Basic Astronomy

Cassini captures a monster hurricane on Saturn

29th April 2013: Cassini spacecraft of NASA captured a stunning picture of a giant hurricane in Saturn. This is the first close-up, visible-light picture of a huge hurricane spining around Saturn's north pole.

Scientists measured with the high-resolution pictures and video the width of the hurricane's eye to be about 2,000 kilometers. This is 20 times larger than the average hurricane eye on Earth.

Saturn Storm

(False-color image from Cassini mission, highlighting the storms at Saturn's north pole. Image credit NASA)

At the outer edge of the hurricane, thin, bright clouds are traveling with 150 meters per second. The hurricane swirls inside a large, mysterious hexagon shaped six-sided weather pattern.

"We did a double take when we saw this vortex because it looks so much like a hurricane on Earth," explained Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena. "But there it is at Saturn, on a much larger scale, and it is somehow getting by on the small amounts of water vapor in Saturn's hydrogen atmosphere."

Scientists will study the hurricane to better understand hurricanes on Earth, which feed off warm ocean water. Although there is no water body close to these clouds high in Saturn's atmosphere, learning how these Saturnian storms use water vapor could tell scientists more about how terrestrial hurricanes are generated and sustained.

Saturn Storm

(False color image of the eye of the Saturn's north polar storm. This is resembling a rose surrounded by green foliage. Image credit NASA)

There are plenty of similarity between terrestrial hurricane and Saturns North polar vortex. Both have a central eye with no clouds or very low clouds. For both of the cases high clouds form an eye wall where other high clouds spirals around the eye.

But all are not same between terrestrial hurricane and Saturns North polar vortex. The most evident difference between the hurricanes is that the one on Saturn is much bigger than its counterparts on Earth and swirls surprisingly fast. At Saturn, the wind in the eye wall runs more than four times faster than that on Earth. Unlike terrestrial hurricanes, which tend to move, the Saturnian hurricane is locked onto the planet's north pole. On Earth, hurricanes tend to drift towards North because of the forces acting on the fast swirls of wind as the planet rotates. The hurricane on Saturn is already as far north as it can be and does not drift.

"The polar hurricane has nowhere else to go, and that's likely why it's stuck at the pole," explained Kunio Sayanagi, a Cassini imaging team associate at Hampton University.

Scientists believe the massive hurricane is swirling for years. When Cassini arrived in the Saturn system in 2004, Saturn was in the middle of its north polar winter and its north pole was dark. During that time, the Cassini spacecraft's composite infrared spectrometer and visual and infrared mapping spectrometer detected a great vortex, but a visible-light view had to wait for the passing of the equinox in August 2009. Only then did sunlight begin flooding Saturn's northern hemisphere. The view required a change in the angle of Cassini's orbits around Saturn so the spacecraft could see the poles.

"Such a stunning and mesmerizing view of the hurricane-like storm at the north pole is only possible because Cassini is on a sportier course, with orbits tilted to loop the spacecraft above and below Saturn's equatorial plane," explained Scott Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena. "You cannot see the polar regions very well from an equatorial orbit. Observing the planet from different vantage points reveals more about the cloud layers that cover the entirety of the planet."

Cassini 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 attentive oversight from navigators. The path requires careful planning years in advance and sticking very precisely to the planned itinerary to ensure enough propellant is available for the spacecraft to reach future planned orbits and encounters.

Source: JPL

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