Investigating the interactions between an atmosphere and an ejecta curtain1. Wind tunnel tests

When an asteroid impacts a planetary surface, ejecta are excavated along ba llistic trajectories whose loci define an inverted cone shape. The outward advancing motion of this curtain displaces atmosphere to generate a ring vo rtex whose winds can entrain, transport, and deposit ejecta and fine-graine d surface materials. Curtain width and velocity, particle concentration, si ze distribution and motion parallel to the curtain, and the density, viscos ity, and compressibility of the surrounding atmosphere all influence the vo rtex circulation strength. As analogs to an advancing ejecta curtain, we te sted the effect of inclined solid and porous plates on vortex formation in a low-speed wind tunnel. We found that hydraulic resistance, a measure of e nergy losses for 1-D porous flow, governs the position along a porous plate where it becomes effectively permeable and flow separation occurs. The res ulting flow field is similar to that over an inclined solid plate of the sa me effective length. Energy losses through the top, permeable portions of t he plate reduce circulation strength by only 7% relative to flow over a sol id plate. The two parameters needed to estimate circulation strength, curta in velocity and impermeable height, can thus be determined by coupling an i mpact model with published hydraulic resistance data. These tests also serv ed to calibrate a numerical model, which we then applied to investigate the influence of atmospheric compressibility and particle motion parallel to t he curtain (see Part 2 [Barnouin-Jha et al., this issue]). These two studie s provide a method to predict the curtain-induced flow and consequent patte rns of debris deposition associated with impacts on planets with atmosphere s.