Scientists spot pentagon pattern of cyclones —and unlock secrets of the planet’s interior

Pentagon of vortices. Mosaic of infrared images of
Jupiter’s south pole. Credit: NASA/SWRI/JPL/ASI/INAF/IAPS

We all recognise Jupiter by its banded pattern of
counter-rotating zones and belts – this can be seen even with
small garden telescopes. These stunning structures are
powered by fast jet streams that are visible in the planet’s
clouds. But what happens near its poles and below its cloud
tops has long been a bit of a mystery.

Thanks to its unique orbit, NASA’s Juno mission has now
revealed some of Jupiter’s best-kept secrets. The results,
published in four papers in Nature, show that the planet
has surprising “polygonal” shapes of cyclones at its poles –
including a pentagon at the south pole – and that its banded
structure persists to depths of 3,000km.

From Earth and spacecraft in certain orbits, we can only see
Jupiter’s equatorial regions well. In fact, this has been the
case for all previous missions to the planet. Images from
Voyager, Cassini and the Galileo orbiter provided magnificent
views of the zone-belt structure and long-lived storms such as
the Great Red Spot. The Galileo probe sampled only down to
160km below the clouds in one location.

Juno has a unique, highly elliptical orbit, giving it the first
good views over Jupiter’s poles. Every 53 days since July 2016,
it has swept as close as 4,100km above Jupiter’s cloud tops,
giving it excellent views of its aurora – a type of “northern
lights” caused by electrical currents in the rapidly rotating
magnetosphere (a magnetic field) interacting with the planet’s
atmosphere – and the polar regions of the atmosphere in
visible, infrared and ultraviolet light.

As well as studying the aurora and magnetosphere, Juno also
helps scientists probe the gravitational field of Jupiter’s
interior in exquisite detail by monitoring small tweaks to the
spacecraft’s orbit – down to 3,000km below the clouds.

Scientists spot pentagon pattern of cyclones – and unlock secrets of the planet's interior
Shot by Juno. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M.
Gill, CC BY-SA

Being the largest planet in the solar system, Jupiter boasts a
radius more than 10 times Earth’s, at nearly 70,000km. The
counter-rotating winds in the zones and belts reach speeds of
100 metres per second. Its main composition is hydrogen and
helium – leftovers from the dense cloud of gas and dust, known
as the solar nebula,
that formed our solar system
4.6 billion years ago.

Below the , the gas pressure is thought to
increase hugely. At just 3,000km below the clouds, the pressure
should reach 100,000 bar, which is the pressure needed to
synthesise diamond on Earth. Further towards the centre, the
pressure and temperature increase even further, and the
hydrogen starts behaving like a metal. Models show that even
further in we would reach an icy and rocky core with a radius
about 20% of Jupiter’s. The models aren’t that reliable though,
and this is where Juno comes in.

Peculiar polar patterns

Scientists were hugely surprised the first time they saw the
poles of another gas giant – Saturn. Cassini confirmed the
Voyager discovery of a peculiar, huge hexagon feature in
Saturn’s atmosphere near the poles. This surrounds a polar
hurricane with a diameter of 1,250km.

At the larger Jupiter, scientists didn’t expect to see this
pattern at all. Instead, theories suggested that the zones and
belts at the centre would weaken towards the poles leading to
chaotic turbulence, rather than structured patterns.

Hexagon at Saturn’s pole. Credit: NASA/JPL-Caltech/Space
Science Institute

But thanks to Juno, scientists
have now discovered
a huge cyclone at each pole, about
4,000km in diameter in the north and 5,600km in the south.
Remarkably, these are surrounded by eight similarly sized
cyclones in the north, and five in the south. These cyclones
seem remarkably stable over the time that Juno has imaged them
in the visible and infrared.

The eight northern cyclones form a “ditetragon” shape (this is
what you get if you connect two pyramids at the base) and the
five southern cyclones form a pentagon shape (see lead image).
We don’t understand yet what causes them and why they are so
persistent. The force from Jupiter’s rotation, combined with
its smaller radius at the pole, would be expected to move many
more cyclones poleward continuously, but this seems not to

Below the clouds

Another of Jupiter’s mysteries was whether its zones and belts
were shallow or deep in the atmosphere. Juno’s answer is deep.

This result came from measurements of its gravity field, which
scientists have now discovered exhibit a
north-south asymmetry
. This was unexpected at Jupiter – a
heavy, fast rotating, oblate (flattened at the poles) planet.
The explanation is that atmospheric flows below the must be present.

Jupiter’s south pole. Credit:
NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles

Another paper reveals that these atmospheric circulate
below each of the zones and belts
, and reach all the way
down to 3,000km. However, the mass of atmosphere involved in
these enormous motions corresponds to only about 1% of
Jupiter’s total mass.

By monitoring how the whole planet rotates, scientists also
discovered that below the 3000km level, Jupiter effectively
spins as a rigid body
– more slowly than the churning gas
above. At this level, the temperature and pressure causes
electric currents to flow, and this creates a magnetic drag
force which starts to slow the wind motion.

The new results can now be put in context with other bodies –
in particular with Saturn, with its zone-belt winds reaching
500 metres per second. Based on what we know now about Jupiter,
it seems likely that Saturn’s jet streams reach even deeper to
9,000km. Comparing models of Jupiter’s persistent cyclones with
Saturn’s hexagon and hurricane could also help us to understand
what causes these mysterious features.

Excitingly, the new data can also help us to understand gas
giant planets in other solar systems. For example, we know now
that those larger than Jupiter would have less deep jet streams
below their zones and belts.

Future missions like ESA’s
and proposed Saturn atmospheric probes may be able to
see deeper than Juno can, to tell us more about the deep
internal structure of this magnificent giant planet –
ultimately helping us to get a full picture of how it formed
and evolved.

Explore further:

Geometric clusters of cyclones churn over Jupiter’s poles

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