DARK DUNES ON MARS

Analyses on Origin, Morphology, and Mineralogical Composition of the Dark Material in Martian Craters

Dissertation zur Erlangung des akademischen Grades „doctor rerum naturalium“ (Dr. rer. nat.)

Eingereicht am Fachbereich Geowissenschaften der Freien Universität Berlin

von Dipl.- Geograph Daniela Tirsch Berlin, 2008

1. Gutachter: Prof. Dr. Ralf Jaumann 2. Gutachter: Prof. Dr. Gerhard Neukum Tag der Disputation: 05.12. 2008 Eidesstattliche Erklärung

Hiermit versichere ich, die vorliegende Dissertation selbständig angefertigt und keine anderen als die angegebenen Quellen und Hilfsmittel verwendet zu haben.

Daniela Tirsch Berlin, 29.10.2008

Acknowledgements

I wish to thank Prof. Dr. R. Jaumann, German Aerospace Center and Freie Universität Berlin, who initiated this work, for interesting, fruitful, and pleasant discussions, for providing excellent technical equipment and for accepting this thesis as a reviewer.

Without Prof. Dr. G. Neukum, Freie Universität Berlin, it would have not been possible to work in the HRSC/MarsExpress project. Moreover, I would like to thank him for useful suggestions and for accepting this thesis as a co-reviewer.

My special thanks go to Dr. F. Poulet, Institut d’Astrophysique Spatiale, CNRS Université Paris- Sud, for developing and providing the tools of spectral mineral detection, and for helpful mineralogical discussions.

Furthermore, I want to thank F. Forget and E. Millour, Laboratoire de Météorologie Dynamique du CNRS, IPSL, Université Paris 6, for providing the modelled MDC wind data and for helpful discussions concerning the database selection.

Very special thanks to Dipl.-Math. K.-D. Matz, German Aerospace Center, who magically and promptly invented programs and tools, whenever I came to him with one of my desires and problems.

My former colleague Dr. D. Reiss, Westfälische Wilhelms-Universität Münster, was a priceless help, particularly at the beginning of my work at DLR. He introduced me to the topic and programs, gave suggestions, and helped whenever he was needed. Thanks for all that help and for being such a good friend.

Furthermore, I want to thank all my colleagues at DLR and the Freie Univerität Berlin, especially Dr. S. van Gasselt, Dipl.-Geogr. T. Kneissl, Dr. P. Saiger, Dipl.-Geol. E. Hauber, Dr. T. Roatsch, Dipl.-Ing. C. Brüßhaber, Dipl.-Phys. M. Wählisch, Dipl. Geogr. M. Langhans, Dipl.-Ing. F. Scholten, M.A. Dipl.-Geol. K. Gwinner, Dipl.-Ing. F. Preusker and I. Vogt for all the suggestions, help, and wonderful discussions, for developing the programs and tools, for the competent technical support and for good and pleasant teamwork. Thank you, Dipl.-Ing. (FH) A. Nass for reviewing this thesis. It is a pleasure to work with all of you.

I wish to thank my trainee J. Glötzl (Dipl.-Ing.-to-be) for adapting the database, crater profiles, figures, and tables. Without his help, it would not have been possible to submit the thesis in time.

My warmest thanks go to my colleague and friend Dipl.-Geol. M. Sowe, German Aerospace Center, who has been the person who enjoyed and suffered the whole time of study with me. I never felt alone because she was always there. She has been my partner, my geologist’s dictionary, my anchor in bad days, and she will always be a good friend of mine.

