The Venus Express orbiter found that Venus now takes 6½ minutes longer to complete a full rotation than it did 16 years ago when NASA's Magellan orbiter measured its spin rate. (European Space Agency)

Scientists have detected a sudden and dramatic slowdown in the rotation of Earth's sister planet Venus.

Data from the European Space Agency's Venus Express orbiter indicates the cloud covered world is now taking 6½ minutes longer to complete a full rotation than it did 16 years ago when NASA's Magellan orbiter measured its spin rate.

Venus Express uses a powerful infrared detector to penetrate the planet's dense shrouded atmosphere.

It found surface features were up to 20 kilometres from where they should be, based on the rotational data collected through surface radar mapping by both the United States Magellan and Soviet Union Venera probes.

The European Space Agency says the Venus Express data also agrees with the most recent long-duration radar measurements from Earth.

Unexpected result

Sue Smrekar, principle research scientist with NASA's Jet Propulsion Laboratory in Pasadena, California said the data came as a surprise.

"At first I was a bit sceptical," Smrekar said. "I thought the 100-kilometre resolution of surface data might not be adequate, considering they were talking about differences of 20 kilometres. But after looking at the data I was convinced the result is correct. It means something has slowed the rotation of the planet and we don't really know what that is."

Over its four-year mission, Magellan was able to watch surface features rotate under the spacecraft, allowing scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days. Venus takes 224.7 Earth days to orbit the Sun, making a day on Venus longer than its year.

But measurements taken from the Venus Express data by Nils Muller of DLR German Aerospace Centre and colleagues found the planet's rotation now takes 243.023 Earth days. Their finding appears in the latest edition of the planetary science journal Icarus.

Internal and external forces

The detailed orbital measurements taken by Venus Express are being used to determine whether Venus has a solid or liquid core, which will help in understanding the planet's creation and evolution.

If Venus has a solid core, scientists believe its mass must be more concentrated towards the centre and the planet's rotation would react less to external forces.

The length of an Earth day can change by roughly a millisecond over the course of a year, depending on winds, temperatures and tides.

Smrekar says a similar thing might be happening on Venus which has a dense atmosphere, with more than 90 times the pressure at sea level on Earth.

"This generates high-speed weather systems on Venus, which are likely to be the primary candidate changing the planet's rotation rate through friction with the surface," says Smrekar.

A team led by Özgur Karatekin of the Royal Observatory of Belgium examined the possibility of short-term random variations in the length of a Venus day, but concluded these should average themselves out over longer timescales.

Atmospheric models have shown Venus could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotational period.

"There are other effects outside the planet itself, including interaction between Venus and other planets that could be playing a role," said Smrekar.

"The closest planets Earth and Mars would have a small pull on the rotation of Venus. But people will have to run the calculations and try to understand if over the course of the last 20 to 30 years, those interactions could have had a sufficient effect to account for this difference."