Modeling of X-ray fluorescence detection by MIXS from the surface and sodium exosphere of Mercury Thesis submitted for the degree of Doctor of Philosophy at Aberystwyth University Rose Cooper Supervised by: Professor Manuel Grande & Dr Heather McCreadie Faculty of Business and Physical Sciences April 2020 Mandatory Layout of Declaration/Statements Word Count of thesis: 37403 DECLARATION This work has not previously been accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. Candidate name Rose Cooper Signature: Date 29/04/2020 STATEMENT 1 This thesis is the result of my own investigations, except where otherwise stated. Where *correction services have been used, the extent and nature of the correction is clearly marked in a footnote(s). Other sources are acknowledged by footnotes giving explicit references. A bibliography is appended. Signature: Date 29/04/2020 [*this refers to the extent to which the text has been corrected by others] STATEMENT 2 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter- library loan, and for the title and summary to be made available to outside organisations. Signature: Date 29/04/2020 i Abstract This thesis concerns the study of Mercury’s surface and exosphere using the Mercury Imaging X-ray Spectrometer (MIXS) onboard BepiColombo, focussing on predicted X-ray fluorescence observations of different regions of Mercury and achieving the limits of what MIXS will be capable of over the course of the mission. X-ray fluorescence (XRF) spectroscopy is a commonly used technique for elemental analysis of a target material; however, it is typically reserved for a laboratory setting with a man-made X-ray source. Due to Mercury’s proximity to the Sun, Solar X-rays can act as the X-ray source. This becomes less practical at greater distances from the Sun due to the flux following the inverse-square law. Because of this, XRF has only been used on missions to the inner solar system such as the Moon and Eros. The Mercury Imaging X-ray Spectrometer (MIXS), which launched on board the ESA/JAXA BepiColombo mission to Mercury in October 2018, uses microchannel plate optics to focus incoming X-rays onto a DEPFET micropixel detector. This allows for improved spatial and spectral detections over previous detectors, as well as imaging in X-ray wavelengths. Chapter 3 aims to investigate how the improvements to the spatial and spectral detections can be fully utilised when observing dayside fluorescence at Mercury. To do this, a program initially created by Bruce Swinyard to model X-ray fluorescence from the Lunar surface is adapted to instead reflect the Mercurian environment. X-ray fluorescence spectra for both MIXS-C and MIXS-T are produced for a range of terrane compositions present at Mercury during different solar conditions. Using the intensity of the detected fluorescence for the lighter elements such as sodium, magnesium, aluminium, and silicon it is possible to determine the type of geochemical terrane being observed by MIXS-C based on the fluorescence detected. In Chapter 4 expands on what MIXS was primarily designed to achieve by investigating the possibility of observing X-ray fluorescence from the sodium exosphere of Mercury. A low- density gaseous target is used in place of the solid planetary surface that has been considered previously. Because of this MIXS-C will be the primary detector considered in this chapter due to its far superior effective area than that of MIXS-T. This high effective area is required to detect such a weak signal. Initial predicted detections give unfavourable spectra and require a high intensity X-class flare along with a prolonged observation time to obtain a significant ii detection. Two different observational orientations are considered to enhance the detections of X-ray fluorescence from exospheric sodium by focusing on high sodium density regions of the exosphere. Taking the limitations of the observations into account such as observation time and pointing angle, the detected fluorescence is too low to be considered significant. This result and its implications are discussed further. Chapter 5 returns to surface-based fluorescence emission, but instead now focussing on fluorescence on the nightside surface of Mercury. Using an energetic electron flux background from Ho, et al., (2011) in the place of solar X-ray flux, predictions of the detected X-ray flux are made for both MIXS-C and MIXS-T for several observation times and electron flux intensities. Following this a comparison between dayside and nightside fluorescence observations is conducted, with particular emphasis on terranes found close to the cusp regions of Mercury’s magnetosphere as that is where Particle Induced X-ray Emission (PIXE) events have been observed previously. It is found that PIXE events produced increased fluorescence flux for heavier elements such as calcium and iron, whereas solar insuded fluorescence is better suited to lighter elements such as sodium and magnesium. The results of this comparison, and the impact they will have on the BepiColombo mission as well as our understanding of Mercury as a whole are discussed futher. Despite being one of the terrestial planets, Mercury is one of the planets in the solar system that is currently the least understood. The work in this thesis aims to fully explore the possibilities of detections that MIXS is capable of to maximise its scientific output upon its arrival at Mercury in December 2025. iii Acknowledgments Thanks to my supervisor Manuel Grande for giving me the opportunity to pursue this PhD and develop my academic career further, as well as my second supervisor Heather McCreadie for providing additional academic assistance. From a more pastoral perspective, thanks to Huw Morgan and Maire Gorman for general advice and support, especially during the latter part of my PhD. I would also like to thank Emma Bunce and Adrian Martindale from University of Leicester for providing MIXS calibration data to aid accuracy of the program. I would like to thank Benjamen Reed, who despite also working his own PhD, and in the final year also a full-time job, managed to still have the time to support me through my PhD and give me the confidence to reach the end. Also, thanks to my family for letting me put off getting a “real job” for a couple more years and pretending to understand what I do when I talk about it. I would also like to thank the other PhD students of both the solar and materials research groups for all the coffee mornings, pub visits, Dungeons and Dragons’ campaigns, game nights, and general meme sharing over the past 4 years. You all made the time I have spent in Aberystwyth incredibly fun and are a large part of what made my PhD so enjoyable. Special thanks to Dr Joseph Hutton for being an IDL wizard and putting up with my general stupidity, and along with Dr Zoe Lee-Payne being my PhD guides and giving me research advice during numerous floating-point meetings or over a box of wine! Of course, I must thank physics team. You guys are part of what gave me the confidence to do a PhD in the first place, and your continued friendship is something I value greatly. Special thanks to Carley and Kirsty for listening to me when things weren’t going well, giving me advice when I needed it and always telling me it would be okay in the end. And finally, I wish to extend the greatest of thanks to the BepiColombo team. This mission has been around for as long as I have been alive, and I am very grateful for having the opportunity to work on it, even if only very briefly. I have my fingers crossed that Bepi reaches Mercury in one piece, and I look forward to all the data that is sure to come from it! iv Table of Contents Abstract ...................................................................................................................................... ii Acknowledgments..................................................................................................................... iv Figures.................................................................................................................................... viii Tables ...................................................................................................................................... xvi Chapter 1 Mercury: A Brief History and Scientific Exploration ............................................... 1 1.1 Planet Mercury ............................................................................................................ 2 1.1.1 Surface of Mercury .................................................................................................... 4 1.1.2 Mercury’s surface bound exosphere .......................................................................... 4 1.1.2.1 The Sodium Exosphere ...................................................................................... 6 1.1.3 Mercury’s magnetosphere ......................................................................................... 7 1.2 Previous missions to Mercury .......................................................................................... 8 1.2.1 Mariner 10 ................................................................................................................. 8 1.2.2 MESSENGER ......................................................................................................... 11 1.3 BepiColombo ................................................................................................................