From the ice of Saturn's rings, the moons emerge

Origins of an Ice Moon 

Part 2

Tethys, Enceladus and Mimas

NASA/Cassini, via Wikimedia Commons

In 1610, Galileo Galilei peered through the eyepiece of his telescope at the giant planet Saturn. He saw more details of the planet than any prior human in history. Galileo's telescope was the best in the world. He still could not fully resolve the rings of Saturn clearly enough to even appreciate that they were rings. Later, astronomers came along with better telescopes. Astronomers like Huygens, Hooke, and Cassini resolved the shadow of Saturn on the ring and the major divisions of the rings. Cassini also discovered a number of moons orbiting Saturn, including Tethys, Iapetus, Rhea, and Dione.


Centuries later, we are still not certain exactly how the rings formed. Modern missions of space exploration to Saturn include: Pioneer 11, Voyager 1, Voyager 2, and the current mission Cassini. Along with extraordinary amounts of data, these space probes returned beautiful images of the Saturn system.

Voyager 2 image of Saturn, the rings and four moons
NASA (1981), via Wikimedia Commons

From the ice of Saturn's rings: moons emerge

We now know that the rings of Saturn consist primarily of ice. Around 90 to 95 percent of the mass of Saturn's rings is water ice. What remains may be in essence pollution from relentless pummeling of the rings by meteors and space debris. The formation of Saturn's rings is a mystery, but we are getting closer to an answer.

Over generations, astronomers developed countless theories on how the rings formed. One theory suggested that Saturn's rings formed when a small moon collided and broke apart. Another suggested that a passing comet shattered under the tidal pull of the giant planet.

In 2010, astrophysicist Robin Canup, writing in the Journal Nature presented numerical models looking at development of the rings of Saturn. She points out that old theories failed to explain why there is so much ice and relatively little rock.

Dr Canup may have found an explanation for not only why the rings of Saturn are so rich in ice. Her mathematical simulation also described the process leading to the birth of icy moons orbiting near the planet. Additional information from Cassini suggests that at least one of these icy moons, Enceladus, could support life.

Daphnis, an inner moon of Saturn, disturbing the rings
NASA/JPL/SSI, via Wikimedia Commons

The lost moon of Saturn

Saturn likely had another moon in the remote past, around the size of Titan. Titan is a giant among moons. Even larger than the planet Mercury, Titan is the second largest moon in the Solar System. The fate of a lost sibling moon of Titan may hold the secret of the rings. At some point, tidal forces of Saturn stripped off the lost moon's thick outer crust of ice. On some level, perhaps, Galileo's joke about Saturn devouring his children may have been true.

As the ice was stripped away, the moon become nothing more than naked rock. This core material, under the pull of gravity, tumbled into oblivion beneath the shroud of Saturn's atmosphere. For a while, the ice stripped away remained spread diffusely across space around Saturn, one vast homogeneous ring.

With time, ripples in the rings developed through small collisions. Larger accumulations of ice formed and continued to collide. Eventually gaps between rings emerged and the Cassini Division came into being. Dr Canup's model predicts that the amount of material in the rings decreases as these events unfold. Meanwhile, the chunks of debris come together and moons are born. Her predictions fit the masses observed of Tethys, Enceladus, and other Ice Moons of Saturn.

Ice Moon Station will publish more about Saturn in future posts.

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