Well, until January 2019, the answer would be ‘Ummmm, I’m not really sure’. And now the answer is, ‘Why, it’s 10 hours, 33 minutes and 38 seconds, of course!’ That is, according to a crackerjack team of astrophysicists at the University of California, Santa Cruz, led by Christopher Mankovich, who finally figured out this variable. Until now, Saturn was the only planet for which we didn’t know how long the day is.
Why does it matter how long a day on a planet is? Well, scientists on earth are trying to find out as much as they can about the planets in our solar system. This is part of a larger effort to understand our universe and our place in it. The length of the day impacts our reading on its gravitational field and on the structure of the planet.
Why was it so difficult to figure this out? Saturn, like Uranus, Jupiter, and Neptune, is a ‘gas giant’. A gas giant is a planet that is composed mostly of gas. So this means that there are no real physical markers or positions that can be tracked as the planet rotates. The other gas giants have magnetic poles that, like those on earth, are tilted from their rotational axes. So when the planet rotates, this causes its magnetic field to wobble due to changes in the planet’s core. So the rotation rate of the magnetic field will roughly equal the rotation rate of the planet. Simplistically, the length of that wobble equals a day on that planet. Apparently Saturn’s magnetic pole and rotational axis are along the same line. So rotation does not cause Saturn’s magnetic field to wobble, so this wasn’t a good marker that could be tracked.
So what changed? Over time, scientists realised that Saturn’s rings do wobble though, in response to changes in Saturn’s gravitational field. What could cause changes in the planet’s gravitational field? A moon or satellite going by, if large enough, tugs at something in the planet’s core, causes a change in the gravitational field, and makes Saturn’s rings wobble. This wobble causes a visible ripple on the rings. The data about these waves or ripples was part of all the information collected on Saturn from spacecraft sent to study it. Cassini collected intricate data on Saturn while circling it for 13 years! It measured all these ripples and waves and gave the scientists a lot of data that told them a lot about Saturn’s core, and was also used to calculate the length of a day on this tricky planet.
References: NASA: https://solarsystem.nasa.gov/news/12955/measuring-a-day/
Sunaina Murthy. Sunaina is a biotechnologist, writer, greedy reader, and amateur photographer.