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Deep gas bands, shapely cyclones — NASA's Juno reveals more of Jupiter's secrets

Deep gas bands, shapely cyclones — NASA's Juno reveals more of Jupiter's secrets
This picture of Jupiter's south pole is a mosaic of images taken by NASA's Juno spacecraft. It shows the heat coming out from the planet through the clouds: yellow indicates the presence of thinner clouds and dark red the thicker ones. (NASA)

NASA's Juno spacecraft has found a strange set of swirling cyclones at Jupiter's northern and southern poles — and found that the stormy bands ringing its surface may extend a whopping 3,000 kilometers deep.

The findings, described in a set of papers published Wednesday in the journal Nature, may help scientists gain a deeper understanding of the gas giant's interior even as it opens up new questions about Jupiter's swirling polar structures.

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The findings could "potentially revolutionize our understanding of the internal dynamics of such gas-giant planets," Jonathan Fortney, a planetary scientist at UC Santa Cruz who was not involved in the research, wrote in a commentary.

Juno was launched in 2011 in an effort to learn more about Jupiter, the king of our planetary pantheon.

This gas giant, whose colorful surface has drawn sky-watchers for centuries, is so massive that it far outweighs all of its fellow planets combined. But in spite of its prominent position in our solar system, very little is known about what lies within the planet, or at its northern and southern ends.

Understanding Jupiter's hidden depths could help us better understand the origins of all the planets in our solar system, said Scott Bolton, Juno's principal investigator at the Southwest Research Institute. After all, the sun is mostly hydrogen and helium. But the planets have a much larger share of heavy elements — particularly rocky planets like Earth.

How that happened remains a mystery, and Jupiter may hold some clues.

"When we look at Jupiter, we see the very first step in that process," Bolton said. "Whatever went on that allowed Earth to get created already started with Jupiter, because Jupiter is already enriched by the things that make us up."

The NASA spacecraft, which arrived at the planet on July 4, 2016, was the first to try and unravel some of these mysteries by swooping over and under Jupiter's poles.

Juno documents the planet in visible, infrared and ultraviolet light; samples electrons and ions that cause the auroras; and measures the planet's magnetic fields. It also measures the gas giant's gravitational field by tracking how strongly Jupiter's mass tugs on it at different points above the surface.

Jupiter's surface — or more accurately, its atmosphere — is covered by a sepia rainbow of gas bands that host powerful winds moving in opposite directions, sometimes faster than 100 meters (328 feet) per second, Fortney said.

But is that surface structure just skin-deep, or is it connected to what goes on deep inside the planet?

To find out, Luciano Iess of the Sapienza University of Rome and his colleagues used those gravitational readings to see beneath Jupiter's cloud-tops. They tracked how the radio signal between Earth and the spacecraft is squeezed or stretched as the spacecraft wobbles in response to Jupiter's gravitational tug. This can be translated to maps of wind patterns on the planet.

"Thanks to the radio link between Juno and the Earth, we could measure the velocity of the spacecraft to exquisite accuracy, to 0.01 mm/s (0.0004 inches per second) or better," Iess said. "This is one hundredth of the speed of a snail!"

The scientists found that while Jupiter is basically a ball of fluid, its wind patterns in the south are not symmetrical to those in the north.

Yohai Kaspi of the Weizmann Institute of Science and his colleagues built on Iess's findings, probing the structure of those jet streams.

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"On a gas-planet, such an asymmetry can only come from the jet streams which are themselves asymmetric between north and south," Kaspi said. "The deeper the jets, the more mass they contain, and therefore have a stronger effect on the gravity field. Thus, the magnitude of this asymmetry allowed us to determine how deep the jets extend."

His paper found that the layers of powerful wind extend surprisingly deep, down to about 3,000 kilometers (1,864 miles) beneath the cloud-tops. At that depth, these layers would make up 1% of the planet's mass.

"Galileo viewed the stripes on Jupiter more than 400 years ago. Until now we only had a superficial understanding of them, and have been able to relate these stripes to cloud features along Jupiter's jets," Kaspi said. "Now, following the Juno gravity measurements, we know how deep these extend and what is their structure beneath the cloud level. It's like going from a 2D picture to a 3D one."

Using another component of the gravitational field, a third study led by Tristan Guillot of the Cote d'Azur Observatory in Nice, France, looked beneath those layers to probe the planet's inner structure.

Their results confirmed that the surface winds extend down around 3,000 kilometers, and found that the deep interior is made of a dense mass of hydrogen and helium that moves together, somewhat like a rigid body, and exerts a drag force on the winds above.

The findings may now shed light on the behavior of Saturn — and even on gas giants outside our solar system, Guillot said.

"Because it is of smaller mass than Jupiter, it is less conductive and we expect its atmospheric zonal flows to extend much deeper, to 9,000 kilometers [nearly 5,600 miles]," he said of Saturn. "We can apply this to better understand exoplanets and analyze their observations even without spatial information."

This composite image shows the central cyclone at the planet's north pole and the eight cyclones that encircle it. It is based on data collected by the Jovian Infrared Auroral Mapper instrument aboard Juno.
This composite image shows the central cyclone at the planet's north pole and the eight cyclones that encircle it. It is based on data collected by the Jovian Infrared Auroral Mapper instrument aboard Juno. (NASA)

A fourth paper, led by Alberto Adriani of the Institute for Space Astrophysics and Planetology in Rome, used infrared and visible-light cameras to take images of the structures at Jupiter's poles. The scientists found that the north pole hosted eight cyclones forming an octagon around a central storm. At the south pole, five storms formed a pentagon around a central tempest.

How these storms started and how they manage to avoid merging with each other remain a mystery. It's even stranger in contrast to the giant hexagonal structure observed at Saturn's north pole.

Next up, Kaspi says he plans on using similar methods to probe the depth and structure of the Great Red Spot, Jupiter's enormous, iconic storm.

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Guillot said he would be using the gravity field measurements to get a more fine-grained look at the planet's composition and mass of its central core.

And Iess said he'd be watching as Juno measures the tides that are raised on Jupiter by volcanic moon Io and its other lunar satellites.

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