For decades, it was a nagging mystery — how long does a day last on Saturn?
Earth pirouettes around its axis once every 24 hours or so, while Jupiter spins comparatively briskly, once in roughly 9.8 Earth-hours. And then there is Venus, a perplexingly sluggish spinner that takes 243 Earth-days to complete a full rotation.
With Saturn, it turns out the answer rippled in plain view, in the planet’s lustrous rings.
After reading small, spiraling waves in those bands, sculpted by oscillations from Saturn’s gravity, scientists reported this month in the Astrophysical Journal that one Saturnian day is a mere 10 hours, 33 minutes and 38 seconds long, measured in Earth time.
“The rings are not only beautiful, they’re very diagnostic of what’s going on inside the planet,” said Linda Spilker, project scientist for NASA’s Cassini mission, which studied Saturn for more than a decade.
Saturn has been stubbornly secretive about its days. Its buttery clouds don’t bear helpful markings that scientists might use to track the planet’s rotation, and they can’t easily use its nearly vertical magnetic axis — as they have for Jupiter’s more off-kilter alignment — to gather clues about the planet’s interior.
Scientists long relied on other, ultimately misleading clues to figure out how fast the ringed world turns. Not until the Cassini spacecraft swooped, flipped and twirled through the Saturn system did scientists realize that the answer was outside the planet itself, etched into its icy rings.
As Saturn spins, its internal vibrations inscribe telltale signatures in its rings; studying those markings is now termed “kronoseismology,” from kronos, the Greek name for Saturn, and seismo, for quakes and vibrations.
In the same way that a bell rings and creates pressure waves that jiggle our eardrums, a spinning Saturn produces gravitational oscillations that herd particles in the rings into filaments. The filaments form visible spiral patterns within the rings, belying motions deep inside the planet that can be linked to its rotation speed.
As Cassini traversed the Saturn system, it used light from background stars, shining through the rings, to capture details of the embedded spirals. Then, a team led by scientists at the University of California, Santa Cruz, worked backward from the spirals to determine how quickly Saturn rotates. The exact answer, in Earth-hours: 10:33:38.
“This a great story, a great picture to see — Saturn kind of almost ringing,” said Dr. Spilker, who was not involved in the study. “This interaction with gravity produces these little ripples that Cassini can see.”
She anticipated that the rate of rotation would be refined once scientists better understood how the churning layers inside the planet affect those oscillations. But it’s clear that one Saturnian day doesn’t leave much time for long, luxurious naps or lazily gazing at the alien moons spangling its skies.