It looks like hurricane season never ends on Saturn. NASA's Cassini spacecraft has spotted a giant hurricane-like whorl at the planet's south pole, a weather pattern previously seen only on Earth. The storm is a monster: 5,000 miles across—big enough to engulf North America—and rising 45 miles above the eye, five times higher than any terrestrial storm, with a top wind speed of 350 miles per hour, faster than the most violent twister ever to plow through Tornado Alley.
Like a terrestrial hurricane, the storm exhibits the characteristic nautilus of rain clouds swirling about a dark central vortex. "But the mechanism on Saturn has to be different," says Caltech planetary scientist Andrew Ingersoll, a member of the Cassini imaging team. This is because terrestrial hurricanes form when an inflow of air along the ocean surface sops up moisture and rises in a halo of updrafts to create towering columns of rain clouds. But Saturn, a gaseous planet, has no oceans. Furthermore, unlike hurricanes on Earth, the Saturnian tempest is stationary, most likely due to the planet's rotation. In this respect, the phenomenon more closely resembles Earth's Arctic vortices—seasonal cyclones that appear above the poles and are driven by temperature gradients in the upper atmosphere. The analogy is not perfect, though, because unlike Earth's Arctic regions, Saturn's south pole is slightly warmer than the rest of the planet.
Ingersoll says the next step is to design computer simulations of Saturn's bizarre meteorology in hopes of better understanding the Cassini footage. Adding to the mystery is the absence of such storms on similar worlds, he says. "There are storms on other planets, such as Jupiter's Great Red Spot, but there is nothing like this one."