Unravelling Jupiter's giant storms

  • Tapio Simula

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Unravelling Jupiter's giant storms

 

The forces driving Jupiter's wild weather are better understood. Belinda Smith reports.

 
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The Great Red Spot is a raging storm twice the diameter of Earth located in Jupiter’s southern hemisphere. This image was taken by NASA's Voyager 1 spacecraft in 1979.
NASA / JPL

Ever since Giovanni Cassini pointed his telescope at Jupiter in the mid-17th century and saw a massive red hurricane, physicists have been perplexed by what generates the gas giant’s storms.

Now, computer modelling suggests that immense bubbles of hot gas deep within Jupiter seed its storms. A team led by Moritz Heimpel from the University of Alberta in Edmonton reached that conclusion after running the first 3-D simulations of Jupiter’s atmosphere, reported in Nature Geosciences.

“Jupiter is an insanely rich dynamical system, and they’ve taken three-dimensional aspects very nicely into account,” says Tapio Simula at Melbourne’s Monash University.

Jupiter’s atmospheric dynamics are different from Earth’s. While both planets have fast-flowing ‘rivers of gas’ known as jet streams, Earth’s flow from west to east, whereas the jet streams on Jupiter’s alternate directions.

This explains the anti-clockwise twist on the Great Red Spot – the hurricane that confronted Cassini. It’s sandwiched between a westward and an eastward jet stream, rolling like a ball bearing in between them.

But working out what fuels Jupiter’s giant storms in the first place has proved tricky.

On the left is a NASA image of Jupiter taken from Hubble Space Telescope. On the right is a 3-D simulation of Jupiter's deep atmospheric flow. The bands of storms predicted by the simulation mirror the weather on the gas giant. – Moritz Heimpel/University of Alberta

NASA has sent spacecraft to look at Jupiter. These include the Galileo probe, which descended to around 100 kilometres below the cloud layer in 2003 and, most recently, the New Horizons flyby in 2007. Together with centuries of telescope images, data from these missions have painted a picture of a ball of hydrogen and helium spinning at breakneck speed – once every nine hours – with a magnetic field so strong it crushes its innermost layers into a hot ocean of hydrogen dense enough to conduct electricity. But they provide few clues to what generates Jupiter’s storms.

Some physicists have tried explaining Jupiter’s storm patterns by modelling the top few hundred kilometres of its atmosphere, while others have tried to model deeper layers – all without success.

So Heimpel and his colleagues modelled them both at once. With the help of supercomputers, they ran 3-D simulations of the churning mass of gas in a 7,000-kilometre-thick slice of Jupiter’s atmosphere to see how lower layers influence storm formation at the top.

They saw hot gas percolating from Jupiter’s simulated interior, becoming trapped beneath the gas giant's coldest layer, a blanket of frozen water and ammonia crystals about 350 kilometres below its outer atmosphere. Once they were big enough, the gas bubbles burst through the ice to the winds whipping above.

As they spread, the bubbles cooled, dropping back again below the icy layer, only to be replaced with more hot gas bubbling from Jupiter's interior, creating a storm-forming convection current.

And as for where the storms erupt? Critically, says Heimpel, the simulation showed the streams of hot gas spiral away from the equator. That matches real life – Jupiter’s biggest storms mostly appear in bands to the equator’s north and south – increasing chances the model is accurate.

One question remains. Jupiter’s storms typically last a few years. But the Great Red Spot hurricane has raged for 350 years, and although the spot is slowly shrinking, it is still twice the Earth’s diameter. That suggests it is powered by some as-yet unidentified mechanism, possibly beneath the layers of the Heimpel team’s model, Simula says.

Fresh clues are coming. NASA’s Juno spacecraft reaches Jupiter mid-2016 to peer 600 kilometres into the gas giant, deeper than we’ve ever seen before.

 
Period14 Dec 2015

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