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.