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dc.contributor.authorChandnani, Mitali
dc.date.accessioned2020-02-13T17:17:57Z
dc.date.available2020-02-13T17:17:57Z
dc.date.issued2019-12
dc.identifier.urihttp://hdl.handle.net/11122/10892
dc.descriptionThesis (Ph.D.) University of Alaska Fairbanks, 2019en_US
dc.description.abstractThe transition from simple to complex crater morphology in impact craters with increase in crater size has been modelled and observed in planetary bodies across the Solar System. The transition diameter depends upon the strength and gravity of the planetary body. On the Moon, this transition takes place over a diameter range of several kilometers. This range spans a diversity of crater morphologies including simple, transitional and complex craters. The diameter range of 15 20 km falls within the lunar simple-to-complex transition. All other impactor properties held constant, the 15-20 km range corresponds to a factor of three in the magnitude of impact kinetic energy. I conducted detailed geologic investigation of 244 well-preserved craters in this diameter range to elucidate the root causes of morphological variations. I used panchromatic data for observing crater and surface morphology, Digital Elevation Models (DEMs) for evaluating crater morphometry and topographic variation of pre-impact terrain, near-infrared (NIR) bands for determining the composition of crater cavity and surrounding terrain, thermal infrared bands for examining rock abundance, and Synthetic Aperture Radar (SAR) data for detecting impact melt deposits. The results of my investigation indicate that the morphological differences are primarily governed by target properties. Simple craters are confined to the highlands, and the mare are more abundant in complex craters. The mare are composed of solidified basaltic lava flows interlayered with regolith. The layering creates vertical strength heterogeneities that drive the destabilization of the transient cavity and its collapse, causing the transition to complex craters at smaller diameters in the mare. The non-layered highlands are more vertically homogeneous in strength and therefore favor simple crater formation. Eight atypically deep simple craters were identified in the highlands near the mare highlands boundaries, the most porous terrains on the lunar surface. After detailed examination of these craters in comparison to their normal-depth counterparts, I conclude that part of the energy from impact on porous target was spent in target compaction. The higher the porosity of the target, the deeper the crater and greater its volume, due to increased compaction. That only some of the craters in the high porosity terrains are deep suggests that those craters are on locally extreme-high porosity patches. However, an unusual impactor property, such as a high velocity impact, a high density impactor, or a near-vertical impact may also be a contributor. The simple craters in the highlands were observed to be located on flat or gradually sloping surfaces or degraded rims and terraces of pre-existing craters. Most craters with localized slumps superpose sharp topographic breaks such as well-developed rims and terraces of pre-existing craters. However, the topographic settings of 35% of the craters with localized slumps appeared to be similar to that of the simple craters. More detailed topographic study of the pre-impact terrains of these two morphologies revealed that the pre-impact terrains of 35% of the craters with localized slumps are gradually sloping or have subtle topographic breaks. Both sharp and subtle breaks are characterized with similar sloping directions as the adjacent craters' walls, which led to over steepening of the transient cavity walls around this part of the rim and their collapse, thereby causing the accumulation of localized slumped material. Several simple craters were also identified to have formed on pre-impact topographic breaks. However, the simple craters' walls that superpose these breaks were observed to be sloping in directions opposite to that of the breaks. So the ejecta around these walls was deposited along the break slopes, and thus syn-impact mass wasting occurred external (and not internal) to the crater cavity.en_US
