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 wind speed “ 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…

1. How fast can an atmosphere travel around its planet?
“Normal” rotation on Earth vs superrotation on Venus.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. Further reading
•You will find more information about Akatsuki and other
lost-then-found satellites in our blog post Where is my
satellite?
•For more technical and detailed information about
superrotation on Venus, Titan and exoplanets please refer
to following papers Dynamics and circulation of Venus and
Titan, About Superrotation in Venus, Superrotation on
Venus: Driven By Waves Generated By Dissipation of the
Transterminator Flow, EQUATORIAL SUPERROTATION
ON TIDALLY LOCKED EXOPLANETS.
If you have any further questions, please contact
our inflatable space dome team.