An Explanation for Saturn’s Hexagon

For over three decades, we’ve been gathering observations of the mysterious hexagonal cloud pattern encircling Saturn’s north pole. Now, researchers believe they have a model that can better explain its formation.

Fascinating Geometry

Saturn’s northern Hexagon is a cloud band circling Saturn’s north pole at 78° N, first observed by the Voyager flybys in 1980–81. This remarkable pattern has now persisted for more than a Saturn year (29.5 Earth years).

Observations by Voyager and, more recently, Cassini have helped to identify many key characteristics of this bizarre structure. Two interesting things we’ve learned are:

1. The Hexagon is associated with an eastward zonal jet moving at more than 200 mph.
   The cause of the Hexagon is believed to be a jet stream, similar to the ones that we experience on Earth. The path of the jet itself appears to follow the hexagon’s outline.
2. The Hexagon rotates at roughly the same rate as Saturn’s overall rotation.
   While we observe individual storms and cloud patterns moving at different speeds within the Hexagon, the vertices of the Hexagon move at almost exactly the same rotational speed as that of Saturn itself.

Attempts to model the formation of the Hexagon with a jet stream have yet to fully reproduce all of the observed features and behavior. But now, a team led by Raúl Morales-Juberías of the New Mexico Institute of Mining and Technology believes they have created a model that better matches what we see.

Simulating a Meandering Jet

The team ran a series of simulations of an eastward, Gaussian-profile jet around Saturn’s pole. They introduced small perturbations to the jet and demonstrated that, as a result of the perturbations, the jet can meander into a hexagonal shape. With the initial conditions of the team’s model, the meandering jet is able to settle into a stable hexagonal shape that rotates with very nearly the same period as Saturn’s rotational period.

The formation of this hexagon depends on factors such as the initial amplitude and curvature of the jet. The model’s treatment of the wind profile within Saturn’s atmosphere is another key component that allowed them to match the observed characteristics of the Hexagon, such as its shape, vorticity behavior, temperature gradient, and seasonal stability.

Bonus

The gif below shows part of an animation the authors produced of the jet evolution in their model. You can see a hexagon begin to develop at around 230 days into the simulation, and by about 400 days it becomes stable and non-rotating (we’re looking at it from a reference frame rotating with Saturn). The full animation can be viewed here. [Morales-Juberías et al., 2015]

Citation

R. Morales-Juberías et al. 2015 ApJ 806 L18 doi:10.1088/2041-8205/806/1/L18