Download - Continental vs Oceanic Asteroid Impacts
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Continental vs Oceanic Asteroid Impacts
Paul S De Carli1,2, Adrian P Jones2, and G David Price2
1 SRI International, Menlo Park, CA94025, USA [email protected] College London, Gower St, London WC1E 6 BT, United Kingdom
2004 GSA; Denver, Co; 8 Nov., T-74, 82-13
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Why Worry About Impacts?
• Consider the surface of the Moon
• For each lunar crater of diameter X, there have been ~ 20 terrestrial craters of that size. (Earth’s gravity helps)
• Impacts must be considered as possible drivers of geologic events on the Earth, particularly on the early Earth, when large impacts were more frequent.
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Possible Effects of Large Impacts
• Formation of large igneous plumes via decompression melting of mantle after excavation of overburden (controversial, but not impossible)
• Modification of plate motion (Speculative, no quantitative studies to date)
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Impact Statistics
• Lunar studies indicate decrease in impact flux ~ 3.5 GA ago.
• Relatively constant flux of impacts since then.
• Terrestrial large (200 km dia) crater forming events are rare, ~ 1/200 MA? Frequency is highly uncertain because corresponding Lunar impact statistic would be ~ 1/4 GA.
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Effects of Large Impacts
• Impact energy enormous, >10 MT (~1017 J)
for the ~1 km diameter Meteor Crater.
• Semi-empirical estimates of crater size, etc., extrapolated from cm-scale lab experiments and nuclear explosions.
• Hydrocode calculations of large impacts. Validity of calculation constrained by modeling and computational limitations.
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Continental vs Oceanic Impacts
• Dallas asked “Could there be a difference?”
• Possibly. One might expect the shock wave to attenuate more rapidly in Continental crust as a consequence of the hysteresis of the phase transitions of tectosilicates to denser phases. Because of the steep release adiabat, rarefaction waves would be faster in Continental than in Oceanic crusts.
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Granite Modeling Problems
• Hugoniot and release adiabats differ; known differences (µs-duration experiments) depend on peak pressure. Shock duration (s- duration cratering events) effects on phase transition behavior are unknown.
• As a first approximation use a compromise Hugoniot, intermediate between load and unload curves.
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Modeling Continental Crust
• Experimental data show qtz (granite) in dense structure (6-3 glass?) during release down to ~7 GPa.
• We use modified Hugoniot to model loading and release behavior.
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0.56 s after Impact
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0.56 s after Impact
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~1.2 s after Impact
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~3.2 s after Impact
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~5 s after Impact
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~12 s after Impact
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Close-in Pressure-time HistoriesTarget Points for histories
0.15 km below surface,
r = 0.15 km, 2.15 km, 4.15 km
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Mantle Pressure-time Histories
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Conclusions and Caveats I
• Peak Pressure >2X higher in mantle (32 km depth) for Oceanic impact than for Continental impact.
• Impulse (area under Pressure-time curve) appears larger for Continental impact.
• CAVEAT- The differences in Peak Pressure and Impulse might be smaller if one used detailed Hugoniot and Release Models.
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Conclusions and Caveats II
• Importance of using appropriate material models is demonstrated by these calculations.
• Present work was hardware and software limited; ~ 100 hrs/calculation on fast PC.
• CAVEAT- A supercomputer result will be no better than the material models used.
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Conclusions and Caveats III
• Conservative interpretation of present results- Suggestive of a possible large difference between Oceanic and Continental impacts.
• Present results too crude to be relevant to Hagstrom’s observations of antipodal craters or volcanism.
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Acknowledgements
Chris Quan, Richard Clegg and Bence Gerber of Century Dynamics for technical assistance.
PSD thanks Century Dynamics for supporting his use of Autodyn™.