Large impacts early in the lifetime of a terrestrial planet can transform its surface environment and interior composition, with the potential to forever change the evolutionary pathways available to that world. The chemically reduced metallic iron cores of these large impactors are key to such processes, as are the vast sums of energy that such impacts deliver.

In this thesis, we explore the environments of such scenarios in the aftermath of impacts, modelling the chemical state of the interacting atmosphere and impact- generated silicate melt. We analyse the production of reduced atmospheric species (e.g., H₂, CH₄, NH₃), which are important to proposed prebiotic chemical pathways. We also analyse the geochemical signatures that such impacts leave in the planet’s mantle, namely in the form of highly-siderophile elements (HSEs), which are commonly cited as evidence for such impacts having occurred on Earth in the time after the Moon-forming impact. We find that the nature of how the impactor core accretes to the impact-generated melt (i.e., as large chemically inaccessible blobs, or as more accessible droplets) is an important but as of yet unanswered question in determining the consequences of these impacts.

To answer this question on the fate of impactor core material, we perform impact simulations using the shock physics code, iSALE, with the primary purpose of determining where impactor metal accretes to within the planet, or indeed escapes the planet altogether. The inclusion of material strength is vital in the ability of these simulations to characterise the accretion process in ways that previous methods have been unable to. From these simulations, we formulate parametrisations of impactor core accretion as functions of impact parameters (mass, velocity, angle), advancing the limited such information currently available in the field for this regime of impacts. We demonstrate that impacts can be divided into two accretion modes: those that can generate a melt column down to the planet core, and those that cannot, and that the consequences of each impact mode are importantly distinct. We thus show that previous estimates of HSE retention in the postimpact mantle are overestimates, with greater mass fractions of the impactor core merging with the planet core than previously thought.

Finally, we combine our simulations with experimentally derived laws governing the interaction between molten metal of the impactor core and the silicate melt through which it sinks. We find that such interactions are inefficient at depositing impactor metal into the planet’s mantle, leaving the rainout of vaporised impactor core material onto the planet surface as the predominant pathway for the retention of such material by the planet mantle. The liquid fragmentation of sinking molten metal, followed by the dissolution of the generated droplets, is an additional mechanism, but with a more minor role played. Therefore, in the absence of alternative proposed mechanisms for retention of impactor core material, we call into question the commonly cited evidence that the present-day abundances of HSEs in Earth’s mantle are geochemical proof of particular masses of large impactors on early Earth, thus challenging the currently held view of Earth’s Late Accretion, with implications for the evolution of the Solar System as a whole.