Formation of spherules in impact produced vapor plumes

We have constructed a numerical model of spherule formation in an impact produced vapor plume. This model tracks the expansion of the vapor plume using a one-dimensional Lagrangian hydrocode coupled with the ANEOS equation of state for silica. We then include the equations for nucleation and growth as described by homogeneous nucleation theory to describe the process of spherule formation. We use this model to determine the number and size of the spherules that an impact creates. We also explore when and where spherules are formed in the vapor plume, and how this affects the size of the spherules. In general we find that smaller spherules form in the outer, faster moving, portions of the vapor plume at earlier times. This work also explores the effect of impactor size and impact velocity on the resultant spherule size. We report a simple linear dependence on impactor size and a complex dependence on impact velocity. We find that a 10 km diameter asteroid impacting at a velocity of ∼21 km/s creates spherules that are ∼250 μm in diameter which is comparable to the spherules found in the K/Pg boundary layer.

► We model the process spherule formation in an expanding vapor plume.
► Our model is consistent with observations of spherules in the K/Pg boundary layer.
► We explore how spherules size depends on impactor size and impact velocity.
► Smaller spherules tend to form in the outer faster moving parts of the vapor plume.
► Spherule formation begins in the outer portions of the vapor plume first.