12,756 km
Mass	4.9 x 1024 kg	5.9 x 1024 kg

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