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ASTRONOMY

Perturbed atmosphere

Meteorologists want to know why the wind is so strong on Venus and Titan

The planet Venus, photographed by the European probe Venus Express: size similar to the Earth's and winds of 400 km/h

ESA/MPS/DLR/IDA, M. PÉREZ-AYÚCAR & C. WILSONThe planet Venus, photographed by the European probe Venus Express: size similar to the Earth’s and winds of 400 km/hESA/MPS/DLR/IDA, M. PÉREZ-AYÚCAR & C. WILSON

Days pass slowly on Venus. The planet rotates very slowly. It is almost the size of Earth, but Venus takes 243 Earth days to turn completely. Given its slow rotation, meteorologists expected that Venus’ atmosphere would be among the calmest in the Solar System. However, probes sent to the planet observed constant wind in the upper atmosphere, where wind speeds reach 400 km/h. Such intense winds only occur on Earth during hurricanes or, sporadically, at high altitudes. On Venus, it is windy all the time, especially at the equator.

In an attempt to solve the mystery, the meteorologist João Rafael Dias Pinto, of the University of São Paulo (USP), and Jonathan Lloyd Mitchell, a planetary scientist at the University of California, Los Angeles, created a simplified computer model of a planet with an atmosphere. Simulations using this model, published in August 2014 in the journal Icarus, are the first to correctly describe how the winds that sweep Venus are maintained, a phenomenon known as atmospheric superrotation, also observed on Titan, Saturn’s largest moon. “We identified new, important mechanisms that help us understand these winds better,” says Mitchell.

The secret of superrotation, according to the new model, is the way in which heat is distributed in the atmosphere of Venus and Titan. Through vertical circulation, heat spreads more slowly upward and toward the poles on Venus and Titan than on Earth. Additionally, a special type of ripple in the atmosphere affects the gas currents.

Venus and Titan are so different from each other that it seems strange that their atmospheres behave similarly. Venus’ surface temperature can reach 477°C as a consequence of the greenhouse effect of its atmosphere, which is rich in carbon dioxide gas. On Titan, the temperature is -180°C and methane rain feeds lakes on the surface. When descending to the surface of Titan in 2005, however, the space probe Huygens discovered that the wind profile was almost identical to that observed on Venus by Venera-series Soviet probes in the 1970s and 1980s. Although weak on the surface, the winds at the equators of Venus and Titan reach 360 km/h at an altitude above 50 km; the winds at the same altitude at the Earth’s equator are under 15 km/h.

Beyond rotation
Dias Pinto explains that, on Earth, the mass of air that circles the globe is driven by the difference in temperature between the equator and the poles, and dragged along by the rotation of the planet. This is why the meteorologists expected weaker winds on planets and satellites that rotate more slowly. The researchers have been seeking an explanation for superrotation since the 1970s and concluded that, in addition to slower rotation, there is probably a specific oscillation pattern in the motion of the atmosphere, called atmospheric waves, that helps create an intense air jet concentrated at the equator and covering almost all of the celestial body. “It is as if the entire atmosphere moved in a single direction,” explains Dias Pinto. “The problem is that most atmospheric models of Venus and Titan, including the most realistic, find it difficult to reproduce superrotation.”

He decided to study superrotation during his PhD and, at a conference in France in 2011, he met Mitchell, an expert on Titan and Venus interested in attacking the problem with a simpler model. “With a more idealized model, I can better control the dynamics of the atmosphere,” says Dias Pinto. He worked under the guidance of Mitchell and the Brazilians Rosmeri Porfírio da Rocha and Tércio Ambrizzi, of the USP Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), and was able to simulate superrotation using an atmospheric model used for weather forecasting.

Modifying some parameters in this model, Dias Pinto discovered that decreasing the rotation of the planet was not enough to accelerate the rotation of the atmosphere. “João demonstrated that the model only develops superrotation if it transports heat from the equator to the poles more slowly,” explains Mitchell, noting that on Venus and Titan, despite the strong winds, air circulates very slowly in the vertical direction.

Dias Pinto also identified a special form of planetary wave in his simulations. It arises from the oscillations in the currents of air at the planet’s equator. “These planetary waves are the main drivers and maintainers of superrotation,” explains Mitchell.

“These aspects of superrotation have never been analyzed in detail,” says Sebastien Lebonnois, a planetary scientist of the National Council for Scientific Research (CNRS) in France, who studies the superrotation of Venus and Titan. “To confirm this analysis, we would need wind and temperature observations with a resolution that is difficult to obtain even on Earth.” Despite the difficulty, he hopes to obtain evidence in data from the Venus Express probe, which is orbiting Venus, or the Cassini orbiter, which is near Titan.

Project
Wave-mean flow interaction and atmospheric superrotation in terrestrial planets (nº 12/13202-8); Grant Mechanism Doctoral Grant – Research Internships Abroad; Principal investigator Tercio Ambrizzi (IAG/USP); Grant recipient João Rafael Dias Pinto; Investment R$40,381.84 (FAPESP).

Scientific article
DIAS PINTO, J. R. e MITCHELL, J. L. Atmospheric superrotation in an idealized GCM: Parameter dependence of the eddy response. Icarus. v. 238. p.93-109. Aug. 2014.

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