Transcript 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

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
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
QuickTime™ and a A nimation decompressor are needed to see this picture.
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
800
600
height (m)
400
200
0
-200
-400
-600
-800
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49.5
50
50.5
51
51.5
52
52.5
53
53.5
distance from impact (km)
800
Example from Fe 1000 m
The particle motion is
clearly not that
expected for a simple
wave
600
height (m)
400
200
0
-200
-400
-600
-800
0
100
200
300
400
500
600
700
800
900
1000
time after impact (seconds)
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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
1.E+05
9.E+06
distance from impact (cm)
amplitude (cm)
8.E+06
1.E+04
1.E+03
7.E+06
6.E+06
5.E+06
max
min
4.E+06
3.E+06
2.E+06
1.E+06
1.E+02
1.E+05
0.E+00
1.E+06
1.E+07
1.E+08
0
200
distance from impact (cm)
Example from Dn 500 m impact
Measure amplitude (line is 1/r slope),
velocity, wavelength and period
400
600
800
time (sec)
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Wave amplitude declines significantly faster than 1/r
(measured indices range from -2.25 to -1.3)
10000
Dn25w tr
Dn25w lsq
Fe25w tr
Fe25w lsq
Dn50w tr
Dn50w lsq
Fe50w tr
Fe50w lsq
Dn1kw tr
Dn1kw lsq
Fe1kw tr
Fe1kw lsq
1/r
amplitude (m)
1000
100
10
1
1
10
100
1000
distance from impact (km)
Only for asteroids > 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
170
Dn 1000
160
Fe 500
velocity (m/s)
150
Fe 1000
Dn 500
140
Fe 250
130
Dn 250
120
110
100
80
100
120
140
160
180
200
period (seconds)
Shallow water wave speed is √(g•depth) ~ 220 m/s
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The mass of water displaced scales directly with
the asteroid kinetic energy
mass of water displaced (grams)
1.00E+19
1.00E+18
1.00E+17
1.00E+16
1.00E+26
1.00E+27
1.00E+28
1.00E+29
Asteroid kinetic energy (ergs)
• 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 < 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
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
• 3 km calcite
45˚ impact
• 30 km granite
• 18 km mantle
Performed with AMR code RAGE
(LANL & SAIC) on ASCI Q
• G Gisler ([email protected]), R Weaver
([email protected]), M Gittings
([email protected])
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