The Cassini spacecraft has relayed to Earth the first close-up, visible-light views of a monster hurricane swirling around Saturn’s north pole. It’s 2,000 km wide, 20 times larger than the average hurricane eye on Earth. The clouds at the outer edge of the hurricane are moving at 150 meters per second as the storm swirls inside a large, mysterious, six-sided weather pattern known as the hexagon. This false-color image highlights the storm at Saturn’s north pole.

A natural color image is at left.

Hurricanes on Earth feed off warm ocean water, but there is no body of water on Saturn. Learning how these Saturnian storms use water vapor might tell scientists more about how terrestrial hurricanes are generated and sustained. Both terrestrial hurricanes and Saturn’s north polar vortex have a central eye with no clouds or very low clouds. Both have high clouds forming an eye wall, other high clouds spiraling around the eye, and a counter-clockwise spin in the northern hemisphere. But the one on Saturn is much bigger than its counterparts on Earth and spins surprisingly fast. On Saturn, the wind in the eye wall blows more than four times faster than hurricane-force winds on Earth. Terrestrial hurricanes move around, but Saturn’s hurricane is parked over the planet’s north pole. On Earth, hurricanes tend to drift northward because of the forces acting on the winds as the planet rotates. Saturn’s does not drift; it’s already as far north as it can be.

Scientists believe the massive storm has been churning for years. When Cassini arrived in the Saturn system in 2004, Saturn’s north pole was in winter darkness. Cassini‘s composite infrared spectrometer and visual and infrared mapping spectrometer detected the great vortex, but a visible-light image had to wait for the equinox in 2009 when sunlight begin falling on the higher latitudes of Saturn’s northern hemisphere. Cassini‘s orbit had to be changed so the spacecraft could see the poles. The spacecraft changes its orbital inclination for such an observing campaign only once every few years. Because the spacecraft uses flybys of Saturn’s moon Titan to change the angle of its orbit, the inclined trajectories require careful planning years in advance to ensure sufficient fuel is available for the spacecraft to reach future planned orbits and encounters.

Image Credits: NASA