Winds of Venus

Print edition : July 12, 2013

fast winds of Venus Photo: dsd

THE most detailed record of cloud motion in the atmosphere of Venus by the European Space Agency’s Venus Express (VEX), which was launched in November 2005, has revealed that the planet’s winds have steadily been getting faster over the past six years.

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. The atmospheric super-rotation of Venus is one of the great unexplained mysteries of the solar system. These results add more mystery to it.

By tracking the movements of distinct cloud features in the cloud tops some 70 kilometres above the planet’s surface over a period of 10 Venusian years (six 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 50° on either side of the equator were measured to be roughly 300 km/hour. Two separate studies have revealed that these already remarkably high-speed winds are becoming even faster, increasing to 400 km/h over the course of the mission.

Such a large variation has never before been observed on Venus, and scientists have not yet understood why this occurred.

The findings have been published in the journal Icarus and the work was led by Igor Khatuntsev from the Space Research Institute in Moscow.

The researchers determined the wind speeds by measuring how cloud features in images moved between frames: over 45,000 features were painstakingly tracked by hand and more than 350,000 further features were tracked automatically using a computer program.

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.

Besides this long-term increase in the average wind speed, both the studies have revealed that there are also 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. The average wind speed seems to undergo dramatic variations between consecutive orbits of Venus Express around the planet. In some cases, wind speeds at low latitudes varied such that clouds completed one journey around the planet in 3.9 days, while on other occasions they took 5.3 days.

R. Ramachandran

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