  The recent discovery of dense rings around the Centaur Chariklo (and possibly
Chiron) reveals that complete dense planetary rings are not only found around
Saturn and Uranus, but also around small bodies orbiting in the vicinity of
those giant planets. This report examines whether there could be a physical
process that would make rings more likely to form or persist in this particular
part of the outer Solar System. Specifically, the ring material orbiting Saturn
and Uranus appears to be much weaker than the material forming the innermost
moons of Jupiter and Neptune. Also, the mean surface temperatures of Saturn's,
Uranus' and Chariklo's rings are all close to 70 K. Thus the restricted
distribution of dense rings in our Solar System may arise because icy materials
are particularly weak around that temperature.
