ESA Venus Express: Super-hurricane-force winds on Venus are getting stronger

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

As the closest planet to Earth, Venus is a relatively easy object to observe.

However, many mysteries remain, not least the super-rotation of Venus' atmosphere, which enables high altitude winds to circle the planet in only four days.

Now images of cloud features sent back by ESA's Venus Express orbiter have revealed that these remarkably rapid winds are becoming even faster.

Similar in size to Earth, Venus has an extremely dense, carbon-dioxide-rich atmosphere and the planet's surface is completely hidden by a blanket of bland, yellowish cloud.

Only at ultraviolet wavelengths (and to a lesser extent in the infrared) do striking cloud streaks and individual cells emerge, due to the presence of some unknown UV absorber in the cloud deck.

By tracking the movements of these distinct cloud features, observers have been able to measure the super-hurricane-force winds that sweep around the planet at the cloud tops, some 70 km above the scorching volcanic plains.

Despite decades of observation from the ground and from spacecraft, a number of mysteries remain.

• What causes the remarkable super-rotation of Venus' atmosphere – so called because the upper winds travel 50 times faster than the planet's rate of rotation? 
• How do the winds vary with latitude and longitude? 
• How much do they change over time?

The answers to some of these questions are being provided by instruments on board Venus Express, such as the Venus Monitoring Camera (VMC), which have been observing the atmosphere for 10 Venus years – equivalent to 6 Earth years.

Venus Monitoring Camera (VMC)

The VMC acquires instantaneous snapshots of Venus at UV and near-infrared wavelengths.

Simultaneous imaging in these wavebands makes it possible to detect and track cloud features, and thus derive wind data, at two different levels - approximately 70 km and 60 km above the surface.

Venus Express follows a 24 hour orbit which approaches to approximately 250 km above the north pole, before moving out to a distance of 66 000 km above the south pole.

This highly elliptical path provides particularly good viewing conditions for the entire southern hemisphere, while enabling higher resolution, small scale images of the northern hemisphere.

These factors combined mean that VMC imagery provides, for the first time, an opportunity to study cloud level winds with high spatial and temporal resolution over a time scale of more than half a decade.

The latest analyses of Venus' cloud motions and wind speeds, based on VMC data, have been made by two independent teams - one led by a Russian group (Khatuntsev et al.) and the other by a Japanese group (Kouyama et al.).

By painstakingly measuring how cloud features in VMC images move between frames, the two groups have been able to reveal new patterns in the planet's circulation.

"We analysed images obtained during 127 orbits with a manual cloud tracking method, and 600 orbits with a digital correlation method," said Igor Khatuntsev from the Space Research Institute in Moscow, lead author of a paper in the journal Icarus.

"Over 45 000 features were tracked by human visual comparisons, and more than 350 000 features were tracked automatically using a computer programme."

The manual method of wind speed measurements consisted of tracking motions of high contrast cloud features in pairs of images taken at different times.

This allowed better recognition of cloud patterns and was more reliable than the digital method in middle to high latitudes, where clouds tend to be streaky, or where contrast was low.

The problem with this method is that it is very time consuming.

On the other hand, the digital tracking technique was capable of streamlining image processing and producing 10 times the number of wind vectors.

Both methods were in good agreement at low latitudes (below 40 degrees), but digital tracking was preferred for studying temporal variations of the mean (average) rate of flow.

The Japanese-Swedish team relied solely upon their own automated cloud tracking method to derive their motion from images taken about one hour apart, at latitudes between 55°S and 70°S.

A specially developed mathematical formula was used to reduce errors in the image analysis. This team's analysis is published in the Journal of Geophysical Research.

More information: I. Khatuntsev et al., Cloud level winds from the Venus Express Monitoring Camera imaging, accepted for publication in the Journal Icarus; doi:10.1016/j.icarus.2013.05.018

T. Kouyama et al., Long-term variation in the cloud-tracked zonal velocities at the cloud top of Venus deduced from Venus Express VMC images. In press at Journal of Geophysical Research - Planets; doi:10.1029/2011JE004013.