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Galen Gisler, Robert Weaver, Charles MaderLANL
Michael GittingsSAIC
LPI Impact Cratering WorkshopFebruary 7, 2003
LA-UR-02-1453
Two- and Three-Dimensional Simulations of Asteroid Ocean
Impacts
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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 kmStart asteroid 30 km above ocean surface
3-D oblique ocean impacts:Iron impactor, diameter 1000mVelocity 20 km/s at 45˚ and 30˚ elevationComputational volume 200 km x 100 km x 60 kmUp to 200,000,000 cells1200 processors on LLNL ASCI White machine1,300,000 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 1000mVertical impact, velocity 20 km/sComputational volume - cylinder 100km radius, 60 km heightUp to 1,000,000 cells, 10,000 cpu-hrs per run
8Maximum cavity
3-d simulation of oblique water impact
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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.
QuickTime™ and aAnimation decompressorare needed to see this picture.
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Wave Dynamics Inferred from Tracer Particles
Example from Fe 1000 mThe particle motion is
clearly not that expected for a simple wave
-800
-600
-400
-200
0
200
400
600
800
0 100 200 300 400 500 600 700 800 900 1000
time after impact (seconds)
height (m)
-800
-600
-400
-200
0
200
400
600
800
49 49.5 50 50.5 51 51.5 52 52.5 53 53.5
distance from impact (km)
height (m)
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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
1.E+02
1.E+03
1.E+04
1.E+05
1.E+05 1.E+06 1.E+07 1.E+08
distance from impact (cm)
amplitude (cm)
0.E+00
1.E+06
2.E+06
3.E+06
4.E+06
5.E+06
6.E+06
7.E+06
8.E+06
9.E+06
0 200 400 600 800
time (sec)
distance from impact (cm)
maxmin
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Wave amplitude declines significantly faster than 1/r(measured indices range from -2.25 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!
1
10
100
1000
10000
1 10 100 1000
distance from impact (km)
amplitude (m)
Dn25w trDn25w lsqFe25w trFe25w lsqDn50w trDn50w lsqFe50w trFe50w lsqDn1kw trDn1kw lsqFe1kw trFe1kw lsq1/r
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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 √(g•depth) ~ 220 m/s
100
110
120
130
140
150
160
170
80 100 120 140 160 180 200
period (seconds)
velocity (m/s)
Fe 1000Dn 1000
Fe 500
Dn 500
Fe 250
Dn 250
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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
1.00E+16
1.00E+17
1.00E+18
1.00E+19
1.00E+26 1.00E+27 1.00E+28 1.00E+29
Asteroid kinetic energy (ergs)
mass of water displaced (grams)
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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 < 500 m has been overstated
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3-D Simulations of “Dinosaur-Killer” asteroid impact
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
Impactor is 10-km diameter granite sphere at 15 km/s
• Kinetic energy ~ 300 TeratonsHorizontal extent of comp volume • 256 km x 128 kmVertical strata in comp volume • 100 km US standard atmosphere • 100 m water• 3 km calcite• 30 km granite• 18 km mantlePerformed with AMR code RAGE
(LANL & SAIC) on ASCI Q• G Gisler (grg@lanl.gov), R Weaver
(rpw@lanl.gov), M Gittings (gittings@lanl.gov)
45˚ impact
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