Abstract I

ABSTRACT

The research presented in this thesis focuses on the morphological, mineralogical, and thermal characteristics of dark material deposits in Martian craters. The information gained will be used to deduce the history and development of the material and to shed light on evolution processes and scenarios of its origin. Special aspects of this topic include revealing the sources of the material, identifying possible mineralogical correspondences between deposits, and examining dune surfaces for mobility or induration. A comparative analysis on a global scale will serve to investigate whether all deposits examined are of common origin, and whether any correlations exist between the characteristics analyzed. Thus, this work is intended to bridge the gap between detailed local analyses and global studies. Carried out on the basis of 70 selected localities comprising individual dark dunes, dune fields and sand sheets, analyzed based on imaging, spectral, and thermal orbiter data, this work aims to derive and compare morphological, compositional and physical properties. Morphological analyses reveal dark layers exposed in a number of crater walls, showing indications of material transport from the layers down-wall to the intra-crater deposits. Further sites show dark material emerging from the floors of several craters, suggesting a dark material source beneath. Spectral analyses showed for the first time that a direct mineralogical analogy exists between the dark layers in the crater walls and the intra- crater dune and sand sheet material. Based on these findings, it could be shown that these layers are the local sources for the dark material inside the craters. Spectral analyses further indicate that all deposits are nearly of the same mafic mineralogical composition, hinting at a similar origin. It is shown that the thermal properties of the dunes point to consolidation on several dune surfaces, whereas the bulk of the dunes seem to consist of unconsolidated sands with coarse grain-size particles. A global consideration reveals no correlation between the geographical location of the deposits and their mineralogical composition in terms of mafic minerals. However, the cluster of hydrated minerals detected in Arabia Terra is supposed to be associated with former water-related processes in this region. Correlation between thermal properties and geographical localities may be assumed for the alignment of immovable deposits along the lowland-highland boundary. To explain their immobility, water-related processes as well as mechanical cementation might be considered. However, a convincing correlation between hydrated minerals and immovable deposits cannot be established. This indicates that mineral hydration does not consequently result in the immobilization of dunes in every case. Two different hypotheses and time ranges of origin will be considered, one suggesting a volcanic origin of the material and the other one claims for impact related processes such as impact glasses and melts. This work provides new insights into the development of deposited layers of dark material, their modification, and their exposition, which led to the distribution of dark materials on Mars. Kurzfassung II

KURZFASSUNG

Diese Arbeit beschäftigt sich mit der Untersuchung morphologischer, mineralogischer und thermaler Eigenschaften von dunklem Material in Kratern auf dem Mars. Die gesammelten Informationen werden dazu genutzt, die Geschichte und Entwicklung des Materials sowie mögliche Zeiträume seiner Bildung abzuleiten und denkbare Entstehungsszenarien zu durchleuchten. Insbesondere sollen die Fragen nach Sedimentquellen des Materials, nach Gemeinsamkeiten in der mineralogischen Zusammensetzung der Materialvorkommen und nach dem Zustand der Dünen- oberflächen hinsichtlich einer Verfestigung oder Mobilität der Dünen beantwortet werden. Eine vergleichende Analyse der Materialeigenschaften im globalen Maßstab soll die Fragen beantworten, ob von einem gemeinsamen Ursprung des Materials ausgegangen werden kann, und ob es Korrelationen zwischen dem Materialeigenschaften unter- einander sowie der geografischen Lage der Materialvorkommen gibt. Somit soll diese Arbeit einen Bogen zwischen Detailstudien und global angelegten Analysen spannen. Als Datenbasis dienen 70 global ausgewählte Lokalitäten, in denen dunkles Material in Form von Dünen, Dünenfeldern oder auch dünnen Sandlagen vorkommt. Die Material- eigenschaften werden mit Hilfe von Bild-, Spektral- und Thermaldatenauswertung verschiedener Satellitenmissionen ermittelt und verglichen. Morphologische Analysen weisen anstehende dunklen Lagen an mehreren Kraterwänden auf, an denen ein Materialtransport beginnend an den Lagen, bergab der Kraterwand, bis hin zu den Ablagerungen im Kraterinneren zu beobachten ist. An anderen Stellen kann ein „Entspringen“ des Materials im Bereich der Kraterböden beobachtet werden, was auf eine Sedimentquelle unterhalb der Kraterböden hinweist. Mit Hilfe spektraler Analysen ist es gelungen, eine direkte Übereinstimmung zwischen der Mineralogie des Materials der anstehenden Lagen und des Dünenmaterials innerhalb der Krater nachzuweisen. Dadurch kann bewiesen werden, dass diese dunklen Lagen als lokale Quellen für das Dünenmaterial in den Kratern dienen. Ebenso kann durch Spektralanalysen festgestellt werden, dass sich die einzelnen Materialvorkommen hinsichtlich ihrer Mineralogie kaum unterscheiden, was auf einen gemeinsamen Ursprung der Ablagerungen hindeutet. Es wird weiterhin dargelegt, dass die thermalen Eigenschaften einiger Dünenoberflächen auf eine Verfestigung hinweisen, während der Großteil der Dünen aus unverfestigten Grobsanden zu bestehen scheint. Die globale Betrachtung deckt keine eindeutige Korrelation zwischen der geographischen Lage der Materialvorkommen und deren Mineralogie auf. Einzig die Detektion von hydratisierten Mineralen weist eine Korrelation mit der Region Arabia Terra auf. Frühere Wasseraktivitäten in diesem Gebiet werden für die Erklärung dieses Zusammenhanges herangezogen. Die Anordnung immobiler und somit wahrscheinlich verfestigter Dünen entlang der Hochland-/Tieflandgrenze wird ebenfalls vergangenen aquatischen Prozessen zugeschrieben, wobei jedoch auch eine mechanische Verfestigung der Dünenoberflächen Kurzfassung III nicht ausgeschlossen werden kann. Eine Übereinstimmung zwischen unbeweglichen Dünen und einer veränderten Mineralogie im Vergleich zu beweglichen Dünen kann nicht festgestellt werden. Dies weist darauf hin, dass der Verfestigungsprozess nicht in jedem Falle zur Bildung neuartiger Minerale führte. Zwei verschiedene Hypothesen über Ursprungsszenarien und Bildungszeiträume werden in der Arbeit vorgestellt. Diese gehen einerseits auf einen möglichen vulkanischen Ursprung des Materials ein und schlagen andererseits vor, dass das dunkle Material impakt-genetische Schmelzen und Gläser darstellen könnte. Abschließend bietet diese Arbeit einen Einblick in die Entwicklungsgeschichte der abgelagerten Lagen aus dunklem Material, deren Mo- difikation, bis hin zu ihrer Freilegung, die damit eine globale Verteilung dieses Materials auf dem Mars zur Folge hatte. Contents IV

