Put a ring on it

Some of the most spectacular images from within our Solar System come from Nasa’s Cassini spacecraft as it orbits Saturn. One of the reasons for this is that many of the vistas are dominated by the planet’s rings.

Saturn and Tethys

Saturn, its rings (and their shadow), and the moon Tethys (NASA/JPL/SSI)


The rings of Saturn are made up mainly of lumps of ice (with a small amount of dust), which while individually are only millimeters up to about a kilometer in size, add up to whole rings structures which whilst only a kilometer deep extend laterally for hundreds of kilometers.

There is still a lot of debate about how they formed and how long they will last. Cassini’s  final orbits are designed to answer some of these questions by flying between the planet and the rings and we should get more answers in the months and years after the end of the mission once the data is analyzed. Current theories suggest that the rings formed around the same time as Saturn did; either from the same planetary nebula which formed the planet or from a moon which was torn apart by Saturn’s gravity. Other researchers have suggested that they are only 100 million years old and transient features which will eventually fall into the planet (over very long time periods). whilst the majority of the evidence points to them being very long lived so far. The exception to this is the outer E rings, these are known to have been generated by the ice volcanoes of Enceladus throwing out icy material into the orbit of Saturn and form a ring.

Rings are dynamic with transient features such as “spokes” of dust caught in the magnetic field chasing around jus above the surface of the rings, disturbances within them like propellers formed by the gravitational distortion of moonlets, or the ripples generated by Daphnis as it travels within a gap in the rings.


Daphnis a small moon causing ripples within the rings of Saturn (NASA/JPL/SSI)

Pan, a moonlet within a gap in the rings has made the gap by collecting up the ice particles, forming a band around its middle.


Pan, which orbits within the rings, has built up ice within its middle (NASA/JPL/SSI)

It is not just Saturn which has rings, Neptune, Uranus, and Jupiter all have rings and even the 250 km diameter small planetoid 10199 Chariklo has a ring system around it. Temporary rings probably occur on all planets as comets and asteroids get too close are torn apart by the gravitational pull. forming a ring of debris which would eventually fall to the surface, Mars may develop a transient ring system, when Phobos is torn apart by tidal forces as it slowly spirals towards the surface. This ring will be transient as the material itself falls onto the surface of Mars.



Infrared image of the rings of Jupiter

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