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A new study suggests that, in the past, the solar system’s moons (like ours) may have had rings. This new discovery deepens the mystery of why there are currently no ringed moons.
Currently, none of the moons in our solar system have rings.
However, a recent study in Astronomy and Astrophysics indicates that, when created, these rings could remain stable for a million years, even being gravitationally pulled by other objects in the solar system.
As , writes, “rings surround many members of our planetary family”. Saturn is perhaps the best-known example, wrapped in eight rings main ones made of thousands of smaller rings.
Composed of pieces of ice and rocks of varying sizes, these ring systems are maintained by small shepherd moons, whose gravitational forces pull the pieces and change their positions.
More recent studies using ground-based telescopes have revealed rings surrounding several centaurs – asteroids beyond Jupiter’s orbit – and smaller planets, including the egg-shaped .
Even Earth and Mars may have had rings. However, to date, no study has definitively identified rings around any of the solar system’s approximately 300 moons.
Hunt for missing lunar rings
These observations led Mario Sucerquiaan astrophysicist at France’s Grenoble Alpes University, and colleagues investigating whether the lunar rings could be stable.
A 2022 study, of which Sucerquia had already co-authored, concluded that, theoretically, Isolated moons could have stable rings around them. But that study did not take into account the gravitational effects of other moons and planets.
To further investigate the phenomenon, in this new study, Sucerquia selected five sets of spherical moons and their neighboring planets, including the Earth and the Moon.
For each set, the team added rings to all the satellites, then simulated the behavior of the rings over a million years as they were gravitationally pulled by the mother moon, other nearby moons, and the planet.
The researchers also calculated the chaotic way the ring particles moved over a millennium, to determine the rings’ stability.
The researchers discovered that Earth’s moon had a 95% chance to support a stable ring system in simulations.
“We did not expect that the moons in a hostile gravitational environment, with many other moons and planets perturbing their rings, would maintain stability,” said Sucerquia, to Live Science.
Instead, “these hostile environments, instead of destroying the rings, endowed them with great beauty, creating structures such as cracks and waves, similar to those observed in Saturn’s rings”, praised the researcher.
Where did the rings go?
The authors suggest that non-gravitational factors, including solar radiation and charged particles from magnetic fields of the moons’ parent planets, caused the previous rings to disintegrate.
Matthew Tiscarenoa planetary scientist at the SETI Institute in California who was not involved in the study, told Live Science that in the long term, the rings will likely have been broken by the gravitational pull of the mother moons themselves.
“Because most moons in the solar system rotate very slowly (keeping the same face toward their planet as they orbit, just as our moon does toward Earth), any ring particles must be orbiting the moon much faster than the the moon rotates,” he said.
Therefore, the gravitational pulls of the progenitor moons, for long periods of time, “would cause the orbits of the ring particles to decay until they end up colliding with the surface of the moon”, he argued. In other words, if our moon once had rings, they collided with the lunar surface a long time ago.