CONTENTS

LIST OF FIGURES ...... VI

LIST OF TABLES...... X

LIST OF ABBREVIATIONS ...... XI

CHAPTER I: INTRODUCTION AND MOTIVATION...... 1

CHAPTER II: PLANET MARS – THE BACKGROUND...... 4

2.1 Martian Geology and Topography ...... 5 2.1.1 Volcanism ...... 8 2.1.2 Impact Cratering and Dating Method...... 9 2.1.3 Erosion and Weathering...... 10 2.2 Martian Climate ...... 12 2.2.1 Atmosphere ...... 12 2.2.2 Wind ...... 12 2.2.3 Water ...... 14

CHAPTER III: AEOLIAN PROCESSES ON MARS ...... 15

3.1 The Physics of Particle Motion ...... 15 3.2 Evidence for Recent Aeolian Transport on Mars ...... 21 3.3 Sand Sources, Formation Processes, and Bed Forms on Mars and Earth...... 23 3.4 Areas of Dark Material Distribution ...... 27

CHAPTER IV: DATASETS, PROCESSING, AND METHODS ...... 30

4.1 HRSC Data...... 30 4.1.1 Selection of Appropriate Craters – The Areas Investigated...... 31 4.2 Visual Interpretation of MOC Data ...... 33 4.3 Derivation of Brightness Temperature from THEMIS Data...... 34 4.4 TES Data - Thermal Inertia Derivation and Theoretical Background...... 35 4.5 Spectral Analysis of OMEGA Data...... 38 4.5.1 Mineral Detection - The Ratio Technique ...... 39 4.6 Additional Orbital Data Sets...... 44 4.7 Wind Direction Data...... 46 4.7.1 Modelled Wind Field Data - The Mars Climate Database...... 46 4.7.2 Morphology-Deduced Wind Directions...... 48 4.8 The Resultant Database...... 50 Contents V

