3D Numerical Modelling of Impact Cratering in the Solar System

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

The violent collision of asteroids and comets with solid planetary surfaces is a fundamental and ubiquitous process in the solar system. In the early history of our Solar System, small rocky particles collided and accreted to form larger and larger bodies until they grew into the planets we see today. For the Earth, the last of these planetary-scale collisions was a giant impact that ultimately formed the Moon. More recently in solar system history, a catastrophic impact event caused a mass extinction on Earth, including the demise of the dinosaurs. Today, the impact of an asteroid or comet poses a real, if poorly understood threat to humanity. Impact craters are also used to study the solar system. Counts of the number of impact craters on a solid planetary surface are used to distinguish older from younger terrains, and the shapes of large craters provide clues to the near-subsurface structure of a planet. The catastrophic potential of impact cratering, and its far-reaching consequences, make it imperative to understand exactly how the impact process works. Central to this are fundamental equations that relate the size of an impact crater to properties of the impacting object (for example, size and velocity) and conditions on the target body (for example, gravity). In other words, equations that give the answer to the fundamental question: how large will the crater be if a given impactor strikes a given target surface? At present we cannot adequately answer this question, because the equations do not properly include all the important impactor and target-material properties. The first aim of this proposal is to better constrain the relationship between all the important variables in an impact event using computer models. As these equations are fundamental tools for estimating the consequences of impact, this work will advance understanding in many areas of planetary science. The major missing pieces in these impact equations are an understanding of how the growth of the crater is affected by pore space in the target, and the angle at which the impactor strikes the target. The effect of impact angle is important to establish because well-studied vertical impacts are much less likely to occur than impacts at an angle greater than 30 degrees to the vertical, about which far less is known. Impact angle is observed to affect the size and shape of the crater in laboratory impacts, but quantifying the effect in larger impacts can only be established through numerical modelling. Porosity is an important property of asteroids, comets and the near-surface of most Solar System bodies. It is known from laboratory experiments that target rocks with a high porosity reduce the volume of material expelled from the crater during growth, but these effects have not yet been properly quantified. The effect of porous compaction during impact, in particular, may have important implications for the evolution of the solar system. Planets grow by the collision of planetesimals. Quantifying the transfer of energy and momentum in such collisions is therefore of vital importance for understanding the thermal, chemical and physical evolution of the solar system. In most previous work and models, collisions during planetary growth were assumed to be exclusively low-velocity and/or between non-porous planetesimals. However, recent work suggests that early planetesimals had very high porosities (up to 80%), and the growth of planetary embryos would have stirred relative velocities between planetesimals to >1 km/s. Experiments and our own modelling work show that target porosity dramatically affects the consequences of impact: increasing heating and reducing ejected mass. A second aim of this work is therefore to use numerical impact simulations to quantify the effect of planetesimal porosity on heating, compaction and ejection during early planetesimal collisions and assess the implications of this for solar system evolution.

Publications

10 25 50
 
Description The violent collision of asteroids and comets with solid planetary surfaces is a fundamental and ubiquitous process in the Solar System. In the early history of our Solar System, small rocky particles collided and accreted to form larger and larger bodies until they grew into the planets we see today. For the Earth, the last of these planetary-scale collisions was a giant impact that ultimately formed the Moon. More recently in Solar System history, a catastrophic impact event caused a mass extinction on Earth, including the demise of the dinosaurs. Today, the impact of an asteroid or comet poses a real, if poorly understood threat to humanity. Impact craters are also used to study the Solar System. Counts of the number of impact craters on a solid planetary surface are used to distinguish older from younger terrains, and the shapes of large craters provide clues to the near-subsurface structure of a planet.
The catastrophic potential of impact cratering, and its far-reaching consequences, make it imperative to understand exactly how the impact process works. Central to this are fundamental equations that relate the size of an impact crater to properties of the impacting object (for example, size and velocity) and conditions on the target planet (for example, gravity). In other words, equations that give the answer to the fundamental question: how large will the crater be if a given impactor strikes a given target surface? Prior to this research, we could not adequately answer this question, because the equations did not properly include all the important impactor and target-material properties. A major output of this research is new, quantitative insight into the effect of impact angle, target strength, planetary curvature and porosity on impact crater size and shape, through the development and application of a new 3D numerical impact model: iSALE-3D.
As well as quantifying the effect of impact angle on crater size and shape, developing a 3D numerical impact model has allowed us to advance understanding of elliptical crater formation, binary asteroid impacts, which account for 15% of all asteroid impacts, and the effect of planetary curvature and deep internal structure on the formation of large impact basins, such as the Moon's South-Pole Aitkin basin. This insight has important implications for the dating of planetary surfaces and the effect of giant impacts on the early evolution of their target planet.
In addition, our research has highlighted the importance of pore-space compaction during impact for the evolution of the solar system. Planets grow by the collision of planetesimals. Hence, knowledge of how energy and momentum is transferred in such collisions is of vital importance for understanding the thermal, chemical and physical evolution of the solar system. In most previous work and models, collisions during planetary growth were assumed to be exclusively low- velocity and/or between non-porous planetesimals. However, it is now known that early planetesimals had very high porosities (up to 80%), and the growth of planetary embryos would have stirred relative velocities between planetesimals to >1 km/s. Our detailed numerical modelling work as part of this research has shown that target porosity dramatically affects the consequences of impact: increasing heating and reducing ejected mass. This has led to a complete re-evaluation of the importance of impacts in the thermal evolution of our solar system.
Exploitation Route As identified in the original KT plan, we have:
* provided access to the iSALE codes strictly for academic use in impact cratering (via the "subversion" version control system; subversion.tigris.org)
* provided access to the results of production 3D simulations and validation simulations for use in code comparison and verification of future model development
publicised as widely as we are able the existence and benefits of this resource for impact cratering
* maintained a web-based user group (http://isale-code.de/software/projects/isale), and encouraged input from all the principal related research groups worldwide

