Aeronomy of Mars: An Observational Study of the Dynamic Martian Upper Atmosphere Alexander Siddle Department of Physics, Imperial College London Submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy December 2020 Declarations I declare that all work presented in this thesis is my own, unless explicitly stated and referenced. The copyright of this thesis rests with the author. Unless otherwise indicated, its contents are licensed under a Creative Commons Attribution-Non Commercial 4.0 International Licence (CC BY-NC). Under this licence, you may copy and redistribute the material in any medium or format. You may also create and distribute modified versions of the work. This is on the condition that: you credit the author and do not use it, or any derivative works, for a commercial purpose. When reusing or sharing this work, ensure you make the licence terms clear to others by naming the licence and linking to the licence text. Where a work has been adapted, you should indicate that the work has been changed and describe those changes. Please seek permission from the copyright holder for uses of this work that are not included in this licence or permitted under UK Copyright Law. Alex Siddle - December 2020 The work presented in this thesis has contributed to two peer-reviewed published papers: • Chapter 5 - A. G. Siddle et al. (2019). ‘Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS’. in: Icarus 333, pp. 12–21 • Chapter 7 - A. G. Siddle et al. (2020). ‘Density structures in the Martian lower thermosphere as inferred by Trace Gas Orbiter accelerometer measurements’. In: Icarus, p. 114109 2 Abstract The Martian upper atmosphere is highly dynamic over both short and long temporal and spatial scales. Our understanding of this region primarily stems from a wealth of data gathered both in-situ and remotely from spacecraft orbiting the Red Planet. In 2014 NASA’s Mars Atmosphere and Volatile Evol- ution (MAVEN) spacecraft began orbiting Mars with the primary objective to probe and characterise the upper atmosphere using composition data. Data from the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board MAVEN has been utilised throughout this thesis. Daily, monthly and seasonal density and temperature variations have been explored. An apparent day-night asymmetry is observed in temperature with the dayside typically 50-100 K warmer due to solar-EUV heating. Seasonal analysis has found densities to be significantly enhanced around perihe- lion compared to other times throughout a Martian year. Perturbations in density and temperature profiles have been interpreted as vertically propagating gravity waves. Diurnal and seasonal variations of gravity wave characteristics have been examined with enhanced activity on the nightside. Fully un- derstanding these features is required if the complexities of the upper atmosphere are to be fully grasped. The Martian science community was fortunate to have a plethora of spacecraft at Mars during the June 2018 global dust storm. This thesis has examined the response of the upper atmosphere to such an important and rare event. A notable expansion of the atmosphere is observed whereby the upper atmosphere is raised by several kilometres, due to heating in the lower atmosphere. It is hoped that results can inform efforts made to model and predict the effects of a dust storm. For the first time, gravity waves at Mars during a global dust storm have been studied. Atmospheric perturbations are found to be significantly enhanced during the dust storm event. During 2017/2018, ESA’s Trace Gas Orbiter (TGO) undertook its aerobraking phase to circularise itself into its science orbit. During this period, density data were able to be retrieved in the lower ther- mosphere from accelerometer measurements. The retrieval process was not part of this thesis; however, these data are used for the first time. Standalone analyses are performed with these data but are also combined with results from MAVEN owing to the two spacecraft sampling similar regions concurrently. This overlapping period is exploited, and densities are hydrostatically connected to understand the en- tire thermosphere structure. By combining wave data, it has been inferred that shorter wavelengths are 3 saturated within the thermosphere, whereas larger wavelengths continue to grow with height. 