CHAPTER V: ANALYSIS OF DARK MATERIAL – THE INTRA-CRATER DEPOSITS...... 52

5.1 Morphology of Dark Intra-Crater Material ...... 52 5.2 Dark Layers - A Possible Local Source? ...... 60 5.3 Mineralogical Composition ...... 68 5.3.1 Uncertainties ...... 69 5.3.2 Results and Discussion ...... 70 5.4 Assessing Dune Activeness by a Wind Direction Analysis?...... 82 5.5 Analysis of Thermal Properties ...... 85 5.5.1 Optical and Thermal Thickness...... 85 5.5.2 Surface Brightness Temperature ...... 86 5.5.3 Thermal Inertia ...... 88 5.5.3.1 Uncertainties ...... 90 5.5.4 Results...... 94 5.6 Combined Interpretation and Comparison of Results ...... 100 5.7 The Immobilization of Dunes - Possible Types of Consolidation ...... 108

CHAPTER VI: ORIGIN AND EVOLUTION OF DARK MATERIAL...... 113

6.1 Possible Scenarios of Origin ...... 114 6.2 Chronology of Evolution – From Deposition to Distribution ...... 119

CHAPTER VII: SUMMARY AND OUTLOOK ...... 122

BIBLIOGRAPHY ...... 126

CURRICULUM VITAE...... i

LIST OF CONFERENCE CONTRIBUTIONS AND PUBLICATIONS ...... ii List of Figures VI

LIST OF FIGURES

Figure 1: The planet Mars: a global view ...... 4 Figure 2: Topographic map of Mars (background: MOLA topography map)...... 7 Figure 3: Artist’s view of Olympus Mons (NASA/MOLA science team)...... 8 Figure 4: Martian impact cratering chronology curve by Hartmann and Neukum (2001) (graph adapted from Werner (2006))...... 9 Figure 5: Global averaged wind field for Mars derived from the Mars Climate Database (MCD) [Lewis et al., 1999]...... 13 Figure 6: Sketch showing the primary forces acting on a spherical particle under aeolian conditions...... 16 Figure 7: Diagram showing the three principal modes of aeolian grain transport (adapted from Greeley & Iversen (1985))...... 17 Figure 8: Average threshold friction speeds (u ) for Earth and Mars for particles of different sizes *t [Edgett and Christensen (1991); adapted from Iversen and White (1982)]...... 18 Figure 9: Particle threshold curve as a function of particle size for representative Martian surface pressures and temperatures [Greeley et al., 1980]...... 19

Figure 10: Ratio of terminal speed (u ) to threshold friction speed (u ) as a function of particle size f *t for Earth and Mars [Edgett and Christensen, 1991]...... 20 Figure 11: Comparison of aggregated dust particles and loose sand at different sites in Gusev crater ...... 22 Figure 12: Classification of major dune types (adapted from Wiggs (2002))...... 26 Figure 13: Distribution of dark dunes on Mars as catalogued in the MCD3 [Hayward et al., 2007b]...... 27 Figure 14: HRSC operating principle and viewing geometry of the CCD sensors [Jaumann et al., 2007]...... 31 Figure 15: Global locations of the 70 areas investigated...... 32 Figure 16: MOC images (2.8 m/pixel) overlaid on HRSC (12 m/pixel) images revealing morphological details...... 33 Figure 17: Comparison of THEMIS grey-scale image and transformed BTR colour image of a crater at 15.5°N, 181.5°E ...... 35 Figure 18: Comparison of gridded (left) and ungridded (right, [Saiger, 2008]) TES data overlaid on HRSC 1223_0000...... 38 Figure 19: Laboratory spectrum derived from the ENVI spectral library (left) and OMEGA spectrum (right) of olivine with wavelength positions (red marks) used for the spectral criteria calculation (see Table 4)...... 42 Figure 20: Laboratory spectra of LCP and HCP from the ENVI spectral library (left) and OMEGA spectrum (right) of pyroxene with wavelength positions (red marks) used for the spectral criteria calculation (see Table 4)...... 42 List of Figures VII

