Ten hours, 33 minutes and 38 seconds. It takes only this much time for a day to complete on Saturn. The finding of Saturn day length by scientists using data from NASA’s Cassini spacecraft solves a long-standing mystery.

The Cassini mission is now extinct but scientists used new data from it and established that one year on Saturn is 29 Earth years, and day lasts only 10 hours, 33 minutes and 38 seconds.

The figure was unknown until now. And NASA says it was because “the gas giant has no solid surface with landmarks to track as it rotates, and it has an unusual magnetic field that hides the planet’s rotation rate”.

The answer, says the space agency, was hidden in the rings.

Christopher Mankovich, a graduate student in astronomy and astrophysics at University of California-Santa Cruz, studied the wave patterns within the rings using the data Cassini stored after examining the icy, rocky rings in unprecedented detail during its orbits of Saturn.

The study says the rings respond to vibrations within the planet, “acting similarly to the seismometers used to measure movement caused by earthquakes”. “The interior of Saturn vibrates at frequencies that cause variations in its gravitational field. The rings, in turn, detect those movements in the field,” says a NASA report that it has uploaded on its website.

“Particles throughout the rings can’t help but feel these oscillations in the gravity field,” the report quotes Mankovich as saying. “At specific locations in the rings these oscillations catch ring particles at just the right time in their orbits to gradually build up energy, and that energy gets carried away as an observable wave.”

Mankovich’s research has been published in the Astrophysical Journal, describing how he developed models of the internal structure of Saturn that would match the rings’ waves. That, in fact, allowed him to track the movements of Saturn’s interiors, and its rotation.

“The rotation rate of 10:33:38 that the analysis yielded is several minutes faster than previous estimates in 1981, which were based on radio signals from NASA’s Voyager spacecraft,” says the report.

Scientists usually rely on magnetic fields to measure planets’ rotation rates. But Saturn is different. It has a unique magnetic field that is “nearly perfectly aligned with its rotational axis”. The magnetic axis of Jupiter, just like Earth’s, is not aligned with its rotational axis. Scientists can measure a periodic signal in radio waves to get the rotation rate of these planets as they swing around while rotating.

Saturn’s rings finding has been key to homing in on the length of day on it. Scientists are now happy as they have got the best answer yet to a central question about the ringed planet.

“The researchers used waves in the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental characteristic of the planet. And it’s a really solid result,” said Cassini Project Scientist Linda Spilker. “The rings held the answer.”

Cassini spacecraft ended its 13-year tour of Saturn on September 15, 2017. Low on fuel, it plunged into the atmosphere of Saturn, to avoid crashing onto the planet’s moons.