Understanding Space Weathering of Asteroids and the Lunar Surface: Analysis of Experimental Analogs and Samples from the Hayabusa and Apollo Missions

Persistent Link:
http://hdl.handle.net/10150/620874
Title:
Understanding Space Weathering of Asteroids and the Lunar Surface: Analysis of Experimental Analogs and Samples from the Hayabusa and Apollo Missions
Author:
Thompson, Michelle
Issue Date:
2016
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Grains on the surfaces of airless bodies are continually being modified due to their exposure to interplanetary space, a phenomenon known as space weathering. This dissertation uses a multi-faceted approach to understanding space weathering of the lunar and asteroidal surfaces. Chapters 1 and 2 provide an introduction to space weathering and a discussion of the methods employed in this work, respectively. Chapter 3 focuses on the analysis of returned samples from near-Earth asteroid Itokawa using the transmission electron microscope (TEM) and contributes to the first-ever comparison of microstructural and chemical features of space weathering in returned samples from two different airless bodies. This research uses high-resolution imaging and quantitative energy-dispersive x-ray spectroscopy (EDS) measurements to analyze space weathering characteristics in an Itokawa soil grain. These analyses confirm that space weathering is operating on the surface of Itokawa, and that many of the resulting features have similarities to those observed in lunar soils. Results show that while there is evidence that both major constituent space weathering processes are operating on the surface of Itokawa, solar wind irradiation, not micrometeorite impacts, appears to be the dominant contributor to changes in the microstructure and chemistry of surface material. Chapter 4 presents a detailed study of nanophase Fe (npFe) particles in lunar soil samples. For the first-time, the oxidation state of individual npFe particles was directly measured using electron energy-loss spectroscopy (EELS) in the TEM. The results show that npFe particles are oxidizing over their time on the lunar surface, and that the amount of oxidized Fe in the nanoparticles is correlated with soil maturity. The EELS data are also coupled to atomic-resolution imaging, which is used to determine the structure of the nanoparticles, confirming their mineral phase. This work challenges the long-standing paradigm that all npFe particles are composed of metallic Fe and that the chemical composition of these features remains static after their formation. A theoretical modeling investigation of the influence that npFe particles of different oxidation states have on the spectral properties of the material is also presented. The model results show that varied Fe-oxidation states of the nanoparticles can produce subtle changes in the optical properties of the soils, including the degree of reddening and the attenuation of characteristic absorption bands. These findings should be accounted for in future modeling of reflectance spectra. Chapter 5 presents a novel technique for simulating space weathering processes inside the TEM. Using an in situ heating holder, lunar soils were subjected to both slow- (~minutes) and rapid-heating (<seconds) events to simulate micrometeorite impacts. The slow-heating experiments show that npFe forms at ~575 ºC, providing a temperature constraint on initial npFe formation. Lunar soil grains that were subjected to a single, rapid, thermal pulse show the development of npFe particles and vesiculated textures near the grain rim. The vesicles were imaged and the npFe particles were imaged and then mapped with EDS. The oxidation state of the npFe particles was confirmed to be Fe^0 using EELS. Several lunar soil grains were subjected to multiple thermal shocks to simulate longer exposure times on the lunar surface. With each heating cycle, the number and size distribution of the npFe particles changed. The average size of npFe particles increased, and the size distribution became more gaussian after multiple heating events, versus the asymmetric distribution present after only one heating event. These results provide insight into the particle growth dynamics for space weathered soils and could offer a new way to place relative age constraints on grains in lunar soil.Chapter 6 provides a summary of the work presented here, discusses its implications for understanding space weathering processes across the solar system, and presents a perspective on the future of space weathering studies.
Type:
text; Electronic Dissertation
Keywords:
Asteroid; Hayabusa; Lunar; Sample Return; Space Weathering; Planetary Sciences; Apollo
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Planetary Sciences
Degree Grantor:
University of Arizona
Advisor:
Zega, Thomas J.

