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The mystery of Jupiter's cyclones

They are powerful, they are the size of Australia and no one imagined them until a few years ago - what is the secret of the storms at the poles of the largest planet in the solar system?

Six cyclones at the south pole of Jupiter, as captured by the infrared lens of the Juno probe in February 2017. Beautifully arranged and shaped like a rose cake. Source: NASA
Six cyclones at the south pole of Jupiter, as captured by the infrared lens of the Juno probe in February 2017. Beautifully arranged and shaped like a rose cake. Source: NASA

Until a few years ago, it had not yet arrived The Juno probe For its orbit around the planet Jupiter, no one knew that powerful cyclones the size of Australia reside at its poles. These cyclones, unlike the storms we are familiar with on Earth, do not pass, they hardly change - and they probably do not appear in combination with flying trees and wet news reports either. Now, in an article published today in the scientific journal Nature Geoscience, the Weizmann Institute of Science scientists managed to explain the mystery of Jupiter's cyclones: what are the forces that set enormous storms of these dimensions around the poles, and why their number and location remain more or less constant over time.

"You can think of Tzedek as an ideal climate laboratory," says Prof. Yohai Caspi, from the Department of Earth and Planetary Sciences at the institute. The Earth is a complicated and branched system: we have oceans and atmosphere, continents, biology, and of course, the effects of humans. Jupiter, on the other hand, the largest planet in the solar system, is composed of gas, so it is a much simpler system - where predictions can be tested and hypotheses confirmed. The data that makes all of this possible is collected by Juno - a research spacecraft that the American space agency NASA launched into space in 2011 and has been orbiting Jupiter since 2016. Prof. Caspi is a co-researcher on the Juno mission, one of whose exciting discoveries is The existence of polar cyclones.

"If you look at pictures of Jupiter until 2016," says Prof. Caspi, "it was common to represent the poles of Jupiter in gray - because no one knew what they looked like." The reason for this lies in the fact that the entire solar system is on the same plane, which is close to the plane of Jupiter's equator, so in observations made from Earth or previous spacecraft, we could mainly see the low latitudes of Jupiter. One of the significant innovations of the Juno mission, then, is its polar orbit, which made it possible to witness for the first time what is happening at the poles. This is how tremendous cyclones, beautifully arranged and rose-cake shaped, were discovered around 84° latitude. Beyond that, Juno's repeated orbits around Jupiter showed that their number remained constant - eight cyclones around the North Pole, and five around the South Pole. "This discovery was particularly surprising at the time," says Prof. Caspi, "because we expected the poles to be more or less symmetrical." In a previous article Prof. Caspi's connection between the asymmetry discovered in Jupiter's gravity field and the depth of the strong jet streams that prevail in the planet's atmosphere.

On Earth, cyclones form in places where the water temperature exceeds 26 degrees - usually, in the central Atlantic and Pacific Oceans - and move in a circular motion towards the poles due to the gravitational force induced on them as a result of the Earth's rotation. On Jupiter, however, strong jet streams do not allow these storms to form below 60° latitude - only above it, the currents are weak enough to allow cyclones to rage. What causes storms in Jupiter to settle at 84° latitude? According to the study, the storms in Jupiter are attracted to the poles as well, but the polar storm that is in the center of the cyclone circle pushes them and prevents them from reaching the pole itself.

"As long as the cyclones are far from the pole - they are attracted to it, but as they get closer - a repulsion occurs," says research student Nimrod Gabriel, from Prof. Caspi's group, whose doctoral thesis focuses on explaining the phenomenon. "The question is whether the resulting repulsion is strong enough to resist the pull of the pole. 84° latitude is the place where the forces of attraction and repulsion are offset." In the article, Gabriel and Prof. Caspi propose a computational model that takes into account the diameter of the polar cyclone (which is larger in the south than in the north), the minimum possible distance between cyclones, the surface area of ​​84° latitude, the size of the cyclones and their speed of rotation, and show that considering the data - around the North Pole Eight cyclones must be assembled. As for the South Pole - the calculation shows that the number of cyclones around it must be 5.62. This number is consistent with the data collected by Juno: although there cannot be such a number of cyclones, the five storms in the south sometimes split into six, as happened in the 18th and 34th flybys of the probe around Jupiter. Also, the explanation they offered explains the absence of the phenomenon in Jupiter's twin planet - Saturn.

Prof. Caspi and his research group are expecting more data to flow to them from Juno in the coming years following the decision by NASA to extend the space mission until 2025: "Slight changes in Juno's orbit gradually bring the probe closer to the North Pole. This way we will be able to receive data from a large number of devices that will help us better characterize the processes taking place in this intriguing area."

Between Jupiter and the Earth

"We are trying to understand the atmospheric dynamics on a large scale, and being able to explain the cyclones at the poles of Jupiter gives us confidence that we really understand the conditions that prevail there," says Prof. Caspi. This confidence may be crucial for us on Earth, since an understanding of cyclones as wide as possible, could help scientists to predict, for example, how the movement of storms will be affected if the Earth warms up - a challenge that it seems humanity will have to face. But for Prof. Caspi, the matter of the forces that prevail in the planet Jupiter is much simpler: "There are no more new islands to discover in the Pacific Ocean, and there is almost no place in the solar system that has not been mapped. The poles of Jupiter and the other gaseous planets are, perhaps, one of the last places in the solar system that remain to be discovered."

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