“If you went below the cloud tops, you would probably find liquid water rain drops, hail, and snow,” says Andy Ingersoll, Caltech professor of planetary science, about the chaotic swirling storms at Jupiter’s South Pole arranged in a strange geometric pattern in the shape of a pentagon or hexagon that have presented a mystery to scientists. “The winds would be hurricane-force winds. Hurricanes on Earth are a good analog of the individual vortices within these arrangements we see on Jupiter, but there is nothing so stunningly beautiful here.”
The awesome immensity of Jupiter’s storms, some measuring up to 1400 kilometers across, that includes the enigmatic Great Red Spot, was captured with discovery of a massive storm system at Jupiter’s South Pole that occurred on Nov. 3, 2019, by NASA’s Juno spacecraft. The colossal 350-year old gaseous vortex, the 16,000 kilometers-wide Great Red Spot –a perfect example of a non-living system that is out of equilibrium with its environment and endure over time– could swallow the Earth whole and still have room for Mars.
Lord Kelvin’s Math
Now, a research team working in the lab of Ingersoll, Caltech’s “planetary weatherman,” has discovered why Jupiter’s storms that are remarkably similar to the ones that lash the East Coast of the United States every summer and fall, just on a much larger scale, that were first spotted by NASA’s Juno space probe in 2019, behave so strangely. They did so using math derived from a proof written by Lord Kelvin, a British mathematical physicist and engineer, nearly 150 years ago.
Gas Giant’s Absence of Land
As on Earth, Jupiter’s storms tend to form closer to the equator and then drift toward the poles. However, Earth’s hurricanes and typhoons dissipate before they venture too far from the equator. Jupiter’s just keep going until they reach the poles.
“The difference is that on the earth hurricanes run out of warm water and they run into continents,” Ingersoll says. Jupiter has no land, “so there’s much less friction because there’s nothing to rub against. There’s just more gas under the clouds. Jupiter also has heat left over from its formation that is comparable to the heat it gets from the sun, so the temperature difference between its equator and its poles is not as great as it is on Earth.”
However, Ingersoll says, this explanation still does not account for the behavior of the storms once they reach Jupiter’s south pole, which is unusual even compared to other gas giants. Saturn, which is also a gas giant, has one enormous storm at each of its poles, rather than a geometrically arranged collection of storms.
A color-enhanced picture above was taken of Jupiter’s south pole by the Juno spacecraft, which has been orbiting Jupiter since mid-2016. Juno is in a highly elliptical orbit, a path that takes the probe up close to Jupiter’s surface once every 53 days. It’s during these close passes that Juno gets within 2,600 miles of Jupiter’s cloud tops, allowing its instruments to collect data more closely than any other spacecraft before.
Mystery of the Geometry Solved
The answer to the mystery of why Jupiter has these geometric formations and other planets do not, Ingersoll and his colleagues discovered, could be found in the past, specifically in work conducted in 1878 by Alfred Mayer, an American physicist, and Lord Kelvin.
Mayer had placed floating circular magnets in a pool of water and observed that they would spontaneously arrange themselves into geometric configurations, similar to those seen on Jupiter, with shapes that depended on the number of magnets. Kelvin used Mayer’s observations to develop a mathematical model to explain the magnets’ behavior.
“Back in the 19th century, people were thinking about how spinning pieces of fluid would arrange themselves into polygons,” Ingersoll says. “Although there were lots of laboratory studies of these fluid polygons, no one had thought of applying that to a planetary surface.”
“We wanted to explore the combination of parameters that makes these cyclones stable,” says Cheng Li, lead author and 51 Pegasi b postdoctoral fellow at UC Berkeley about using used a set of equations known as the shallow-water equations to build a computer model of what might be happening on Jupiter, and began to run simulations.”There are established theories that predict that cyclones tend to merge at the pole due to the rotation of the planet and that’s what we found in the initial trial runs.”
Eventually, however, the team found that a Jupiter-like stable geometric arrangement of storms would form if the storms were each surrounded by a ring of winds that turned in the opposite direction from the spinning storms, or a so-called anticyclonic ring. The presence of anticyclonic rings causes the storms to repel each other, rather than merge.
“Other planets provide a much wider range of behaviors than what you see on Earth,” he says, “so you study the weather on other planets in order to stress-test your theories.”
Source: Cheng Li et al. Modeling the stability of polygonal patterns of vortices at the poles of Jupiter as revealed by the Juno spacecraft, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2008440117
The Daily Galaxy, Sam Cabot, via California Institute of Technoloy