The hidden depths of Jupiter

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Jupiter is not only the largest planet in our solar system, it still holds many secrets. Now, for the first time, data from the NASA spacecraft Juno provide an indirect look deep into the inner workings of the gas giant. They reveal, among other things, that the raging windbreaks of Jupiter’s gas can reach up to 3,000 kilometers into the depths. Underneath, the pressure is so high that hydrogen and helium are liquid and rotate like a solid core.

Already Galileo Galilei could have seen the striking cords of Jupiter, because the atmosphere of the gas giant is in permanent turmoil. Bands of raging winds, moving in different latitudes and partially opposite directions around the planet, form a light-dark pattern that shapes the appearance of the planet. The speeds of these mighty jetstreams can be up to 100 meters per second. However, what exactly drives them and how far they reach into the interior of the gas planet, was previously unknown. No wonder: the pressure under the cloud cover of Jupiter is so enormous that in 1995 a sample probe exposed in NASA’s Galileo mission was crushed a good hundred kilometers after entering the Jupiter atmosphere. Direct glances or measurements of the planet’s interior are thus denied us for the time being.

Asymmetrical gravity field
But since July 2016 around the gas giant circling NASA space probe Juno has now opened up new possibilities of Jupitererkundung. As part of the Juno Gravity experiment, researchers have used the changing speed of the probe to measure the attraction of Jupiter and thus its gravitational field. For this they determined whether and how much a radio signal sent to the probe and back was distorted by the Doppler effect. “The Juno probe is the first space mission to use the Ka-band radio system for planetary geodesy,” report Luciano Iess of Sapienza University of Rome and his colleagues.

The result was surprising: “Models of Jupiter structure predict that the planet’s gravity will be dominated by a component that is symmetrical with respect to both the hemispheres and the axis,” say the researchers. “But the Juno data has now provided evidence that there are north-south asymmetries in the gravity field of the gas giant.” For a fast-rotating gas planet like Jupiter, such an asymmetry is rather unusual, as it suggests that it is also low The surface must give dynamic and diverging currents, as Iess and his colleagues explain. The windbands of Jupiter can therefore not be a purely superficial phenomenon, as one of the common theories postulates.

Deep currents and solid liquids
What’s up with the deep currents of Jupiter has been studied in detail by two other research teams. Yohai Kaspi of the Weizmann Institute of Science in Rehovot and his colleagues analyzed certain features in Juno’s asymmetric gravity field data that reveal how deep the raging wind currents reach into the Jupiter’s gas envelope. The result confirms that the windbands are by no means just a superficial phenomenon: “The jet streams reach from the top of the cloud cover to a depth of around 3,000 kilometers,” the researchers say. As they calculated, in these winds around one percent of the massive mass of the planet moves.

This depth estimate is confirmed by the analysis of the team led by Tristan Guillot from the University of the Côte d’Azur in Nice. They had searched the symmetric component of the Juno gravity field data for clues to the nature and dynamics of the planet’s interior. These data also speak for zonal gas flows up to a depth of around 3,000 kilometers. Below that, however, the differential rotation of the gases ceases, as the researchers found out. Because there begins the zone in which hydrogen and helium are liquid because of the high pressure. The surprising thing is that this area does not seem to rotate like a liquid, but behaves like a solid instead. “At this depth, the conductivity and the resulting magnetic drag are so strong that they force fluid motion into a solid-state rotation,” explain Guillot and his colleagues.

These new insights into the inner life of Jupiter throw a whole new light on the processes inside gas giants. This is not only important and exciting, in order to better understand the gas planets in our own solar system. It also helps in exploring the many gas giants that circle around strange stars.