Saturn's rings gave scientists the information they needed to finally determine how long a day on the gas giant lasts.
Credit: NASA / JPL-Caltech / Space Science Institute
Set your timers for 1
According to newly published research using data collected by NASA's Cassini mission prior to the spacecraft's destruction in September 2017, the new calculation shaves several minutes based on previous estimates of a Saturn day made by scientists for decades based on data from Cassini. mission and its predecessor, Voyager.
"They used scientists waves in the rings to look into Saturn's interior and exhibited this long-sought-after, fundamental characteristics of the planet. And that's a really solid result," Cassini Project Scientist Li nda Spilker said in a statement. "The rings kept the answer." [In Photos: Cassini Mission Ends with Epic Dive into Saturn]
It may seem easy to measure the length of a day on a planet – just wait and see the world spin. But Saturn's precise day length has blunted scientists for decades. Because the planet is a gas giant, scientists cannot see stable landmarks through the clouds as they could with a rocky planet.
Researchers can also typically use the tilt of a planet's magnetic field to measure its length. But it didn't work for Saturn, because the field adjusts almost perfectly with the planet's axis of rotation, and stymies their calculations. A scientist who has studied the planet's magnetic field said that the uncertainty of day length is "a little embarrassing" in an interview with Space.com on research published in October.
These challenges left researchers with rough estimates falling between 10 hours, 36 minutes and 10 hours, 48 minutes – not particularly satisfactory.
The research that was published today took a completely different approach – not looking at the planet itself, but at its delicate rings. This idea was proposed in 1982, but not before the Cassini mission, the scientists had the data to see if the technique would work.
The idea is that when Saturn turns, its internal angles are slightly skipped and cause small changes in the plane's gravity. These small changes ripple to the ice clumps in the rings that decorate the giant gene and cause small waves in the rings.
"Particles through the rings cannot help but feel these fluctuations in gravity," head author Christopher Mankovich, a graduate student in astronomy at the University of California, Santa Cruz, said in a statement. "At certain points in the rings, these oscillations capture ring particles at the right time in their circuits to gradually build energy and that energy is carried away as an observable wave."
Then Mankovich and his colleagues studied them observable waves and used them to track inward to the planet itself. This was how the researchers came up with measurements in 10 hours, 33 minutes and 38 seconds. It is still not set in stone – the error bars on this calculation extend between one minute and 52 seconds longer and one minute and 19 seconds shorter. But the new calculation range turns a window in 12 minutes.
The research is described in a paper published yesterday (January 17) in The Astrophysical Journal.