Saturnian day deduced from ring formations

Data from NASA’s Cassini spacecraft has enabled a more accurate estimate of the length of a day on Saturn.

The new estimate of 10 hours, 33 minutes and 38 seconds, is based on imagery of the formation of the Saturn’s rings, which respond to vibrations in the planet’s interior.

Using this imagery, Christopher Mankovich, an astronomy student at UC Santa Cruz, developed models of Saturn’s internal structure to match the wave patterns. These then allowed him to track movements of the planet’s interior, including its rotation.

"Particles throughout the rings can't help but feel these oscillations in the gravity field," Mankovich said. "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."

Due to having a magnetic axis that is nearly aligned with its rotation axis, as well as the lack of a solid surface that can be tracked as it rotates, the length of a Saturnian day has been difficult to measure. Previous best estimates of 10 hours 39 minutes from 1981 were based on radio signals from NASA’s Voyager spacecraft.

NASA’s Cassini space probe was active in space for nearly 20 years. Its mission ended in September 2017 when, shortly before running out of fuel, it was flown into the planet’s upper atmosphere.

By coasting between the planet and its rings, Cassini was also able to estimate the amount of material in the planet’s rings, based on the strength of their gravitational pull. The estimate is only about 40% of the mass of Saturn’s moon Mimas, which itself is 2,000 time smaller than the Earth’s moon.

This suggests that the rings may have originated as recently as 10 million years ago, more recently than previously thought. Lower mass correlates with lower age, because rings are initially made of ice and are contaminated and darkened by interplanetary debris over time.

Scientists have also calculated that surface clouds at Saturn’s equator rotate 4 percent faster than the layer that’s around 9,000km deep."The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet," said Cassini project scientist Linda Spilker of NASA's Jet Propulsion Laboratory in Pasadena, California.

"The question is what causes the more rapidly rotating part of the atmosphere to go so deep and what does that tell us about Saturn's interior."