Earth and Planetary Science
EPS Atmosphere, Oceans, and Climate

How did Saturn Become the Lord of the Rings in our Solar System?

Thursday, September 15, 2022

 Artistic rendering of the moon Chrysalis disintegrating in Saturn’s intense gravity field. The chunks of icy rock eventually collided and shattered into smaller pieces that became distributed in the thin ring we see today. (Image credit B. Militzer and NASA)


On September 15, a new manuscript appeared in the journal Science in which Jack Wisdom, Rola Dbouk, EPS Professor Burkhard Militzer, William Hubbard, Francis Nimmo, Brynna Downey, and Richard French explain how Saturn's received its prominent set of rings. For this joint project, Professor Militzer computed Saturn's moment of inertia and angular momentum by constructing models for the planet's interior structure that matched the gravit measurements of the Cassini spacecraft. The results from four independent calculations in the two graphs below show the planet's angular momentum is close but just outside the range for Saturn to be in a spin-orbit resonance with planet Neptune today. However, Saturn would have been in resonance if the planet once had an additional moon. In this case, the Neptune could have tilted Saturn's spin axis by 27 degrees, which is far too large to have arisen when the planet form out of the protosolar disk. The dynamical simulations by Jack Wisdom predict that Saturn's rings formed in the following way:

(1) The Saturnian system formed with an additional moon, Chrysalis. Saturn's spin axis was perpendicular to its orbital plane.

(2) Chrysalis gave Neptune an extra "handle" to tilt Saturn's spin axis (via a spin-orbit resonance) to the large value that we see today, 27 degrees.

(3) Saturn's moon Titan started to migrate out. About 160 million years ago, it entered into an resonance with the moon Chrysalis destabilizing its orbit.

(4) As a result, Chrysalis came so close to Saturn that it was sheared appart by Saturn's intense gravity (tidal disruption, illustrated in the figure above). Most of the material fell into Saturn but out of 1%, the rings formed.

(5) With Chrysalis gone, Neptune no longer could change Saturn's spin axis. So the planet was left spinning at an angle of 27 degrees.

(6) Over time the rings became thinner and thinner because particles, that do not share the orbit with the majority, will eventually collide with others. This 'grinding' process broke up ring particles and thus greatly increased their number. The ring particle also became slightly dark over time because of a slow but steady bombardment with meteorites. This process led us to conclude in 2019 that the rings were surprisingly young, only about 100 million year old. So next time, when you look at Saturn's rings, we recommend you appreciate them a little more because will not be there forever. (Saturn's rings are being slowly eroded as particles fall into Saturn.)

Here is a press release from UCB Science Communications Manager Bob Sanders.

Our calculation of Saturn's angular momentum as function of rotation period.
Enlarged version of the graph in the middle that confirms that Saturn's is not in a spin-orbit resonance with Neptune today but it is very close...