Galen Gisler, Robert Weaver, Charles Mader LANL Michael Gittings SAIC LPI Impact Cratering Workshop February 7, 2003 LA-UR Two- and Three-Dimensional Simulations of Asteroid Ocean Impacts

2 Outline The SAGE / RAGE hydrocode –Physics, implementation Simulations of Asteroid Impacts –Oblique water impacts (three dimensions) –Vertical water impacts (two dimensions) Scaling of impact phenomenology –Tsunami hazards from small asteroids?

3 The RAGE hydrocode RAGE = Radiation Adaptive Grid Eulerian Originally developed by M.L. Gittings for SAIC & LANL Continuous adaptive mesh refinement (CAMR): cell-by-cell and cycle-by-cycle High-resolution Godunov hydro Multi-material Equation of State with simple strength model 1-D Cartesian & Spherical, 2-D Cartesian & Cylindrical, 3-D Cartesian Unit aspect ratio cells (squares & cubes) Implicit, gray, non-equilibrium radiation diffusion SAGE is RAGE without radiation

4 Parallel Implementation of code Message passing interface (MPI) for portability, scalability Adaptive cell pointer list for load leveling –Daughter cells placed immediately after mother cells –M total cells on N processors gives M/N cells per processor Gather/scatter MPI routines copy neighbor variables into local scratch Excellent scaling to thousands of processors Used on SGI, IBM, HP/Compaq, Apple, and Linux Clusters

5 Physics included in simulations Fully compressible hydrodynamics AMR resolves shocks & contact discontinuities Godunov - Riemann solvers track characteristics 2 nd -order in space, close to 2 nd -order in time (except at shocks) Courant-Friedrich time-step limit applies on smallest cell in problem Constant vertical gravity EOS SAGE is routinely used with multiple EOSs SESAME tables for air, crust (basalt) & mantle (garnet) PACTECH table for water includes dissociation Mie-Gruneisen EOS for projectile avoids early time-step difficulties Strength Elasto-plastic model with tensile failure and pressure hardening used for crust and mantle

6 Validation of RAGE/SAGE codes Water cratering simulations: Gault & Sonnet laboratory experiments of small projectile water impacts LANL Phermex experiments of underwater explosive detonations Lituya Bay landslide-generated tsunami - lab experiment and the real thing More tsunami comparisons are in progress - source terms uncertain See recent issues of the Journal of the Tsunami Society, Mader et al. Strength & EOS: Taylor anvil and flyer-plate experiments (in progress) Underlying hydrodynamics: Weekly regression testing on well-known standard problems (shock tube, Noh, Sedov blast wave, wind tunnel, …) Still, extrapolation is always uncertain …

7 Characteristics of Simulations All simulations: Atmosphere 42 km, ocean 5 km, basalt crust 7 km, mantle 6 km Start asteroid 30 km above ocean surface 3-D oblique ocean impacts: Iron impactor, diameter 1000m Velocity 20 km/s at 45˚ and 30˚ elevation Computational volume 200 km x 100 km x 60 km Up to 200,000,000 cells 1200 processors on LLNL ASCI White machine 1,300,000 CPU-hours 2-D Parameter study of six vertical ocean impacts: Material dunite (3.32 g/cc) and iron (7.81 g/cc) Diameters 250m, 500m, and 1000m Vertical impact, velocity 20 km/s Computational volume - cylinder 100km radius, 60 km height Up to 1,000,000 cells, 10,000 cpu-hrs per run

8 Maximum cavity 3-d simulation of oblique water impact

9 Density visualization in 45˚ water impact

10 Wave trains from water impacts are complex This movie is of a small portion (50 km wide by 15 km tall) of the simulation volume for a vertical 1km iron impact. The viewing window moves to the right at a speed close to that of the final wave. The horizontal red lines have a spacing of 1 km, but disappear when the movie plays. The development of the wave train is affected by shocks reflecting between the sea floor and the surface.

11 Wave Dynamics Inferred from Tracer Particles Example from Fe 1000 m The particle motion is clearly not that expected for a simple wave

12 Wave Dynamics Inferred from Tracer Particles Example from Dn 250m Here the motion is relatively simple, though we must compensate for tracer drift

13 Amplitude and propagation from tracer plots Example from Dn 500 m impact Measure amplitude (line is 1/r slope), velocity, wavelength and period

14 Wave amplitude declines significantly faster than 1/r (measured indices range from to -1.3) Only for asteroids > 1km diameter is an ocean-wide tsunami a significant hazard (ignoring seafloor topography). There are other reasons to fear smaller asteroids!

15 Impact tsunamis are slower than “shallow-water” waves, and their periods are short compared to earthquake tsunamis Shallow water wave speed is √(gdepth) ~ 220 m/s Fe 1000 Dn 1000 Fe 500 Dn 500 Fe 250 Dn 250

16 The mass of water displaced scales directly with the asteroid kinetic energy A fraction (~5-20%) of this mass is vaporized in the initial encounter

17 Summary SAGE is a sophisticated CAMR hydrocode developed for large parallel simulations under ASCI - collaborations are invited! SAGE may prove useful for determining important dynamical effects of major asteroid impacts Risk of ocean-wide tsunami damage from asteroids < 500 m has been overstated

18 3-D Simulations of “Dinosaur-Killer” asteroid impact Impactor is 10-km diameter granite sphere at 15 km/s Kinetic energy ~ 300 Teratons Horizontal extent of comp volume 256 km x 128 km Vertical strata in comp volume 100 km US standard atmosphere 100 m water 3 km calcite 30 km granite 18 km mantle Performed with AMR code RAGE (LANL & SAIC) on ASCI Q G Gisler R Weaver M Gittings 45˚ impact

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