superrotation is observed in the atmospheres of some planets

“The Earth spins around once every 24 hours. What about its atmosphere? Does it travel with our planet? Stays still? Or maybe goes faster than the Earth?”

WonderDome Planetarium visitor, age 10

 

Wouldn’t it be wonderful to live on a planet with a non-rotating atmosphere to be able to travel places by just jumping into the air and hovering long enough? Sounds fantastic, right? The problem is, when a planet turns and the air does not, things get really windy.  As in “a thousand miles per hour winds peed “ windy.

Luckily for us, we don’t experience it here on Earth. The Earth’s atmosphere travels in sync with our planet and completes one turn around its axis in roughly 24 hours. Phew!

but…

Fast currents in the Earth’s atmosphere (aka jet streams)

Have you ever noticed that flights from the US to Europe are often shorter than the other way around? Sometimes they even arrive well ahead of schedule! That’s jet streams’ work! Jet streams are high altitude eastbound currents that flow at speeds of 100 miles per hour and faster. They occur in a layer of the atmosphere called tropopause because of the big temperature differences in the air. Each hemisphere has two jet streams: a polar stream and a subtropical stream. Consequently, if an eastbound airplane flies with a jet stream, it can cut travel time and save fuel. On the other hand, the westbound airplane encountering a jet stream will take longer to reach its destination and at higher fuel costs.

Jet streams can be found in the atmospheres of other, rocky and gas, planets, both in the Solar System and beyond.

Conclusion: There is a lot of “movement” in the Earth’s air and there are some pretty fast currents, but as a whole, the atmosphere does a good job of keeping up with the surface.

Superrotation

Now, imagine a planet, where the jet streams are fast and the planet’s rotation is slow. Do such planets exist?

It turns out that Venus, Saturn’s moon Titan (the only moon we know to have a substantial atmosphere) and several newly discovered exoplanets, do have atmospheres that rotate very fast with winds in some zones blowing faster than the planet rotates! Scientists call this phenomenon “superrotation”. They have lots of ideas about how celestial objects maintain superrotation, but no definite theory.

Venus

Venus is the slowest rotating planet in the Solar System. It really takes its time to turn around:  one day on Venus lasts a whooping 243 Earth days. Venusian atmosphere moves in the same direction as the planet but much faster! The speed of the air is the highest at the top of a 20 kilometer thick acid cloud layer. There, the atmosphere rotates 60 times faster than the ground beneath, taking only 4 days to complete a turn. Scientists compare that with steering a coffee with a spoon, only the coffee moves faster than the spoon itself. It is not clear what powers this fast rotation and prevents the air from “catching up” with the solid planetary body.

What causes superrotation on Venus

One of the most widely accepted explanations of Venusian superrotation has to do with the solar wind. Because Venus, unlike the Earth, does not have its own protective magnetosphere, the solar wind particles arrive unchallenged to the upper layers of the planet’s atmosphere. The collision between solar wind and atmosphere particles (and, therefore momentum transfer) is thought to be responsible for speeding up the upper atmosphere. It is quite likely that the faster top layers, in turn, speed up the inner layers.

Other factors that contribute to Venusian superrotation might include temperature variations between day and night, tidal effects caused by Venus’ proximity to the Sun and (our favourite) difference in the reflective properties of the planet’s day and night sides.

Akatsuki

For the past three years Japanese probe Akatsuki has been exploring Venusian atmosphere and trying to solve the mystery of its superrotation (and, more generally, answer the question what makes the Earth and Venus so different).

Recently, Akatsuki spotted the air current that looked very much like jet streams we observe on Earth. The venusian air stream, JAXA scientists nicknamed it Venusian equatorial jet, was only seen for several months in 2016. In the original publication the authors say the jet has to do with the planet’s superrotation. Hopefully, more data from Akatsuki will help astronomers understand what is going on in the Venusian atmosphere.

Other places where we can observe superrotation are Saturn’s moon Titan, and, outside the Solar System, some short-period tidally locked exoplanets.

Further reading

If you have any further questions, please contact our inflatable space dome team.

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