REHOVOT, ISRAEL—July 19, 2021—Until recently, before NASA’s Juno space probe entered its orbit around the planet Jupiter, no one knew that powerful cyclones approximately the size of Australia rage across its polar regions. Jupiter’s storms, as opposed to the earthly variety, do not disperse and rarely change. In an article published in Nature Geoscience, researchers from the Weizmann Institute of Science reveal the mysteries of Jupiter’s cyclones: which forces affix these gargantuan storms to their polar locations and why the storms’ numbers and locations remain more or less constant over time.
“We can think of Jupiter as an ideal climate laboratory,” says Prof. Yohai Kaspi of the Department of Earth and Planetary Sciences. Earth is an intricate and multivariable system: it has oceans and an atmosphere, continents, biology – and of course, human activity. Jupiter, on the other hand, the largest planet in our solar system, is composed of gas and is therefore a far easier system to study, one that we can create predictions for and test hypotheses on. The data required for such analysis is being collected by Juno, a research probe that was launched by NASA in 2011 and entered Jupiter’s orbit mid-2016. Prof. Kaspi, a NASA co-investigator on the Juno mission, witnessed one of its more exciting findings: the cyclone storms swirling around the planet’s poles.
“If we look at older images of Jupiter taken before 2016,” says Prof. Kaspi, “we see that the poles were commonly represented as large grey areas because no one knew then what they actually look like.” This is because the solar system is organized on the same plane, which is very close to the plane of Jupiter’s equator. Therefore, past observations of the planet that were carried out from Earth, or from earlier space missions, could – for the most part – only capture Jupiter’s lower latitudes. Hence, one of the Juno mission’s noteworthy innovations is its polar orbit, which allows researchers to observe Jupiter’s tumultuous poles in detail for the first time. This is exactly how the cyclones were exposed, surprisingly organized and resembling a round tray of cinnamon rolls, along 84⁰ latitude north and south. Moreover, data gathered from Juno’s many orbits around Jupiter indicate that the number of cyclones remains fixed – eight are active around the north pole and five around the south. “This discovery was very surprising at the time,” says Prof. Kaspi, “because we expected the poles to be more or less symmetric.” In a previous study, Prof. Kaspi used the lack of symmetry in Jupiter’s gravitational field to determine the depth of the strong east-west wind belts that are characteristic of the planet’s atmosphere.
On Earth, tropical cyclonic storms form in areas where the water temperature 26⁰ Celsius (about 79⁰ Fahrenheit) – usually in the center of the Atlantic and Pacific oceans – and drift in a circular motion toward the poles, owing to a pull resulting from the planet’s spin. On Jupiter, however, strong jet streams prevent these storms from forming below 60⁰ latitude – only above that line are the currents weak enough to allow cyclones to rage. What causes these particular storms on Jupiter to settle at 84⁰ latitude? According to the new study, the planet’s cyclones are indeed attracted to the poles, but the polar storm located in the center of the ring of cyclones pushes them away, preventing them from reaching the pole itself.
“As long at the cyclones remain at a distance from the pole, they are attracted to it. But the nearer they venture – the more strongly they are repelled,” says doctoral student Nimrod Gavriel from Prof. Kaspi’s research group, whose thesis focuses on elucidating this phenomenon. “The question is whether the repulsion effect is strong enough to resist the pole’s attraction. 84⁰ latitude is where these forces even up.” Gavriel and Prof. Kaspi propose a mathematical model that considers the diameter of the polar cyclone (which is larger at the south pole than in the north), the possible minimal distance between each cyclone, the surface area around 84⁰ latitude, and the size of the cyclones and their spin, and that accurately predicts the presence of eight cyclones across the north pole. As for the south pole, based on their calculations, the number of cyclones should be 5.62. This number is consistent with the data collected by Juno – and while in reality this number cannot exist, the five southern storms often separate into six storms, as observed during Juno’s eighteenth and thirty-fourth orbits around Jupiter. The proposed model also explains why this phenomenon does not occur on Jupiter’s closest neighboring planet – Saturn.
“We are trying to understand atmospheric dynamics at a large scale, and providing a successful explanation for the phenomenon of Jupiter’s polar cyclones gives us the confidence that we truly know what’s going on there,” says Prof. Kaspi. This confidence may be crucial for us here on Earth, as a deeper understanding of cyclones could help meteorologists predict, for example, how the warming of our planet will affect the movement of storms across it – a challenge that humanity will most likely face in the near future. But Prof. Kaspi’s fascination with the exploration of Jupiter is more straightforward: “There are no new islands to discover in the Pacific, and most planetary bodies in the solar system have already been mapped. The poles of Jupiter and the other gaseous planets are, perhaps, the last spots in the solar system that are still left to explore.”
“We are expecting more valuable data to come in from Juno during the next couple of years,” Prof. Kaspi adds, given the Juno mission’s recent extension to 2025. “Owing to gradual changes in the spacecraft’s polar orbit, it is now getting closer and closer to Jupiter’s north pole, allowing us to gain information about this polar region from several specialized instruments,” he concludes.
Prof. Yohai Kaspi is head of the Helen Kimmel Center for Planetary Science.
