Investigation of impact crater processes using experimental and numerical techniques

• Main Author: Baldwin, Emily Clare
• Published: University College London (University of London) 2008
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.505116

Impact events throughout the history of the Solar System have occurred at all scales, from craters produced by the hypervelocity impact of cosmic dust observed on lunar return samples, to the giant planet-sculpting impacts that have shaped the solid bodies of the Solar System. Investigating the impact process in the laboratory allows us to understand crater formation at a small scale where strength effects dominate; however, it is difficult to scale directly to planetary sized impacts because gravity governs the cratering process at this large scale. Through computer modeling, it is possible to bridge the gap from small to large scale impact events. The influence of target porosity, saturation and an overlying water layer on crater morphology is investigated in the laboratory using a two-stage light gas gun to fire 1 mm diameter stainless steel projectiles at ~5 km s^{-1} into sandstone targets. Larger craters were formed in the higher porosity targets and saturated targets. A critical water depth of 11.6\pm 0.5 times the projectile diameter was required to prevent cratering in an unsaturated target, compared with 12.7\pm 0.6 for saturated targets. The sensitivity of this critical water depth to impact velocity, projectile diameter and density is examined through use of the AUTODYN numerical code, for both laboratory and planetary scale impact events. Projectile survivability into water and sand targets is investigated in the lab for stainless steel and shale projectiles impacting at 2-5 km s^{-1}; up to 30% of the projectile is found to survive. AUTODYN simulations shows that basalt or sandstone meteorites impacting a simulated lunar surface survive the impact at velocities <5 km s^{-1} and at a range of angles, which has positive implications for detecting terrestrial meteorites on the Moon. Groundwork has also been laid for the modelling of the deliberate collision of the SMART-1 spacecraft into the Moon. Finally, lunar and terrestrial impact events are simulated in order to quantify the depth of excavation as a function of transient crater diameter for a range of crater and basin sizes. The output is found to lie in the range 0.08-0.15, with the South Pole Aitken basin excavating material to a depth comparable to the thickness of the farside crust.