Saturn’s inner moons may have formed only recently, from a giant ring

It's common to think of the Solar System as a fairly static place. Yes, everything is moving around all the time, but the same basic system of planets and most of their moons seems to have more or less existed as-is since the relatively early days of the Solar System. Earth’s Moon, for example, is thought to have formed less than 100 million years after the Solar System coalesced, coming into existence when Earth collided with a Mars-sized body. And so it has gone for the last 4.5 billion years.

In contrast, a series of observations suggests some of Saturn’s moons may be a mere 100 million years old. That is the hypothesis put forth in a new paper published Thursday in the Astrophysical Journal, authored by scientists from the SETI Institute and the Southwest Research Institute. Based upon observations made by the Cassini spacecraft, the researchers think that the interior moons could only have existed in their relatively pristine orbits for a short time. “I think we are at a point where we can confidently say that the inner moons are not as old as the planet,” Matija Ćuk, lead author of the new research paper, said in an interview.

Saturn has a complicated system of at least 62 moons in addition to its famous rings, making it the busiest planetary system in the Solar System. Titan, the largest and best-known of Saturn’s moons, is covered in exotic hydrocarbon seas. But equally interesting to scientists is one of its inner moons, the much smaller Enceladus. Cassini found the moon has geysers at its southern pole, blasting water from its interior into outer space. Since then, scientists have wondered whether life may exist in the warm, interior oceans of Enceladus. Ćuk and his colleagues, Luke Dones and David Nesvorný, have an answer that astrobiologists may not like.

Orbital mechanics

Ćuk has made a career of studying the orbits of natural satellites, including asteroids and moons in the Solar System. He won a prestigious award in 2014 for his mastery of planetary dynamics, including the concept of orbital resonance, which explains how objects in orbit around another object sometimes exert a gravitational influence on one another.

This happens all over the Solar System. Most famously, Pluto and Neptune have an orbital resonance of 2:3, meaning that Pluto completes two orbits during the same amount of time that Neptune completes three. In this case, Pluto, the smaller object, has its orbit driven by Neptune. Some resonances are stable and others are unstable, even to the point where a body can be kicked out of the Solar System.

Orbital resonances becomes quite a bit more complicated with a very large planet and a sprawling system of more than five dozen moons. One might think that the moons of Saturn would, over time, fall into more elongated orbits or get knocked out of their orbital planes due to unstable resonances. But when one looks at the inner moons of Saturn, those inside the orbit of Titan, the planet’s satellites are found to be in relatively good order. They question is, should they be?

Saturn is a gas giant, of course. The gases at its surface and more exotic types of matter in its interior collectively act as a fluid. Because there are all these moons orbiting around Saturn, most of them in a plane, they exert a gravitational pull on the planet. It's therefore a bit egg shaped; as it flexes, there is some friction, which causes the rotation of Saturn to slow down slightly. The tidal forces, in turn, cause the moons to slowly move away.

Moons moving out faster?

In 2012, French researchers published an analysis of 130,000 data points from 1874 to 2007 showing the position of Saturn’s inner moons. These scientists found that, due to tidal forces, the inner moons had moved away from Saturn 10 times faster than previously thought. This movement was confirmed in 2015 using data from the Cassini spacecraft.

This revelation really made Ćuk wonder how the complex Saturn system evolved over time, so he began trying to model the movement of Saturn and its moons. What he pretty quickly found is that some of the most significant inner moons, such as Tethys and Dione, did not have orbits that were altered as much as they should have been over billions of years. In other words, there hadn’t been enough orbital resonances to perturb their orbits.

That idea meant they had to be pretty young. But how young? To answer this question, the researchers focused on Enceladus and its geysers, which Cassini has repeatedly flown by during its 12 years in the Saturn system. Ćuk and his colleagues assumed the geysers were powered by tidal energy from Saturn, which gives the small moon the heat it needs to maintain internal water oceans. They also assumed the geothermal activity is relatively constant. Based on this assumption, the simulation suggested that Enceladus has only moved a very short distance from Saturn since its formation, about the amount it would be expected to move in 100 million years.

If this is true, it means that, as dinosaurs roamed the Earth, a system of moons not unlike those today fell into some unstable resonances, which began a string of collisions that created a giant ring. And then that ring, in relatively short order, re-formed back into the moons we see today.

A crazy idea

As remarkable as it is to think about witnessing the formation of a moon system so late in the Solar System’s history, it's not such a crazy idea, says Erik Asphaug, a planetary scientist at Arizona State University who is familiar with this area of research. “There are other big problems with the idea of a steady state,” he told Ars. “For one thing, the rings of Saturn can't be more than a few 100 million years old, at most, or they would have gone away. So we've known for some time that small, ten-kilometer or so satellites have been crashing together and breaking up to replenish the rings.”

The simulations by Ćuk may or may not point to the final answer, Asphaug said, noting other ideas about how the Saturn system evolved to they way it is now, with small-, medium-, and large-sized moons. “But this paper takes it to the next level, [saying] that some of the largest moons formed in a recent collisions,” he added. “At any rate, this is a very exciting and controversial paper.”

Additional observations may provide clarity. For one thing, Ćuk believes some answers may actually lie on Pluto, where the New Horizons spacecraft was able to observe a world where virtually every crater has been caused by bombardments from comets and other Kuiper Belt objects. If the size and distribution of craters on the inner moons of Saturn look like that, he said, it means the inner moons are very old, like Pluto, and “our theory is toast.”

Asphaug believes several lines of evidence, from impact craters to tectonic and thermal histories of Saturn’s moons, will eventually crystallize into a single acceptable theory. And if Enceladus is indeed young, it would bode poorly for the prospects of life on Enceladus, as it would've had to spring into existence over the course of a few dozen millions of years.

Astrophysical Journal, 2016. DOI: 10.3847/0004-637X/820/2/97 (About DOIs).