  Spokes in Saturn's rings are radially-extended structures consisting of dust
grains. Although spacecraft and space telescope observations have revealed
various detailed features of the spokes and their time variation, their
formation mechanism is still under debate. Previous models examined charging
mechanisms to attempt at explaining dust release from cm-sized ring particles;
however, the attempt has been unsuccessful, because the electrostatic force
caused by such charging mechanisms is much weaker than the cohesive force
acting on dust grains at ordinary conditions in the ring environment. Here we
propose a novel model for the formation of the spokes, where the temperature
dependence of cohesion plays an essential role. Ring particles with a
temperature below 60K adsorb an O2 ring atmosphere, which facilitates release
of dust grains from them by a reduction in the cohesive force between the
grains and the particles on the morning ansa. Then, intense electrostatic
forces sufficient to overcome the cohesive force are generated on the surface
of ring particles and the released dust grains form the structure of spokes.
Our model explains observational features of the spokes including their
longitudinal location, lifetime, radial expansion velocity, and seasonality.
