Galen Gisler, Robert Weaver, Charles Mader

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Two- and Three-Dimensional Simulations of
Asteroid Ocean Impacts

• Galen Gisler, Robert Weaver, Charles Mader
• LANL
• Michael Gittings
• SAIC
• LPI Impact Cratering Workshop
• February 7, 2003

LA-UR-02-1453
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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?

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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

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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

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Physics included in simulations

• Fully compressible hydrodynamics
• AMR resolves shocks contact discontinuities
• Godunov - Riemann solvers track characteristics
• 2nd-order in space, close to 2nd -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

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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

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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

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3-d simulation of oblique water impact
Maximum cavity
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Density visualization in 45 water impact
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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.
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Wave Dynamics Inferred from Tracer Particles

• Example from Fe 1000 m
• The particle motion is clearly not that expected
  for a simple wave

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Wave Dynamics Inferred from Tracer Particles

• Example from Dn 250m
• Here the motion is relatively simple, though we
  must compensate for tracer drift

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Amplitude and propagation from tracer plots

• Example from Dn 500 m impact
• Measure amplitude (line is 1/r slope),
• velocity, wavelength and period

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Wave amplitude declines significantly faster than
1/r(measured indices range from -2.25 to -1.3)

• Only for asteroids gt 1km diameter is an
  ocean-wide tsunami a significant hazard (ignoring
  seafloor topography).
• There are other reasons to fear smaller asteroids!

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Impact tsunamis are slower than shallow-water
waves, and their periods are short compared to
earthquake tsunamis

• Shallow water wave speed is v(gdepth) 220 m/s

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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

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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
  lt 500 m has been overstated

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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 (grg_at_lanl.gov), R Weaver (rpw_at_lanl.gov),
  M Gittings (gittings_at_lanl.gov)

45 impact
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