In theory, determining the length of a day on a planet is pretty straightforward—just measure how long it takes to make one full rotation. Doing that with the gas giant Saturn, however, has proven to be an astronomical headache.
The surface of the planet is more or less featureless, meaning there are no mountains or craters to mark a full rotation, and Saturn's magnetic field is unusual, preventing scientists from using electromagnetic measurements to determine the rotation. But using data from the gone-but-not-forgotten Cassini spacecraft, researchers have studied the planet’s iconic rings to determine a day length of 10 hours, 33 minutes and 38 seconds, the teams reports in the Astrophysical Journal.
Meghan Bartels at Space.com reports that researchers used the planet’s system of rings, made of space dust, rocks and ice particles, like a seismometer. The idea was first proposed in 1982 and expanded in 1990, but it couldn’t be tested until the Cassini craft began exploring the rings up close. The theory was that the interior of Saturn vibrates at frequencies that cause variations in its gravitational field. The particles in the rings, researchers surmised, would register those tiny changes in the tug of gravity as wave pattern, which could then be used to determine how quickly the planet rotates.
“The researchers used waves in the rings to peer into Saturn's interior, and out popped this long-sought, fundamental characteristic of the planet,” Cassini project scientist Linda Spilker says in a NASA press release. “And it’s a really solid result. The rings held the answer.”
When the Voyager probes flew past Saturn in 1980 and 1981, researchers used magnetic field readings to estimate a day length of 10 hours, 39 minutes and 23 seconds. When Cassini reached the planet, it also used magnetic field measurements to estimate the length of a day, getting a range of results from 10 hours and 36 minutes to 10 hours and 48 minutes.
The slight discrepancies arise because Saturn’s magnetic field is almost perfectly aligned with its rotational axis, unlike the magnetic fields of planets like Earth, Jupiter, Uranus and Neptune, which are a bit askew. When an off-kilter magnetic field swings around the rotational axis, it creates periodic radio signals that can be used to calculate its rotation. But Saturn’s top-like rotation doesn’t give off the same signals, making it difficult to get an accurate rotation time.
The situation frustrated astronomers for decades. Even after Cassini studied Saturn from orbit for over a decade, scientists still couldn’t solve the day-length conundrum.
“It’s a bit embarrassing,” Michele Dougherty, a physicist at Imperial College London who studies Saturn’s magnetic field, told Bartels in another story last October. “We were in orbit there for 13 years and we still can't say how long a day on Saturn is.”
Now researchers don’t have to be so ashamed, but there is still room for improvement. Bartels reports there is a margin of error on the latest calculation, and the actual day length could be between a minute and 52 seconds longer or up to 19 seconds shorter—though that's a huge improvement on the previous 12-minute margin of error.