Mars and Phobos Related Impact Processes: the formation and degradation of craters in the Northern Lowland Plains and the transfer of Martian material to the surface of Phobos

Despite extensive efforts to study Mars, with more than 40 spacecraft returning data from it to date, our red neighbour remains a subject of intense study within the field of planetary science. The work presented in this thesis focuses on the process of impact cratering at Mars and its moons, conducting experimental impacts using the one and two stage light gas gun at the University of Kent to provide insights into two topics: 1. The quantity of ice within the Northern Lowland Plains on Mars, and 2. The transfer of material to the Martian moon Phobos.

It has long been thought that the morphological features of an impact crater can be related to the surface properties of the target, including whether or not ice is present within it. Past experimental investigations into the influence of target ice content, however, have typically focused on ice-silicate mixtures in the range of 0−50 wt% silicate content. Whilst this may be appropriate for ice-rich bodies in the outer solar system, it leaves a wide range of silicate-dominated mixtures unstudied (ones which may be more appropriate for Mars for example). Here, 80 wt% JSC Mars-1 content targets are impacted with the Kent Light Gas Gun over the velocity range of 1 − 5 km/s with 1.5 mm diameter spherical copper projectiles. For comparison to this, targets with a 50 wt% content of either JSC Mars-1 or kiln dried sand are impacted over the same velocity range with either 1.5 mm copper of 1.5 mm basalt (for JSC Mars-1 targets) spherical projectiles.

The results of this showed that, whilst little difference is present when considering the size of craters formed in silicate-dominated mixtures compared to those formed in 50 wt% content targets, subtle differences could be observed between the interior morphologies of the produced craters. Additionally, Martian simulant craters formed by the impact of a copper projectile presented central pit features, forming craters similar in appearance to terraced craters on Mars. In contrast, those formed by a basalt projectile showed a linear-to-bowl shaped profile. This suggests that the formation of terraced craters of Mars may be due to the impact of a metallic (i.e., relatively dense) projectile, rather than the properties of the target surface.

To investigate the transfer of Martian material to Phobos, a series of shots was carried out using custom 3D-printed shells containing a europium-doped MGS-1 Martian simulant material. These were fired over a velocity range of 620−1620 m/s into cemented PCA-1 Phobos simulant targets. An ejecta collection system was designed for this series of shots with samples being collected from four shots, whilst two failed to collect a measurable quantity of ejecta. Projectiles are thought to have passed through three distinct stages of emplacement: at the lowest speeds projectiles rebounded producing highly contaminated ejecta, while at higher speeds the projectile was largely emplaced, decreasing the level detected within ejecta and at the highest speeds, projectile material was ejected along with target material. Collected ejecta showed the presence of projectile material with the quantity ranging from 260 − 7833 ppm. This indicates that the levels of Martian material planned to be detected on Phobos by the future MMX mission may be more localised than previously assumed.

The thesis then concludes by recommending improvements to the design of the various experiments reported, along with suggestions for how the work can be taken further to tackle the questions studied in more depth and detail.