![]() ![]() The amount of porosity generated by in situ fragmentation, however, has not been determined. This dynamically fragmented material represents a large volume within the lunar crust that should have some porosity 19. The rarefaction wave can produce strong tensile stresses and high strain rates, triggering dynamic fragmentation 18. The succession of a shock and rarefaction sets up the flow that excavates the crater 17. During a hypervelocity impact an initial shockwave propagates into the projectile and target and is followed by a tensile rarefaction wave, or tensile relief wave, that propagates from free surfaces. Here we show that appreciable porosity deep in the lunar crust and outside of crater rim crests is likely the result of in situ tensile fragmentation during impacts. While the porosity of the upper few kilometers can be explained by ejecta deposition, another mechanism is required to explain porosity found deep within the lunar crust, as the maximum thickness of large-scale basin ejecta is about 5 km closer to the basin’s rim 3, 14– 16. On the other hand, observations indicate that a large fraction of the porosity generated during impact events is created outside the crater rim crest 6, 14. ![]() Previous impact simulations with dilatancy demonstrated how shear deformation can create pore space throughout the crust inside the crater and in the ejecta blanket, but this process cannot produce significant porosity at depth outside the crater rim crest 12, 13. The spatial distribution of porosity in the lunar crust suggests that impacts are the likely source of porosity 6, 9– 11 however, it is unclear how impacts produce substantial porosity at depth and outside the crater. The uppermost ~4 km of the lunar highland crust has an average crustal porosity of 12%, and this porosity decreases to 4% at depths of 20 km 6, 8. ![]() NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission provided an unparalleled look into the interior structure of the Moon revealing that the lunar crust is much less dense and therefore more porous than previously thought 6, 7. The Moon provides a unique window into the evolution of ancient planetary crusts because its early porosity has mainly only been affected by subsequent impacts, especially on the lunar farside which has experienced less widespread volcanism than the nearside. The result of these endeavours was a versatile hydrocode, now known as SALEB, capable simulating impact events from first contact of the impactor with the target, to cessation of the final gravity driven collapse of the crater.Understanding the origin and evolution of planetary crustal porosity is of particular interest because crustal porosity has a strong effect on thermal, magmatic, and hydrothermal processes occurring early in planetary history 1– 5 as well as on the prospect of habitable niches outside of Earth. Concurrently, Ivanov substantially advanced SALE’s underlying solution algorithm by incorporating free-surface and material-interface tracking in Eulerian mode, greatly improved the constitutive model by incorporating (among other things) damage accumulation and strain-weakening, and implemented into the code the semi-analytical equation of state ANEOS. Melosh began improvements to the code by implementing an elasto-plastic constitutive model in tandem with the viscous model, and incorporated the Grady-Kipp fragmentation algorithm and several equations of state for impacts, including the Tillotson equation of state. The original SALE code was capable of simulating only single-material, Newtonian-fluid flow. iSALE includes extensions, corrections and enhancements to the original SALE code made by several workers since the early 1990s. ISALE (impact-SALE) is a multi-material, multi-rheology shock physics code based on the SALE hydrocode (Simplified Arbitrary Lagrangian Eulerian1). ![]()
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