The rotation of the planet Venus is slowing down, according to recent data gathered by the European Space Agency's Venus Express satellite. Peering through the planet's dense atmosphere with infrared imaging, the orbiter saw surface features up to 20 km (12.4 miles) from their expected location. The discrepancy could be explained if the Venusian day has lengthened by six and a half minutes since the planet's speed of rotation was established 16 years ago.

Venus' accepted rate of rotation was established by NASA's Magellan mission in the 1990s, when a day on Venus (or one rotation) was clocked at being equal in duration to 243.0185 Earth days. Slow, in other words. But it would appear that the rotation has slowed further still in recent years.

"When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of," said planetary scientist Nils Müller of the DLR German Aerospace Centre. Long-duration radar measurements from Earth appear to corroborate the findings, according to the ESA.

Fluctuating weather patterns are believed to affect planetary rotation through friction with the surface. An Earth day can vary by a millisecond, depending on seasonal wind patterns and temperatures. But this phenomenon has been discounted as the cause for the Venusian discrepancy because such small, random fluctuations should average out over longer periods.

However, the ESA claims that other atmospheric modeling has shown that Venus might undergo decades-long weather cycles which might perhaps cause more lasting changes to the planet's rotational speed. It's an explanation that might account for the 390-second lengthening of the Venusian day.

Pinning down the precise rate of the planet's rotation is not without practical application. "An accurate value for Venus' rotation rate will help in planning future missions, because precise information will be needed to select potential landing sites," said Venus Express scientist Håkan Svedhem.

Venus Express has monitored the planet's atmosphere since 2006 and will continue to do so until at least December 31 2014, though further project funding may be allocated before then.

Source: ESA