For years, passing spacecraft have observed mysterious weather patterns at the poles of Jupiter and Saturn. The two planets have very different types of polar vortices. A polar vortex is a giant atmospheric vortex that rotates over a planet’s polar regions. On Saturn, a single giant polar vortex appears to cover the north pole in a strange hexagonal shape, while on Jupiter, the central polar vortex is surrounded by eight smaller vortices, like a swirling pan of cinnamon rolls.

Given that both planets are similar in many ways – roughly the same size and made of the same gaseous elements – the distinct differences in polar weather patterns have been a long-standing mystery.

Now, MIT scientists have identified a possible explanation for how two different systems evolved. Their findings could help scientists understand not only the weather patterns on the planet’s surface, but also what lies beneath the clouds and deep inside.

In a study published in this week’s journal, Proceedings of the National Academy of Sciencesthe team simulated different ways in which well-organized vortex patterns can form from random stimuli on a gas giant planet. A gas giant is a large planet, like Jupiter or Saturn, that is made up mostly of gaseous elements. Among the various possible planetary configurations, the researchers found that in some cases the streams coalesced into a single large vortex similar to Saturn’s pattern, while in other simulations they produced multiple large circulations similar to Jupiter’s vortices.

By comparing the simulations, the researchers found that the pattern of vortices, and whether a planet produces one or more polar vortices, ultimately comes down to one key property: the “softness” of the vortex’s base. This is related to internal configuration. Scientists liken individual vortices to swirling cylinders that rotate through the planet’s many atmospheric layers. If the base of this swirling cylinder is made of a softer and lighter material, the evolving vortex can become very large. The final pattern allows for multiple small vortices, similar to Jupiter. In contrast, if the base of the vortex is made of harder, denser material, the vortex can grow even larger and then swallow other vortices, forming a single giant vortex similar to Saturn’s monster cyclone.

“Our study shows that depending on the nature of the interior and the softness of the vortex bottom, it influences the type of fluid pattern observed at the surface,” says study author Wanying Kang, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “I don’t think anyone has made the connection between the fluid patterns on the surface of these planets and the properties of their interiors. One possible scenario is that Saturn’s bottom is harder than Jupiter’s.”

The study’s lead author is MIT graduate student Jial Shi.

spin up

Kang and Shi’s new work is inspired by images of Jupiter and Saturn taken by the Juno and Cassini missions. NASA’s Juno spacecraft has been orbiting Jupiter since 2016, capturing stunning images of Jupiter’s north pole and its multiple spirals. From these images, scientists estimated that each of Jupiter’s vortices is huge, spanning about 3,000 miles in diameter (almost half the width of Earth itself).

The Cassini spacecraft orbited Saturn’s ringed planet for 13 years before intentionally burning up in Saturn’s atmosphere in 2017. Observations of Saturn’s north pole recorded a single hexagonal polar vortex about 18,000 miles wide.

“People have spent a lot of time deciphering the differences between Jupiter and Saturn,” Shi says. “These planets are about the same size, and both are made primarily of hydrogen and helium. It’s unclear why the polar vortices are so different.”

Shi and Kang set out to identify the physical mechanisms that explain why some planets evolve a single vortex while others generate multiple vortices. To do this, they used a two-dimensional model of surface fluid dynamics. Although polar vortices are inherently three-dimensional, the researchers reasoned that the rapid rotation of Jupiter and Saturn forces uniform motion along their rotational axes, allowing them to accurately represent the evolution of the vortices in two dimensions.

“In high-speed rotating systems, fluid movement tends to be uniform along the axis of rotation,” Kang explains. “So we were motivated by the idea that because fluid patterns do not change in 3D, we could reduce 3D mechanics problems to 2D problems. This would make simulating and studying the problem hundreds of times faster and cheaper.”

reach the bottom

Following this reasoning, the research team developed a two-dimensional model of vortex evolution on gas giant planets, based on existing equations that describe how swirling fluids evolve over time.

“This equation is used in many situations, including modeling mid-latitude cyclones on Earth,” Kang says. “We adapted this equation to the polar regions of Jupiter and Saturn.”

The researchers applied a two-dimensional model to simulate how fluids evolve over time on the gas giant planet under different scenarios. For each scenario, the team varied parameters such as the planet’s size, rotational speed, internal heating, and the softness or hardness of the rotating fluid. They then set up random “noise” conditions in which the fluid initially flows in a random pattern across the planet’s surface. Finally, we observed how the fluid changes over time given the specific conditions of the scenario.

Through multiple different simulations, we observed that some scenarios evolve to form a single large polar vortex, like Saturn, while others form multiple smaller polar vortices, like Jupiter. After analyzing the combination of parameters and variables for each scenario and how they relate to the final outcome, they arrived at a single mechanism that explains whether a single or multiple vortices evolve. As the random fluid motions begin to coalesce into individual vortices, the size that the vortices can grow is limited by the softness of the vortex base. The softer, or lighter, the gas spinning at the bottom of the vortex, the smaller the vortex will eventually become, allowing multiple smaller vortices to coexist at the planet’s poles, as is the case with Jupiter.

Conversely, the harder and denser the vortex base, the larger the system can eventually grow to a size where it can follow the curvature of a planet as a single planet-scale vortex like Saturn.

If this mechanism is indeed at work in both gas giants, it would suggest that Jupiter may be made of softer, lighter material and that Saturn may harbor heavier material inside.

“What we see from the surface, the fluid patterns of Jupiter and Saturn, may tell us something about the interior, such as how soft the bottom is,” Shi says. “And this is important because beneath Saturn’s surface, the interior is probably richer in metals and contains more condensable material, allowing for stronger stratification than on Jupiter.”

“Jupiter and Saturn are otherwise very similar, which makes the differences in their polar weather a mystery,” says Yohai Caspi, a professor of geophysical fluid dynamics at the Weizmann Institute of Science and a member of the Juno mission’s science team. “Dr. Shih and Dr. Kang’s research reveals a surprising link between these differences and the ‘softness’ of the planet’s deep interior, providing a new way to map the key internal properties that shape the atmosphere.”

This research was supported in part by a Mathworks Fellowship and a gift from the MIT Department of Earth, Atmospheric, and Planetary Sciences.