A model spot for Jupiter.

A Model Spot for Jupiter

Jupiter's Great Red Spot, a gigantic mass of circulating fluid in the planet's thick atmosphere, has survived for centuries despite the turbulence surrounding it. How such flow features can exist has been a longstanding scientific puzzle. Now researchers have managed to construct both a simple computer model and an analogous laboratory experiment that reproduce the Red Spot's main features.

"We've reduced the problem to a very simple situation," says graduate student Steven D. Myers. "The actual Jovian atmosphere is more complicated, but we think we now understand the fundamental mechanism involved in the Red Spot -- why it has such a long lifetime, why it exists at a particular latitude, and why it is the shape it is." Myers is a member of the team that did the experimental work at the University of Texas at Austin.

The experiments were inspired by the theoretical studies of Philip S. Marcus of the University of California at Berkeley. Several years ago, Marcus had suggested that organized features could appear in the midst of chaotic fluid flow (SN: 6/2/84, p.340). Subsequent numerical simulations showed that large, stable vortices arise naturally from solutions of the equations of motion governing Jupiter's atmosphere. Marcus's latest results, along with the experimental work done in Texas, are reported in the Feb. 25 NATURE.

The experiments were done in a rapidly rotating, circular tank, nearly 1 meter in diameter. A sloping bottom, highest near the center and lowest at the tank's rim, mimicked effect of latitude on the forces responsible for skewing liquid flow. Water was pumped into the tank through an inner ring of six inlets, and it drained out through a corresponding outer ring.

At a sufficiently high pumping rate, with the tank spinning at 4 revolutions per second, the researchers found that a jet of water begins to flow in a direction opposite to that normally expected in a rotating system. As a result, some water moves in one direction while the rest moves in the opposite direction, establishing a shear zone. A large, stable vortex, bounded by the zone's edges, forms within this layer.

This result matches the observation that Jupiter's Red Spot also sits in a shear zone, rolling like a giant ball between a westward current to the north and an eastward current to the south. The l aboratory model's pumping action, which is responsible for establishing the shear zone, may imitate planetary convection currents that carry fluid into and out of the layer containing the Red Spot.

Both the computer simulations and the experiments show that a large vortex may initially form by the amalgamation of many smaller vortices. Furthermore, despite forces that dissipate its energy, a large spot seems to maintain its size over a long time period by absorbing tiny vortices that happen to form in its vicinity.