Saturn’s Polar Mystery

Deep inside Saturn, buried under more than 10,000 miles of clouds, a sea of liquid hydrogen blisters and boils, racing with electricity that transforms the whole planet into a giant magnet.

Planetary scientists believe this phenomenon, called a dynamo, is what generates magnetic fields around all the solar system’s gas giants. And deep in Earth’s core, the same process also creates the magnetic field that protects our planet from the ravages of “space weather.” But new magnetic field readings from the Cassini spacecraft are forcing some planetary scientists to reconsider what’s beneath the obscuring clouds of the great ringed planet.

Follow Cassini's final days as it skims the cloud tops before plunging into the planet.

On April 26, after almost 13 years in orbit around Saturn, Cassini began its “grand finale,” a series of 22 thread-the-needle orbits that pass inside Saturn’s rings. Now, as Cassini polishes off these final orbits, it is preparing for an even more dramatic climax: on September 15, it will make a swan-song swan-dive right into Saturn. Before it loses its communication link with Earth, the spacecraft is expected to give scientists their closest-ever look at the planet’s magnetic field. The results could help resolve a longstanding mystery—or widen the chasm between theory and observation.

This image of the northern polar region of Saturn, captured by Cassini, shows both the aurora and underlying atmosphere seen at two different wavelengths of infrared light.

“The results so far are showing that the planetary magnetic field at Saturn is very different from the planetary magnetic field generated at other planets,” says Michele Dougherty, a professor of space physics at Imperial College in London. Dougherty leads the 40-person team responsible for Cassini’s magnetometer, an instrument that measures the strength and direction of a magnetic field.

The earliest hint that something was unusual about Saturn’s magnetic field came from the very first probe to fly by Saturn, Pioneer 11, in 1979. Scientists were keen to use Pioneer’s magnetometer readings to figure out the tilt angle between Saturn’s “true north”—it’s geographic north pole—and magnetic north. As here on Earth, most dynamo models show a sizable tilt between the poles. But Pioneer measured Saturn’s tilt angle to be puzzlingly small, somewhere between zero and one degree. Voyager 1 and 2 confirmed the measurement when they flew by in 1980 and 1981.

That was a jolt for scientists. “A planetary dynamo, in its simplest terms, consists of two separate things,” Dougherty explained in a 2016 interview with NOVA producer Terri Randall, “an overturning, bubbling motion,” called convection, plus fast rotation inside the planet. Think of it as a pot of oatmeal being stirred and simmered at the same time, she says. The combination of the two types of motion causes currents to flow, and those currents generate a magnetic field. The details are notoriously abstruse, but, Dougherty told NOVA, “All the theory to date tells you that, for a planetary dynamo to continue to operate, you need the resultant magnetic field to not be symmetric.”

Earlier this year, Cassini inserted itself into a series of orbits between Saturn and its rings.

Not everyone agrees that Saturn’s uncannily aligned poles are a fundamental problem for dynamo theory. To David Stevenson, a planetary scientist at Caltech the result is a puzzle but not a deal-breaker. In 1980, Stevenson proposed that, deep within Saturn, the magnetic field does have a tilt, but that the tilt is “screened out” closer to the top of the atmosphere. Still, he says, he suspects that the screening is, at best, a partial explanation.

For reasons that scientists still don’t understand, Saturn operates differently from its most closely-related planetary cousin, Jupiter, where the poles are 10˚ apart. Other planets with dynamos also register tilts. Earth’s magnetic and geographic poles are about 11˚ apart, and the tilts are even bigger on Uranus (59˚) and Neptune (47˚).

Now, as Cassini prepares to make its final suicide plunge into Saturn, scientists have their best chance yet to solve the riddle of the missing tilt. The reason, Stevenson says, is simple: “Proximity. It helps enormously to get closer.”

Mystery Remains

But the closer Cassini gets to Saturn, the harder Dougherty’s team has to work to make sense of the data. The magnetic field gets stronger as the spacecraft approaches the planet, meaning the magnetometer needs to switch into a little-used high-field mode, and set its “zero” level by rolling the spacecraft in two perpendicular directions, a dangerous maneuver. The team also has to reconstruct the spacecraft’s path with extreme precision. “Close to the planet, we are in very high magnetic field, so even if we have a very small error in our knowledge of the pointing, that’s going to give us a huge error in our knowledge of the magnetic field,” Dougherty says.

So far, though, the results are as mystifying as ever. “We thought if we got inside the rings we would somehow see a clear tilt, and we don’t see that,” Dougherty says. By late July, Dougherty and her team had whittled the tilt measurement to under 0.06˚. She hopes that the full end-of-mission measurements will give them the data they need to measure a tilt as small as 0.015˚—maybe even less.

Planetary scientists are exploring at least three explanations for the near-zero tilt. The first possibility, Dougherty says, is that Saturn’s planetary dynamo is dying. Dynamos aren’t forever. Mars’ dynamo, for instance, petered out billions of years ago, leaving the planet without a strong magnetic bubble to protect it; now Mars’ atmosphere is almost entirely gone, eroded away by the solar wind. No one knows exactly what would happen on Saturn if that planet’s dynamo-generated magnetic field were to vanish.

Saturn faces an uncertain fate if its magnetic field were to disappear.

If Saturn’s dynamo is dying, its magnetic field will not shut off all at once. Instead, like a complex harmony fading away into a solo voice at the end of a song, the field will gradually lose complex features physicists call “higher-order moments” before settling down into the simple north-south form of a bar magnet. Cassini will be looking for those higher-order moments. If they are are missing, that suggests that the dynamo is dying. But if they are still present, Dougherty says, something else must be going on.

Perhaps, as Stevenson suggests, the tilt is there, but it is being masked by other effects higher up in Saturn’s atmosphere. To help investigate that possibility, researchers will combine magnetometer data with gravitational measurements from Cassini’s Radio Science Subsystem instrument, which exploits the Doppler effect to map out Saturn’s gravitational field and probe the motion of its inner layers. One thing they will be looking for: strong, deep winds that could enhance the screening effect and help explain why it might be more powerful on Saturn than on Jupiter, Stevenson says.

Otherwise? “We’ll have to come up with a new idea,” Dougherty says. “I don’t know what that idea is, but there are lots of theoreticians out there that are champing at the bit.”

Cassini will send its last data transmission on September 15 before it starts tumbling toward oblivion. Dougherty expects it will take another six months to fully calibrate and analyze the “grand finale” data. “The instrument in the spacecraft was not designed to do what we’re doing, so it’s taking us a long time to do this,” Dougherty says. “We’re working our damnedest to get it all sorted”

Don’t expect to wait too long, though. As Dougherty puts it, “I’m not renowned for my patience.”