NASA’s Cassini spacecraft observed Saturn’s ring system in unprecedented detail, and a team of planetary researchers from the University of California Santa Cruz and NASA’s Ames Research Center has now used those observations to revise the gas giant’s rotation period. According to their calculations, the length of a day on Saturn is 10 hours 33 minutes and 38 seconds — 53 seconds longer than an estimate made three years ago.
“We now have the length of Saturn’s day, when we thought we wouldn’t be able to find it,” said Cassini project scientist Dr. Linda Spilker, a planetary scientist at NASA’s Jet Propulsion Laboratory.
“They used the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental quality of the planet. And it’s a really solid result. The rings held the answer.”
University of California Santa Cruz’s researcher Christopher Mankovich and colleagues studied wave patterns created within Saturn’s rings by the planet’s internal vibrations.
In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.
Similar to Earth’s vibrations from an earthquake, Saturn responds to perturbations by vibrating at frequencies determined by its internal structure. Heat-driven convection in the interior is the most likely source of the vibrations.
These internal oscillations cause the density at any particular place within the planet to fluctuate, which makes the gravitational field outside the planet oscillate at the same frequencies.
“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.”
“Most of the waves observed in Saturn’s rings are due to the gravitational effects of the moons orbiting outside the rings,” said University of California Santa Cruz’s Professor Jonathan Fortney, co-author of the study.
“But some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet’s internal oscillations and internal structure.”
The team developed a set of models of the internal structure of Saturn, used them to predict the frequency spectrum of Saturn’s internal vibrations, and compared those predictions with the waves observed by Cassini in Saturn’s C ring.
One of the main results is the new calculation of Saturn’s rotation rate, which has been surprisingly difficult to measure.
“As a gas giant planet, Saturn has no solid surface with landmarks that could be tracked as it rotates,” the scientists explained.
“Saturn is also unusual in having its magnetic axis nearly perfectly aligned with its rotational axis.”
“Jupiter’s magnetic axis, like Earth’s, is not aligned with its rotational axis, which means the magnetic pole swings around as the planet rotates, enabling astronomers to measure a periodic signal in radio waves and calculate the rotation rate.”
The results appear in the Astrophysical Journal.
Christopher Mankovich et al. 2019. Cassini Ring Seismology as a Probe of Saturn’s Interior. I. Rigid Rotation. ApJ 871, 1; doi: 10.3847/1538-4357/aaf798