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Physical Processes of the CO2 Hydrate Formation and Decomposition at Conditions Relevant to Mars

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Physical Processes of the CO2 Hydrate Formation and Decomposition at Conditions Relevant to Mars Physical processes of the CO2 hydrate formation and decomposition at conditions relevant to Mars Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultäten der Georg-August-Universität zu Göttingen vorgelegt von Georgi Yordanov Genov aus Varna, Bulgarien Göttingen 2005 D 7 Referentin/Referent: Prof. Dr. W. F. Kuhs Korreferentin/Korreferent: Prof. Dr. S. Webb Tag der mündlichen Prüfung: 14.01.2005 Abstract This thesis is concerned with the formation and decomposition kinetics, as well as with the microstructure of CO2 hydrate at conditions relevant to those on the Martian surface and in the Martian interior. It was conducted in the framework of DFG-project Ku 920/11 – part of the larger German research initiative (Schwerpunktprogramm 1115) “Mars and the terrestrial planets”. Here, the results from neutron diffraction and gas consumption measurements of the CO2 hydrate growth in the temperature range 185 K – 272 K are gathered and checked for consistency. Also first data from in situ neutron diffraction runs on CO2 hydrate decomposition are presented. A sigmoid reaction development (higher order kinetics) was observed in a number of runs in both – formation and dissociation, suggesting for concomitant nucleation and growth processes taking place. The asymmetry, found in the sigmoid shape of the reaction curves, suggests that diffusion also plays an appreciable role. A new two-stage method for data interpretation (stage A – nucleation-and-growth transformation and stage B – diffusion controlled transformation), trying for the first time to unify the theoretical description of both – formation and decomposition processes on macroscopic level is suggested. The previously reported anomalous preservation for the CO2 hydrate case is confirmed and first hints to explaining this problem are given. Thus, valuable information on the physics of the CO2 hydrate formation and dissociation is obtained. On this basis it can be calculated that a volume of ice with a specific surface area of around 0.1 m2/g, exposed to Martian conditions, i.e. temperatures of about 150 K and pressures around 6 mbar, will be half transformed into CO2 hydrate in approximately 10 000 yr and fully transformed in approximately 90 000 yr, disregarding the initial reaction- controlled part and allowing only the diffusion to control the transformation. For its part, the anomalous preservation may, on one hand, serve as an inhibitor or at least as a slow-down factor for some catastrophic processes involving CO2 hydrate decomposition; on the other hand it may cause such processes, once the ice-hydrate phase boundary is crossed. Special attention is paid to the hydrate microstructure. For the first time an attempt for its quantification is presented on the basis of a partly-open 3D clathrate foam structure. An estimate of the connectivity between the foam cells (bubbles), important for different model simulations, is also given. Moreover, a general image processing algorithm, allowing for fast quantification of foam structures established by SEM is outlined. i Auszug Diese Doktorarbeit befasst sich mit der Kinetik der Bildung und der Zersetzung sowie mit der Mikrostruktur von CO2-Hydrat unter p-T Bedingungen der Marsoberfläche und des Marsinneren. Sie wurde im Rahmen des DFG Projektes Ku 920/11 als Teil einer DFG-finanzierten Forschungsinitiative "Mars und die terrestrischen Planeten" (Schwerpunktprogramm 1115) durchgeführt. Die Wachstumskinetik wurde mit Neutronenbeugungs- und Gasverbrauchs-Messungen im Temperaturbereich von 185 K bis 272 K untersucht und die Ergebnisse der beiden Methoden auf Konsistenz geprüft. Darüber hinaus werden erste Ergebnisse von in situ Neutronbeugungsmessungen der CO2-Hydrat-Zersetzung präsentiert. Eine sigmoide Reaktionsentwicklung (Kinetik höherer Ordnung) wurde mehrfach sowohl bei der Bildung, als auch bei der Zersetzung beobachtet. Diese weist darauf hin, dass teilweise gleichzeitig Keimbildungs- und Wachstumsprozesse stattfinden. Die Asymmetrie der sigmoiden Form der Reaktionskurven zeigt zudem, dass Diffusionsprozesse eine wesentliche Rolle spielen. Mit einer erstmals hier vorgeschlagenen zweistufigen Methode für die Dateninterpretation (Stufe A: Kernbildung- und Wachstumstransformation und Stufe B: Diffusionskontrollierte Transformation) wird zum ersten Mal versucht, die theoretische Beschreibung von Bildungs- und Zersetzungsprozessen auf phänomenologischem Niveau zu vereinheitlichen. Die von anderen Autoren berichtete „anormale Erhaltung“ von CO2-Hydrat wird bestätigt und erste Überlegungen zur Erklärung dieses Phänomens werden gegeben.. Die experimentellen Untersuchungen erlauben erstmals Vorhersagen des Umwandlungsverhaltens von CO2-Hydraten unter Marsbedingungen. So kann berechnet werden, dass ein Volumen von Eis mit einer spezifischen Oberfläche von ca. 0.1 m2/g bei Marsbedingungen, d. h. bei Temperaturen von 150 K und einem Druck um 6 mbar, in ca 10 000 J. zur Hälfte in CO2-Hydrat umgewandelt sein wird und in