Tigers may not change their stripes, but Jupiter will. The giant planet’s surprisingly neat, alternating bands of dark and light clouds periodically change their appearance, but the reason for these cyclical variations is a mystery.
Now, after studying data on Jupiter’s magnetic field collected by the Juno probe, a team of scientists from Japan, Spain and the UK think they’ve cracked it. The changes in Jupiter’s stripes coincide with magnetic oscillations in the gas world.
“It’s possible to get wave-like motions in a planetary magnetic field called torsional oscillations,” explains mathematician Chris Jones from the University of Leeds in the United Kingdom.
“The exciting thing is that when we calculated the periods of these torsional oscillations, they matched the periods you see in the infrared radiation on Jupiter.”
From a distance, Jupiter appears as serene as a sunrise of pale creams and burnt caramels. These dark stripes are known as girdles, the lighter ones as zones.
Quiet as they may seem, the belts and zones are part of Jupiter’s wild weather system. They circle the giant planet in opposite directions – bands move against Jupiter’s rotation, zones travel with it – and at different altitudes. The bands are regions of upwelling, so the cloud tops in the bands are higher than the cloud tops in the declining zones.
In infrared, the color scheme is reversed. The light bands darken and the dark bands shine bright, suggesting that the bands have much thinner cloud cover than the zones.
Earth has comparable (if fewer and fainter) alternating bands of atmospheric circulation, suggesting some similarity in how each world generates these atmospheric features.
But the weather on Jupiter differs so greatly from that on Earth that it’s impossible to extrapolate what one planet does on another. In addition, Jupiter has cyclic variations in its clouds related to variations observed in infrared data from 50 kilometers (30 miles) below the surface, which has puzzled scientists.
“Every four or five years things change,” Jones says. “The colors of the bands can change and sometimes you see global upheavals when the whole weather pattern gets a little crazy for a while, and it’s a mystery why that happens.”
Juno has been studying Jupiter since 2016 and has collected a lot of data about Jupiter’s different structures and properties. One is the planet’s magnetic field — a massive magnetic structure generated by Jupiter’s dynamo, a convective and conductive fluid that rotates in the planet’s interior and converts kinetic energy into magnetic energy.
Earth also has a dynamo. One thing the Earth’s dynamo produces is magnetic torsional oscillations – a kind of magnetic “wave” that oscillates in and out – symmetrically about the planetary axis, influenced by rapid rotation.
By studying years of magnetic field data collected by Juno, Hori and her colleagues were able to identify the signatures of similar oscillations in Jupiter. And, excitingly, these seemed to be related to the changes in the planet’s stripes and variations in the infrared.
“We propose here,” they write in their newspaper“that torsional oscillations give rise to a shear that disrupts the slow convective currents in the deep interior that carry heat flow to the visible troposphere.”
This could cause major weather disruption and alter the upwelling and downturn patterns in the Jupiter clouds. The team also tracked a highly concentrated region of the magnetic field called the Great Blue Spot near the equator. They found that it is slowing down, suggesting a new oscillation is starting.
By continuing to observe the planet to see how the clouds change in the near future, the team can validate or refine their theory and understand how it happens.
“Uncertainties and questions remain, particularly how exactly the torsional oscillation produces the observed infrared variation, which likely reflects complex dynamics and cloud/aerosol responses. Those need more research,” says Hori.
“Nevertheless, I hope that our paper can also open a window to explore the hidden deep interior of Jupiter, just as seismology does for Earth and helioseismology does for the sun.”
The research has been published in Nature.