Why didn't Saturn eat Titan, its biggest moon?

An image provided by JPL/NASA taken by the Cassini spacecraft in 2015 of Saturn and its largest moon, Titan. Photo / JPL/NASA, The New York Times

In classical mythology, the titan Cronus, who was reinterpreted by the Romans as Saturn, devoured his newborn children to prevent a prophesied coup. (He did not succeed, and Zeus became the king of the gods.)

In planetary science, a similar scenario emerges when scientists re-create the evolution of large planets like Saturn, which has a satellite system dominated by one massive moon, Titan. Typically those simulated planets either eat their orbital retinue, or multiple sizable moons survive into adulthood, like the four Galilean moons of Jupiter.

How, then, did Saturn end up with massive Titan and a multitude of tinier moons? Using a set of simulations detailed Monday in the journal Astronomy and Astrophysics, a team of planetary scientists identified an explanation for how a moon like Titan could have avoided straying too close to its murderous parent.

"Titan is one of the largest moons in our solar system," said Yuri Fujii of Nagoya University and lead author of the new study. "I would like to reveal its origin."

Large planets, in their infancy, are swaddled by a swirling disk of gas and dust. Usually, most of their moons, if there are any, form in tandem with the planet from that disk.

Some may be small and icy, while others might end up like Titan, the second-largest moon in our solar system. Bigger than the planet Mercury, Titan is a hulking mass with liquid hydrocarbon seas and a thick froth of nitrogen atmosphere. But when scientists try to simulate how these moons grew and evolved, they end up with satellite systems that don't replicate reality.

Fujii said that these simulations oversimplify how moons move and how those gassy, circumplanetary disks behave.

She and a colleague attempted a more detailed simulation of moon formation for a planet at Saturn's distance from the sun. Instead of considering a large world surrounded by a simplified, gassy disk, she modelled an environment with more intricate, fine-scaled temperatures and densities. Then, she tweaked the amount of turbulence in the gas itself.

They then simulated more than 100,000 years of moon evolution, adding in the gravitational jostling caused by migrating satellites and accounting for the disk's dissipation over time.

She found that a swath of space emerges within the disk that acts like a safety zone. That zone is a dusty expanse with a steep temperature change between its warmer inner edge, closer to the planet, and the colder outer edge, nearer to the void. Warm gas inside that band, which fluctuates between 20 and 100 planetary radii from the infant planet, prevents farther-flung moons such as Titan from moving inward and becoming snacks for the young planet. But moons that already live on the interior of the safety zone are out of luck.

The simulation, Fujii said, reproduces a satellite system with a single, large moon around a Saturn-like planet — although that outcome largely depends on how long it takes the gas in the disk to vanish and how far from the planet the large moons initially form.

"We found, for the first time, a couple of cases and conditions when the number of large moons becomes one," she said.

Luke Dones, an expert in planetary rings and solar-system dynamics at the Southwest Research Institute in Boulder, Colorado, said the simulations were "interesting, but preliminary" and was skeptical about whether they could reveal much about how moons form and grow more broadly. He wondered, for instance, whether the work would reproduce a satellite system like Jupiter's, given that Jupiter-like starting conditions will be substantially different from a simulated Saturn.

Studying the origins of Saturn and Titan continues to be an outstanding quest, as moons in the outer solar system are now at the forefront of the search for life beyond Earth. And, as with any good mystery, hazy, orange Titan — a world whose riches might include life-forms with bizarre, unearthly chemistries — is consistently outwitting scientists and demanding more than a clever gaze from faraway to solve.

Written by: Nadia Drake
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