ca 90000 J. völlig transformiert. Im wesentlichen ist die Umwandlungskinetik dabei von der Diffusion der Bestandteile durch das kristalline Gashydrat bestimmt. Die „anormale Erhaltung“ steht zwar zunächst den mehrfach zur Erklärung geomorphologischer Strukturen herangezogenen katastrophalen Zersetzungsprozessen von Gashydraten entgegen, der Effekt kann andererseits aber auch solche katastrophalen Prozesse fördern, indem er großen Mengen von Gashydraten metastabil erhält, die sich dann beim Überschreiten des Eisschmelzpunkts in katastrophaler Weise zersetzen. Spezielle Aufmerksamkeit wird in der Arbeit auch auf die Mikrostruktur der Gashydrate gerichtet. Zum ersten Mal wird ein Versuch für die Quantifizierung der Mikrostruktur basierend auf einer Beschreibung als teilweise offen-porigem Schaum präsentiert. Außerdem wird ein allgemeiner Bildverarbeitungsalgorithmus, der die schnelle Quantifizierung von im Rasterelektronenmikroskop beobachteten Schaumstrukturen zulässt, entworfen. ii Table of contents Abstract i Table of contents iii Chapter I – CO2 clathrate hydrates on Mars I-1 § 1. A few words about Mars I-1 1.1. Martian atmosphere I-1 1.2. Martian inner structure I-4 § 2. Ice and clathrate hydrates I-6 2.1. Ice Ih I-6 2.2. Hydrate structures and phase diagram I-7 2.3. Hydrate formation and decomposition kinetics I-10 § 3. CO2 hydrates on Mars I-13 Chapter II – Methods and instrumentation II-1 § 1. Neutrons – basic physics and instruments II-1 1.1. Neutrons – basic physical properties II-1 1.2. Neutron interactions II-3 1.3. Neutron production II-10 1.4. Neutron detection II-13 1.5. D 20 – a high-intensity 2-axis neutron diffractometer II-14 1.6. Radiation protection II-16 § 2. pVT method II-18 2.1. Main principles II-18 2.2. Experimental setups II-20 § 3. Field Emission Scanning Electron Microscopy (FE-SEM) II-22 3.1. Electron – basic physical properties II-22 3.2. Principles of the scanning electron microscopy II-23 3.3. LEO 1530 Gemini – one FE-SEM with cryo stage II-25 § 4. BET method II-27 Chapter III – Modeling approaches III -1 § 1. Multistage Model of Gas Hydrate Growth from Ice Powder III -1 1.1. The model III -1 § 2. JMAKGB – a combined Avrami-Erofeev and Ginstling-Brounshtein way of data interpretation III -12 2.1. The approach III -12 Chapter IV – Experiments, results and conclusions IV-1 § 1. Experiments on CO2 hydrate formation IV-1 1.1. The starting material IV-1 1.2. The experiments IV-2 1.3. Data analyses and discussion IV-4 § 2. Experiments on CO2 hydrate decomposition IV-18 2.1. Starting material and experiments IV-18 2.2. Data analyses and discussion IV-19 § 3. Topological observations – hydrate foam structure IV-22 CO2 clathrate hydrates on Mars - yes or no? 1 References 5 iii Appendix I 15 Appendix II Sheet 1 Appendix III 24 Acknowledgements 39 Lebenslauf 40 iv Chapter I CO2 clathrate hydrates on Mars The aim of this chapter is to give the reader a general idea about the planet of Mars with its atmosphere and inner structure, since the atmospheric conditions and the vertical thermal profile of the Martian interior are of major importance for the existence of CO2 hydrates on the Red Planet (see § 2 and § 3). Also the ice Ih, as well as the clathrate hydrates with their structure and thermodynamics are conversed. The possible significance of the gas hydrates for the Universe, the Solar system, and certainly for our target – Mars is being discussed. Of course, this cannot be done in very detail for the reason of limited space. Nevertheless, this is supposed to be one enjoyable reading. § 1. A few words about Mars1 1.1. Martian atmosphere Being the fourth planet in the Solar system, Mars is the last of the inner planets, characterized by their rocky composition, unlike the gaseous and icy outer ones. The history of Mars exploration starts in the year 1608 with the first observations of Galilei. In 1659 Huygens saw a dark area on its surface (Syrtis Major). It helped for defining the Martian rotation period. In 17-th and 18-th century were found the polar ice regions and their seasonal variations, as well as the giant dust storms. The attempts to map the Martian surface date from the 1830 when Mars was close to the Earth. In 1877 Schiaparelli, using the 22-cm refractor in Milan, observed and mapped his famous “canale” (Fig.I.1). He had won his fame first showing that the Perseides were linked to the Swift-Tuttle comet, a discovery that earned him his own observatory. Therefore his peculiar Martian map was taken seriously and that was the beginning of the speculations for the existence of intelligent life there. Some people even went further as for instance Clara Goguet Guzman, a French widow, who established the “Guzman Prize” (100 000 FFr) for the one who first established a contact with another civilization. By that time the scientific community got divided into two fractions - “canalists” and “anticanalists”. This delusion lasted till the beginning of the XX-th century when better telescopes with higher resolution appeared. Since 1960, 36 unmanned missions were sent to Mars, 20 of them by USSR/Russia, one by Japan, one by EU and the rest by USA. A huge amount of climate data, spectroscopic observations, pictures etc was gathered.
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