Bad news for any slackers on Saturn: The days on the ringed planet are shorter than the number you'll find in most books—6 minutes briefer, to be exact. The faster spin drastically changes how scientists think Saturn's winds blow.
"We used to think that we knew the rotation period," says Ravit Helled, a planetary scientist at Tel Aviv University in Israel. After all, she notes, spin is a basic planetary property. In the early 1980s, the Voyager spacecraft flew past Saturn and pegged a day as 10 hours and 39 minutes, based on periodic changes it detected in the planet's radio waves. But the Cassini spacecraft, which arrived in 2004, found a longer period of 10 hours and 47 minutes, based on a similar technique.
The problem is that Saturn doesn’t reveal this information easily, unlike the solar system's other gas giant. No one disputes Jupiter’s spin rate of 9 hours and 55 minutes, because deep beneath that planet’s atmosphere, its core generates a magnetic field, the axis of which is tilted compared with the spin axis; therefore, as the planet turns, the magnetic field sweeps around like the beam of a lighthouse, sending out radio waves that make it easy to measure how fast the hidden core spins.
But this method won't work for Saturn, because its magnetic and spin axes are aligned. Indeed, the conflicting numbers from Voyager and Cassini demonstrated that the periods they measured weren't reflecting the planet's rotation. So Helled's team deduced the spin from the planet's gravity field. Saturn spins fast, making its equator bulge outward, which distorts the gravity field in ways Cassini has measured. By modeling this effect, Helled's team reports online today in Nature that Saturn spins every 10 hours and 32 minutes and 45 seconds with an uncertainty of 46 seconds. That's more than 6 minutes faster than Voyager found.
"Six minutes makes a big difference," says Andrew Ingersoll, a planetary scientist at the California Institute of Technology (Caltech) in Pasadena who was not part of the research team. Because Saturn is so big, the equator at the level of the atmosphere must spin nearly 400 kilometers per hour faster than the Voyager value implied. Scientists measure wind speeds relative to the spinning planet, so the new rotation rate alters the estimated wind speeds by that same amount. The old rotation rate suggested that most winds blew only one way, whereas the new number means about half of the winds blow east and the other half blow west, "which seems a little more sensible," Ingersoll says, because it's the same pattern Jupiter has.
The new number is valuable but not revolutionary, says David Stevenson, another planetary scientist at Caltech. "It is what people have been suspecting for several years," he says, based on assuming Saturn's winds resemble Jupiter's. But he cautions that the rotation period is probably more uncertain than the researchers claim and could still be off by 2 minutes.
Fortunately, the situation will soon improve. During the final months of its life, in 2017, the Cassini spacecraft will skim close to Saturn, better probing the planet's gravity field. "I'm hoping that we're going to detect some tilt" of the magnetic field, Ingersoll says, because then scientists will be able to measure Saturn's spin period just as they have Jupiter's.