Vertical gas flow may be key to massive storm’s longevity
Gases flowing vertically from the top and bottom of Jupiter’s Great Red Spot keep the huge storm swirling, according to a new mathematical simulation. No previous work has explained how the cyclone has survived for the nearly two centuries that astronomers have observed it.
The Great Red Spot is a giant vortex: a mass of swiftly rotating gas. Nearly twice the diameter of Earth, the spot has been roiling since at least 1831, when it was first described. Early telescope observations in the 1660s of a large vortex on Jupiter may even have been views of the Red Spot.
Despite this long observational record, scientists have struggled to explain why the spot did not long ago radiate away its energy and disappear. Astronomers’ best attempts involve the storm gaining energy by swallowing smaller vortices in Jupiter’s jet streams. But observations over the last few decades revealed that the planet does not produce enough of these smaller storms to power the Great Red Spot. As a result, computer simulations predict the pancake-shaped spot should last at most a few decades. “We have lots of publications that show how the Red Spot dies,” said Philip Marcus, a computational physicist at the University of California, Berkeley.
In the new simulation of the spot’s churning winds, Marcus and Harvard physicist Pedram Hassanzadeh included vertical as well as horizontal gas motion. Most previous analyses have either ignored or approximated the spot’s vertical gas velocities because they are far smaller than horizontal winds and including vertical flow makes such simulations slower and more cumbersome.
But when Marcus and Hassanzadeh included precise vertical speeds, the Great Red Spot lasted up to 800 years, far longer than it had in any other researchers’ simulations. That was a big surprise, Marcus said.
Exploring their results further, Marcus and Hassanzadeh found that gases flowing out of the top and bottom of the spot looped around, picked up energy from nearby jet streams and plunged back into the vortex, continuously replenishing the storm. The two scientists believe that nearby vortices provide additional energy. Together, the vertical gas flow and swallowed vortices seem to explain the Great Red Spot’s longevity, the scientists say. They reported their findings November 25 in Pittsburgh at the annual meeting of the American Physical Society’s Division of Fluid Dynamics.
The idea “seems very reasonable,” said Robert Ecke, a physicist at Los Alamos National Laboratory in New Mexico. He stresses that the work is preliminary and still needs to go through peer review. But Marcus and Hassanzadeh have already provided a new way for scientists to think about self-sustaining vortices, Ecke said.
P. Hassanzadeh and P. Marcus. On the unexpected longevity of the Great Red Spot, oceanic eddies, and other baroclinic vortices. Annual Meeting of the American Physical Society Division of Fluid Dynamics, Pittsburgh, November 25, 2013.