|Abstract: ||Understanding of basin-scale crater formation is limited; only a few examples
of basin-scale craters exist and these are difficult to access. The approach
adopted in this research was to numerically model basin-scale impacts with
the aim of understanding the basin-forming process and basin structure.
Research was divided into: (1) investigating early stage formation processes
(impactor survivability), (2) investigating later stage formation processes
(excavation and modification) and basin structure, and (3) constraining an
impact scenario for the largest lunar crater, the South Pole-Aitken Basin.
Various impact parameters were investigated, quantifying their effect on the
Simulations showed impactor survivability, the fraction of impactor remaining
solid during the impact process, greatly increased if the impactor
was prolate in shape (vertical length > horizontal length) rather than spherical.
Low (≲15 km/s) impact velocities and low impact angles (≲30 ) also
noticeably increased survivability.
Lunar basin-scale simulations removed a significant volume of crustal material
during impact, producing thinner post-impact crustal layers than those
suggested by gravity-derived basin data. Most simulations formed large, predominantly
mantle, melt pools; inclusion of a steep target thermal gradient
and high internal temperatures greatly influenced melt volume production.
Differences in crustal thickness between simulations and gravity-derived data
could be accounted for by differentiation of the voluminous impact-generated
melt pools, predicted by the simulations, into new crustal layers. Assuming
differentiation occurs, simulation results were used to predict features such as
transient crater size for a suite of lunar basins and tentatively suggest lunar
thermal conditions during the basin-forming epoch. Additional simulations
concerned the formation of the South Pole-Aitken Basin. By constraining
simulation results to geochemical and gravity-derived basin data, a best-fit
impact scenario for the South Pole-Aitken Basin was found.|