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Jupiter’s wind bands have deep roots

7 March 2018

Gravity measurements by the Juno spacecraft resolve a long-standing debate about the structure of gas giants.

Jupiter's stripes
Credit: NASA, Kevin M. Gill

Aside from the Great Red Spot, Jupiter’s most prominent feature is its sequence of alternating bands of east–west winds. For decades, planetary scientists have debated how far those jet streams penetrate Jupiter’s interior: Some suspected the movement is confined to a thin outer layer (see Physics Today, November 2002, page 9), while others pinned the flow on a deeply rooted rotational mechanism. The Voyager 1, Voyager 2, and Galileo probes offered clues but no definitive evidence (see Physics Today, July 1996, page 17). Now, in a trio of papers, scientists with NASA’s Juno mission have used gravimetry to determine that Jupiter’s zonal flow stretches downward for an impressive few thousand kilometers.

Because of Jupiter’s rapid rotation and gaseous composition, its equatorial radius is 7% larger than its polar radius. Thanks to that equatorial bulge, spacecraft like Juno can measure how the gas giant’s gravitational field strays from spherical symmetry (see the article by Tristan Guillot, Physics Today, April 2004, page 63). The probe acts as a free-falling test particle, with subtle tugs detected via Doppler shifts in radio signals exchanged between the spacecraft and an antenna on Earth. If all of Jupiter rotated uniformly, then Juno would have detected only even-number gravitational harmonics, which reflect gravitational deviations that are symmetric with respect to the equator. Instead, researchers discovered subtle differences between Jupiter’s northern and southern hemispheres, shown in the figure below; those differences are created by the redistribution of mass in the banded atmosphere. Using the magnitude of the odd-number harmonics, the researchers determined that the zonal flow must extend about 3000 km beneath the cloud deck, a distance equivalent to about 4% of the planet’s mean radius. The observation bolsters a 1976 theory positing that convection drives a series of rotating cylinders in Jupiter that manifest as bands on the planet’s exterior.

Jupiter gravity measurements

As the temperature and pressure increase at greater depths, so does the electrical conductivity of hydrogen. That hastens the influence of Jupiter’s powerful magnetic field, which wipes out the banded structure and causes the entire interior to rotate uniformly. Analysis of gravity and magnetic field data could reveal the density and structure of that interior and its stockpile of heavy elements. For now, researchers can apply their Jovian lessons to gas giants and brown dwarfs throughout the galaxy. Saturn, for example, likely sheds its banded structure about 9000 km beneath its golden cloud tops. (L. Iess et al., Nature 555, 220, 2018; Y. Kaspi et al., Nature 555, 223, 2018; T. Guillot et al., Nature 555, 227, 2018.)

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