Ever since Cassini captured its first images of Saturn’s north polar region, scientists and the public alike have been fascinated by one of the planet’s most prominent idiosyncrasies: the hexagon.
Scientists first observed the six-sided jet stream when the two Voyager spacecraft passed by Saturn in the early 1980s, but the Voyager flybys' geometries prevented the spacecraft from capturing the entire monstrous hexagon in a single image from above.
Twenty years later, when Cassini arrived at Saturn in 2004 during Saturn’s northern winter, the hexagon was in shadow. Scientists had to wait a few more years to see the hexagon in visible wavelengths of light because Saturn’s seasons last longer than those on Earth.
Saturn is farther from the sun than Earth is. As a consequence, a Saturn year — the time it takes to complete one solar orbit — is about 29 Earth years, and each Saturnian season lasts more than seven Earth years. So Cassini first observed the hexagon in 2006 (still Saturn northern winter), using the Visual and Infrared Mapping Spectrometer (VIMS) instrument.
VIMS can see the hexagon in the dark because it detects infrared wavelengths of light, which are generated in the warm interior of Saturn. (Humans also emit light at infrared wavelengths, just like Saturn).
Finally, when Saturn approached its August 2009 equinox, the start of northern spring, the wait was over. Cassini began studying the hexagon with its main imaging cameras, the Imaging Science Subystem (ISS). The ISS instrument has higher resolution than the VIMS instrument and produced images of the hexagon in unprecedented detail.
The ISS images revealed a stunningly symmetric jet stream about 20,000 miles (30,000 kilometers) across with winds around 300 miles per hour (500 kilometers per hour). The hexagon is centered around a vortex that marks Saturn’s north pole.
The hurricane-like vortex has an eye about 50 times larger than a typical Earth hurricane and its maximum wind speed pushes 300 miles per hour (500 kilometers per hour).
“This is the belly button of Saturn,” said VIMS scientist Kevin Baines, a planetary scientist at NASA’s Jet Propulsion Laboratory. Baines emphasizes that, although it resembles a hurricane, Saturn’s north polar vortex is not the same thing because hurricanes on Earth feed on water and heat energy from Earth’s surface. “But it does have at least the structure of a hurricane,” he said. “It’s what we call the classic vortex structure.”
The hexagon itself, however, perplexed scientists. “This is one of the biggest mysteries of the dynamics of Saturn,” Baines said.
To humans, familiar exclusively with weather on Earth, the hexagon can seem exotic or alien. It has a diameter more than double the width of Earth. The hexagon’s sides are roughly equal in length, and the angles where the sides meet are roughly equal to one another as well, producing a seemingly unnatural order. But the hexagon is very natural indeed.
On Earth, jet streams seem to slither and meander with no pattern or symmetry. But while the atmospheres of both Saturn and Earth follow the same natural laws, different conditions produce different results.
Sunlight drives wind and weather patterns on Earth, and our planet is nearly 10 times closer to the sun than Saturn is, which means we receive nearly 100 times more sunlight than Saturn. As the Earth spins, the sun’s energy input on Earth varies more sharply from day to night than on Saturn. Earth’s surface is also non-uniform. We have water, land, ice, and snow, which heat and cool at different rates.
Further still, Earth also has an extremely shallow atmosphere compared to Saturn’s, and towering mountains reach miles or kilometers into the sky and disturb atmospheric flow the same way a scattering of large stones in a shallow creek cause water to churn and roll.
In short, Earth lacks the conditions for a jet stream to settle into a symmetric shape.
But Saturn is a different story. The gas giant has no solid surface, and its composition is far more uniform. And although Saturn spins, it receives about 1 percent of the sunlight that Earth does, so the difference in energy input between dayside and nightside is far less sharp than on Earth.
There’s apparently nothing in Saturn’s atmosphere to keep a perfectly natural symmetric shape from forming. Scientists even duplicated the hexagon in a laboratory on Earth using a cylindrical tank of water on top of a slowly rotating table. They also produced other shapes. “We can also get squares, octagons and other polygons — it depends on the speed and the different properties of the fluid,” Baines said.
That’s not to say scientists have Saturn’s hexagon all figured out. For some reason, Saturn has a hexagonal jet stream only in the north, not in the south. And while scientists have reproduced the hexagon in the lab, they don’t know why it’s so long-lived.
"We have already learned an immense amount about the hexagon during the Cassini mission, but we still don't know what maintains its unusual shape and extremely high winds,” Baines said. “The polar vortex at the hexagon's center — which was discovered by Cassini — is a bit of an enigma as well.”
Baines hopes Cassini’s Grand Finale orbits, which carry the spacecraft astoundingly close to Saturn, might provide further insights. “These last 22 orbits are giving us a unique chance to observe and analyze the hexagon in unprecedented detail,” Baines said. “Over and over during the Cassini mission, Saturn has surprised us with fascinating new discoveries, and I'm thrilled to see what we might find out between now and the end of the mission."