Modeling of X-ray fluorescence detection by MIXS from the surface and sodium exosphere of Mercury

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 lowdensity 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 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.

Date of Award	2020
Original language	English
Awarding Institution	Aberystwyth University
Sponsors	Science and Technology Facilities Council
Supervisor	Manuel Grande (Supervisor) & Heather McCreadie (Supervisor)