Figure 21: Laboratory spectra of different hydrated minerals derived from the ENVI spectral library (left) compared to OMEGA spectrum (right) of hydrated minerals with wavelength positions (red marks) used for spectral criteria calculation (see Table 4)...... 42 Figure 22: Aeolian features adequate for determining effective wind directions...... 48 Figure 23: Morphology-deduced wind direction derived from slip-face orientation data: the Rabe Crater dune field example (43.9°S, 34.8°N)...... 49 Figure 24: Different types of dark material deposits in Martian craters...... 53 Figure 25: Differences in the appearance of sand sheets...... 54 Figure 26: Diverse dark-material dune types...... 55 Figure 27: Linear dunes associated with barchans in Dawes Crater (9.2°S, 38.0°E)...... 56 Figure 28: Global view of dune type and sand sheet distribution in the localities analyzed...... 58 Figure 29: Cross-section of a probable collapse depression in Ophir Planum (3.0°S, 307.8°E)...... 59 Figure 30: Schematic model of dark material transport into and out of craters...... 60 Figure 31: Indication of craters acting as a local material source...... 61 Figure 32: Dark layers exposed in the walls of a pit within Rabe crater...... 62 Figure 33: Dark layer exposed in the wall of a possible collapse formation in Ophir Planum (3°S, 307.8°E)...... 63 Figure 34: Sketch of two possible cases of exposure of dark material layers indicated by multiple image data (see text for discussion)...... 63 Figure 35: Dark layers exposed in the sun-facing walls of small impact craters in Arabia Terra (near 16.1°N, 344.5°E)...... 64 Figure 36: Dark features exposed in the walls of channels and craters close to Ares Valles showing remarkable similarity to the dark layers described above (6.8°N, 336.9°E; subset of MOC 2000929)...... 64 Figure 37: Dark features exposed in the sun-facing walls of channel-like structures...... 65 Figure 38: Function diagram of probable processes leading to the exposure of dark layers in crater walls...... 65 Figure 39: Sketch of probable situations of coverage and exposure of dark layers in crater walls. . 66 Figure 40: Analysis of the elevations of dark material craters and their surrounding...... 67 Figure 41: (a) OMEGA I/F atmospherically corrected spectra; black curve: spectrum of a dark dune field (ORB0349_2, flattened); blue curve: reference spectrum from a dusty terrain (same orbit, flattened)...... 69 Figure 42: Results of the OMEGA spectral analysis of dark material in Dawes Crater (Terra Sabea, 9.2°S, 38°E; ORB2384_4)...... 71 Figure 43: Results of the OMEGA spectral analysis of dark material in Trouvelot Crater (western Arabia Terra, 16.3°N, 346.5°E; ORB1260_2)...... 72 Figure 44: Glacial features and hydrated minerals in a crater in Ismenius Lacus, near Mamers Valles (34.0°N, 17.0°E)...... 74 List of Figures VIII

Figure 45: Different scenarios for hydration caused by glacial processes in the crater shown in Fig. 44...... 75 Figure 46: Analysis of the floor topography of the crater shown in Fig. 44 revealing a depression as a potential site for a melt water pond close to the dark patch...... 75 Figure 47: Dark material covered by an ice cap in a crater in Vastitas Borealis (70.5°N, 103.0°E). This example proves the coexistence of ice and dark material on Mars, which might possibly be analogues with the former situation in the crater in Fig. 44, following the scenario in Fig. 45, case Ia and Ib (HRSC perspective view 1343_0000; © ESA/DLR/FUB (G. Neukum))...... 76 Figure 48: Spectral analysis of a dark layer exposed in an intra-crater pit at Rabe Crater (43.9, 34.8°E)...... 77 Figure 49: CRISM spectral analysis of dark material emanating from a dark layer exposed in a crater wall...... 79 Figure 50: Global consideration of the mineralogical composition of dark intra-crater deposits. .. 81 Figure 51: Statistic of the mineralogical analysis...... 81 Figure 52: Wind direction comparison based on main dune-forming winds (morphology-deduced, red arrows) and modelled current wind data (MCD data, blue arrows) with corresponding rose diagrams showing the morphology-deduced wind direction...... 83 Figure 53: Results of the wind direction analysis showing the global distribution of dune fields with concurrent, partialy concurrent or divergent wind directions...... 84 Figure 54: THEMIS brightness temperature analysis of dune fields. The mosaics show normalised surface temperatures. Dune fields are outlined by black dotted lines...... 87 Figure 55: Varying diurnal temperature (a) and apparent seasonal thermal inertia (b) for a rock surface caused by a layer of dust representing different dust coating thicknesses [Mellon et al., 2008]...... 91 Figure 56: Thermal inertia versus latitude and elevation (TI is given in J m-2 K-1 s-½)...... 93 Figure 57: Thermal inertia versus latitude and mineralogy (TI is given in J m-2 K-1 s-½)...... 93 Figure 58: Comparison of low and high thermal inertia dunes...... 95 Figure 59: Results of thermal inertia analysis of Kaiser Crater dune field (46.5°S, 18.8°E)...... 96 Figure 60: Thermally thin dark sand sheet in a crater in Thaumasia Planum (16.0°S, 296.3°E)...... 97 Figure 61: Thermal inertia versus grain size demonstrating the correlation of both parameters.... 98 Figure 62: Global distribution of night-time warm and night-time cold dunes/sand sheets...... 99 Figure 63: Global distribution of localities showing high and low night-time thermal inertia values...... 99 Figure 64: Validation of thermal inertia interpretation by means of brightness temperature results in Trouvelot Crater (16.3°N, 346.5°E)...... 100 Figure 65: Combined interpretation of thermal property analyses...... 101 Figure 66: Statistics of the mobility analysis showing the number of locations classified by interpretation results...... 102 Figure 67: Global distribution of probably movable and immovable dunes as inferred from the thermal property analysis...... 106 List of Figures IX