Full metadata record

DC FieldValue Language
dc.language.isoen_USen
dc.titleUnderstanding Space Weathering of Asteroids and the Lunar Surface: Analysis of Experimental Analogs and Samples from the Hayabusa and Apollo Missionsen_US
dc.creatorThompson, Michelleen
dc.contributor.authorThompson, Michelleen
dc.date.issued2016-
dc.publisherThe University of Arizona.en
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en
dc.description.abstractGrains on the surfaces of airless bodies are continually being modified due to their exposure to interplanetary space, a phenomenon known as space weathering. This dissertation uses a multi-faceted approach to understanding space weathering of the lunar and asteroidal surfaces. Chapters 1 and 2 provide an introduction to space weathering and a discussion of the methods employed in this work, respectively. Chapter 3 focuses on the analysis of returned samples from near-Earth asteroid Itokawa using the transmission electron microscope (TEM) and contributes to the first-ever comparison of microstructural and chemical features of space weathering in returned samples from two different airless bodies. This research uses high-resolution imaging and quantitative energy-dispersive x-ray spectroscopy (EDS) measurements to analyze space weathering characteristics in an Itokawa soil grain. These analyses confirm that space weathering is operating on the surface of Itokawa, and that many of the resulting features have similarities to those observed in lunar soils. Results show that while there is evidence that both major constituent space weathering processes are operating on the surface of Itokawa, solar wind irradiation, not micrometeorite impacts, appears to be the dominant contributor to changes in the microstructure and chemistry of surface material. Chapter 4 presents a detailed study of nanophase Fe (npFe) particles in lunar soil samples. For the first-time, the oxidation state of individual npFe particles was directly measured using electron energy-loss spectroscopy (EELS) in the TEM. The results show that npFe particles are oxidizing over their time on the lunar surface, and that the amount of oxidized Fe in the nanoparticles is correlated with soil maturity. The EELS data are also coupled to atomic-resolution imaging, which is used to determine the structure of the nanoparticles, confirming their mineral phase. This work challenges the long-standing paradigm that all npFe particles are composed of metallic Fe and that the chemical composition of these features remains static after their formation. A theoretical modeling investigation of the influence that npFe particles of different oxidation states have on the spectral properties of the material is also presented. The model results show that varied Fe-oxidation states of the nanoparticles can produce subtle changes in the optical properties of the soils, including the degree of reddening and the attenuation of characteristic absorption bands. These findings should be accounted for in future modeling of reflectance spectra. Chapter 5 presents a novel technique for simulating space weathering processes inside the TEM. Using an in situ heating holder, lunar soils were subjected to both slow- (~minutes) and rapid-heating (<seconds) events to simulate micrometeorite impacts. The slow-heating experiments show that npFe forms at ~575 ºC, providing a temperature constraint on initial npFe formation. Lunar soil grains that were subjected to a single, rapid, thermal pulse show the development of npFe particles and vesiculated textures near the grain rim. The vesicles were imaged and the npFe particles were imaged and then mapped with EDS. The oxidation state of the npFe particles was confirmed to be Fe^0 using EELS. Several lunar soil grains were subjected to multiple thermal shocks to simulate longer exposure times on the lunar surface. With each heating cycle, the number and size distribution of the npFe particles changed. The average size of npFe particles increased, and the size distribution became more gaussian after multiple heating events, versus the asymmetric distribution present after only one heating event. These results provide insight into the particle growth dynamics for space weathered soils and could offer a new way to place relative age constraints on grains in lunar soil.Chapter 6 provides a summary of the work presented here, discusses its implications for understanding space weathering processes across the solar system, and presents a perspective on the future of space weathering studies.en
dc.typetexten
dc.typeElectronic Dissertationen
dc.subjectAsteroiden
dc.subjectHayabusaen
dc.subjectLunaren
dc.subjectSample Returnen
dc.subjectSpace Weatheringen
dc.subjectPlanetary Sciencesen
dc.subjectApolloen
thesis.degree.namePh.D.en
thesis.degree.leveldoctoralen
thesis.degree.disciplineGraduate Collegeen
thesis.degree.disciplinePlanetary Sciencesen
thesis.degree.grantorUniversity of Arizonaen
dc.contributor.advisorZega, Thomas J.en
dc.contributor.committeememberZega, Thomas J.en
dc.contributor.committeememberByrne, Shaneen
dc.contributor.committeememberLauretta, Dante S.en
dc.contributor.committeememberReiners, Peter W.en
dc.contributor.committeememberSwindle, Timothy D.en
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