4 Acknowledgements First and foremost, to Ingo. You taught me so much over the years from trivial things like atmospheric physics to the more essential skills in life, such as how to think like a scientist, and more importantly how to have a laugh whilst you’re doing it. I could not have asked for a more friendly, knowledgeable and humourous supervisor. The pleasure is all mine. Thanks must go to my original office mates – David, Ewen and Lars - who took me under their wing and provided much joy and humour throughout the many years we shared an office together. The arrival of the next cohort – Joe, Emma, Harry, Maks and Ned - saw us kicked out of our office. Truly unforgivable. Despite that, you’ve all added something unique to the research group, from the revival of SPAT walks to the introduction of MarsBallTM. A special mention must go to Joe, who followed my path from Lancaster to Imperial. As such, the patent profession awaits your imminent arrival. Next up are Earn, Pete and Sadie. You three were always up for a drink, whether it be coffee or alcohol. Both of which were crucial during the final months of my PhD. And finally my newest office mates Adrian, Ronan and Tom. I may have only spent half my time in the office, but it’s been a pleasure (and please keep the office plants alive!) I want to thank Roger Yelle and Shane Stone for their guidance with everything and anything MAVEN/NGIMS related. It was a pleasure to meet you, and thank you for your support on my first paper. Thanks must go to Sean Bruinsma and J-C Marty for undertaking the unenviable task of retriev- ing data from Trace Gas Orbiter. Such meticulous work often goes underappreciated. This wonderous dataset was the basis of my second paper, so for that and your support, I am incredibly grateful. Thank you to Adam and Apostolos for assessing me throughout the years and thereby ensuring I had a chance of achieving a PhD. Thank you to Adam, Bob and Stephen for taking the time to read this thesis and asking me all the wrong questions. Joking aside, your pertinent comments have made this thesis a much stronger piece of work. Thank you to all my family for the endless support over the past few years. It really does mean the world. There’s undoubtedly an art to asking about a PhD and then always remembering something needs doing within seconds of me opening my mouth. 5 Finally, thanks to Southeastern for rarely sticking to their timetable, allowing me more time to work on the train. Those extra few minutes certainly added up over the years - as did the price hikes. 6 Contents List of Figures 11 List of Tables 14 Abbreviations 15 1 Introduction 17 1.1 Introduction......................................... 17 1.2 Motivation for Studying Mars................................ 18 1.3 Previous Observations of Mars............................... 18 1.3.1 Pre-Spacecraft Observations............................ 18 1.3.2 Previous Spacecraft Observations.......................... 19 1.4 Mars Planetary Properties.................................. 21 1.5 Atmospheric Structure.................................... 24 1.5.1 Density Structure.................................. 24 1.5.2 Temperature Structure............................... 26 1.6 Martian General Circulation Models............................ 33 1.6.1 Laboratoire de Météorologie Dynamique Mars Climate Database......... 33 1.6.2 Other Martian General Circulation Models..................... 34 1.7 Gravity Waves........................................ 36 1.7.1 Gravity Wave Generation.............................. 36 1.7.2 Gravity Wave Evolution............................... 37 1.7.3 Gravity Wave Observations............................. 39 1.7.4 Modelling Gravity Waves.............................. 40 1.8 Dust Storms......................................... 41 7 CONTENTS 1.8.1 Dust Storm Observations.............................. 41 1.8.2 Dust Storm Theory................................. 44 1.9 Response of Upper Atmosphere to Dust Storms...................... 46 1.10 Open Questions....................................... 48 1.11 Summary........................................... 49 2 Instrumentation and Data 50 2.1 Introduction......................................... 50 2.2 The Mars Atmosphere and Volatile Evolution Mission................... 50 2.2.1 Spatial and Temporal Coverage........................... 52 2.2.2 The Neutral Gas and Ion Mass Spectrometer................... 52 2.2.3 Accelerometer.................................... 56 2.2.4 Comparison Between NGIMS and ACC Data.................... 58 2.3 The ExoMars Mission.................................... 61 2.3.1 Trace Gas Orbiter.................................. 62 2.3.2 Rosalind Franklin Rover............................... 64 2.4 Concurrent MAVEN and ExoMars Data.......................... 64 2.5 Summary........................................... 66 3 Data Analysis and Model Comparison 67 3.1 Introduction......................................... 67 3.2 Deriving Temperature Profiles From Density Data..................... 67 3.3 Drawbacks of Current Temperature Derivation Technique................. 70 3.4 Comparison Between Derived