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Comet’s death uncovers powerful winds on Jupiter

The aftermath of a historic Jupiter comet impact enabled scientists to directly study the giant planet’s middle atmosphere’s winds for the first time. Surprisingly powerful winds near the polar region point to a presence of a giant vortex with a diameter of up to four times that of Earth.

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Jupiter is a giant cosmic vacuum cleaner. The Shoemaker-Levy 9 knew that only too well after the planet trapped the comet with gravity, shredded it to pieces, and eventually consumed the remains. But there is a silver lining in every cosmic horror story. The comet’s death helped astronomers learn about the winds in the so-far elusive part of Jupiter’s atmosphere.

The talk about the weather on Jupiter would sure be something. Swirling red and white strips of clouds, embellished by giant spotty storms, occupy the lower atmosphere. The planet’s upper atmosphere near the poles glows with auroras. Scientists use the shapes and motions of the clouds and auroras to study winds. But there are no clouds in the stratosphere, the middle part of Jupiter’s atmosphere. Properties of stratospheric winds have therefore only been inferred from models.

When the Shoemaker-Levy 9 fell into Jupiter’s atmosphere in 1994, the impact produced new molecules in the stratosphere. Molecules like hydrogen cyanide (HCN) and carbon monoxide (CO) travel with the wind in the stratosphere and emit light at specific frequencies in the radio band. In essence, these molecules are the perfect tracers of the winds.

A team of astronomers used the ALMA radio array, located in the Atacama desert in Chile, to observe Jupiter in search of the two molecules. In the absence of motion, the signatures of the two molecules are centered around 354.5 GHz (HCN) and 345.8 GHz (CO). Just like a siren’s pitch of a moving vehicle changes upon passing a listener, so do the motions in Jupiter’s atmosphere shift the frequencies of the molecular lines1The good old Doppler effect.. Most of the shift comes from the fast rotation of Jupiter itself. That can easily be accounted for, and any additional frequency shift is due to the winds.

Wind velocity map of Jupiter's rim (the observational setup didn't allow a reconstruction of the velocity field across Jupiter's disk). Only the south pole was completely visible during the observation. The image shows the coincidence of the auroral region and the strong winds in the stratosphere. Credit: Cavalié et al. 2021

Scientists used the molecular signatures to measure the wind speed as a function of latitude2Latitude is the coordinate running from the south to the north pole, with the equator at latitude zero.. To everyone’s surprise, they found very strong winds in the polar regions with a peak velocity of 350 meters per second. This is more than twice as fast as the fastest wind in the Great Red Spot and more than three times the wind speed of Earth’s tornadoes.

The powerful winds in the polar regions are located in about the same regions as auroras, only a few hundred kilometers lower in the atmosphere. Auroral regions are also known for their strong winds. The discovery could point to the existence of a giant vortex with a diameter of up to four times that of Earth and 900 kilometers in height. “A vortex of this size would be a unique meteorological beast in our Solar System,” says Thibault Cavalié, the lead author of the study.

Amazingly, it took only 30 minutes of observation with ALMA to obtain these results. Astronomers are already planning additional radio observations to learn more about the winds, especially in the polar regions. But they also have their eyes on the next generation of ground and space instruments, such as the ESO’s Extremely Large Telescope and the European Space Agency’s JUICE mission.

What else has the giant planet in store for us? Stay tuned.

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