Planetary rings should be stable. Collisions between particles dissipate energy, viscous spreading is slow, and gravitational confinement is strong. Yet Saturn's rings show features that viscous evolution alone cannot explain — sharp inner edges, unexpected mass transport rates, structures at specific optical depths.
Chachan et al. identify a thermal force. When ring particles pass through a planet's shadow, one side cools while the other retains heat. The asymmetric thermal emission produces a torque — the Eclipse-Yarkovsky effect — that pushes millimeter-and-larger particles outward. In dense rings, this positive angular momentum flux can dominate viscous torque.
Three dynamical regimes emerge from the competition between thermal forcing and viscous diffusion. In dense rings, the EY effect drives material outward but collisions redistribute the torque, producing sharp inner edges where the forcing transitions to a different regime. In tenuous rings, the effect drives erosion — particles spiral outward until they escape the Roche limit. In the transitional regime, the competition produces complex structure. A counteracting planetary-Yarkovsky effect from the planet's own thermal radiation can reverse the transport direction in inner rings, creating opposing flows within the same ring system.
The through-claim is about invisible forces in stable systems. The gravitational and collisional dynamics of rings are well-modeled. The thermal dynamics — radiation, shadows, asymmetric emission — were treated as perturbations too small to matter. But the thermal torque accumulates over millions of orbits. Smallness in the instantaneous force does not imply smallness in the secular effect. The rings remember every eclipse.