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Copyright by Parvathy Prem 2017 The Dissertation Committee for Parvathy Prem certifies that this is the approved version of the following dissertation: DSMC Simulations of Volatile Transport in a Transient Lunar Atmosphere and Ice Deposition in Cold Traps after a Comet Impact Committee: David B. Goldstein, Supervisor Philip L. Varghese, Co-Supervisor Laurence M. Trafton, Co-Supervisor Laxminarayan L. Raja Richard C. Elphic DSMC Simulations of Volatile Transport in a Transient Lunar Atmosphere and Ice Deposition in Cold Traps after a Comet Impact by Parvathy Prem, BE; MSE Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2017 Epigraph “All that survives after our death are publications and people. So look carefully after the words you write, the thoughts and publications you create, and how you love others. For these are the only things that will remain.” – Susan Niebur (1973-2012) Acknowledgements First and foremost, I would like to thank my advisors – Prof. David Goldstein, Prof. Philip Varghese and Dr. Laurence Trafton – for their guidance, encouragement and support over almost seven years of graduate study. They have my gratitude and respect not only for their (willingly shared) scientific expertise, but also for the high standards to which they hold their work (and that of their students), for the balance of guidance and independence that they provide as advisors, and for the consideration with which they treat others. One of the main reasons I wished to complete this PhD was to be able to pay forward what I’ve gained from their training; I would be more than satisfied if I could ever advise a student even half as well as they have. I would also like to thank the other two members of my Dissertation Committee – Prof. Laxminarayan Raja for his interest in my progress and professional advice over the years, and Dr. Richard (Rick) Elphic for his enthusiasm and support. The encouragement that I’ve received from Rick and several other members of the planetary science community has made a world of difference to me, and I am deeply appreciative of all that they do to make this a welcoming and inclusive field. The work described in this dissertation would not have been possible without Dr. Bénédicte Stewart’s previous work. I cannot thank Bénédicte enough for all her help in getting started with the DSMC code. I would also like to thank the late Dr. Elisabetta Pierazzo and Dr. Natalia Artemieva for their collaboration, which has been instrumental to this work. Many thanks are also due to several current and former members of the Computational Fluid Physics Lab at UT Austin – Dr. William (Billy) McDoniel, Dr. Seng Yeoh, Dr. Chris Moore, Dr. Andrew Walker, Dr. Aaron Morris, Jeff Chu and Will Hoey. Billy’s improvements to code performance helped my work greatly; Chris, Andrew and v Aaron were always ready to help when I was getting started; Yeoh and Jeff were great office mates, and all of them have been great company to work with. I was fortunate to have several excellent professors during the course of my graduate studies, but special thanks go to Prof. John Lassiter and Prof. John (Jack) Holt. John’s class on Meteoritics and Early Solar System Processes not only provided a broad overview of the field that was as interesting as it has been useful, but also taught me how to read scientific literature critically. As for Jack’s Planetary Geology & Geophysics class – I don’t think I’ve ever enjoyed a course more; for that, and for all his support in the years since, Jack has my lasting gratitude. Despite the fascinating science and the wonderful people I’ve had the privilege to work with, the last seven years have also been the hardest of my life. I don’t think I could have pulled through without the support of several old friends, who deserve much more thanks than I can give in a few lines. The same goes for my parents and my little brother, who have always had more faith in me than I think is rational, and for my husband, Anindya, who has been my best friend for eleven years. I would also like to thank Dr. Margarita Leonor Diaz for her invaluable professional help over the last year and a half – I don’t think I could have written this dissertation without it. Last, but by no means least, I would like to acknowledge Tina Woods, Geetha Rajagopal and Scott Messec for their help with all things administrative and electronic. The simulations presented here would not have been possible without the generous computational resources provided by the Texas Advanced Computing Center, and the assistance of the technical support teams at TACC and ICES. This work was also supported in part by the NASA LASER program under Grant NNX09AM60G. vi DSMC Simulations of Volatile Transport in a Transient Lunar Atmosphere and Ice Deposition in Cold Traps after a Comet Impact Parvathy Prem, Ph.D. The University of Texas at Austin, 2017 Supervisor: David B. Goldstein Co-Supervisors: Philip L. Varghese and Laurence M. Trafton Over the years, a number of missions have detected signs of water and other volatiles in cold, permanently shadowed regions near the lunar poles, where temperatures are sufficiently low that volatile ices can remain stable over geological timescales. Several observations suggest that comet impacts may have played a role in delivering these cold-trapped volatiles. In this work, I use Direct Simulation Monte Carlo (DSMC) simulations to investigate the transport and sequestration of water in the aftermath of a lunar comet impact, focusing on developing a broad understanding of the physical processes that govern the fate of impact-delivered volatiles (particularly water), in order to better interpret remote sensing data. The sheer amount of vapor generated by a volatile-rich impact can transform the Moon’s tenuous, surface-bound exosphere into a collisional, transient atmosphere with characteristic gas dynamic features that influence the redistribution of impact-delivered vii volatiles. Notably, the simulations indicate that reconvergence of vapor antipodal to the point of impact may result in preferential redistribution of water in the vicinity of the antipode; in some circumstances, water may be distributed non-uniformly between different cold traps. It is also found that atmospheric self-shielding from photodestruction significantly increases the amount of water that reaches the shelter of cold traps. Volatile transport in an impact-generated atmosphere is also influenced by gas phase interactions with solar radiation and the lunar surface. The Moon has a distinctive surface thermal environment, characterized by large gradients in temperature over very small scales. In this work, I develop a stochastic rough surface temperature model that is then coupled to volatile transport simulations. It is found that surface roughness reduces the mobility of water at high latitudes, while also increasing the concentration of atmospheric/exospheric water molecules around the poles. I also implement a coupled DSMC-photon Monte Carlo method to model radiative heat transfer in the evolving, three-dimensional, rarefied atmosphere. The trapping of radiation within the optically thick gas slows the rate of cooling of the expanding vapor cloud, and also affects near- field atmospheric structure and winds. Ultimately, the fate of impact-delivered water is determined by the interplay between these factors. viii Table of Contents List of Tables ....................................................................................................... xii List of Figures ...................................................................................................... xiii Chapter 1: Introduction ............................................................................................1 1.1. Motivation ................................................................................................1 1.2. Objectives ................................................................................................4 1.3. Dissertation Overview .............................................................................6 Chapter 2: Literature Review ...................................................................................7 2.1. Chapter Overview ....................................................................................7 2.2. Distribution, Abundance and Origin of Lunar Water ..............................7 2.2.1. The Search for Cold-Trapped Water............................................7 2.2.2. Comets as a Source of Lunar Water ..........................................10 2.2.3. Modeling ....................................................................................12 2.3. Thermal Modeling of Rough Planetary Surfaces...................................16 2.4. Computational Methods and Models for Radiative Transfer ................21 Chapter 3: Computational Methods and Models ...................................................28 3.1. Chapter Overview ..................................................................................28 3.2. Review of the Hybrid SOVA-DSMC Method .......................................28 3.3. Photodestruction and Atmospheric Self-shielding.................................34 3.4. Rough Surface Temperature Model .......................................................36 3.4.1. Development of a Rough Surface Temperature Model .............37 3.4.2.