OBLIQUE IMPACT AND ITS EJECTA NUMERICAL MODELING

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OBLIQUE IMPACT AND ITS EJECTA NUMERICAL
MODELING

• Natasha Artemieva and Betty Pierazzo
• Houston 2003

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Content

• Oblique impact in nature and in modeling
• 3D modeling brief history
• Hydrocodes in use
• Melt production
• Fate of the projectile
• Distal ejecta tektites and martian meteorites

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Impact angle
Probability of the impact within the angle (?,
?d?) dP2sin? cos? d?
50 - (30? -60?) 7 - ( 0? -15?) 7
- (75? -90?)
Vertical impact (? 90?) - 0 Grazing impact (?
0?) - 0 Most probable angle ?45?
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Earth craters
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Elliptical craters on the planets
5-6 of the craters (Moon, Mars, Venus) Impact
angle lt 12?
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Asymmetrical ejecta
Venus, Golubkina, 30 km Magellan photo
Mars, small fresh craters Mars Global Surveyer
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3D Hydrocodes versus 2D

• More complex? Or simpler?
• Time and computer capacity expensive
• Widely used in impact modeling
• CTH Sandia National Laboratories
• SALE Los-Alamos National Laboratory
• SAGE Los-Alamos National Laboratory
• SOVA Insitute for Dynamics of Geospheres,
  Russia
• SPH various authors
• AUTODYN - commercial

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Shoemaker-Levy 9 Comet

• July 1994
• Impact velocity 60 km/s
• Impact angle - 45?
• 21 fragments
• Size, density - unknown
• Observations telescopes, HST, Galileo
• Modeling CTH, SOVA, SPH et al.

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3D modeling of fireball
Space Telescope Science Institute, 1994
Crawford et al., 1995
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Melt production comparison with geology
From Pierazzo et al, 1997
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Melt production
From Pierazzo and Melosh, 2000
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Ries real and model stratigraphy
Stoffler et al., 2002
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Melt for the Ries
150
5
50
Shock modified molten partially
vaporized
Stoffler et al., 2002
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Is it useful to geologists?

• Not all the melt remains within the crater
• What is the final state of the melt?
• What is the final crater?

More work is needed..
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Scaling for oblique impact
Vtr 0.28 ?pr/?t Dpr2.25g-0.65V1.3sin1.3?
Schmidt and Housen, 1987 Gault and Wedekind,
1978 Chapman and McKinnon, 1986
Dpr (sin?)-0.55
Ivanov and Artemieva, 2002
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Experiments and modeling (DYNA) for strength
craters

• increase of oblique impact cratering
  efficiency at higher velocities in experiments
  (Burchell and Mackay, 1998) and modeling
  (Hayhurst et al., 1995)

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Natural impacts high efficiencyLaboratory
low efficiency
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Projectile fate
From Pierazzo and Melosh 2000
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Distal ejecta

• Tektites
• Meteorites from other planets

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SNC size and shape
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Three stages for distal ejecta evolution

• Compression and ejection after impact
• disruption into particles
• flight through atmosphere and final deposition
  (or escape)

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Melt disruption into particles

• Pure melt ( 50 ltP lt 150 GPa) disruption by
  tension and instabilities.
• Particle size is defined by balance of surface
  tension and external forces.
• Particle size cm
• Two-phase mixture
• (P gt 150 GPa) partial vaporization after
  decompression
• Particle size is defined by amount of gas.
• Particle size - ?m mm.

Melosh and Vickery, 1991
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Particles in flight
Melt vapor - 700 Mt Ejecta - 540
Mt Tektites - 140 Mt Mtektites -
400 Mt
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Particles in post impact flow
u
ug
DRAG
ug
ug
GRAVITY
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First 2 s (trajectory plane)
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Moldavites first 20 s
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Trajectory in atmosphere
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Pressure-temperature along trajectory
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Strewn field
Real
Modeled
Deposited outside ejecta blanket 15
Mt Geological estomates 5 Mt
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Last minute results
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Initial stage
High-velocity unmelted material is ejected at the
stage of compression t Dpr/V
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Where are they from?
Excavation depth 0.1 Dpr Distance from impact
point 1.5 - 2 Dpr
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Ejection velocity vs. shock
No SNC without shock compression!
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Deceleration by atmopshere
Only particles with d gt20 cm may escape Mars
! Independent confirmation 80Kr (Eugster et
al., 2002)
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Impact conditions

• Impact velocity 10 km/s
• Impact angle 45
• Asteroid diameter 200 m
• Final crater 1.5 - 3 km
• Maximum particles size -1m

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Conclusions

• 3D modeling is becoming possible thanks to
  computer improvements
• We need 3D for
• scaling of impact events
• melt production estimates
• investigation of projectile fate
• vapor plume rising in atmosphere
• distal ejecta description

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Problems

• Computer expensive
• Spatial resolution limitations
• More physics is needed
• EOS

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Connection with observations

• Melt and its final distribution
• Shock effects in SNC meteorites
• Tektites strewn field

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Connection with experiments

• ?