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17 March 2017

Gentle breeze may help Venus’s atmosphere spin like crazy

Venus with stripy features in atmosphere
False-colour image secured by a camera on the Venus Express probe


Venus has a second wind. Not only do winds whip around our planetary neighbour’s equator, they also blow from the equator toward the poles, something never conclusively observed before. Their existence could help solve the biggest mystery about the planet’s atmosphere: how it rotates so quickly.

Venus rotates once every 243 Earth days, but its atmosphere does so every four days, with wind speeds in excess of 400 kilometres per hour parallel to the equator. Energy from sunlight is needed to maintain this frenzy. But with more sunlight hitting near the equator than at the poles, it wasn’t clear how enough energy could arrive where it was needed.

The newly detected meridional winds, blowing at a relatively leisurely 80 kilometres per hour, could pull some of that energy away from the equatorial regions, spreading it more evenly throughout the atmosphere.


“It was difficult to understand how the atmosphere could maintain these speeds at high latitudes,” says Pedro Machado at the Astronomical Observatory of Lisbon, Portugal. “But with these meridional winds, we have energy and momentum transported from equatorial regions to higher latitudes.”

You measured that?

Machado and his colleagues were able to “see” the wind using the Doppler effect. Just as a siren’s sound changes pitch as it moves toward or away from you, light waves reflected off Venus’s atmosphere are compressed or elongated depending on the atmosphere’s motion.

But the Doppler shift is tiny in the case of Venus’s winds. To observe it, the team studied sunlight reflected from Venus’s clouds, which are being dragged along by those winds. We already know how different atoms and molecules absorb light in different ways, leaving a distinctive signature. To detect the clouds’ motion, and hence the associated wind, Machado and colleagues compared the Doppler-shifted light they were seeing with that known signature.

“It is amazingly hard to make these kinds of measurements,” says Glyn Collinson at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “I just read the paper and thought, holy smoke, you measured that?”

We already had hints of meridional circulation from the Venus Express spacecraft, but its orbit only allowed it to study winds in Venus’s southern hemisphere. Using the Canada-France-Hawaii Telescope in Hawaii, Machado and his team were able to measure the winds in both hemispheres, finding them to be symmetrical.

“This is very important, because we don’t know how the atmosphere of Venus works,” says Ricardo Hueso at the University of the Basque Country in Bilbao, Spain. “Understanding meridional circulation is one of the key elements to solving this problem.”

With the Doppler method attaining precision close to what spacecraft can achieve, without leaving Earth, Machado hopes to use it to probe the atmospheres of other nearby worlds like Saturn and its hazy moon Titan.

Journal reference: Icarus, DOI: 10.1016/j.icarus.2016.12.017

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