The length of Saturn’s day is still a mystery, but a doomed space probe may soon find out

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Sunlight spills across the rings of Saturn in this photo by the Cassini spacecraft. NASA/JPL-Caltech/Space Science Institute
  • The length of Saturn’s day isn’t quite certain, though it’s thought to be about 10.5 hours.
  • That’s because the planet is shrouded in clouds that go thousands of miles deep.
  • However, the gas giant does have a core that can be measured through magnetic fields.
  • When NASA kills its Cassini probe in September 2017, it may help solve the mystery of Saturn’s rotation.

If you look up the length of a day on Saturn, search engines will lie to you.

Google proclaims it’s 10 hours 42 minutes. Same goes for Microsoft’s Bing, though in a “10.7 hours” format. (a search engine for brainiacs) narrows the rotation down to 10 hours, 39 minutes, and 22 seconds. Meanwhile, the latest theoretical estimates go as low as 10 hours 33 minutes.

The truth is no one really knows how long a day “on” Saturn is. This in part because the world, which is 95 times the heft of Earth, is not at all like the rocky inner planets that humans can imagine walking upon.

“When it comes to a giant planet like Saturn, it’s a bit strange to ask, ‘What is the rotation?'” Ravit Helled, a planetary scientist at Tel-Aviv University, told Business Insider. “It’s not solid all the way through, it’s a gaseous planet.”

But Saturn does have a solid core, and while it’s possible to measure the rotation of that structure, it has proven very difficult to do so.

Luckily, in the weeks leading up to the spectacular death of NASA’s $US3.26 billion Cassini spacecraft, we may soon get a definitive answer that researchers have waited for decades or perhaps centuries to receive.

Saturn’s slippery day

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This illustration shows the interior of Saturn, including its core of heavy elements (orange), liquid metallic hydrogen layer (grey), and gaseous molecular hydrogen envelope (brown). The planet’s magnetic field is thought to originate from the top of the metallic hydrogen layer. NASA/JPL-Caltech

How fast a planet spins says a lot about that world. It can provide hints of how it formed, whether or not a proto-planet smacked into it shortly after formation, what drives its weather, and even what exact shape it has.

Figuring out a rocky planet’s rotation is straightforward: You just watch and see how long it takes for a distinctive point on the surface to come back around, like a mountain or crater.

Anything resembling a liquid or solid surface on Saturn, however, is smothered by thousands of miles of swirling cloud cover — so researchers have resorted to more creative methods.

Saturn is thought to have a rocky inner core, plus an outer core made of metallic hydrogen on top that generates a magnetic field. Solar radiation that pummels Saturn’s atmosphere also generates a constant crackle of radio waves, and those emissions change as the planet rotates.

However, it’s proven notoriously difficult to trust a reading of either the magnetic field or radio emissions.

For example, NASA’s Voyager 1 probe measured Saturn’s radio waves as it flew past in 1980, hinting that one “day” on the planet lasted 10 hours, 39 minutes, and 22.4 seconds. But in 2006, after the Cassini spacecraft fell into orbit around Saturn, its magnetic field measurements suggested a rotation of 10 hours 47 minutes.

Researchers eventually learned that radio emissions from Saturn change over time, so they can’t really be trusted.

Enceladus plume

They also discovered two issues with the planet’s magnetic field lines. First, they line up almost perfectly with the planet’s axis of rotation, which makes it devilishly hard to use as an invisible “landmark” that goes around the poles as the planet spins.

Second, researchers eventually figured out that Saturn’s rings interfere with the planet’s magnetism, as do geysers of water and other material spewed out by Enceladus — an icy moon of Saturn that hides a warm, salty ocean.

In 2015, Helled and her colleagues tried to get around both issues with a mathematical model. They fed several ideas about what Saturn’s internal structure might be, and worked in the magnetic data. That model suggested a Saturnian day lasts 10 hours 33 minutes, but Helled says this is only theoretical.

“What is really going on, we’ll have to wait and see,” Helled said.

Decades after the first attempt to precisely measure Saturn’s day, that moment could come by the end of summer 2017: when NASA plunges its nuclear-powered Cassini probe into the clouds of Saturn.

Cassini’s scientific ‘roller coaster’ of death

During a NASA press conference on April 4, researchers explained why they’re killing off their cherished spacecraft with what they call the “Grand Finale.”

The manoeuvre will use up the dwindling reserves of Cassini’s fuel and put the robot on a collision course with Saturn on September 15, 2017.

“Cassini’s own discoveries were its demise,” said Earl Maize, an engineer at NASA’s Jet Propulsion Laboratory (JPL) who manages the Cassini mission. He was referring to the probe’s discovery of Enceladus’ potentially habitable ocean, and the need to destroy Cassini to avoid crashing into and contaminating “that pristine body.”

So on April 22, Cassini will use the gravity of Titan, another moon of Saturn, to slingshot toward the gas giant. This will put Cassini on a series of orbits that slip through a 1,200-mile-wide (1,930-kilometer-wide) gap between the planet’s atmosphere and its innermost rings.

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An illustration of the Cassini spacecraft over Saturn’s north pole with its hexagon-shaped storm. NASA/JPL-Caltech

“That last ‘kiss goodbye’ will put Cassini into Saturn,” Maize said. “This is a roller-coaster ride. We’re going in, and we are not coming out — it’s a one-way trip.”

Luckily, NASA has a lot of science planned for that wild ride.

Since Cassini will pass within all of Saturn’s rings — and beyond their magnetic interference, as well as that of Enceladus — the probe should get the most precise magnetic readings ever taken of Saturn.

“With the magnetic field we’ll be able to get, for the first time, we hope, the length of a day for the interior of Saturn,” Linda Spilker, a Cassini project scientist and a planetary scientist at NASA JPL, said during the press briefing. “If there’s just a slight tilt to the magnetic field, then it will wobble around and give us the length of a day.”

Helled hopes the daredevil manoeuvre works and solves a longstanding and important mystery of Saturn.

“I think there’s a good chance it will, and I’m really excited about it,” Helled said. “If not, we’ll just have to have another mission to Saturn.”

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