We Finally Know The Length Of A Day On Saturn

This view from NASA's Cassini spacecraft shows Saturn's northern hemisphere in 2016. NASA/JPL-Caltech/Space Science Institute 

The concept of a “day” seems so obvious to humans that it often comes as a surprise that day length on other planets can be very different from our own. Perhaps even more shocking is that we didn’t know the length of a day on Saturn until now.

Thanks to the data collected by the Cassini mission we finally have an answer to this long-standing mystery. Saturn rotates on its axis in 10 hours, 33 minutes and 38 seconds. The findings are published in the Astrophysical Journal.

Planetary scientists have often used magnetic fields to estimate planetary rotation. If the magnetic field and the rotation axis of a planet are not aligned, it is possible to measure periodic signals in radio waves that repeat every day. But on Saturn, the magnetic field is almost in perfect alignment, so this has not been possible.

Therefore, researchers had to come up with a different approach and hypothesized that studying the rings of Saturn could be key to finally pin down a number. As the planet rotates, subtle differences in gravity generate waves within the rings, and these can be used to estimate the length of a day. This was first proposed in 1982 and then planned in more detail in 1990 by study co-author Dr Mark Marley, long before it was actually possible to see the phenomenon.

"Particles throughout the rings can't help but feel these oscillations in the gravity field," lead author Christopher Mankovich, a graduate researcher at the University of California, Santa Cruz, said in a statement. "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."

This result was possible thanks to the Cassini spacecraft's observations of Saturn's rings. The mission team made the decision to crash the probe into Saturn as it began to run short of fuel, but not before it was moved into a closer and more daring orbit. The “Grand Finale” phase took Cassini between Saturn and its rings, giving researchers unparalleled views. The mission, a collaboration between NASA, the European Space Agency, and the Italian Space Agency, ended in September 2017.

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