Venus's rotation is slowing down - and scientists aren't at all certain as to why.
Infrared observations from ESA’s Venus Express spacecraft have shown surface features some 20km away from where they should be, given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s.
This can be explained if the planet's day has lengthened by six and a half minutes in the meantime.
"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," says Nils Müller of the DLR German Aerospace Centre.
The team's ruled out short-term random variations as the cause, as these should average themselves out over longer timescales.
More likely is long-term weather patterns. Recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period.
Even on Earth, changes in winds and tides can alter the length of our day by a millisecond or so, through friction with the surface. On Venus, which has an atmosphere more than 90 times the pressure of Earth’s and higher-speed weather systems, the effects could be much greater.
Other effects could also be at work, including exchanges of angular momentum between Venus and the Earth when the two planets are relatively close to each other.
More work is needed, says the team.
"An accurate value for Venus’ rotation rate will help in planning future missions, because precise information will be needed to select potential landing sites," points out Håkan Svedhem, ESA’s Venus Express project scientist.