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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