William sent in this link to https://phys.org/print423907645.html … where researches from the University of Idaho and Wellesley College say they have found evidence two pairs of moons orbiting Uranus are on a collision course. The paper has been uploaded to the arXiv preprint server – see www.arxiv.org/abs/1708.07566
Uranus is thought to be an ice giant, like Neptune, as opposed to a gas giant. It is also thought to have an extremely cold atmosphere. However, it has a ring system and many moonlets orbiting the planet. It is some of these that are claimed to be on a collision course – a sort of worlds in collision around Uranus. The rings of Uranus have an odd non circular orbit thought to be due to the gravitational pull of one of its moons, Cressida. The particles in the ring are moving faster than the moon which causes Cressida to tug at the rings as it passes by. The peculiar thing is that Cressida is small – and porous (or that is what has been calculated). Where does the gravitational pull come from?
At https://tallbloke.wordpress.com/2017/09/08/cassini-captures-wave-structu… … another amazing find from the Cassini Mission. A wave structure in Saturn's rings known as the Janus 2:1 spiral density wave. This is said to result from the same process that creates spiral galaxies – but spiral density waves in Saturn's rings are much more tightly wound. Every second wave crest is actually the same spiral arm which has encircled the entire planet multiple times. It seems to affect the innermost portion of the B ring. At his location ring particles orbit Saturn twice for every time the moon Janus orbits once, creating an orbital resonance. The wave propagates outward from the resonance (and away from Saturn) and for unknown reasons damping of waves by the larger ring structure is very weak at this location. Even more peculiar is that the Moon Janus exchanges position with another moon once every 4 years – spawning further waves.