Fragmentation of Carbon-bearing Projectiles and the Effects on their Raman Spectra due to Hypervelocity Impacts

The term hypervelocity refers to something that is travelling at speeds in excess of a few km/s. Impacts within the Solar System generally occur at these speeds, hence they are referred to as Hypervelocity Impacts.

Typical impact speeds in the Solar System depends on their location. Within the main asteroid belt, the average impact speed is generally considered to be 5 km/s. Moving to impacts on Earth, the Moon and Mars, an asteroid average impact speed is approximately 22, 19 and 9 km/s respectively. Move to the outer Solar System and the average impact speed on Pluto is thought to be approximately 2 km/s

Generally, research into hypervelocity impacts looks into cratering and the ejecta from these craters. However, the fate of the projectile is relatively neglected. Hence, this topic was explored in this thesis. To achieve this, experiments were performed using the University of Kent's two-stage light gas gun.

Along with a mechanical effect on the projectile material, the propagating shock wave can cause effects on the molecular structure. This can be investigated using Raman Spectroscopy, which is an inelastic scattering effect resulting from the laser light interaction with the molecules of the sample. By comparing a before and after spectrum of an impacted material, it is possible to determine the effects of shock pressure.

An important biomarker is carbon. The Raman spectra of carbon often contain a D (disorder) and G (graphite/order) band. The amplitude and area ratios of these two bands denote the structural organisation of the carbon-bearing materials. Impacting these materials can affect the Raman spectra. Changes in the spectra can reflect the effect shock pressures have had on the molecular structure of the material.

Firstly, the mechanical effect of a hypervelocity impact was investigated for basalt and shale. The materials were filed into 1.5 mm cubes. These cubes were then fired into water at speeds up to 6.13 km/s (peak shock pressure of 30.9 GPa). The water was then filtered through a 0.1 um filter membrane and a scanning electron microscope used to image the entire filter paper. ImageJ was then used to analyse the fragments. From this, information on the morphology, cumulative fragment size distribution, survival percentage and the energy density at the catastrophic disruption threshold are obtained. Catastrophic disruption is where the shock wave of an impact is sufficiently intense that the largest fragment is equal to or less than 50% of the original mass of the body (exactly 50% is the threshold limit). Over 400,000 fragments were measured per shot, providing fragment sizes down to 10^-3 of the original projectile size. When excluding the partially disrupted projectiles (impacts not sufficient enough to surpass the catastrophic disruption threshold limit), the average semi-minor to semi-major axis ratio (b/a) for basalt and shale were 0.58 +/- 0.16 and 0.59 +/- 0.14 respectively. This suggests that the difference in morphology does not have an effect on this ratio at higher impact speeds. From the results an estimate of percentage survival at Pluto, the Moon, in the asteroid belt and on Mars is 76 +/- 11 %, 39 +/- 8 %, 17 +/- 5 % and 10 +/- 4 % respectively. It was found that for basalt and shale the catastrophic disruption energy density was (24.0 +/- 2.1) x 10^4 and (9.4 +/- 5.0) x 10^4 J/kg respectively.

The work then moved on to investigating the effect of the shock upon the fragments of the projectile. An additional material (graphite) was used with basalt and shale. The materials were shot using the same method used to investigate the mechanical effect of the hypervelocity impact. Pre-shot the projectile was mapped using a 532 nm laser in a Raman spectrometer. These spectra were then compared to the spectra of 40 separate randomly chosen fragments in each shot. From this, it is possible to determine the shock pressure effects of the impact. Although no trends were identified positive shifts were observed for the D band peak position for basalt and shale, the G band peak position of basalt experienced a positive shift while graphite experienced both a positive and negative shift. Additionally, the G band width for basalt and shale experienced an absolute narrowing of 12.2% and 8.1% respectively, while graphite exhibited an absolute broadening of 17.6%. Overall, all the materials displayed an increased structural disorder after impact, as suggested by plotting the Raman spectra R1 and R2 values. These are ratios of the D and G band amplitudes (R1) and the bands' areas (R2). Furthermore, it was found from this work that there is a possibility of misinterpreting a sample when attempting to determine whether it is biotic carbon if from a shocked environment.

The samples were also subjected to static pressure up to a maximum pressure of 3.59 GPa using a diamond anvil cell, and heating/cooling (temperature range 173 to 773 K) using a Linkam temperature stage. This was done in order to ascertain the effects of flash heating upon Raman spectra during hypervelocity impacts. It was found that the effects of temperature are mostly opposite to the effect of shock and static pressure on a carbon Raman spectrum. Increasing static pressure led to the G band peak position for shale and graphite shifting a total of 19.2 and 15.0 cm^-1, at 3.48 and 3.23 GPa respectively. In contrast, for shale and graphite the D and G band peak position were shifted to lower wavenumbers at high temperature (>300 K), and to higher wavenumbers at low temperature (