Figure 68: Result image of the mineralogical analysis for comparison with Fig. 67...... 106 Figure 69: Crusting of aeolian bed forms observed at MER Spirit landing site...... 108 Figure 70: Dune shape as evidence of induration...... 111 Figure 71: Gullies as indicators of indurated dune surfaces...... 111 Figure 72: Yardangs and dust mantles as morphological evidence of indurated dune surfaces..... 112 Figure 73: Possible periods of dark material formation incorporated in the sequence of the global geological evolution of Mars (from Werner (2006), modified)...... 115 Figure 74: Comparison between calculated impact melt thickness and global albedo map from Wrobel & Schultz (2007)...... 117 Figure 75: Sketches of scenarios of the volcanic origin hypotheses and the impact melt/glasses theory...... 118 Figure 76: Example of the development of dark layers...... 121

List of Tables X

LIST OF TABLES

Table 1: Parameters of Mars [Kieffer et al., 1992a]...... 5 Table 2: Martian stratigraphic time-system of geological epochs...... 6 Table 3: Major events in the geological history of Mars (adapted from Head et al. (2001))...... 11 Table 4: Spectral parameters used for mineral detection with corresponding detection thresholds adapted from Poulet et al. (2007)...... 43 Table 5: Overview of all data sets used in this study...... 45 Table 6: Modelled yearly mean and maximum MCD wind direction (amplitude) and wind speed data (condensed abstract of the comprehensive database)...... 47 Table 7: Overview of parameters measured for this analysis assigned to the respective data sets. . 50 Table 8: Extract of the database (to be found in the Appendix) which covers one crater but is representative of the whole set of localities...... 51 Table 9: List of analyzed localities with corresponding dune types...... 57 Table 10: Thermal inertia values and corresponding particle sizes...... 89 Table 11: Interpretation of dune surface condition derived from brightness temperature and thermal inertia analysis...... 107 List of Abbreviations XI

