An artist's rendition shows the Venus Express probe orbiting the planet. ((ESA/Image by AOES Medialab))

Long before Venus became a hot, dry and barren planet with a choking mass of carbon dioxide for an atmosphere, it might have once been home to shifting continents and an ocean of water, according to the latest data from a European space probe.

Using infrared images of the planet's southern hemisphere taken in 2006 and 2007 from the Venus Express probe, German astronomers say they have produced a map that reveals the planet's highland plateaus were likely once ancient continents surrounded by water.

The researchers relied on infrared imagery because the planet's thick atmosphere of carbon dioxide and sulphuric acid makes visual imagery of the planet's surface difficult, and probes landing on the planet haven't lasted long on the surface, where temperatures can get up to 477 C.

The imagery showed that different areas of the planet had different surface temperatures, information that provided insight into the chemical composition of different areas of the planet.

A look at the highland plateaus suggests they are made of a substance similar to granite, which is formed on Earth when ancient rocks made of basalt are driven down into the planet by shifting continents, mix with water and then re-emerge through volcanic activity.

The presence of granite-like rock on the plateaus suggest a similar early history on Venus, Nils Muller, a researcher with the Joint Planetary Interior Physics Research Group in Berlin, said in a statement.

Liquid water on Venus?

"If there is granite on Venus, there must have been an ocean and plate tectonics in the past," said Muller.

Past theories of the history of our sun suggest that about four billion years ago, it might have been just 70 per cent as bright, raising the possibility temperatures on Venus might have once been cool enough to have liquid water.

But as the sun's power grew, the thinking goes, conditions on Venus changed, and liquid water would have evaporated, leaving carbon dioxide in the atmosphere instead of dissolving in water. The increased carbon dioxide would in turn trap more heat on the planet.

The planet's lack of a significant magnetic field — a sign of declining activity at its core — also exposed it to the solar wind, the high-energy particles emanating from the sun.

The readings from Venus Express seem to support this theory, but Muller cautions that the results are preliminary and for now simply suggest that the plateau rock looks different from elsewhere.

But he and his colleagues suggest the plateaus might be a good place to try to land a probe on Venus, as these areas might turn up evidence of past or even current volcanic activity.