Saturn’s rings are a dynamic system , constantly changing and evolving. We usually imagine them as something static, fixed, immovable (beyond their orbit around the gas giant). Observations in recent decades, which reached an unprecedented level of detail with the visit of the Cassini spacecraft , have shown us that this is not the case.
First of all, we think that Saturn’s rings formed relatively recently . According to observations made with telescopes located both on land and in orbit around the planet, these rings should be no more than 100 million years old . In addition, a constant flow of material has been observed falling towards Saturn, which tells us that the rings will not last more than 300 million years , perhaps even less.
But in addition to being affected by the planet they surround, Saturn’s rings are also influenced by the moons that orbit the planet . Some of them come to orbit within the rings themselves, creating holes in them , such as those known as Encke and Keeler holes, located in the A ring of Saturn. These holes are nothing more than that, parts of the ring that are especially empty of the dust and ice that make up the rest of the structures that form it. Encke’s gap is created by the moon Pan, while Keeler’s gap is created by the moon Daphnis.
Both satellites are small, with Pan being between 20 and 30 kilometers in size and Daphnis around 7 kilometers in size. This difference in size is also responsible for the difference in width of the gaps they form. While the Encke gap is about 325 kilometers thick , the Keeler gap is only 42 kilometers wide. These types of moons, which control and distribute the dust particles that form the rings , are known as shepherd moons , referring to how a shepherd controls his herd of cattle.
But shepherd moons not only clean up Saturn’s rings, creating gaps in them, they are also capable of disturbing and creating structures on the boundaries of those gaps . A clear example of this is the waves created by Daphnis in the rings that are found on both sides of the Keeler gap. These waves have a horizontal component, parallel to the plane of the rings themselves, caused by the passage of the moon through the region . These waves lead Daphnis in the inner part of its orbit and lag behind it in the outer part, due to differences in the speed of their orbits . Objects closer to Saturn will orbit faster than those further away, causing this effect.
In addition, they have a vertical component, due to the fact that Daphnis does not orbit in exactly the same plane as the rings , so its perturbation extends up and down as well. The height of these undulations can reach up to a kilometer and a half , a considerable distance considering that the average thickness of Saturn’s rings is approximately 10 meters.
But the most impressive perturbations of the dust that make up these rings are those seen outside the B ring (which, despite what it might seem, lies inside the A ring). These do not look like waves, with ordered troughs and crests, like those created by Daphnis, but are completely irregular . The exact cause of this kind of ring mountains is not entirely clear, although it is believed that they could be due to the presence of several submoons within the rings themselves .
These vertical structures reach two and a half kilometers “height” above the plane of the ring they inhabit. Photos like this, in which these mountains cast long shadows on the rest of the material, are only possible around the time around Saturn’s equinox . In the weeks before and after the equinox, the Sun sits just a few degrees off the ring plane , allowing any object in the ring to cast shadows like these. This particular photograph was taken just two weeks before this equinox, which took place on August 11, 2009 . Since Saturn’s orbit takes almost 30 years to complete, these equinoxes only repeat once every 15 years.
A submoon (like the ones that created these structures) would be any object large enough to be distinguishable from the rest of the ring material , but without enough mass (and thus gravity) to clean out the rings and create a hole . These objects would have sizes of a few hundred meters, like the submoon S/2009 S1 , which is 400 meters in diameter and is capable of casting shadows on the rings (in fact, it was thanks to its shadow that we discovered it), but which cannot Completely clean your orbit.
References:
Daphnis Up Close, 2017, NASA – Cassini: Mission to Saturn
JN Spitale, 2010, Free Unstable Modes and Massive Bodies in Saturn’s Outer B Ring, Astron.J.140, https://doi.org/10.48550/arXiv.0912.3489