LIST OF ABBREVIATIONS

ASCII American Standard Code for Information Interchange ASU Arizona State University BTR Brightness Temperature Record CCD Charged Coupled Device CRISM Compact Reconnaissance Imaging Spectrometer for Mars DLR Deutsches Zentrum für Luft-und Raumfahrt (German Aerospace Center) DTM Digital Terrain Model ESRI Environmental Systems Research Institute GCM Global Circulation Model GIS Geographic Information System GML Geography Markup Language HCP High Calcium Pyroxene HiRISE High Resolution Imaging Science Experiment HRSC High Resolution Stereo Camera IAU International Astronomical Union IDL Interactive Data Language ILD Interior Layered Deposits ISIS Integrated Software for Imagers and Spectrometers LCP Low Calcium Pyroxene MarsGRAM Mars Global Reference Atmospheric Model MAWD Mars Atmospheric Water Detector MCD Mars Climate Database MER Mars Exploration Rover MGD³ Mars Global Digital Dune Database MIMOS 2 Miniaturized Mössbauer Spectrometer MOC Mars Orbiter Camera MOLA Mars Orbiter Laser Altimeter NA near angle NIR near infrared OMEGA Observatoire pour la Minéralogie, l'Eau, les Glaces et l’Activité SI Système International d'unités SRC Super Resolution Channel TAR Transverse Aeolian Ridges TES Thermal Emission Spectrometer THEMIS Thermal Emission Imaging System TI Thermal Inertia VICAR Video Image Communication And Retrieval VIS visual WA wide angle

CHAPTER I: INTRODUCTION AND MOTIVATION

Mars, the 'Red Planet', is not as red as its name implies. Numerous huge dark patches that consist of a fine-grained dark material dissect the reddish-brown colour of its surface. It is a big motivation to analyze this material and to know why it is so different from the other surface materials on Mars.

The dark features on Mars were first discovered from data provided by Mariner 6 and 7 [Christensen, 1983]. Very soon, Mariner 9 and the Viking orbiters revealed that these dark features are frequently associated with impact craters at all latitudes and in a great variety of regions [McCauley et al., 1972; Sagan et al., 1972; Arvidson, 1974]. Before it was ascertained that the intra-crater features are aeolian sand dunes coupled with wind streaks in most cases, dark 'splotches' was the term most commonly used to designate these deposits [e.g. Christensen, 1983; Thomas, 1984].

Dark aeolian material is not only concentrated in impact craters, it can cover extensive regions on Mars. However, dark regions on Mars are not necessarily associated with dark saltating sands. Many authors thought the dark areas were exclusively due to the absence of bright dust and covered by saltating sand, which keeps the surface clean of bright dust [e.g. Sagan et al., 1972; Christensen, 1983; Thomas, 1984; Edgett and Christensen, 1994]. Today it is well known that this interpretation is too simple, and that the albedo alone cannot be used as an indicator in generic surface interpretation. According to Edgett and Malin (2000b), next to bright dust and dark sand, bright saltating sands and dark fines are to be found as well. Transported by suspension, dark fines are unable to build any bed forms but cover extensive regions on Mars. However, the object of investigation in this study is the dark saltating sand because of its dune-forming function.

Early investigations described the distribution, colour, and morphology as well as the physical properties of dark material, arriving at a particle size range for which the suggestion of aeolian transport might be proven [e.g. Breed, 1977; Breed et al., 1979; Christensen, 1983; Thomas, 1984; Edgett and Blumberg, 1994]. Further investigations determined the effective particle size of the dark Martian dunes to be in the medium to coarse-grained range, discovering that Martian dunes seem to be coarser-grained than typical terrestrial dunes [Edgett and Christensen, 1991; 1994]. Early spectral analyses of the dark material were performed by Singer and McCord (1979) and Singer (1980a; b), for example, revealing a basaltic composition of pyroxenes and olivines disclosing the unweathered nature of the material. These spectroscopic analyses have been continued and refined by a number of authors [e.g. Christensen et al., 2000; Bibring and Erard, 2001; Christensen et al., 2001; Bandfield, 2002; Christensen et al., 2003; Bonello et al., 2004; Bibring et al., 2005; Mangold et al., 2007; Mustard et al., 2007; Poulet et al., 2007; Poulet et al., 2008; Christensen et al., 2004b], who similarly concluded that composition is mafic. Chapter I Introduction and Motivation 2

