In a study published in the Proceedings of the National Academy of Sciences, scientists have discovered that planetary rings, such as those orbiting Saturn, have a universally similar particle distribution. The study also suggests that Saturn’s rings are essentially in a steady state that does not depend on their history.
In this simulated image of Saturn’s rings, color is used to present information about ring particle sizes in different regions based on the measured attenuations of three radio signals. Image credit: NASA.
“Saturn’s rings are relatively well studied and it is known that they consist of ice particles ranging in size from centimeters to about 10 m,” said study lead author Prof Nikolai Brilliantov from the University of Leicester, UK.
With a high probability these particles are remains of some catastrophic event in a far past, and it is not surprising that there exists debris of all sizes, varying from very small to very large ones.
“What is surprising is that the relative abundance of particles of different sizes follows, with a high accuracy, a beautiful mathematical law of inverse cubes,” Prof Brilliantov said.
“That is, the abundance of 2 m-size particles is 8 times smaller than the abundance of 1 m-size particles, the abundance of 3 m-size particles is 27 times smaller and so on.”
“This holds true up to the size of about 10 m, then follows an abrupt drop in the abundance of particles.”
The reason for this drastic drop, as well as the nature of the amazing inverse cubes law, has remained a mystery until now.
This is an artist concept of a close-up view of Saturn’s ring particles. The planet Saturn is seen in the background (yellow and brown). The particles (blue) are composed mostly of ice, but are not uniform. They clump together to form elongated, curved aggregates, continually forming and dispersing. The space between the clumps is mostly empty. The largest individual particles shown are a few meters across. Image credit: NASA.
Prof Brilliantov and his colleagues from the United States, Europe, and Japan, have finally resolved the riddle.
“Our study shows that the observed distribution is not peculiar for Saturn’s rings, but has a universal character,” Prof Brilliantov said.
“In other words, it is generic for all planetary rings which have particles to have a similar nature.”
The same size distribution is expected for any ring system where collisions play a role, like the Uranian rings, the recently discovered rings of Chariklo and Chiron, and rings around extrasolar objects.
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Nikolai Brilliantov et al. 2015. Size distribution of particles in Saturn’s rings from aggregation and fragmentation. PNAS, vol. 112, no. 31, pp. 9536-9541; doi: 10.1073/pnas.1503957112
