Why is Saturn’s F ring getting bigger?

Saturn, the sixth planet in the Solar System, is famous mostly for its beautiful rings, of which there are 17. The F Ring is one of a group of four denser rings, the so-called main rings, and it hides a very interesting mystery. It is twice as bright and three times as wide today as it was in 1980-81 when the Voyager spacecraft flew past Saturn. Why did this happen?

Why is Saturn's F ring getting bigger?

Saturn – the sixth planet of the Solar System is mainly famous for its beautiful rings, of which it has as many as 17. The F ring is one of a group of four denser rings, the so-called main rings, and it hides a very interesting mystery. It is twice as bright and three times as wide today as it was in 1980-81 when the Voyager spacecraft flew past Saturn. Why did that happen?

Saturn’s rings are not only beautiful but also mysterious – to this day, it’s not entirely clear how they came to be and how they’re able to stay in orbit in such perfect shape – models for the systems of such small particles that comprise them clearly indicate that over time, the rings should simply fall to the planet’s surface. And with the F ring – one of the four main rings – the opposite is happening – it seems to be getting bigger.

A team of astronomers led by Robert French of the SETI Institute set out to analyze the exact data on this structure – they collected images from the Cassini probe’s flybys past Saturn in 2004 and 2009 and used them to make precise measurements, comparing them to data collected by the Voyager probe, which was near Saturn in the very early 1980s.

And it turned out that this ring has increased its width from about 200 to almost 600 kilometers and it is twice as bright.

The main suspect here is Prometheus, one of Saturn’s inner moons, which has a very large influence on this ring. It has already been observed that it can gravitationally twist the F ring a bit, drawing matter from it, and it is closer to the ring today than it was in the 1980s.

According to the researchers, the key is a phenomenon observed in 2006 when the ring suddenly increased in brightness, which is most likely due to the fact that it flew into a dust cloud and its particles collided with it fragmenting.

A problem arises, however, because such shredded dust would fall toward the planet, so the F ring must have a good source of new matter. Astronomers think that a self-perpetuating mechanism could be responsible for this – as the ring gets new material from passing moons, it gets thicker and its orbit is destabilised. As a result, some of the dust goes into further orbits and disperses a bit, and then it starts to fall back towards the ring and collides with it – what we see as a brightening of the ring.

If this theory is correct, then the brightening and darkening of the F ring is cyclic, so observations should confirm it. According to this theory, the ring should be in a similar configuration in 2 years as it was in 1980, so it should look similar to when Voyager saw it.

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