Why Jupiter and Saturn Have Different Polar Storms

Jupiter and Saturn: similar giants, contrasting poles

Jupiter and Saturn, the Solar System’s two biggest planets, share many characteristics. Their compositions are broadly alike, they rotate at comparable rates, and they emit internal heat in much the same fashion. They even accumulate their moons in similar ways.

Yet one longstanding difference has perplexed researchers: the immense swirling storms positioned over their poles.

Saturn hosts a single enormous storm at each pole.

At Jupiter’s poles, by contrast, one large storm is encircled by a crown of smaller vortices.

Why the polar storms take different forms

Two planetary scientists now believe they may have an explanation for this puzzle. The answer may lie in the way these storms develop and link to the planet’s interior: whether the atmosphere permits them to expand without restriction, as on Saturn, or instead effectively limits their size, as on Jupiter.

In the researchers’ model, the determining factor is how firmly the storms are connected to deeper layers.

"Our study shows that, depending on the interior properties and the softness of the bottom of the vortex, this will influence the kind of fluid pattern you observe at the surface," says planetary scientist Wanying Kang of MIT.

"I don't think anyone's made this connection between the surface fluid pattern and the interior properties of these planets. One possible scenario could be that Saturn has a harder bottom than Jupiter."

The weather systems of Jupiter and Saturn are renowned. Their thick, gas-rich atmospheres are churned by turbulent storms, intense wind bands and dense clouds, which twist into formations that resemble abstract expressionist art.

Both worlds have received focused observation from spacecraft: Cassini at Saturn and Juno at Jupiter. These pioneering missions showed that, despite the planets’ numerous similarities, their polar storm arrangements are distinctly individual.

"People have spent a lot of time deciphering the differences between Jupiter and Saturn," says atmospheric scientist Jiaru Shi of MIT. "The planets are about the same size and are both made mostly of hydrogen and helium. It's unclear why their polar vortices are so different."

To investigate, Shi and Kang created a two-dimensional surface-fluid-dynamics model designed to reproduce the vortices observed at both planets’ surfaces.

"In a fast-rotating system, fluid motion tends to be uniform along the rotating axis," Kang says. "So, we were motivated by this idea that we can reduce a 3D dynamical problem to a 2D problem because the fluid pattern does not change in 3D. This makes the problem hundreds of times faster and cheaper to simulate and study."

Atmospheric layers and vortex growth

On gas giants, vast storms emerge from smaller moving components, including convection, and progressively increase in size. Their eventual scale, however, is set by several constraints: the depth of atmospheric layering, the strength with which the atmosphere is stirred - known as ‘forcing’ - and how rapidly friction dissipates energy.

Shi and Kang determined that the sequence in which these constraints are encountered has a major effect on the vortex patterns appearing across the atmosphere’s visible surface.

Jupiter’s atmosphere is sufficiently deep and energetic to generate numerous vortices. But turbulence arising early on stops them from merging into a single giant vortex, leaving its poles with a strikingly geometric, pepperoni-pizza-like arrangement of storms.

Put another way, the model indicates that Jupiter has weaker layering, stronger forcing as heat escapes from its centre, and slower energy loss through friction. Together, these conditions allow the separate storm structures to endure at the surface.

Saturn, in comparison, has more deeply layered atmosphere. In its case, weaker forcing may lessen turbulence at depth, more energy may be removed through friction, or both processes may occur. That eliminates the obstacle that prevents vortices from combining, allowing every storm to merge into one immense system.

What Jupiter and Saturn’s storms may reveal

The density of the lower layer where a vortex develops could also influence the result. Although this is not definitive proof, the team’s work suggests that each planet’s polar-storm pattern may contain clues to the environment in which those storms formed.

"What we see from the surface, the fluid pattern on Jupiter and Saturn, may tell us something about the interior, like how soft the bottom is," Shi says.

"And that is important because maybe beneath Saturn's surface, the interior is more metal-enriched and has more condensable material, which allows it to provide stronger stratification than Jupiter. This would add to our understanding of these gas giants."

The research was published in the Proceedings of the National Academy of Sciences.