The most detailed record of cloud motion in the atmosphere of Venus chronicled by ESA's Venus Express has revealed that the planet's winds have steadily been getting faster over the last six years.

False-colour image of cloud features seen on Venus by the Venus Monitoring Camera (VMC) on Venus Express. The image was captured from a distance of 30 000 km on 8 December 2011. The VMC was designed and built by a consortium of German institutes lead by the Max-Planck Institute for Solar System Research in Katlenburg-Lindau. Credit: ESA/MPS/DLR/IDA
 
Venus is well known for its curious super-rotating atmosphere, which whips around the planet once every four Earth days. This is in stark contrast to the rotation of the planet itself - the length of the day - which takes a comparatively laborious 243 Earth days.

By tracking the movements of distinct cloud features in the cloud tops some 70 km above the planet's surface over a period of 10 venusian years (6 Earth years), scientists have been able to monitor patterns in the long-term global wind speeds.

When Venus Express arrived at the planet in 2006, average cloud-top wind speeds between latitudes 50º on either side of the equator were clocked at roughly 300 km/h. The results of two separate studies have revealed that these already remarkably rapid winds are becoming even faster, increasing to 400 km/h over the course of the mission.

"This is an enormous increase in the already high wind speeds known in the atmosphere. Such a large variation has never before been observed on Venus, and we do not yet understand why this occurred," says Igor Khatuntsev from the Space Research Institute in Moscow and lead author of the Russian-led paper to be published in the journal Icarus.

Long-term studies of the atmosphere of Venus by ESA's Venus Express find that the average wind speeds at low latitudes (between the equator and 50º north or south) have increased from roughly 300 km/h to 400 km/h over the first six years of the mission. In this graph, the white line shows the data derived from manual cloud tracking, and the black line is from digital tracking methods. Credit: Data: Khatuntsev et al; background image: ESA
 
In a complementary study, a Japanese-led team used their own automated cloud tracking method to derive the cloud motions: their results are to be published in the Journal of Geophysical Research.

On top of this long-term increase in the average wind speed, however, both studies have also revealed regular variations linked to the local time of day and the altitude of the Sun above the horizon, and to the rotation period of Venus.

One regular oscillation occurs roughly every 4.8 days near the equator and is thought to be connected to atmospheric waves at lower altitudes.

But the research also unveiled some harder-to-explain curiosities.

"Our analysis of cloud motions at low latitudes in the southern hemisphere showed that over the six years of study the velocity of the winds changed by up 70 km/h over a time scale of 255 Earth days - slightly longer than a year on Venus," says Toru Kouyama from the Information Technology Research Institute in Ibaraki, Japan.