Research Collection Doctoral Thesis Polarimetry of Planets in the solar system and beyond Author(s): Bazzon, Andreas Publication Date: 2013 Permanent Link: https://doi.org/10.3929/ethz-a-010087118 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Diss. ETH No. 21541 Polarimetry of Planets in the solar system and beyond A dissertation submitted to ETH Zürich for the degree of DOCTOR OF SCIENCES presented by ANDREAS BAZZON MSc ETH Zürich born July 16, 1983 citizen of Lohn GR, Switzerland accepted on the recommendation of Prof. Dr. M. R. Meyer Prof. Dr. H. M. Schmid Dr. D. Mouillet Zürich, 2013 — To my family — Contents Abstract ........................................ ix Zusammenfassung .................................. xi Abbreviations .................................... xiii 1. Introduction ................................... 1 1.1 The hunt for extra-solar planets ....................... 1 1.2 Polarized light ................................ 2 1.2.1 Stokes-Formalism ......................... 4 1.2.2 Mueller calculus .......................... 6 1.2.3 Basic polarimeter design ...................... 8 1.3 Polarimetry of planets ............................ 9 1.3.1 Observables ............................. 10 1.3.2 Solar system observations ..................... 12 1.4 The SPHERE instrument .......................... 14 1.4.1 CPI ................................. 15 1.4.2 IRDIS ................................ 16 1.4.3 IFS ................................. 16 1.4.4 ZIMPOL .............................. 17 1.5 About this thesis .............................. 17 Bibliography ................................ 19 2. Polarization of Earth .............................. 21 2.1 Introduction ................................. 22 2.2 Instrumentation ............................... 24 2.2.1 Instrument requirements ...................... 24 2.2.2 The earthshine polarimeter ..................... 26 2.3 Observations ................................ 27 2.4 Data reduction ................................ 28 2.4.1 Polarimetric reduction ....................... 28 2.4.2 Extracting the earthshine polarization ............... 29 2.5 Earthshine polarization results ....................... 35 2.5.1 Data ................................. 35 2.5.2 Fits for the phase dependence ................... 35 2.5.3 The moonshine polarization .................... 38 v Contents 2.6 A correction for the depolarization due to backscattering at the lunar sur- face ..................................... 39 2.7 Polarization of planet Earth ......................... 42 2.7.1 Fractional polarization derived from the earthshine ........ 42 2.7.2 Comparison with previous measurements ............. 42 2.7.3 Comparison with the models from Stam (2008) .......... 46 2.7.4 Polarization flux contrast for the Earth - Sun system ....... 46 2.8 Summary and discussion .......................... 48 Bibliography ................................ 51 3. Limb polarization of Titan ........................... 53 3.1 Introduction ................................. 54 3.2 Observations ................................ 55 3.3 Basic data reduction ............................. 57 3.3.1 Calculation of Stokes parameters for ACS ............. 58 3.3.2 Calculation of Stokes parameters for NICMOS .......... 59 3.4 Intensity images ............................... 59 3.5 Stokes Q and U images for Titan ...................... 60 3.6 The radial polarization ........................... 62 3.6.1 Radial Stokes Qr and Ur images for Titan ............. 62 3.6.2 Polarization as function of radius ................. 63 3.6.3 Disk-integrated radial polarization ................. 63 3.6.4 Correction for the PSF smearing effect ............... 66 3.7 Comparison with limb polarization models ................. 69 3.7.1 Atmospheric parameters ...................... 69 3.7.2 Radiative transfer code ....................... 71 3.7.3 Results ............................... 72 3.8 Discussion and conclusions ......................... 75 3.8.1 Detection of the limb polarization. ................. 75 3.8.2 Comparison with model calculations ................ 76 3.8.3 Prospects for extra-solar planets .................. 77 3.9 Appendix: Titan scattering model parameters ............... 79 3.9.1 Number densities and column density ............... 79 3.9.2 Rayleigh scattering optical depth .................. 79 3.9.3 Methane absorption ......................... 79 3.9.4 Haze optical depth and single scattering albedo .......... 80 3.9.5 Effective single scattering albedo ................. 80 Bibliography ................................ 81 4. ZIMPOL/SPHERE ............................... 83 4.1 Introduction ................................. 