Friday Night Science: Why Does Saturn Have Rings?

 Why Does Saturn Have Rings? 

It was 1610. Galileo Galilei was turning his telescope towards Saturn. Though he believed in a much larger universe and the more divine Copernican ordering of the cosmos, what he was about to see would be unlike what he could've imagined. Instead of a single planetary orb, Saturn had come with two tiny stars on either side, almost touching. Galileo continued his observations the next night, the night after, and noted that the little Saturnian stars do not steer away from the central body. His primary conclusions were that Saturn must be triple-bodied. In a letter to his patron, also the Grand Duchy of Tuscany, Galileo wrote that if the observations are made at a high magnifying power (through the telescope), the little stars appear to be a hair's breadth apart. However, at a lesser power, the sharpness disappeared, and Saturn took the shape of an elongated olive. 

Galileo's drawing of Saturn in a letter to the
Grand Duke of Tuscany. Padua, July, 1610. 
Credit: Public Domain. 

In 1670, Giovanni Domenico Cassini discovered that Saturn's ring didn't represent a uniform solid body but contained a gap, which would later be tagged as the Cassini Division. As Mimas keeps its path free from icy debris, the Cassini Division leaves about five thousand kilometers of empty space between the outer A ring and the inner B ring. 

During 1789 - 1790, William Herschel discovered two new moons of Saturn, and with Cassini's discovery of four more satellites earlier, Saturn's moon count had hit eleven. Herschel was the first to put an estimate on the thickness of the ring at 300 miles.

Saturn through an 80 mm objective diameter
telescope provides an idea of what Galileo
could have seen.
Credit: Public Domain. 

As the years rolled on, bit by bit, Saturn had already revealed itself to be an enigmatic world. Pressed with the question of the nature of Saturn's rings and their plausible origins, multiple schools of thought emerged.  

In his seminal volume, Systema Saturnium (1659), Huygens proposed that Saturn's ring seemed to be a solid (broad) annulus, nowhere attached to the planetary globe, and rotated in tandem with the parent body, and kept the same distance.  

In 1787, Pierre Simon Laplace gave the world a definitive insight into the true nature of Saturn's rings. Laplace shows that Newton's fundamental laws of motion and the way gravity works do not allow a solid ring or pairs of them. For the system to remain stable, the rings have to revolve at a much higher rate to avoid crashing into the planet. Backing upon observations, Laplace abandons the idea of a solid ring and proposes the existence of numerous ringlets. By 1859, James Clerk Maxwell had built a solid theory stating that Newton's laws only allow for a ring made up of an aggregate of an uncountably large number of minute particles. 

Five years before the turn of the century, the first spectroscopic study revealed that the rings are mostly water ice, while its constituent particles are no larger than a grain of sand, as Maxwell had guessed. Fast forward a hundred years, and the first probe arrives at the crown jewel of the solar system, sending back thousands of photos of a world that has captivated the human senses for five thousand years. 

On June 20, 2019, the Hubble Space Telescope snapped this spectacular portrait of Saturn. Mimas is responsible for carving out the Cassini division, which separates the outer A-ring from the wider B-ring. At the outer rims of the A-ring, there's the Encke Gap carved out by another of Saturn's moons, Pan.  
Credit: ESA and Hubble/NASA. 

The origin of Saturn's rings remains an open question. There are two possibilities. One theory, initially proposed by Edouard Roche sometime during the 19th century, suggests that the rings formed in a disruptive event when one of Saturn's moons migrated too close to the planet. In other words, when the moon had fallen beyond Saturn's Roche limit. The Roche limit is a maximum radius around a planet where a second body gets shredded to bits under the immense pull-and-push effected by the planet onto its little moon. Astronomers also like to consider the possibility of a moon hit by a passing comet. The second theory, forwarded by Immanuel Kant in 1775, the so-called Nebular Hypothesis, gives the idea that the rings hail back from the very early days of the solar system when planets were condensing from the nebular cloud. 

Spectroscopic analysis of the water ice suggests that the rings can not be a billion years old. Relatively fresh research published in the Astrophysical Journal speculates a much recent origin in the aftermath of two moons crashing onto each other less than 100 million years ago. In the abstract, the team of researchers writes, quoting their exact words, ''We simulate the collision of precursor ice moons analogous to Dione and Rhea as origins for Saturn's remarkably young rings . . . triggered a few hundred million years ago by resonant instabilities in a previous satellite system''. Taking away the jargon, the latter half of the sentence reads as follows: when multiple satellites orbit a parent (planetary) body, they push and pull each other's orbit. Such pulling and pushing over millions of years may either stabilize the system and result in a harmonious dance of moons, or worst-case scenario, fall out of sync and crash into each other. As two worlds shatter into mere crumbs and go helter-skelter, the planet's gravity binds them into a ring of debris. But it is not a permanent decoration. Over hundreds of millions of years, the ring dumps its materials into the planet and eventually disappears. 

Seen up close, Saturn's rings resembles the grooves on a vinyl record. 
Photographed by the Cassini spacecraft. 
Credit: Public Domain. 

Thus, rings are not just static artifacts of planets. Planets because Saturn isn't the only one with rings. Jupiter has a faint dust ring. Uranus has a ring whose exact composition is unknown. Even Neptune has a coronet. And that's not the end of it. In other distant star systems, located light-years away in our galaxy, and in other galaxies, there are planets with extensive ring systems. On that note, J170b, an exoplanet approximately 450 light-years away, looking into the direction of the constellation of Centaurus, has an enormous ring system extending over 640 times that of Saturn. 

 

Planetary ring systems are constantly changing. They take shape over millions of years, and once they form, it is only a matter of time before they disappear. The cosmos, however, to use the phrase is stranger than fiction. When one kingdom falls, another king or queen rises to power. Which begs the question: Can Earth have rings? Or did it already have one in the remote past?