Open Research Online The Open University’s repository of research publications and other research outputs Planet Mercury: volatile release on a contracting world Thesis How to cite: Thomas, Rebecca (2015). Planet Mercury: volatile release on a contracting world. PhD thesis The Open University. For guidance on citations see FAQs. c 2015 The Author Version: Version of Record Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk Planet Mercury: volatile release on a contracting world Rebecca J. Thomas BSc, MA (Hons) Submitted 4th September 2015 A thesis submitted to the Open University in the subject of Planetary Science for the degree of Doctor of Philosophy. Department of Physical Sciences Open University 2 χαῖρ᾽. Ἑρμῆ χαριδῶτα, διάκτορε, δῶτορ ἐάων. Hail, Hermes, giver of grace, guide, and giver of good things Anonymous (English translation by H. G. Evelyn-White) (1914), Hymn 18 to Hermes, in The Homeric Hymns and Homerica, Harvard University Press, Cambridge, MA. 3 4 Abstract This thesis investigates the geological processes by which materials containing high concentrations of volatile substances have been delivered to the surface of the planet Mercury throughout its history, despite global contraction, which could be expected to impede the replenishment of these materials from depth. With the aid of high-resolution data from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, I perform detailed analyses of the two types of geological features most indicative of the action of volatiles at Mercury’s surface: flat-floored depressions known as ‘hollows’, and pits and deposits thought to be the products of explosive volcanism. For hollows, I seek to clarify the nature of the substance lost to form them and the mechanisms by which this has been introduced to, and lost from, the surface recently enough to explain their pristine appearance. I produce a global catalogue of hollows, study their local associations, and investigate their spectral character, finding evidence that they form primarily by sublimation of a moderately volatile substance, potentially a sulfide, from a specific low-reflectance substrate exhumed and exposed by meteorite impacts. To better understand explosive volcanism, I investigate its longevity and the processes promoting it at specific locations. Through identifying, dating and characterising proposed pyroclastic deposits and vents, I show that putative explosive volcanism was more long-lived than voluminous effusive eruptions, that its occurrence is tectonically controlled, and that its eruptive style indicates magma storage prior to eruption, at greater depths than on the more tectonically-neutral Moon. This indicates that, rather than precluding it, global contraction favoured volcanic explosivity by promoting magma storage during which volatiles could become concentrated in the magma. Furthermore, because processes concentrating volatiles have been important in the genesis of both hollows and explosive volcanism, their occurrence does not necessarily indicate a high planetary bulk volatile fraction. 5 Acknowledgements I extend my deepest thanks to my PhD supervisors: David Rothery, for curing me of ‘which’- ery, Susan Conway, not only a GIS-guru but bringer of pork scratchings, Mahesh Anand for his gems of insight, and all of three for their unfailing help and support. Thanks also to the MESSENGER team for the stunning data I’ve had the privilege to work with in the course of my PhD. Credit for excerpts from global mosaics used in figures in this thesis goes to NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, and for NAC and WAC images, to NASA/JPL-Caltech. In particular, my thanks to Noam Izenberg, Dave Blewett and Larry Nittler for the collaboration that inspired Chapter 3, and to Peter James for providing me with his global crustal thickness model, so useful for Chapter 6. My thanks go to Matteo Massironi, Alice Lucchetti and Gabriele Cremonese at the University and Astronomical Observatory of Padova for our fascinating and fruitful collaboration, and particularly to Matteo for his invaluable guidance and for taking such good care of me in Italy. Thanks also to Simone Marchi of the Solar System Exploration Research Virtual Institute, Southwest Research Institute, Boulder, CO for his invaluable help in producing model production function ages for Chapter 4. Thanks to my fellow Mercury Girls, Valentina Galluzzi and Emma Fegan. You are both awesome. Last but very much not least, my thanks to my family, Jeremy, Yvonne and Giles, and to Brigid, Fran, Robyn, Anya, Kirstin and Colin for keeping me sane and reminding me of the big picture. And finally, a dedication: For Jean Inspiring woman, beloved grandmother. I know you’d be proud. 6 Table of Contents Abstract ................................................................................................................................................................................ 5 Acknowledgements ......................................................................................................................................................... 6 List of Figures ................................................................................................................................................................... 14 List of Tables .................................................................................................................................................................... 18 List of Supporting Digital Media ............................................................................................................................... 19 List of Abbreviations ..................................................................................................................................................... 19 Chapter 1. Introduction ......................................................................................................................................... 20 1.1 The overarching question and approach .......................................................................................... 20 1.2 Background.................................................................................................................................................... 20 1.3 Specific objectives ....................................................................................................................................... 24 1.4 Methodology ................................................................................................................................................. 25 1.5 Thesis structure ........................................................................................................................................... 25 Chapter 2. Hollows on Mercury: materials and mechanisms involved in their formation ....... 28 2.1 Introduction .................................................................................................................................................. 28 2.2 Background.................................................................................................................................................... 28 2.3 Methods ........................................................................................................................................................... 31 2.3.1 MESSENGER imagery ........................................................................................................................... 31 2.3.2 Data collected .......................................................................................................................................... 32 2.4 Results ............................................................................................................................................................. 35 2.4.1 Global variations in the areal extent of hollows ....................................................................... 35 2.4.2 Preferred slope aspect ......................................................................................................................... 39 2.4.3 Hollow depth ........................................................................................................................................... 40 2.4.4 Geological settings ................................................................................................................................ 40 7 2.4.5 Association with pyroclastic features ...........................................................................................46 2.4.6 Association with regional substrates ............................................................................................46 2.5 Discussion ......................................................................................................................................................48 2.5.1 Hollow formation mechanisms ........................................................................................................49 2.5.2 Means of transfer of hollow-forming volatiles to the surface .............................................54 2.5.3 Nature of the hollow-forming material ........................................................................................59 2.6