Numerical modelling of large scale asteroid impact events: Crater formation on continental and oceanic targets, environmental pertubations and code development
Final Report Abstract
The impact-scarred surface of the moon testifies to the violent history of the terrestrial planets and their satellites. Despite the apparent importance of impact processes a quantitative understanding of the formation of crater structures is still lacking. Numerical modelling is the key to filling this void in current understanding. The goal of the project was (1) to develop an appropriate model to simulate impact crater formation and (2) to use the model for "numerical experiments" aiming at quantifying the crater formation process in several respects. Prior to the project the available code iSALE was only capable to model axisymmetric vertical impacts on a two-dimensional numerical grid. However, the vast majority of impacts on planetary surfaces occur at an angle of incidence, most likely 45°. Moreover, important material properties such as porosity and brittle fracturing were not taken into account, previously. To eliminate those shortcomings the impact model iSALE was completely revised and most importantly extended to 3D. Additionally, a porosity compaction model was implemented to account for the effect of open pore space in the target rocks. The code was rigorously tested against laboratory experiments and other numerical models. The newly developed model was made available for a broad user community for a variety of applications beyond modelling of impact processes and it is planned to distribute the model as "open source" package in the future. The project was subdivided into four major application fields: (a) An extensive parameter study has been carried out addressing the question how much energy is required to form a crater of a given size. In particular so-called scaling laws relating the properties of the projectile (velocity, density, size, angle) and the target (density, cohesion, coefficient of friction, porosity) with the size (diameter, depth, volume) of the resulting crater were improved. The results enable for the first time to predict crater size as a function of material properties (friction, porosity, cohesion) and angle of impact. Additionally, the effect of the impact angle on crater morphology and potentially detectable asymmetries was investigated and compared with the terrestrial and lunar crater record. The numerical models show that a critical impact angle for the formation of elliptical craters varies between approximately 10°-30°' and depends on material properties and the size of the event. For larger angles crater morphology appears at first glance relatively symmetric which is in agreement with observations on planetary surfaces. However, clear evidence for the direction of impact can be derived from the asymmetric rise of the crater floor and inwards slumping and folding of strata during the formation of a central peak. (b) Porosity was identified as an important material property of the target affecting significantly shock wave propagation and, thus, crater formation. The latter was investigated in (a). The crushing of pores due to shock compression results in a faster attenuation of the waves and enhanced post-shock heating due to the additional amount of plastic work consumed by the closure of pores. The enhanced shock heating in pores materials results in an increased melt production which was quantified for the first time in the framework of the project. (c)The geophysical signature at impact craters resembles the structural deformation beneath impact craters and fracturing and brecciation of rocks induced by the passage of the shock wave creating open pore space between fragments due to shear bulking (dilatancy). Numerical models of the formation of the Waqf as Suwwan crater (Jordan) and the Ries crater (Germany) were used as constraints to model the observed gravity anomalies. Besides structural information such as the amount of stratigraphic uplift the models provide the extent of the zone where brittle fracturing occurs. Further quantification of the increase in porosity due to shear bulking as a function of plastic strain could not be implemented successfully; however a promising approach was developed that will be further investigated in the future. (d)The strike of a meteorite in one of the ocean basins is the most likely scenario for future impacts on Earth. As an additional threat the generation of extreme tsunami waves was identified. The numerical models were used to simulate the generation and propagation of impact induced waves. In comparison to earthquakes the impact-generated waves are very steep with an order of magnitude higher amplitudes, they attenuate relatively quickly due to dispersion, and may break during shoaling far away from the coastline. These facts led to the conclusion that the tsnuami hazard by meteorite impacts was previously overrated. The numerical models provide attenuation rates as a function of the relative water depth and projectile size. It was also noticed that impact-waves have much more in common with tsunamis generated by landslides and that the same modelling technique (the iSALE model) is also applicable to simulate gravity driven mass movements generating waves. In summary the outcome of the project are (1) significant improvements in numerical modelling as an important tool to study impact cratering and (2) advances in understanding and quantifying impact processes on continental and oceanic targets.
Publications
- (2008). Numerical modelling of impact melt production in porous rocks. Earth and Planetary Science Letters 269, 529-538
Wünnemann K., Collins G.S., Osinski G.R.
- (2009). Hybrid modeling of the mega-tsunami runup in Lituya Bay after half a century. Geophysical Research Letters 36
Weiss R., Fntz H. M., Wünnemann K.
(See online at https://doi.org/10.1029/2009GL037814) - (2009). Scaling of oblique impacts in frictional targets: Implications for crater size and formation mechanisms. Icarus
Elbeshausen D., Wünnemann K., Collins G.S.
(See online at https://doi.org/10.1016/j.icarus.2009.07.018) - (2010). The impact of a cosmic body in Earth's ocean and the generation of large tsunami waves - insight from numerical modeling. Reviews of Geophysics, 48
Wünnemann K., Collins G.S., Weiss R.
(See online at https://doi.org/10.1029/2009RG000308)
