Permalink : http://hdl.handle.net/2115/34714

リング・衛星系は巨大惑星形成過程の自然な副産物であると考えられる．本稿では，土星リングを構成する氷粒子の物性がリングの力学進化や構造形成に及ぼす影響について述べる．氷粒子の反発係数はリングの力学進化を考える上で重要な物理量であり，室内実験結果が理論研究に使われてきた．粒子表面の摩擦は粒子自転を引き起こし，土星リングからの熱輻射の観測結果を解釈する上で重要となる．また粒子間の非重力的吸着力がリングの進化や構造形成に及ぼす影響，隕石衝突によるリング組成進化に関する研究等も行なわれているが，これらについては今後さらに詳しく調べる必要がある．
All the four giant planets in our Solar System have rings, thus ring-satellite systems seem to be a natural byproduct of giant planet formation process. Therefore, studies on the origin and evolution of ring-satellite systems would constrain the formation processes of giant planets. Saturn's rings are mainly composed of water ice, and particles in the main rings have sizes between centimeters to several meters. We first describe effects of normal and tangential coefficients of restitution of icy ring particles on dynamical evolution of planetary rings. Restitution coefficients for icy particles for low-velocity impacts relevant to Saturn's rings have been measured by laboratory impact experiments. Restitution coefficients influence random velocity evolution of ring particles, their rotational states, and accretional evolution. We will also discuss possible effects of adhesive forces between ring particles on accretional evolution and structure formation in rings, although it is not clear at present whether nongravitational cohesive forces would play a major role in the dynamics of planetary rings. Finally, we will discuss effects of meteoroid impacts on the creation and evolution of planetary rings. Dilute dust rings including the Jovian rings are created by meteoroid impacts on small satellites. Such impacts would also cause compositional evolution of Saturn's rings. Further theoretical as well as observational studies of compositional evolution of Saturn's rings would provide a unique constraint on the origin and evolution of the rings.