These activities were ongoing throughout the duration of the project.

Beyond providing access to iSALE, we have published both the technology and the science outcomes in the international peer-reviewed literature (see publications), and presented these same results at international conferences. In particular, the scaling equations derived from this research were disseminated to the scientific community via an upgrade to a popular interactive web program already in place. The upgrade to the web program was achieved between December 2009 and November 2010.

As identified in the KT plan, iSALE model development has also engaged industrial partners. 3D numerical modelling of dynamic impact processes has obvious and important industrial and defence applications.
Sectors Aerospace, Defence and Marine

 
Description AWE 
Organisation Atomic Weapons Establishment
Country United Kingdom 
Sector Private 
PI Contribution access to software and data
Collaborator Contribution significant intellectual input into research
Impact CASE student ship and PhD studentship on software development
 
Description Impact experiments 
Organisation University of Kent
Department School of Physical Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution Access to software, intellectual input into research
Collaborator Contribution access to gas-gun impact experiment data, performing impact experiments using their equipment
Impact One paper in preparation
Start Year 2009
 
Description Impacts 
Organisation Purdue University
Department Department of Earth, Atmospheric and Planetary Sciences
Country United States 
Sector Academic/University 
PI Contribution access to software, training of staff, significant intellectual input into research
Collaborator Contribution significant intellectual input into my research, access to software
Impact Several papers and conference abstracts; Impact: Earth! web page (impact.ese.ic.ac.uk)
 
Description LPI 
Organisation University of Arizona
Department Lunar and Planetary Laboratory
Country United States 
Sector Academic/University 
PI Contribution access to software, computer hardward, and significant intellectual input into your research.
Collaborator Contribution access to data, computers, training of staff and significant intellectual input into your research.
Impact 4 conference abstracts; three papers in preparation
Start Year 2009
 
Description Museum fur Naturkunde 
Organisation Natural History Museum
Department Department of Mineralogy
Country United Kingdom 
Sector Public 
PI Contribution access to data, computers, software development and significant intellectual input into our research.
Collaborator Contribution access to data, computers, software development and significant intellectual input into our research.
Impact Several papers; many conference abstracts; iSALE software for simulating impacts
 
Description Planetesimal evolution 
Organisation Planetary Science Institute - Arizona
Country United States 
Sector Charity/Non Profit 
PI Contribution access to data, software, and significant intellectual input into your collaborator/partners research.
Collaborator Contribution Own time and computational resources data
Impact One published paper; several conference abstracts; several papers in preparation
Start Year 2009
 
Description Planetesimal evolution 
Organisation University of Chicago
Department Department of the Geophysical Sciences
Country United States 
Sector Academic/University 
PI Contribution access to data, software, and significant intellectual input into your collaborator/partners research.
Collaborator Contribution Own time and computational resources data
Impact One published paper; several conference abstracts; several papers in preparation
Start Year 2009
 
Description Shock Physics 
Organisation Imperial College London
Department Institute of Shock Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Numerical model development; numerical simulation of shock wave propagation
Collaborator Contribution Funding of PDRA and PhD student for numerical model development
Impact Several conference abstracts and papers in preparation
Start Year 2009
 
Title iSALE shock physics code 
Description iSALE (impact-SALE) is a multi-material, multi-rheology shock physics code for simulating high speed impacts and other violent geophysical phenomena. iSALE includes constitutive and porous-compaction models specifically developed for impact simulations. The code is being continually developed, improved and maintained by research groups at the Museum für Naturkunde, Berlin and Imperial College London. 
Type Of Technology Software 
Year Produced 2006 
Open Source License? Yes  
Impact iSALE has been used in pioneering studies of the formation of large impact craters on the Earth and the influence of target property variations on crater formation, the influence of a water layer on crater formation, as well as investigating the mobility of large rock avalanches.The software has been extensively validated against laboratory experiments and used to show, for the first time in numerical simulations, the important effect of friction and porosity on crater growth in granular materials. 
URL http://www.isale-code.de
 
Description Chicxulub Impact 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact A Science article that I co-wrote, describing evidence that a meteorite impact was the cause of the KT mass extinction and the demise of the dinosaurs was widely reported by various media sources, including newspapers, TV and radio.

I received many emails from the general public with questions about the impact. In addition, my Impact Effects web page, received a large increase in visits.
Year(s) Of Engagement Activity 2010
URL http://www3.imperial.ac.uk/portal/page/portallive/people/g.collins/media