The Solar System is home to a fascinating array of moons some hot, some cold, some with liquid surfaces, and others covered in dust. Among these nearly 300 known moons, we have “walnut moons,” “potato moons,” and even a few with geysers. However, one curious absence has puzzled scientists: none of these moons have rings.
Rings are not uncommon in the Solar System. Four of the eight planets Jupiter, Saturn, Uranus, and Neptune boast rings of dust and ice orbiting their equators. Even Mars is believed to have once had a ring, and some dwarf planets and asteroids still do. Given the prevalence of rings around other celestial bodies, the lack of ringed moons dubbed “cronomoons” has led astrophysicists to question why they don’t exist.
This question began to bother astrophysicist Mario Sucerquia from Adolfo Ibáñez University in Chile and his colleagues. “If giant planets in our Solar System have rings, and if centaurs (asteroids beyond Jupiter’s orbit) and trans-Neptunian objects also have them, why don’t moons in the Solar System have rings?” Sucerquia explained to ScienceAlert.
To explore this mystery, the researchers conducted a series of simulations to determine whether there might be underlying reasons preventing the formation or long-term stability of rings around moons.
The team discovered that there is no shortage of raw material from which rings could form around moons. Many moons are covered in craters that could eject dust during impacts, while others, like Saturn’s moon Enceladus, eject vapor and gas from geysers. However, instead of forming rings around the moons, this material often gets transferred to the planet’s orbit due to intense interactions with nearby moons and the planet itself.
“In other words, while moons generate part of the raw material for rings, the surrounding environment ensures that the more massive planet retains it, preventing the formation of rings around the moons themselves,” Sucerquia explained.
NASA currently lists 293 moons orbiting planets in our Solar System, with most circling Jupiter and Saturn. The researchers simulated these moons over a million years of evolution to study the stability of potential ring systems and how they might change over time.
The results were surprising. “I initially expected the rings to be completely unstable, which would have directly answered the question. However, contrary to our expectations, we found that these structures were quite stable in many cases,” said Sucerquia.
The simulations revealed that rings could indeed form and remain stable around moons, even in hostile gravitational environments with many other moons and planets exerting influence. In fact, these challenging conditions could even enhance the beauty of the rings by creating structures like gaps and waves, similar to those observed in Saturn’s rings.
Some features on Solar System moons indicate that rings may have existed in the past. For instance, debris orbiting Saturn’s moon Rhea could be the remnants of a former ring system, and the equatorial ridge on Saturn’s moon Iapetus might be the remains of a ring that fell onto the moon.
These findings suggest that the reason we don’t see moon rings today could be simply due to timing. Factors such as radiation pressure from the Sun, magnetic fields, internal heating, and magnetospheric plasma may have contributed to the loss of any moon rings that once existed.
“I believe we might have been somewhat unlucky in this regard, as we started observing the Universe during a period when these structures are no longer present. After conducting this study, I am convinced that these rings likely existed in the past,” Sucerquia said.
The researchers propose that further simulations including more variables, such as radiation pressure and magnetic fields, could help to better understand the absence of moon rings. Additionally, studying moons for evidence of past rings could yield further insights.
Meanwhile, Sucerquia and his colleagues are expanding their search to look for ringed moons that might exist around exoplanets orbiting distant stars. “I can’t help but wonder about the mythology and stories that might emerge from inhabitants of other worlds where their moons have rings,” Sucerquia mused. “How different might our own stories and culture have been if our Moon had rings? The possibilities are endless.”
The research has been accepted into the journal Astronomy & Astrophysics and is available on arXiv.
Read Now:Discovery of New Microbe Species in Inhospitable Lake Sheds Light on the Origins of Complex Life
