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Modeling Complex Crater Collapse. Gareth Collins and Zibi Turtle Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. Motivation. To summarize the current state of numerical modeling of complex crater formation. - PowerPoint PPT Presentation
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Modeling Complex Crater Collapse
Gareth Collinsand
Zibi Turtle
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
Motivation• To summarize the current state of numerical modeling of complex crater formation.
• To highlight the major avenues for further research, both observational and in modeling.
Overview
• Why model impact crater collapse?
• The fundamentals of modeling collapse
• Dynamic rock strength during an impact
• Major results from modeling collapse
• Where to go from here?
Why model crater collapse?
Why model crater collapse?
• No direct observation of crater collapse.• Laboratory and nuclear test experiments of
limited use to study of crater collapse.• Means of studying dynamics of large crater
collapse.• Best instrumented experiment.
Fundamentals of Modeling
Importance of the initial conditions
• Late stage impact cratering is a very different process to the contact-early excavation stage – sensible to model separately.
• Z-model with static starting conditions is an approximation: not appropriate in all cases.
• Late stages controlled by gravity and strength – need to model strength differences.
• Final crater-transient crater scaling laws not always appropriate either.
Fundamentals of Modeling
Importance of the Constitutive Model
• Crater collapse is controlled by the competition between gravity and the strength of the target.
• The constitutive model describes the response of a material to deformation.
• It combines the concepts of:– Elasticity (strain proportional to stress)– Plasticity (elastic until yield stress)– Fluid flow (strain rate a function of stress)
Constitutive Model Used in Impact Simulations
Target Rheology
• The most commonly used constitutive model for rock material is elastic-plastic.
• Yield strength is a function of pressure:
• Damage:• and internal energy (temperature):
Coulomb-Von Mises model
Target Rheology
Pressure
Yield strength
Cohesion
D = 0
D = 1
Current strength models do not allow sufficient collapse
Target Rheology
• For standard strength models of rock materials, the transient crater is stable in a gravity field.
• First determined using analytical modeling by Dent (1973), then by Melosh (1977) and McKinnon (1978).
• All numerical modeling work echos this result.
Standard Strength Model
Target Rheology
Movie courtesyof Boris Ivanov
Target Weakening Facilitates Crater Collapse
Target Rheology
Movie courtesyof Boris Ivanov
Something is missing from current strength models
Target Rheology
• Some form of temporary target weakening is required to facilitate collapse.
• Candidates include:– Fragmentation (during shock release or deformation) – Heat (shock or friction melting, thermal softening)– Pressure vibrations (remnant from passing shock)– Dynamic weakening (bulking, strain localization)
Modeling has constrained the required weakening effects
Target Rheology
• The target’s strength must be reduced by an order of magnitude or more.
• A volume of material at least equivalent to the transient crater volume must be weakened.
• The weakened material must be mobile enough to overshoot the target surface (<109 Pa-sec, for largest terrestrial crater).
Modeling has constrained the required weakening effects
Target Rheology
• For external ring formation in multi-ring basins there is an additional constraint.
• There must be a weak, mobile layer at depth (Melosh and McKinnon, 1978).
• Supported by numerical modeling (Turtle, 1998) and analogue modeling (Allemand and Thomas, 1999).
Major recent results
Results
• Melosh and Ivanov, 1999• O'Keefe et al., 2001• Collins et al., 2002• Ivanov and Artemieva, 2002• Shuvalov et al., 2002• Turtle, 1998 • Allemand and Thomas, 1999
Model for Peak-Ring Formation
Results
Model for Peak-Ring Formation
Results
Model for Peak-Ring Formation
Results
Model for Peak-Ring Formation
Results
Model for Peak-Ring Formation
Results
Model for Peak-Ring Formation
Results
Peak-Ring Formation Model Supported by Seismic Data
Comparison with observations
• Peak ring identified as a topographic high at ~40km radius, reaching a max. height ~500m.
slump blocks
Weak reflector• Weak, shallow-dipping
reflector beneath peak-ring
• Peak ring overlies the base of the slump blocks
Peak ring
Peak-Ring Formation Model Supported by Seismic Data
Comparison with observations
• Peak-ring formation due to the collision between the two regimes:
Inward collapse of transient crater
Outward collapseof central uplift
• Inwardly collapsing crater rim
• Outwardly collapsing central uplift.
Subsurface Structure Model for Generic Peak Ring Crater
Results
Fate of the Melt?
Results
Simulations by Boris Ivanov
Fate of the Melt?
Results
Chicxulub Formation Model(Courtesy of Dugan O’Keefe)
Results
Chicxulub Formation Model
Results
Chicxulub Formation Model
Results
SummaryResults
Results
Key Results• Collapse requires temporary weakening:
– Order of magnitude reduction in strength.– Volume of weakened material > Vtc
– Material mobile enough to overshoot surface.– External rings also require mobile sub-surface layer
• Significant central structural uplift ~ 0.1D• Modeling suggests “over-thrusting” model for
peak-ring formation.• Majority of melt lies within the peak ring.
Further Work
What is the weakening mechanism?
• Current state of modeling cannot distinguish between weakening mechanisms.
• How can one distinguish between these mechanisms in the field?
• More experimental work needs to be done to understand dynamic rock strength!
How can we test the models?
Further Work
• Best test is still morphometry.• Need to test peak-ring and structural-uplift
models with geological, geophysical and drill core data.
• Test predictions of damaged region dimensions.
• Test predictions of melt volume and distribution.
How can we test the models?
Further Work
• Need for code benchmarking.
• Test problem comparison for early-stage calculations.
• Compare strength models in late-stage codes.