Three-Dimensional Shapes of Cosmic Spherules: Deformation of Dust Particles Molten in the Earth Atmosphere

Introduction Cosmic spherules are extraterrestrial-origin round-shaped dust particles collected from the stratosphere, polar ice, and ocean floor sediments. When extraterrestrial dust particles enter the Earth atmosphere, they are heated by the gas friction and melted. Because of the surface tension, the molten particles become spherical and form cosmic spherules when they solidify. Tsuchiyama et al. have examined 3-D structures of cosmic spherules and found that there are both prolate and oblate shapes. The ram pressure, the surface tension, and the centrifugal force acting on the particles deform the shape when they are molten. Thus, it seems natural to consider that the variation of the observed cosmic spherule shapes may originate from the shape of dust particles when they solidified. In this study, we evaluate the ram pressure and the centrifugal force when the dust particles solidify by solving the equation of motion and the energy equation of the dust particles entering the Earth atmosphere. And we calculate the magnitude of deformation of the dust particles by using analytic solutions for the shape of the molten particles. Finally, we compare the results of calculations with observations.