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Ever since the invention of the telescope over 400 years ago, humans have observed the tapestry of Jupiter’s swirling, colorful cloud bands and storms. But astronomers have long wondered how deep these vibrant zones, belts, and other features penetrate the atmosphere of the giant planet.
Thanks to the Juno spacecraft, which has orbited Jupiter since July 2016, we now know these churning, swirling features extend — surprisingly — over 3,000 kilometers deep. But below that, scientists say, Jupiter’s gas and liquid interior appears to move more like a solid body.
In four papers published March 7 in the journal Nature, scientists provide the most detailed look yet into Jupiter’s complex atmospheric depths and flows, its gravitational field, and its interior composition. They also provide a look at the mysterious cyclones seen by Juno at the planet’s poles.
“Until now we only had a superficial understanding of these stripes and bands,” Yohai Kaspi from the Weizmann Institute of Science in Israel told Seeker. He is the lead author of a paper detailing Jupiter’s atmospheric jet streams. “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.”
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Jupiter’s colorful atmospheric patterns flow alternately east and west, at speeds that differ by up to 100 meters per second. Determining the depths of these flows has been one of the main goals of the Juno mission, and by studying the complicated interaction between Jupiter's winds and gravitational field, researchers have now been able to probe thousands of kilometers into the planet.
They do this by using Doppler data of how the spacecraft reacts to the gravity of the planet.
As the spacecraft operates in its 53-day highly elliptical polar orbit of Jupiter, scientists can study the minute amount of acceleration and deceleration Juno experiences as it moves around Jupiter. These measurements provide clues to the structure of Jupiter’s interior and how the gravity field varies with depth: the bigger the gravity signal, the stronger the flow of gas deep within Jupiter.
Juno detected a gravity signal strong enough to indicate that material in the bands is flowing as far down as 3,000 kilometers. Additionally, the spacecraft’s motion varies from pole to pole, meaning that Jupiter’s gravitational field is “askew” — differing patterns exist in its northern and southern hemispheres.
“The fact that we were able to detect a north-south asymmetry was most surprising,” Tristan Guillot from Côte d’Azur University in France said in an email to Seeker. He is the lead author of a paper describing the rotation in Jupiter’s deep interior. “This is a fluid planet, it should be in equilibrium.”
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Guillot and his colleagues say the gravitational asymmetry is the result of complex atmospheric and interior wind flows.
“The winds (or flows) determine the gravity asymmetry,” Luciano Iess from Sapienza University of Rome said in an email to Seeker. “Jupiter is a huge, fast spinning, gas ball. Rotation makes it oblate (squashed at the poles and fatter at the equator), but how much so depends on the internal density distribution.”
However, the planet’s rotation should deform the body in a symmetric way and so, Iess said, the asymmetric part of the gravity field can only be due to dynamical phenomena, such as flows in the atmosphere.
Where winds are blowing east, that motion adds speed to the planet's already high rotational speed of about 43,000 kilometers per hour (27,000 miles per hour). But where winds are blowing west, it's akin to slowing the spin of that part of Jupiter, changing the shape of the planet in different places. Both of these motions effectively either add or subtract mass in different areas and that has an effect on the planet’s gravity field.
“How large these north-south asymmetries are depends on the depth of the flows,” Iess said. “Flows are associated to density variations, pretty much as winds on Earth are caused by high and low pressure areas. These density variations and the ensuing gravity signatures are measured by Juno when it passes by the planet.”
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If the winds on the surface were shallow — 200 miles, for example — the asymmetry is small. If the winds were deep — say, 2,000 miles — the asymmetry of the gravity field is large. The models used by the researchers indicate the winds are about 2,000 miles deep.
Deeper than that, Guillot and his coauthors found that below the massive cloud level, Jupiter’s deep interior is made up of a fluid mixture of hydrogen and helium, rotating as a solid body.
“It’s thanks to the exquisite accuracy of Juno’s gravity field measurements and the great work of my colleagues Luciano Iess and William Folkner from NASA’s Jet Propulsion Laboratory who analyzed the radio signal from the probe to extract the gravity field information that we were able to extract that asymmetry,” said Guillot.
Getting that information, Guillot said, allowed them to get a handle on the depth of the flow. The findings were confirmed with all the researchers getting the same measurements. The Juno team hope to build on these studies with subsequent findings to get the full picture of Jupiter’s interior.
“This is important for understanding the nature and possible mechanisms driving these strong jet streams,” Kaspi said. “In addition, the gravity signature of the jets is entangled with the gravity signal of the interior (e.g., Jupiter’s core). Now that we know the gravity signature of the atmosphere it will help us in better understanding the interior structure, core mass and eventually the origin of Jupiter.”
Kaspi added that the results were surprising because they indicate that the atmosphere of Jupiter is more massive and extends much deeper than theorized. The researchers say Jupiter’s atmosphere makes up about 1 percent of the planet’s mass. While that may not sound like much, Earth’s atmosphere, by comparison, is less than one millionth the mass of the planet. Also, Jupiter’s atmosphere is equivalent to about three Earth masses.
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Another surprise found by Juno is the cluster of cyclones at each pole, seen by Juno’s cameras in visible and infrared wavelengths. The polar regions of Jupiter haven’t been studied previously because they are difficult to see from Earth. Juno is the first spacecraft to fly over the polar regions.
There are eight cyclones around the north pole and five around the south pole. They all seem to be quite stable and unmoving, which is mysterious, because computer modeling suggests that small storms would be unable to survive the polar winds that swirl around them.
“The manner in which the cyclones persist without merging and the process by which they evolve to their current configuration are unknown,” the researchers wrote. Lead author Alberto Adriani of Italy’s National Institute for Astrophysics told Seeker that he suspects the cyclones could be as long-lived as Jupiter’s other famous storms, such as the Great Red Spot.
Juno is currently scheduled to remain in orbit around Jupiter until July 2018, but NASA is looking at ways to extend the mission.
The researchers said that in future orbits they plan on using similar gravitational studies to investigate the depth and structure of Jupiter’s iconic spot. Other research topics include better understanding the origin and driving force of the jets in Jupiter’s atmosphere and measuring how Jupiter’s axis moves in time, which will provide information on how density varies in the deepest layers of the planet. Juno is also set to measure tides raised by Io and other moons, which may provide new insights into other internal dynamics on Jupiter.
“We hope by the end of the Juno mission to be able to address all these topics,” Kaspi said.