Despite being close to Earth and almost the same size, Venus is another world. Under its thick mantle of acidic sulfur clouds, surface temperatures of 460 degrees Celsius are the rule. This temperature is maintained by the greenhouse effect of an almost carbon-only atmosphere. Seventy kilometers above, one must withstand a perpetual windstorm.
A study published in the journal Atmosphere, led by Pedro Machado, of IAstro and Faculdade de Ciências da Universidade de Lisboa (Ciências ULisboa), presents the most detailed and complete set of measurements ever made of wind speed in Venus parallel to the equator (wind zonal) and the altitude of the bottom of the cloudy bridge. One of the new results was the simultaneous measurement of wind speed at two different heights 20 kilometers apart. The team recorded a difference in wind speed of around 150 kilometers per hour faster at the top of the clouds, which strengthens the hypothesis that energy is transferred from the heat of the lower layers to the general circulation of the atmosphere.
“The winds accelerate as we move upwards to increasing altitudes, but we still don’t know why,” says Pedro Machado. The ground-level temperature reaches 460 degrees Celsius and produces infrared radiation (called thermal emission), which heats the air and causes it to rise. This radiation passes through the more transparent regions of the cloud floor, approximately 48 kilometers above the surface. When Venus is observed in the infrared, we see this light radiate from the heat of the surface and the cloud shapes, dull and dark, become visible.
By observing and following the clouds at hourly intervals and using an improved tracking technique by Javier Peralta, from the Universidad de Sevilla and co-author of this study, the researchers indirectly calculated the speed of the wind pushing those clouds. This speed is about 216 kilometers per hour at the bottom of the cloudy bridge and in mid-latitudes, decreasing by half closer to the poles.
This work was undertaken on the night side by recovering the images that the team captured in infrared with the Galileo National Telescope (TNG), in La Palma, Canary Islands, between 11 and 13 July 2012. In those same days and In a coordinated strategy, the Venus Express probe, of the European Space Agency (ESA), then in orbit around the planet, observed in visible light the top of the cloudy bridge, about 20 kilometers above, at 70 kilometers of altitude.
Even by monitoring those clouds, the researchers achieved speeds on the order of 360 kilometers per hour. Other studies and computer simulations indicate that the wind speed at the bottom of the clouds is almost constant, with no significant difference between day and night. The team was then able to assume that the recorded wind speed at night is the same in the lower layers of the atmosphere on the daytime side.
The researchers thus collected, for the first time, measurements of the differences in wind speed between two altitudes from simultaneous observations, concluding that, on the diurnal side and in just 20 kilometers, the wind parallel to the equator undergoes an increase in speed of about 150 km / h more. The heat from the surface may be the engine that sustains these cyclonic speeds of the winds on top of the clouds.
The accuracy of the data collected with telescopes on Earth is comparable to infrared cameras on space probes, thanks to the method brought to this study by Javier Peralta. “We used the same geographic reference method as the images obtained with space probes, which was developed by NASA and supplemented by the European Space Agency,” explains Pedro Machado. “It’s as if the telescope on the ground is a spaceship.”
With the success of this approach, the team will now expand their research on the vertical component of winds with new ground-based observations coordinated with the spacecraft currently in Venus’ orbit, the Akatsuki mission, from the Japanese space agency JAXA. This study shows that observations conducted from Earth complement the data collected simultaneously by space missions. Despite the lower spatial resolution, due to the distance of our planet from Venus, it is generally possible to have a global view of our neighbor, which space probes, due to their orbits, are not always able to obtain.
The super-rotation of the atmosphere of Venus is a phenomenon in which, due to winds parallel to the equator, or zonal winds, the atmosphere surrounds the planet in just over four Earth days, i.e. 60 times faster than the period of rotation of the solid globe, which is 243 Earth days.
The next ESA mission dedicated to Venus, the EnVision, is scheduled. It will study the surface of the planet and try to know its past. Portugal is involved in the mission and Pedro Machado leads the Portuguese consortium, as well as being the co-investigator in charge of one of the instruments, an infrared spectrograph.
