  Spacecraft and ground-based observations show that the main rings of Saturn
lack particles larger than 10 m. Tidal or collisional destruction of
satellites/comets have been proposed as the origin of the main rings; however,
Saturn's tide alone cannot grind km-sized fragments into submeter-sized
particles because of the high mechanical strength of water ice and rock. The
question arises as to why such large particles are not left in the current
ring. It is known that thermal stress induced by diurnal and seasonal
temperature variations can cause weathering and fragmentation of boulders and
contribute to dust and regolith production on the Moon and terrestrial planets,
and then such thermal stress can break particles larger than a critical radius
while cannot smaller than the critical radius. In this study, we examined the
role of thermal stress acting on Saturn's ring particles. We found that thermal
stress can grind porous ring particles larger than 10-20 m, which explains the
lack of particles larger than 10 m in Saturn's ring. Also, fragmentation by
thermal stress can be adoptable for the Epsilon rings of Uranus. Furthermore,
thermal stress caused by diurnal or seasonal temperature variation acting on
boulders on surfaces of icy satellites and asteroids may play an important role
in the evolution of their sizes. Our calculations explain the lack of boulders
on icy satellites, except in the geologically active provinces such as the
tiger stripes of Enceladus, where boulders are supplied by recent geological
activity. We predict that future observations can find numerous boulders around
Europa's geologically active cracks.
