Gray, Zuri; (2025) Polarimetry as a Remote Sensing Tool for the Investigation of Small Bodies of the Solar System. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

I explore the polarimetric properties of small bodies of the Solar System, focusing specifically on asteroids and comets. By analysing the polarisation of light scattered by regolith surfaces or dust particles in cometary comas, we can gain crucial insights into their physical properties, often unobtainable through traditional methods. My original contribution to the field include the observation, reduction and analysis of polarimetric data of asteroids and comets, along with their interpretation based on light scattering studies. These studies include aperture polarimetry, the analysis of polarimetric phase curves, as well as polarimetric and imaging maps. I begin with a review of literature and theory of polarimetric studies, highlighting inconsistencies in the polarimetric maps of comets and their interpretation. I then discuss the experimental light scattering studies conducted at the Instituto de As trof´ısica de Andaluc´ıa, relevant to polarimetric interpretations, and my contribution to the testing of a new levitator device in these experiments. After presenting the principles and methods of polarimetric observations at large telescopes, I discuss in detail the data reduction techniques I developed throughout my PhD to ensure correct analysis of raw polarimetric data. My first science contribution involves polarimetric observations of comet 67P/Churyumov-Gerasimenko. The quality of these measurements are, by far, higher than any previous cometary observations. Our measured polarimetric phase curve is in good agreement with previous studies of 67P, and is consistent with that of other Jupiter Family Comets (JFCs). Comparing the phase curves of the pre- and post-perihelion data, we find only a small difference between the two stages, pro- viding marginal evidence to the evolution in dust properties. In our imaging maps, we detect various linear structures in the inner coma, produced by localised areas of cometary activity. Despite this, we find a homogeneous spread of polarisation throughout the coma and tail, which contrasts with previous studies. Based on light scattering modeling by Markkanen et al., 2018, we attribute the result to the preva lence of dust grains in the geometric optics regime whose polarisation converge to a single curve irrespective of particle size. Further, we explore the consequences of image misalignments on both polarimetric maps and aperture polarimetric measure ments, potentially providing an explanation for some inconsistencies in published polarimetric maps of comets. My second science contribution involves polarimetric observations of Didymos Dimorphos, the target of the Double Asteroid Redirection Test (DART). As well as causing a change in Dimorphos’ orbital period, the impact caused a massive dust cloud to be ejected from the surface. Our polarimetric observations of Didymos Dimorphos show a dramatic drop in polarisation following the impact, suggesting differences between the material in the ejecta and that on the original regolith sur face. We interpret this drop to be due to the ejected particles being smaller and/or brighter than those on the pre-impact surface. Remarkably, even months after the impact, the polarisation remained persistently lower than pre-impact, suggesting the presence of residual ejecta material still within the system months after the impact. We propose two hypotheses to explain this: either a layer of fresh material settled on the surface, forming a thin ’dust-blanket’, or there was still a lingering dust cloud in orbit around the system at the time of the observations. Finally, we confirm that the polarimetric characteristics of Didymos-Dimorphos resembles that of an S-type asteroid, and we explore some explanations for small variations in polarisation.

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