Venus is a planet similar in size to the Earth. Nevertheless, it is quite different in other aspects. On the one hand, it spins very slowly on its axis, taking 224 terrestrial days and, moreover, it does so in the opposite direction to that of our planet, i.e. from East to West.

Its dense atmosphere of carbon dioxide with surface pressures 90 times that of Earth (equivalent to what we find at 1000 metres below the surface of our oceans), causes a runaway greenhouse effect that raises the surface temperatures up to 450 degreesC, to such as extent that metals like lead are in a liquid state on Venus.

At a height of between 45 km and 70 km above the surface there are dense layers of sulphuric acid clouds totally covering the planet.

It was in the 1960s that they discovered, by means of telescopic observations, that the top level of cloud layers moved very rapidly, orbiting the planet in only four days, compared to the planet's own orbit of 224 days. This phenomenon was baptised the "superotation" of Venus: the winds carrying these clouds travel at 360 km/h.

The various space missions that explored the planet in the 70s and 80s showed that the "superotation" was a permanent phenomenon and, moreover the probes that descended through its atmosphere indicated that, in a number of places, the winds decreased in speed to zero at Venus's surface.

New observations carried out with the Venus Express mission of the European Space Agency, in orbit around Venus since April 2006, have enabled the team of scientists from the University of the Basque Country (UPV/EHU) to determine in detail the global structure of the winds on Venus at its level of clouds while, at the same time, to observe unexpected changes in the wind speeds, and which will help to interpret this mysterious phenomenon.

The team was led by Agust�n S�nchez Lavega with team members being Ricardo Hueso, Santiago Perez Hoyos and Javier Peralta, from the Planetary Sciences Group at the Higher Technical Engineering School of Bilbao. The article, entitled "Variable winds on Venus mapped in three dimensions", was published with front page coverage in the Geophysical Research Letters.

This journal is published by the US American Geophysical Union (AGU) and is the most prestigious in its sphere of research. Moreover, the article was one of eight selected amongst hundreds for publication by the AGU in all journals as being the most outstanding in the EOS Transactions bulletin - sent to 50,000 AGU members at research centres all over the world.

Novel aspects of the rotation
Using images recorded by both day and night on Venus with the VIRTIS spectral camera on board the Venus Express, the UPV/EHU scientists have succeeded in measuring these clouds over several months and have discovered new aspects of the "superotation". Firstly, between the equator and the median latitudes of the planet there dominates a superotation with constant winds blowing from East to West, within the clouds decreasing speed with height from 370 km/h to 180 km/h.

At these median latitudes, the winds decrease to a standstill at the pole, where an immense vortex forms. Other aspects of the superrotation that observations with VIRTIS have made possible are that the meridional (North - South) movements are very weak, about 15 km/h, and, secondly, unlike what was previously believed, the superotation appears to be not so constant over time: "We have detected fluctuations in its speed that we do not yet understand", stated the scientists.

Moreover, for the first time they observed "the solar thermal tide" effect at high latitudes on Venus. "The relative movement of the Sun on the clouds and the intense heat deposited on them makes the superotation more intense at sunset than at sunrise", they stated.

"Despite all the data brought together, we are still not able to explain why a planet than spins so slowly has hurricane global winds that are much more intense than terrestrial ones and are, moreover, concentrated at the top of its clouds" stated Mr S�nchez Lavega. This study has enabled advances to be made in a precise explanation of the origin of superotation in Venusian winds as well as in the knowledge of the general circulation of planetary atmospheres.