By Jonathan Amos
Science reporter, BBC News
Venus Express has been studying our near neighbour since 2006
A European probe orbiting Venus has new data that indicates the planet may once had a lot of water on its surface and even had a system of plate tectonics.
The Venus Express craft has returned infrared maps that show heat variations among the surface rocks.
Scientists say some highland areas are slightly cooler, suggesting they have a different composition.
The German researchers working on the mission say these rocks could be akin to the continental rocks seen on Earth.
Such rocks would be granitic in nature.
On our own planet, granites are made during the process of rock recycling that goes on at the edges of the great geologic plates that cover the Earth. At the boundaries of these plates, ancient rock is pulled deep into the planet, reworked with water and then re-surfaced at volcanoes.
Critically, then, if there is granite on Venus, there must also have been an ocean and a process of plate movement in the past, say the team which publishes its map data in the Journal of Geophysical Research.
The new evidence has been obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on Venus Express. The instrument's data has been combined with previously obtained maps of surface elevation.
VIRTIS can see through the thick clouds that shroud the surface and detail the variations in the amount of heat energy coming off the rocks.
Different geological compositions will radiate at slightly different wavelengths.
The new maps of Venus' southern hemisphere show that the rocks on the Phoebe and Alpha Regio plateaus are lighter in colour and look old compared with the majority of the planet. On Earth, such light-coloured rocks are usually granites.
This contrasts with basaltic rocks - characteristic of oceanic basins - seen by the Russian landers of the 1970s and 1980s which touched down away from the highlands.
"We know from modelling what the temperature of the surface should be and we compare that to what we measure, and then get an idea of what is the different heat emissivity of different types of rocks we see on the surface," said Joern Helbert from the German Aerospace Centre's (DLR) Institute of Planetary Research.
"We see some of the highlands, especially the very young volcanic areas, look different. It's not all highlands that are different; it's mainly very young volcanic areas that look different," he told BBC News.
The observations fit neatly with the theory that the highland plateaus of Venus are ancient continents, once surrounded by ocean and produced by past volcanic activity.
The only way to tell for sure whether the plateaus were made from granites would be to send landers to those locations.
Venus Express was despatched from the Baikonur Cosmodrome in Kazakhstan in November 2005.
It was sent to the planet to try to understand why Earth's near neighbour has evolved into a very different world.
In size, mass and composition, Earth and Venus are remarkably similar. Venus is closer to the Sun, but this alone does not explain the differences with Earth.
Scientists wonder whether small volcanoes may still be active
It has undergone runaway greenhouse warming, where trapped solar radiation has heated the surface to an average temperature of 467C (872F).
The scientists think that Venus may once have held copious amounts of water on its surface. But it is likely the solar wind removed most of it during the first billion years or so after the formation of the Solar System.
The question remains open as to whether volcanoes are still active on Venus. They would need to be small, though, otherwise their presence would have been detected by now.
"I don't expect nowadays on Venus the 100s of km of lava flows we had in the past, but I would not be surprised if there was still some small volcanic activity going on," said Dr Helbert.
"We look from day to day for differences. You look for something that is warm and then gets cold again. It's just a matter of statistics; you just have to observe long enough."