The formation of distal impact ejecta

Brandon C Johnson, Purdue University

Abstract

Here we present two models for the dynamics of ejection and formation of distal impact ejecta. The first model focuses on the most highly shocked material that forms a massive expanding vapor plume or fireball. In this model molten droplets or spherules condense from the vapor. We model the expanding vapor plume using a one dimensional Lagrangian hydrocode. The condensation of droplets is treated by directly coupling the equations for homogeneous nucleation and growth with our hydrocode. The second model is focused on less energetic material ejected as part of the excavation flow. Using the iSALE hydrocode, we determine the details of the excavation flow and formation of the ejecta curtain. Using this information and some simple analytical approximations we produce a model for the formation of melt droplet spherules, melt fragments, and accretionary impact lapilli, within this flow. Using our model for spherules produced in the vapor plume, we create a method to estimate the size of an impactor and impact velocity required to create a spherule layer. The impactor size depends on the thickness of the layer and the impact velocity depends on the size of the spherules within the layer. Using observations of known spherule layers and the derived dependence on impactor size, we show that the impactor flux on Earth was significantly higher ∼2 – 3.5 Gyr ago than it is today. Our model for the less energetic material ejected as part of the ejecta curtain predicts how ejecta particle sizes depend on impactor size and ejection velocity. In the future, this model can also be used to estimate the scale of an impact required to make observed distal impact ejecta layers.

Degree

Ph.D.

Advisors

Melosh, Purdue University.

Subject Area

Geophysics|Planetology

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