Just how old are the moons and the rings of Saturn? A new study from researchers at the SETI Institute claims that the planet’s satellites and features are relatively modern, and may not have even been around when most of the dinosaurs died out millions of years ago.
As lead investigator Matija Cuk and his colleagues report in a paper scheduled for publication in the Astrophysical Journal, their analysis of the evolution of Saturn’s moons indicated that a predicted orbital resonance (an event in which one moon’s orbital period ends up being a simple fraction of another moon’s) involving the moons Tethys and Dione did not appear.
In these unusual configurations, even moons with weak gravity can have a significant impact on the orbits of another moon, causing them to become more elongated or to tilt outside of their original orbital plane, the researchers explained. Since the anticipated resonance involving Tethys and Dione did not occur, it implies that the system is less evolved than experts previously believed.
Their computer simulations did reveal, however, that the orbital inclinations of Tethys, Dione, and Rhea suggest the system experienced a 5:3 orbital resonance involving Dione and Rhea, which was followed closely by a secular resonance involving Tethys and Dione. Based on these findings, the authors conclude that the moons are either “significantly younger” than Saturn, or that their tidal evolution is extremely slow.
The slow-evolution suggestion is not compatible with the intense tidal heating discovered on the moon Enceladus, Cuk’s team noted. In light of the evidence, they propose that the planet’s mid-sized moons re-accreted from a disk roughly 100 million years ago. This disk likely was formed from the remains of an earlier generation of mid-sized moons, they added.
Yes, there may be dinosaur fossils older than Saturn’s moons
“Moons are always changing their orbits. That’s inevitable,” Cuk said in a statement, adding that this fact allowed them to conduct the computer simulations that ultimately revealed that Saturn’s mid-sized moons were likely “born during the most recent two percent of the planet’s history.”
“While Saturn’s rings have been known since the 1600s, there’s still debate about their age,” he explained. “The straightforward assumption is that they are primordial – as old as the planet itself, which is more than four billion years. However, in 2012, French astronomers found that tidal effects – the gravitational interaction of the inner moons with fluids deep in Saturn’s interior – are causing them to spiral to larger orbital radii comparatively quickly.”
This discovery implied, based on the moons’ present positions, that they were relatively young, and that the formation of the rings was a relatively recent phenomenon. The study authors used their computer simulations to compare the current orbital tilts of the moons and to compare them to previously projected ones to determine how much the moons have grown over time.
They found that the orbits of Tethys, Dione and Rhea all changed less dramatically than had been predicted, indicating that they had not crossed many orbital resonances and had formed close to their current locations. By combining this information with data from NASA’s Cassini mission, they determined that the moons (with the exception of Titan and Iapetus) likely were formed during the Cretaceous Period, when dinosaurs still roamed the Earth.
“So the question arises, what caused the recent birth of the inner moons?” Cuk pondered. “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the Sun. Eventually, the orbits of neighboring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.”
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Image credit: NASA
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