Saturn is Making Waves

The ringed planet has been making news for a hurricane at its north pole, digestive troubles in its interior, and "rain" of water ice from its rings.

Right now our solar system's signature ringed planet is putting on a show. Saturn reached opposition just two days ago, so it rises at sunset and takes all night to arc across the sky.

But I'm not referring to that kind of show, as beautiful and visually satisfying as telescopic views of Saturn might be. Instead, I mean Saturn's performance for planetary scientists: it's been in the news three times recently.

Two days ago, the Cassini imaging team released dramatic views of the planet's north pole. Seasonal change happens slowly on Saturn, which takes 29½ years to circle the Sun. So ever since equinox came and went in August 2009, sunlight has been gradually getting stronger around the north polar region.

The waxing northern spring has allowed Cassini scientists to resume their tracking of the "hexagon," a broad, wave feature centered at 76° north that has six straightish segments. Roughly two Earths across, the hexagon has been around at least since its discovery in 1981 by Voyager 2 and appears to be a fast-moving jet, though the exact cause remains a mystery.

The researchers had hoped to gain some insight by seeing what lies in the hexagon's center, at the north pole, and the surprising answer is … a hurricane! Its ring-shaped "eye" spans some 1,250 miles (2,000 km), 20 times larger than its counterparts on Earth. Winds moving around the outer edge are racing at 330 miles (530 km) per hour.

It's unclear what's driving this vortex of motion or how long it's existed. On Earth, hurricanes are powered by heat drawn from tropical ocean water. The one at Saturn's north pole "is somehow getting by on the small amounts of water vapor in Saturn's hydrogen atmosphere," notes Andrew Ingersoll (Caltech) in a NASA press release.

Cassini took these revealing images when it glided over the north polar region last November 27th at a distance of 260,000 miles (418,000 km). Especially dramatic is the "red rose" close-up created from images taken at three near-infrared wavelengths. Seen at right, the blue channel corresponds to light at 890 nm, green at 728 nm (sensitive to high-altitude clouds), and red at 752 nm (sensitive to low clouds).

Meanwhile, closer to Saturn's equator, another investigation has turned up evidence that water derived from the rings is raining into the planet's upper atmosphere. This process had been suspected for decades — especially after Cassini found evidence when it arrived in 2004 that the rings are immersed in a tenuous water "atmosphere" but confirmation took a while. In the April 11th issue of Nature, James O’Donoghue and seven others describe infrared observations of ionized trihydrogen (H₃⁺) in Saturn's ionosphere made two years ago with the Keck I telescope. The observers expected to see an even glow across the planet, induced by a steady bombardment charged particles from Saturn's magnetosphere. Instead, they found a series of bright and dark bands at the middle latitudes of both hemispheres.

Apparently, water molecules from the rings are being ionized by sunlight and then travel along magnetic field lines into Saturn's upper atmosphere, where they "quench" the expected infrared emission by reacting with H₃⁺. The ionospheric glow is strongest at the locations that map back to two big "holes" in the rings — the Cassini Division and Colombo Gap — where icy ring particles are sparse.

As the team explains in a Keck Observatory press release, this banding effect is not seen in the ionosphere of Jupiter. That's likely because the Jovian rings are far more tenuous and contain little water.

Observers have long realized that Saturn's entire globe has a healthy infrared glow, thanks to roughly 10¹⁷ watts escaping from its interior. All the solar system's planets have radiated heat as they've gradually cooled over 4½ billion years, but Saturn's glow is brighter than it should be for a planet of its age. For decades, the most reasonable explanation seemed to be that helium was separating from hydrogen in the planet's deep interior, "raining out" toward the core as droplets that liberate energy as they descend.

But maybe that's not the right explanation. Jérémy Leconte (Laboratory of Dynamic Meteorology, Paris) and Gilles Chabrier (University of Exeter, England) think Saturn's youthful glow comes about because deep-seated layers of dense gas prevent the internal heat from making its way outward efficiently. Instead of traveling via giant "conveyor belts" of buoyant gas, the heat must diffuse outward — and that takes longer.

"Our calculations show that Saturn appears young because it can’t cool down," Chabrier noes in an Exeter press release. "These separate layers effectively insulate the planet and prevent heat from radiating out efficiently. This keeps Saturn warm and bright.”

Helium rainout is probably still happening, Leconte and Chabrier note in the April 21st issue of Nature Geoscience, but it probably can't account for all of Saturn's infrared excess. Jupiter and Neptune also release a tremendous amount of internal heat. But Uranus, a curious planet in many ways, does not.