Most of the analyses of dark dunes concentrated on distinct dune fields or regions on Mars, analyzing local morphology, physical properties, and aeolian processes and revealing a variety of dune shapes, the existence of possibly active and non-active dunes, and the complexity of dune-forming winds [e.g. Greeley et al., 1992; Greeley et al., 1993; Greeley et al., 1999; Fenton and Bandfield, 2003; Bourke, 2005; Fenton, 2005b; a; 2006; Fenton and Mellon, 2006]. As far as the source of the dark material is concerned, many questions are still open. Edgett (2002) observed correlations between dark dunes associated with exposures of eroded layered material in several craters at Arabia Terra. He speculated that these layers might contribute a portion of the dark sediments. However, he realised that the light-toned colour of the layers contrasted with the dark tone of the dune and wind streak sediments. Although some darker layers are also present at some sites, he found that this difference in colour from the deep dark material rules out the suggestion that a generic connection might exist between the eroded layers and the dark sediment. Fenton (2005b) found tentative indications of potential dark sand sources in dark layers exposed in the pit walls of a big crater at Noachis Terra. Although a number of previous studies were concerned with dark dunes and dark material as such, the scenario of the origin and evolution of these dark sediments is not resolved yet.

The aim of this study is to refine the body of knowledge about dark material, especially in terms of its local sources, and to shed light on its origin and evolution by focusing on a widespread selection of dark material deposits in Martian craters. The difference to former studies lies in the global view adopted as well as in the analysis of possible regional or global correlations between specific parameters and features (e.g. dune type, composition, dune surface condition, i.e. indurated or unconsolidated) of the dark material deposits. Therefore, as many characteristics of the dark dunes as possible, including their morphology, mineralogy, and thermal properties, will be analyzed and compared with each other. Although these parameters have been analyzed before in specific dune fields, it is not clear whether differences between the dunes and sand sheets exist on a global scale. Combined, this information will be used to propose possible scenarios of origin and a possible time range of genesis and deposition.

Therefore, various high-resolution image data sets, terrain elevation information, spectral data sets, and thermal data were considered in this investigation. The close view afforded by high-resolution image and elevation data might permit tracing the pathway of the material to its arrival at the crater, to learn more about its relative position inside the crater, and to disclose evidence on the mechanism of material mobilization. The question whether all dunes and sand sheets are made of the same material or not, pointing to a consistent or different genesis, will be answered by a mineralogical analysis. A physical property analysis based on thermal data will provide information about the dune surface condition, i.e. about grain size and induration, which indicates both dune immovability and the relative dune age. A combined consideration of mineralogy and dune surface condition might reveal clues about the formation of specific minerals by induration. Chapter I Introduction and Motivation 3

This work is structured so that the reader is first introduced to the fundamentals of Martian geology and its climatic conditions (Chapter II) that are of relevance to the topic of this study, including references to more detailed examinations. The fundamental knowledge about the physics of particle motion and the current status of aeolian processes and bed forms on Mars presented in Chapter III is necessary to understand the problems discussed in this study. An introduction to the datasets and processing methods employed is given in Chapter IV. Given these fundamentals, detailed analyses of the dark intra- crater deposits for the parameters and properties mentioned above will be presented in Chapter V. At this juncture, the methods applied to derive information and the results obtained will be discussed together with the uncertainties and problems involved. Chapter VI provides the result of this work, combining, interpreting, and discussing the information presented in the analysis chapter and deducing from it a final picture of the possible origin and evolution of the dark material. The last chapter presents a comprehensive review of this work, listing its results and summarizing the main conclusions. Furthermore, some important notes on open questions and future investigation proposals will be made.

This work is not intended to analyze the dark material on Mars in its entirety. Several deposits having been omitted, especially in the polar regions. It is probably advisable to analyze these polar dunes separately because of their enigmatically different physical and thermal properties [Putzig et al., 2008] and their different mineralogy [Langevin et al., 2005; Fishbaugh et al., 2007], which might lead to misinterpretations in a global context. The results of such a separate analysis can be compared later with the results of the present analysis so as to arrive at conclusions applying to the whole set of material. Dark material deposits in Valles Marineris have been analyzed by Lucchitta (1990; 2001), for example. The results from other sites presented in the current literature have been regarded in this study. Thanks to the huge amount of samples, it should now be more feasible to apply conclusions from the samples discussed to the collectivity of intra-crater deposits, and to make proposals for interpreting further studies at other localities. Due to the comprehensive amount of data and localities analyzed here, it cannot be the intention of this work to go into the details of every single locality, but merely to bridge the gap between detailed local and generalised global analyses, with the aim of being as exact as possible.