Categories: Saturn

Saturn Got Its Tilt From Its Moons

One of the fundamental tenets of physics is that two objects, now matter how different their size, exert a force on each other. In most cases the size makes a big difference, with the larger objects enacting a much greater force on the smaller one. 

However, over long periods of time, even much smaller objects can have an effect on the larger object in the pair.  Recently a team of researchers from CNRS, the Sorbonne, and the University of Pisa have found an example of the smaller object, or in this case group of objects, having an outsized impact on the larger one.  They have discovered that Saturn’s moons actually caused its famous tilt.

Saturn, the second largest planet in the solar system, has a total of 82 discovered moons.  Though that is a lot of moons, over 96% of the mass in orbit around the planet exists in one moon in particular: Titan. The other six “ellipsoidal” moons make up almost another 4% of the mass. However, Saturn itself masses over 4000x all of its moons combined.  

A top-down image of the orbits of some of Saturn’s larger moons, with Titan in red. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas.
Image Credit: By !Original: Rubble pileVector: Mysid. – Own work based on: Titan’s orbit.jpg., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4819125

Recently scientists have begun taking a closer look at Saturn’s moons, in part because of their potential as harbors of life as well as them being targets of some upcoming missions.  During this closer look, they realized that the moons were moving away from their parent planet at a much faster rate than previously thought.  When they recalculated some models using this increased speed, they realized that Saturn’s tilt can be almost entirely explained by the gravitational effects of its moons.

About one billion years ago, the ring planet’s moons forced it into a resonance state with themselves which continues to this day.  That resonance, along with a slight disruption by Neptune as it moved through the early solar system, resulted in the 27° incline we can see on the planet today.

Saturn, with its noticeable axial tilt, and its largest moon, Titan.
Credit: Credit: NASA / JPL-Caltech / Space Science Institute

Even more interestingly, it seems this axial tilting will continue for some time yet.  By the authors’ calculations, it could more than double over the next few billion years.

Saturn is not the only gas giant in the solar system to get the same push from its moons.  Jupiter is currently in the process of being tilted by its moons, with some help from Uranus along the way.  Five billion years from now, the biggest planet in the solar system is expected to have an axial tilt 10x what it is today (3°), making it even more angled than Saturn is currently.

Unfortunately no one alive today will be around to witness that.  But the fact that models can tell us that is what will happen is always impressive.  So is a small force maintained over a long period of time.

Learn More:
CNRS: Saturn’s tilt caused by its moons
ZME Science: Saturn is tilted. The fault lies with its moons
Tech Explorist: Saturn’s tilt – caused by its moon – will increase over the next billion years

Lead Image:
Artist’s impression of Saturn’s tilt and Titan’s migration.
Credit: Coline Saillenfest / IMCCE

Andy Tomaswick

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