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Cracking the mystery

Physics researcher creates model to explain Saturn's space storms


By Kelly Foss

Pictures of Saturn have been mystifying scientists ever since they were beamed back to Earth from NASA’s Voyager mission in 1981 and, more recently, the Cassini mission in 2006.

The strange, hexagon-shaped jet stream circling the planet’s north pole and a huge, hurricane-like vortex at the pole have inspired many theories. The pictures also show much smaller vortices covering the entire surface of the gas giant.

Saturn’s cloud belts generally move around the planet in a circular path, but one feature is slightly different. The planet’s wandering, hexagon-shaped polar jet stream breaks the mold, a reminder that surprises lurk everywhere in the solar system.
Photo: NASA/JPL-Caltech/Space Science Institute

Dr. Iakov Afanassiev, a professor in the Department of Physics and Physical Oceanography at Memorial, was likewise inspired by those photographs. A paper featuring Dr. Afanassiev’s experiments that are described below was recently published in Nature Geoscience.

Known for his research on the dynamical processes that govern the behaviour of stratified and rotating fluids that comprise the Earth’s oceans, he believes the smaller storms are due to tilted convection.

Convection currents are formed when heated, and therefore less dense, air rises to the surface while colder, denser air sinks to the bottom – a cycle which can lead to hurricanes. Tilted convection occurs when buoyancy forces do not align with the planet’s rotation axis.

“My graduate student, Yang Zang, and I started doing experiments in the lab,” he explained. “Our experiments were conducted with a cylindrical water tank that is heated at the bottom, cooled at the top and spun on a rotating table we normally use for modelling the rotating ocean.

Lab image
An image of the tank from above showing small vortices due to convection.
Photo: Iakov Afanassiev

“Convection is a very familiar thing, but when everything is rotating, different effects come into play,” he continued. “We saw that the warm rising plumes and cold sinking water generated small anti-cyclonic and cyclonic vortices similar to Saturn’s storms.”

‘Collective dynamics’

The experiments were supported by numerical simulations which further showed small-scale convection leads to larger-scale cyclonic flow at the surface and anti-cyclonic circulation at the bottom of the fluid layer, with a polar vortex forming from the merging of smaller cyclonic storms that are driven polewards.

“Basically, we are saying the whole circulation is created by the collective dynamics of these tiny vortices,” he said.

“Merging of small vortices into a large polar vortex was previously explained by an effect of a planet’s rotation and sphericity, but we believe that they join together because they are attached to convecting air parcels.”

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