The MESSENGER Ultraviolet and Visible Spectrometer: Calibration, Mercury Sodium Observation Strategies, and Analysis of In-flight Lunar Observations by Eric Todd Bradley B.S., University of Alabama (Birmingham), 1997 M.S., University of Florida, 2001 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Aerospace Engineering Sciences 2007 This thesis entitled: The MESSENGER Ultraviolet and Visible Spectrometer: Calibration, Mercury Sodium Observation Strategies, and Analysis of In-flight Lunar Observations written by Eric Todd Bradley has been approved for the Department of Aerospace Engineering Sciences ___________________________________________ Prof. William Emery ___________________________________________ William McClintock ___________________________________________ Prof. Nick Schneider ___________________________________________ Prof. Larry Esposito ___________________________________________ Prof. Dan Baker Date________________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Bradley, Eric Todd (Ph.D., Department of Aerospace Engineering Sciences) The MESSENGER Ultraviolet and Visible Spectrometer: Calibration, Mercury Sodium Observation Strategies, and Analysis of In-flight Lunar Observations Thesis directed by Dr. William McClintock The MESSENGER Ultraviolet and Visible Spectrometer (UVVS) is one channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) that is presently on the MESSENGER spacecraft en-route to Mercury. The goal of this thesis is the calibration of the UVVS and development of observation strategies for Mercury’s exosphere during the MESSENGER orbital phase. The calibration of the UVVS is crucial for the analysis of flight data. Both pre-launch laboratory and post- launch stellar calibration experiments that characterize the performance of the UVVS are described and results presented. Mercury has been shown to have a tenuous exosphere. Careful observation planning of the exosphere before orbit insertion is crucial in order to devise strategies to address key scientific questions concerning the nature of the Mercurian exosphere. A software planning tool has been developed that combines instrument performance, spacecraft orbital geometry, and predictions of sodium in Mercury’s exosphere in order to develop observational strategies and set detection limits for sodium. Using the observation strategies, signal and SNR predictions based on a sodium exospheric model are made that characterize the ability of the UVVS to distinguish between sodium exospheric production processes. In 2005, MESSENGER flew by the Earth-Moon system and gave an opportunity to validate instrument detection limit predictions. A description of lunar limb iv observations is given along with a comparison of detection limits based on real and model predicted data. Dedication This thesis is dedicated to my 2 sons Hayden Walt Bradley and Heston Jacob Bradley vi Acknowledgements I would like to thank the members of my thesis committee, Bill McClintock, Bill Emery, Nick Schneider, Larry Esposito, and Dan Baker for the professional manner in which they contributed to my thesis as committee members. In particular, I thank my thesis advisor, Bill McClintock, for the opportunity and support to work on this project and for an extensive review of my thesis. I also give a special thanks to Nick Schneider for extremely helpful advice and guidance. This project was supported by the National Aeronautics and Space Administration’s Discovery program through a contract to the University of Colorado from the Carnegie Institution of Washington. I thank Rosemary Killen for her Mercury sodium exospheric model that I used for my thesis research. I thank Greg Holsclaw for reviewing my defense presentation and for all the many conversations that we have had. I thank Dewey Anderson for his helpful insights concerning my defense presentation and for our lengthy discussions concerning scientific matters as well as many other subjects I thank my parents, Roy and Shelby Bradley, for giving me a good childhood, for always being reliable, and for their many years of support. I especially thank my wife, Tara Bradley, who made as many sacrifices during the years that I worked on this thesis as I have made and who was always there to give me moral and emotional support. vii Contents Chapter 1 Introduction 1 1.1 Thesis outline 1 1.2 Mercury Overview 3 1.2.1 Mercury characteristics 3 1.2.2 3:2 spin-orbit resonance 5 1.3 The MESSENGER mission 9 1.3.1 MESSENGER instrumentation 10 1.3.2 The MESSENGER orbit 10 1.3.3 MESSENGER orientation constraints 15 1.4 Mercury observations 16 1.4.1 The Rayleigh 17 1.4.2 