Posted: September 22, 2008
Click for animation of Venus' wind circulation, the first 3D picture of the venusian winds for an entire planetary hemisphere. Image: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/ Universidad del País Vasco (R.Hueso).Since 2006, the Venus Express Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) has been studying the thick blanket of clouds that shroud Venus above the southern hemisphere, spanning altitudes of 45 to 70 kilometres (km) above the surface. It is above the southern hemisphere that Venus Express reaches its highest point in orbit (about 66,000 km), providing a global view of the planet's atmospheric phenomena. The study focused on the clouds and their movement, tracking them for long periods of time to determine the variation in the speeds of the winds.
"We have determined simultaneously for the first time the vertical structure of the zonal winds of Venus within their main cloud decks," lead researcher Agustin Sanchez-Lavega from the Universidad del Pais Vasco in Bilbao, Spain, tells Astronomy Now. "This has been done for the whole southern hemisphere, from equator to pole. These and the other measurements (meridional wind, wind variability) will help to constraint existing models of Venus' general circulation."
Because VIRTIS operates at different wavelengths, clouds at different altitudes can easily be tracked. The team studied three atmospheric layers and followed the movement of hundreds of clouds in each: 625 clouds at 66 km altitude, 662 at 61 km altitude, and 932 at 45-47 km altitude, on the day and night sides of the planet. The individual cloud layers were imaged over several months for about 1-2 hours each time, a first for atmospheric study of Venus, which has never been done before at such large temporal and spatial scales and with multi-wavelength coverage.
VIRTIS measured wind speeds and cloud movements at three different altitudes using different infrared wavelengths to penetrate each layer. Image: R.Hueso, Universidad del Pais Vasco."We have learnt that between the equator and 50-55? latitude south, the speed of the winds varies a lot, from about 370 km/hour at a height of 66 km down to about 210 km/hour at 45-47 km," says Sanchez-Lavega. "At latitudes higher than 65?, the situation changes dramatically - the huge hurricane-like vortex structure present over the poles takes over. All cloud levels are pushed on average by winds of the same speed, independently of the height, and their speed drops to almost zero at the centre of the vortex."
Sanchez-Lavega and colleagues observed that the speed of the zonal winds (which blow parallel to the lines of latitude) strongly depend on local time, which could be related to differences in heat reaching Venus in the mornings and in the evenings. This effect, called the solar tide effect, influences the atmospheric dynamics greatly, making winds blow more strongly in the evenings.
"Solar radiation is the only driver of Venus atmospheric motions," explains Sanchez-Lavega. "Most radiation is deposited at cloud level, although the high temperatures at the surface are due to the runaway greenhouse effect. The problem is that we do not know how this heating translates into the strong observed motions. This is an open problem in Venus."
The study shows that on average, the winds regain their original speeds every five days, but the mechanism that produces this periodicity is unclear. VIRTIS is expected to provide the missing answers during its next few years of operation.
Observatório Astronômico Monoceros