Statistics – Computation
Scientific paper
Jan 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997lpico.922...35m&link_type=abstract
Large Meteorite Impacts and Planetary Evolution, p. 35
Statistics
Computation
Meteorites, Plumes, Vapors, Meteorite Craters, Cratering, Ejecta, Projectile Cratering, Hypervelocity Impact, Equations Of State, Kinetic Energy, Simulation, Lagrangian Function
Scientific paper
When a meteorite strikes a planetary surface at speeds greater than a few kilometers per second, the kinetic energy of the meteorite is partially converted into heat by irreversible processes. The meteorite and some target material may vaporize after release from high pressure. In the past, a model of vapor-plume expansion based on the expansion of a spherical cloud with a perfect gas equation of state was used to model the expansion of these vaporized gases. A comparison between this model and detailed numerical calculations of the Chicxulub impact shows that the vapor plume in the more realistic numerical model takes far longer than predicted to accelerate out of the crater (more than 30 s, compared to a predicted time of a few seconds). We verified this delayed expansion for an impact of an Al projectile onto an Al target to check that problems of chemical speciation in the vapor plume were not affecting our results. Based on the study of several such computations, we propose that this long delay is due to a combination of the liquid-vapor phase transition in the realistic (ANEOS) equation of state used in this simulation and the nonspherical geometry of the expanding projectile. To examine these effects on plume expansion, we employed a highly simplified equation of state (the Van da Waals equation) that nevertheless exhibits a liquid-vapor phase transition. Using a one-dimensional Lagrangian hydrocode, we investigated the qualitative effect of the phase transition on vapor plume expansion and demonstrated that the expansion is affected by (1) a rapid decompression (compared to a perfect gas) that cools the supercritical rock vapor until it reaches the phase boundary, followed by (2) a very slow phase of acceleration fueled mainly by the latent heat of the two-phase mixture. This slow acceleration occurs mainly because of the extremely low sound speed in the two-phase liquid-vapor system. The final velocity for the realistic equation of state and a perfect gas of the same initial internal energy is nearly the same, but it takes much longer to achieve this velocity with a realistic equation of state. In addition, studies of the expansion of cylinders and planes of hot gas show that the expansion is greatly affected by the geometry of the initial gas cloud. Since an impacting projectile is quickly distorted from its initial spherical shape to a pancake shape lining the growing crater cavity, geometric effects may also strongly affect the expansion rate. Current numerical simulations of impacts do not extend to late enough times to capture correctly the dynamics of this plume expansion; as a result, these simulations greatly underestimate the amount and velocity of ejected debris. This very slow plume expansion may also affect the final phases of melt deposition on top of the crater, leading to a normally graded, apparently air-fall type of deposit consisting of melted rock droplets.
Melosh Henry Jay
Pierazzo Elisabetta
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