84 4.2 The ZIMPOL principle ........................... 84 4.3 ZIMPOL/SPHERE polarimetric concept .................. 86 4.3.1 ZIMPOL in SPHERE ........................ 86 4.3.2 Compensation of the telescope polarization ............ 87 vi Contents 4.3.3 The derotator polarization and the role of HWP2 and HWPZ ... 87 4.3.4 Polarization switch and the compensation of the CPI polarization 88 4.4 Polarimetric calibration of ZIMPOL/SPHERE ............... 89 4.4.1 Overview of the calibration principle ............... 89 4.4.2 ZIMPOL/SPHERE calibration optics ............... 90 4.4.3 Alignment of the polarization axis ................. 92 4.4.4 Calibration and monitoring measurements ............. 94 4.5 Tests and performance of the FLC-modulator package ........... 96 4.5.1 Definitions: polarimetric efficiency and cross-talks ........ 96 4.5.2 Basic properties of the ferro-electric liquid crystal modulator ... 98 4.5.3 FLCeffects vs. Detector Effects .................. 99 4.5.4 Performance of the FLC modulator ................100 4.6 Conclusions .................................102 Bibliography ................................104 5. Final remarks and Outlook ...........................105 5.1 Titan observations with SPHERE ......................105 5.2 Extra-solar planets with ZIMPOL/SPHERE ................106 5.3 Pushing ZIMPOL/SPHERE to the limits ..................108 5.3.1 The Gliese 876 system .......................108 5.4 An imaging polarimeter for the E-ELT ...................111 Bibliography ................................112 6. Acknowledgments ................................113 7. List of Publications ...............................115 8. Curriculum Vitæ .................................117 vii Contents viii Abstract Since ancient times men have been speculating about the possibility of alternative worlds other than Earth, and the ultimate question whether we are alone in this vast and myste- rious universe. At least the first part of this question is now solved. We are now rapidly approaching a number of over a thousand of confirmed exoplanets, revealing an over- whelming diversity and complexity of planetary systems. However, direct imaging of extra-solar planets remains very challenging, and the majority of the known exoplanets could only be detected by indirect methods where the presence of the planet has been inferred by its impact on the flux and motion of its host star. This thesis is devoted to the direct detection and characterization of scattered light from extra-solar planets. The challenge in measuring the scattered light from extra-solar planets is to overcome the huge brightness contrast between the central star and the planet. Even when using the largest available telescopes, equipped with extreme adaptive optics systems, the planetary signal is still buried below the bright halo of the host star. Thus, a dedicated instrument with some kind of differential measuring principle is required to disentangle the planetary signal from the bright stellar halo. Differential polarimetry is a powerful method to achieve this, making use of the fact that the light reflected at the plane- tary atmosphere is polarized, whereas the light of the central star is generally unpolarized. Furthermore, polarimetry is also a powerful diagnostic tool for the characterization of the planetary atmosphere and surface. The first part of this thesis investigates the polarimetric signatures of Earth and Titan, i.e. two showcase examples for planets with atmospheres dominated by strongly polariz- ing Rayleigh scattering and scattering at small aggregate haze particles respectively. The second part discusses the polarimetric calibration and performance of the upcoming high- performance imaging polarimeter ZIMPOL for the new high-contrast SPHERE planet finder instrument that is currently built for the ESO-VLT. In Chap. 2 new benchmark values for the broad-band earthshine polarization in the B, V, R, and I band for phase angles α = 30◦ 110◦ are presented. For the measurements we built a dedicated wide-field imaging polarimeter− with a focal plane mask to suppress the light scattered from the bright lunar crescent. The depolarization introduced by the backscattering at the lunar surface is investigated, and for the first time we derive and apply a polarization efficiency correction based on lunar soil measurements from the lit- erature. The polarization of Earth at quadrature phase α = 90◦ is as high as 25 % in the B band and decreasing with wavelength. The polarization contrast between Sun and 11 Earth at quadrature is at the level of a few times 10− , and the best phase to observe an extra-solar Earth-Sun system is at α 65◦. In Chap. 3 disk resolved