Resonance scattering 20 1.4.3 g-values 21 1.4.4 Radiation pressure 24 1.4.5 D2 to D1 line ratio 27 1.5 Observation of Mercury’s exosphere by MESSENGER 30 1.5.1 Tangentially integrated column abundances 30 1.5.2 Attitude control capabilities 32 1.6 Summary 34 viii 2 MASCS UVVS Pre-launch and Post-launch Calibration 35 2.1 Introduction 35 2.1.1 Overview of instrument calibration 35 2.1.2 Calibration facilities 36 2.1.3 Stellar observation summary 41 2.2 Instrument description 42 2.3 Telescope boresight 47 2.4 Dark counts 54 2.5 Wavelength scale 56 2.6 Point spread function 60 2.7 Field of view 70 2.8 Spectrometer scattered light 72 2.9 Spectrometer stray light 77 2.10 Spectrometer polarization 77 2.11 Telescope off axis response 81 2.12 Window transmission 89 2.13 Radiometric sensitivity 91 2.13.1 Overview 91 2.13.2 Pre-launch calibration 94 2.13.3 Post-launch stellar calibration 96 2.14 Dead time correction 102 2.15 Summary 107 ix 3 Mercury’s Surface-Bounded Sodium Exosphere 110 3.1 Introduction 110 3.1.1 General description of the exosphere 110 3.2 Overview of the sodium exosphere 113 3.2.1 High latitude enhancements 113 3.2.2 Dawn dusk asymmetries 114 3.2.3 Extended sodium tail 116 3.2.4 Smoothly varying sodium component 118 3.3 Source process physics 121 3.3.1 Ion sputtering 121 3.3.2 Photon stimulated desorption 125 3.3.3 Meteoritic vaporization 130 3.3.4 Thermal vaporization 135 3.4 Loss process physics 138 3.4.1 Photoionization 139 3.4.2 Dayside surface capture 142 3.4.3 Gravitational escape 143 3.4.4 Cold trapping 144 3.5 Dynamics 145 3.5.1 Radiation pressure 145 3.5.2 Magnetospheric transport of ions 146 3.6 Unresolved issues with the sodium exosphere 146 3.7 Summary 148 x 4 Observation software planning tool 150 4.1 Introduction 150 4.2 MESSENGER trajectory database 152 4.3 Sodium exospheric emissions 153 4.3.1 Sodium exospheric model 153 4.3.2 Distribution of density with height 154 4.3.3 Tangentially integrated column densities 157 4.3.4 Limitations of the constant gravity assumption 159 4.3.5 Conversion of tangentially integrated column abundances to radiances 161 4.4 Predicted count rate and uncertainty 161 4.4.1 Count rates from sodium exospheric emission 163 4.4.2 Solar continuum background 167 4.4.3 Addition of dark counts 170 4.4.4 Addition of random noise 172 4.4.5 Determination of predicted signal and SNR 172 4.5 Summary 184 5 Observation opportunities, Signal, and SNR predictions 186 5.1 Introduction 186 5.2 MESSENGER orbital constraints 187 5.2.1 Geometrical pointing constraints 187 5.2.2 Projected field of view 193 xi 5.2.3 Off-axis angle 193 5.2.4 Spacecraft velocity 198 5.2.5 Sub-spacecraft latitude 201 5.2.6 Summary of observation constraints 201 5.3 Observations that address science questions while obeying s/c geometrical and temporal constraints 206 5.4 Detection limits 213 5.5 Test cases that address physical processes associated with the exosphere 221 5.5.1 Trends for each source process 223 5.5.2 Observation strategies to test physical processes 229 5.5.3 Distinguishing between temperatures for high energy processes 229 5.5.4 Distinguishing between dominant source processes 237 5.5.5 North-south asymmetries 248 5.5.6 Summary of test cases 260 5.6 Optimum altitudes for observations 260 5.7 Error analysis 270 5.8 Summary 270 6 Lunar flyby 276 6.1 Introduction 276 6.2 Observations 277 6.2.1 Full disk observations 277 6.2.2 Limb scans 279 xii 6.3 Data reduction 279 6.3.1 Extra grating step removal 279 6.3.2 Dark count and spectrometer scattered light removal 281 6.3.3 Integration time and non-linearity correction 283 6.3.4 Pointing correction 283 6.3.5 Removal of off-axis light 284 6.3.6 Removal of grating scans with noise spikes 284 6.3.7 Disk integrated radiance and reflectance 284 6.3.8 Determination of background for limb observations 286 6.4 Analysis of lunar limb observations 289 6.4.1 Comparison of lunar data off-axis light with the off-axis model 289 6.4.2 Comparison of lunar data detection limits with model predicted detection limits 294 6.4.3 Variation of off-axis light during a grating scan 296 6.4.4 Comparison of detection limits with D1 + D2 counts 298 6.4.5 Comparison with other datasets 302 6.5 Uncertainty analysis 307 6.6 Summary 310 7 Conclusions 315 Bibliography 323 xiii Tables Table 1.1 Instruments on the MESSENGER spacecraft that will perform scientific investigations of Mercury 12 2.1 List of pre-launch calibration experiments 36 2.2 Stellar observation summary 43 2.3 Characteristics of the 3 UVVS spectral channels 45 2.4 Relation of UVVS atmospheric and surface slit boresights to the alignment cube 48 2.5 Boresight