Uncovering Jupiter’s secrets

Massive amounts of new data gathered by NASA’s Juno spacecraft have drawn back the veils on Jupiter’s cloudy surface, revealing unparalleled insights into the planet’s mysterious depths and the continent-sized cyclones at its poles.

In a suite of four papers published this week in the journal Nature, an international team of astronomers reports Juno’s new findings about Jupiter’s gravitational field, atmospheric flows, interior composition and polar cyclones. These discoveries solve an almost 50-year-old puzzle in planetary science — what goes on beneath the planet’s swirling clouds.

Although Jupiter’s surface has been studied extensively, its interior remained unexplored until 2016, when Juno successfully slid into orbit around the gas giant. According to Tristan Guillot, one of the lead authors from the Observatoire de la Cote d’Azur in France, astronomers previously didn’t know whether gaseous planets like Jupiter, Saturn and giant exoplanets rotate “with zones and belts all the way to the centre, or whether on the contrary the atmospheric patterns are skin-deep”.

To tackle this question, Juno measured Jupiter’s gravitational field. The team expected the winds in Jupiter’s interior to be affected by the planet’s density distribution, similar to how winds on Earth are caused by low and high pressure areas. Changes in density in turn cause the planet’s gravitational field to fluctuate.{%recommended 1245%}

Alan Duffy, an astrophysicist from the Swinburne University of Technology in Australia, who was not part of the research team, explains: “As the spacecraft orbits it builds up a map of regions that pull more strongly than others. The data can then be split into spherical harmonics or moments, similar to the notes on a drum, that trace out the scale and type of the gravitating structure beneath.”

This technique allowed Luciano Iess from the Sapienza Università di Roma in Italy and colleagues to measure Jupiter’s gravitational field 100 times more accurately than before. They showed that the field changes from the north to the south pole. This “north‒south asymmetry” is driven by powerful flows of gas in the atmosphere and the interior. The deeper the winds go, the larger the asymmetry. 

Two further papers used these results to show that the wind flows extend 3000 kilometres below the cloud tops — much deeper than previously expected. 

“It’s like going from a 2D picture to a 3D one,” says Yohai Kaspi from the Weizmann Institute of Science in Israel, lead author of one paper. “This is important for understanding the nature and possible mechanisms driving these strong jet streams.”

Kaspi’s team also calculated that Jupiter’s atmosphere makes up 1% of the planet’s total mass. In comparison, the Earth’s atmosphere is less than one millionth of our planet’s mass.

Guillot and co-authors confirmed the depth of the wind flows by using models to interpret Juno’s gravity measurements. They further discovered that the planet’s deep interior is composed of a liquid mixture of hydrogen and helium that acts like a solid.

According to Guillot, this means 99% of Jupiter’s mass “rotates uniformly — rather than with zones and belts rotating at differential speeds as in the atmosphere”.

The fourth and final paper, led by Alberto Adriani from INAF–Istituto di Astrofisica e Planetologia Spaziali in Italy, reports that the continent-sized cyclones at Jupiter’s poles — discovered by Juno last year — are not a chaotic jumble, but instead form polygonal patterns. 

Using optical and infrared observations, the team found eight cyclones rotating around a single central cyclone at Jupiter’s north pole, while at the south pole five such circumpolar cyclones dance around a central one. 

How these structures formed and how they continue to survive without merging are still open questions. 

But the research is far from over. Juno will continue to gather data until July 2018. Next, Iess says, the spacecraft “is set to measure tides raised by Io and the other moons, which may provide new insight into dynamical phenomena ongoing inside Jupiter”.

The team also aims to use Juno to further study the Great Red Spot, the jet streams, and to measure the planet’s moment of inertia to find out more about the variations in density. This same measurement was performed on Saturn just nine months ago by the Cassini spacecraft, giving scientists the opportunity to compare the two planets. 

These new insights will go a long way towards building a better understanding of gas giant planets, including those in distant solar systems.