dc.description.tableofcontentsChapter 1 Introduction -- 1.1 Chapter 2: Geologic analyses of the causes of morphological variations in lunar craters within the simple-to-complex transition -- 1.2 Chapter 3: Geologic investigation of deep simple craters in the lunar simple-to complex transition -- 1.3 Chapter 4: Influence of target properties on wall slumping in lunar craters within the simple-to-complex transition -- 1.4 References. Chapter 2: Geologic analyses of the causes of morphological variations in lunar craters within the simple-to-complex transition -- Abstract -- 2.1 Introduction -- 2.1.1 Lunar simple-to-complex impact crater transition -- 2.1.2 Objectives -- 2.2 Methods and data -- 2.2.1 Morphological characterization -- 2.2.2 Close-proximity analyses -- 2.2.3 Data sets used -- 2.3 Results -- 2.3.1 Morphological characterization -- 2.3.1.1 Simple crater -- 2.3.1.2 Crater with localized slumps -- 2.3.1.3 Crater with localized slumps and terraces -- 2.3.1.4 Crater with localized slumps and central uplift -- 2.3.1.5 Crater with localized slumps, terraces, and central uplift -- 2.3.1.6 Floor-fractured crater -- 2.3.1.7 Concentric crater -- 2.3.2 Highlands versus mare craters -- 2.3.3 Close-proximity craters -- 2.4 Discussion -- 2.4.1 Mare-highlands differences -- 2.4.2 Unusually deep craters -- 2.4.2.1 Impact into a high-porosity target -- 2.4.2.2 The highlands are more coherent in these locations and less susceptible to minor slumping of the transient cavity -- 2.4.3 Potential impactor-caused variations -- 2.4.4 Comparison with other planets -- 2.5 Conclusions -- 2.6 Acknowledgments -- 2.7 References -- Appendix A. Chapter 3: Geologic investigation of deep simple craters in the lunar simple-to-complex transition -- Abstract -- 3.1 Introduction -- 3.1.1 Deep simple craters -- 3.1.2 Objectives -- 3.1.2.1 Greater compaction of target with increase in porosity results in larger crater depths -- 3.1.2.2 The highlands are more coherent in these locations and more resistant to minor slumping of the transient cavity -- 3.2 Methods and data sets -- 3.2.1 Crater depths from SLDEM and LOLA tracks -- 3.2.2 Depths of proximity craters -- 3.2.3 Trends in crater morphometry -- 3.2.4 impact melt deposits -- 3.2.5 Visual examinations of crater cavities -- 3.3 Results -- 3.3.1 Crater depths from SLDEM and LOLA tracks -- 3.3.2 Depths of proximity craters -- 3.3.3 Trends in crater morphometry -- 3.3.3.1 Association of crater depth with porosity -- 3.3.3.2 Rim height -- 3.3.3.3 Wall slope -- 3.3.3.4 Floor size -- 3.3.3.5 Cavity profiles -- 3.3.3.6 Ejecta-cavity volume ratio -- 3.3.4 Impact melt deposits -- 3.3.5 Visual examinations of crater cavities -- 3.4 Discussion -- 3.4.1 Greater compaction of target with increase in porosity results in larger crater depths -- 3.4.2 The highlands are more coherent in these locations and more resistant to minor slumping of the transient cavity -- 3.5 Conclusions -- 3.6 Acknowledgements -- 3.7 References -- Appendix B. Chapter 4: Influence of target properties on wall slumping in lunar craters within the simple-to-complex transition -- Abstract -- 4.1 Introduction -- 4.1.1 Wall slumping -- 4.1.2 Objectives -- 4.1.2.1 Localized slumping occurred post-crater formation -- 4.1.2.2 Localized slumping occurred on weaker target -- 4.1.2.3 Impact cratering on a slope causes the localized slumping -- 4.2 Methods and data sets -- 4.2.1 Testing hypothesis 1: Comparison of crater densities on slumps and ejecta units of craters with localized slumps -- 4.2.2 Testing hypothesis 2: Frequencies of proximal simple craters and craters with localized slumps -- 4.2.3 Testing hypothesis 3: Topographic variation in pre-impact terrains of the simple craters and craters with localized slumps along with rim circularity -- 4.3 Results -- 4.3.1 Comparison of crater densities on slumps and ejecta units of craters with localized slumps -- 4.3.2 Frequencies of proximal simple craters and craters with localized slumps -- 4.3.3 Topographic variation in pre-impact terrains of the simple craters and craters with localized slumps along with rim circularity -- 4.4 Discussion -- 4.5 Conclusions -- 4.6 Acknowledgements -- 4.7 References -- Appendix C. Chapter 5: Conclusion -- 5.1 Target properties -- 5.2 Impactor properties.en_US
dc.language.isoen_USen_US
dc.subjectlunar cratersen_US
dc.subjectlunar geologyen_US
dc.subjectlunar basinsen_US
dc.titleEffects of target properties on the formation of lunar impact craters in the simple-to-complex transitionen_US
dc.typeThesisen_US
dc.type.degreephden_US
dc.identifier.departmentDepartment of Geosciencesen_US
dc.contributor.chairHerrick, Robert
dc.contributor.committeeKramer, Georgiana
dc.contributor.committeeLarsen, Jessica
dc.contributor.committeeDehn, Jonathan
refterms.dateFOA2020-02-13T17:17:58Z


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