[P-PS07] Planetary Sciences

[PPS07-P16] Experimental study on low-velocity glancing collision of ice ball using 2D laser displacement meter

*Yukari M Toyoda¹, Masahiko Arakawa¹ (1.Kobe University)

Keywords:Saturn's ring, Ice, Impact experiment

Introduction: It has been well established that Saturn’s ring system consists of numerous icy particles orbiting the Saturn on nearly circular, almost co- planar orbits. The dense rings such as A and B ring are composed of particles ranging from several mm to several tens of m in size. These rings are optically thick, and the mean free path of these particles is comparable to or shorter than their particle sizes. Therefore, the most important physical process for the dynamics of the dense rings should be inelastic collisions among ring particles. From ground telescope observations and active microwave observations, it is suggested that the particles of the rings could be porous water ice balls with a high porosity of >50%. The glancing collisions commonly occur among ring particles, and clarifying the energy dissipation during the glancing collision described as the restitution coefficients for normal and tangential impacts are important when we consider the dynamics of Saturn’s rings. In Supulver et al., 1995, they conducted the low-velocity glancing collision experiments at the impact velocities of 0.02-1 cm/s using the pendulum apparatus. The restitution coefficients of the ice balls colliding on the ice block were measured at the temperature of ~100 K. From these experiments, it was found that the tangential restitution coefficients converged to ~0.9 with the increase of the impact velocity but the normal restitution coefficients decreased with the increase of the impact velocity. In this study, we conducted low-velocity glancing collision experiments using not only non-porous ice balls but also porous ice balls. We observed the motion of the ice ball by using a 2D laser displacement meter in order to measure rotation of the ball and the displacement in the direction of horizon and height.

Experimental method: We conducted the low-velocity glancing collisions by dropping the frost-free smooth ice ball and the porous ice ball onto the granite plate freely. The radius of the ice and porouos ice ball were 1.5 cm and the mass of the granite plate was sufficiently larger than those of the balls. The collision experiments were conducted for the impact angle of 30, 45, 60 degree (the impact angle of 90 degree corresponds to the head-on collision) and the impact velocity of ~50 cm/s. The motion of the ice ball was observed by using the 2D laser displacement meter, and the setup of experiments is illustrated in Fig. 1. The 2D laser displacement meter can measure the profile of the ball (horizontal position) for 4 seconds at every 75 μm height with the horizontal resolution of 5 μm and the time resolution of 1 ms. The side surface of the ice ball where the laser irradiated was cut flat so that we can observe the rotation of the ball.

Experimental results: Fig. 2 shows the measured profiles of the ice ball at each time before/after the collision by using the 2D laser displacement meter. In this figure, the horizontal axis shows the distance from the laser displacement meter and the vertical axis shows the height within the measurement range. The red/blue profiles show those before/after the collision. From this figure, it is found that the ice ball dropped almost vertically before the collision and then fell down while moving horizontally and rotating after the collision. From this result, it was confirmed that the 2D laser displacement enables us to observe the time variation of the ball position. In the presentation, we will show the analysis method to derive the normal/tangential restitution coefficients and the rotation, and discuss these results.