In-situ exploration of Titan's atmosphere Adapting SPEX for a balloon mission to Titan Nicoletta Silvestri August 31, 2010 Faculty of Aerospace Engineering - Delft University of Technology In-situ exploration of Titan's atmosphere Adapting SPEX for a balloon mission to Titan MASTER of SCIENCE THESIS Nicoletta Silvestri Supervisor: dr. D.M. Stam SRON - Netherlands Institute for Space Research Sorbonnelaan 2, 3584 CA, Utrecht The Netherlands Co-Supervisor: dr. L.L.A. Vermeersen Astrodynamics & Space Missions Faculty of Aerospace Engineering - TU Delft Kluyverweg 1, 2628 CD Delft The Netherlands Faculty of Aerospace Engineering - Delft University of Technology Preface The Aerospace Engineering Masters at the Delft University of Technology is con- cluded with the development of a final project. The research and the development of the project are documented in a thesis work. This report presents the thesis work of the author. The project hereby presented has been developed at SRON - Netherlands Institute of Space Research - in Utrecht at the division of Earth and Planetary Science. The last stage of this research has been carried out at the chair of Astrodynamics and Space Missions at the aforementioned faculty. The thesis is addressed to people who are interested in both atmospheric science and instrument engineering. This thesis will provide the reader with basic but thorough insight into the related disciplines of radia- tive transfer in planetary atmospheres and spectropolarimetry. The reader that will go through the report will read about how to characterize the microphysical properties of aerosol particles contained in the atmosphere of Titan by using the spectropolarimeter SPEX for in-situ exploration aboard a hot-air balloon. This thesis can serve as a means for broadening ones knowledge on the subject or as a start for further research. The background knowledge on radiative transfer and light scattering phenomena is given by Chapters 4 to 6. Here, the single scattering and the multiple scattering tak- ing place in the atmosphere of Titan are explained together with numerical simulations of radiative transfer. If the reader is more interested in spectropolarimetry, the author would recommend to take a look at Chapter 7. Here, SPEX (Spectropolarimeter for Planetary Exploration) and its novel spectropolarimetric technique is treated in detail. In this chapter, the advantages of its new method are compared to classical polarimet- ric techniques. In addition, the author would like to bring the attention of the reader to the content of appendix A. Here it is possible to find the article presented by the author at the 7th edition of the International Planetary Probe Workshop (IPPW-7) held in Barcelona on June 14-18 2010. The article is a concise version of the thesis work. Together with the article, I would definitively recommend to read Chapter 1 where a detailed introduction to the whole project is given. I would like to thank my thesis supervisor Dr. Daphne Stam in specific for her constant and fruitful guidance in the 'new world' of atmospheric radiative transfer. I would like to thank her for her help, patience and constant availability during the research. A spe- cial thanks goes to Dr. ir. Martijn Smit and Dr. ir. Jeroen Rietjens, without their help this thesis work would not have been possible. I would like thank my co-supervisor dr. Bert Vermeersen for his guidance during the last stage of the thesis work. A particular thank goes also to the EPS division members for the incredible year spent together at SRON. In particular I would like to thank Dinand Schepers, my desk-mate at SRON VI and friend in life. A special word of appreciation goes out to the students from the 9th floor for keeping me company during this work. Specifically, in random order, Guido Ridolfi, Antonio Pagano, Stefan van Doorn, Hermes Jara Orue, Tom de Groot, Mirjam Boere, Vivek Vittaldev. Moreover, I am particularly grateful to my'dutch family' and to Federica and Manuela for their precious friendship. Last but not least, the biggest thank goes to my boyfriend Bob who supported me during the IPPW-7 and during most of the thesis work. I would like to thank him for helping me out with drawing the new design of SPEX and making the movie about the Titan-balloon. This thesis work is dedicated to my Family, who gave me the possibility to become who I am. Nicoletta Silvestri August 23, 2010 Summary Titan is the biggest moon orbiting the planet Saturn. The peculiarity of Titan lies in its thick nitrogen atmosphere with a surface pressure of 1.5 bar. The atmosphere consists of two thick haze layers thought to be composed of fractal-shaped aerosols. Knowing the properties of the hazes is crucial for understanding the existence and evolution of Titan's atmosphere and the dynamical processes that take place therein. This work investigates how we can characterize the hazes in Titan's atmosphere using a balloon- borne spectropolarimeter SPEX, the Spectropolarimeter for Planetary EXploration. First, we focus on the haze particles: what is the knowledge that has been acquired so far and what more do we need to know about the aerosols? Next, we explain the concept of spectropolarimetry, which is the method we plan to use to get more information about the haze. Then, we explain how to measure the aerosol's properties. We describe the SPEX instrument and its novel polarimetric method, and compare it with classical polarimetric methods. SPEX measures simultaneously the radiance and the degree and direction of linear polarization of sunlight scattered inside a planetary atmosphere. We present numerical simulations of sunlight scattered inside Titan's atmosphere and numerical simulations of realistic performances of SPEX as payload on the hot-air balloon of the Titan Saturn System mission (TSSM) which was submitted for the ESA's Cosmic Vision Program. In its current design, SPEX has been optimized as payload for a Mars orbiter. For Titan we would prefer a higher spectral resolution to resolve the methane features that show up in the spectra of scattered sunlight inside the atmosphere. Moreover, compared to a Mars SPEX, we would prefer different viewing angles due to its placement on a balloon instead of on an orbiter. We conclude that if we increase the thickness of the birefringent crystals in the SPEX' pre-optic system from 4 mm (Mars design) to 16 mm, we can resolve the strong methane bands present in Titan's spectra, which will allow us to retrieve aerosol information contained in these bands. An interesting solution for the viewing angles would be to have a rotating disk placed on a boom beneath the balloon's gondola and inclined under an angle of 40◦ with respect to the vertical axis. The disk would contain a number of apertures looking outward along its rim. We conclude that SPEX on the TSSM balloon would be a powerful tool for characterizing Titan's haze. Contents Preface .......................................V Summary ...................................... VII List of Figures . .XI List of Tables . XIV List of Symbols . XVI 1 Introduction 1 2 Titan's structure 9 2.1 Titan's Interior and methane outgassing . .9 2.2 Titan's atmosphere . 11 2.2.1 Atmosphere's origin . 11 2.2.2 Hydrocarbon cycle . 13 2.2.3 Seasonal haze circulation . 17 3 Light scattering 23 3.1 Whyistheskyblue?............................. 23 3.2 Describing radiation and polarization . 24 3.2.1 Stokes vector . 25 3.3 Mueller matrix and Poincar´esphere . 29 4 Single scattering 31 4.1 Analytic representation . 31 4.2 Mie theory . 33 4.3 T-matrix . 37 4.4 Single scattering properties of Titan aerosols . 38 4.4.1 Complex refractive index . 38 4.4.2 Influence of monomer size . 38 4.4.3 Influence of aerosol size . 39 5 Modeling Titan's atmosphere 51 5.1 Analytic representation . 51 5.2 The Doubling-Adding method . 56 5.3 DAP input parameters . 57 5.4 Setting up the atmospheric scenario . 59 5.4.1 Surface albedo . 59 5.4.2 Atmospheric pressure and temperature . 60 5.4.3 Aerosols' Refractive index . 62 X CONTENTS 5.4.4 Methane absorption . 62 5.4.5 Aerosol optical thickness . 66 5.5 Sensitivity tests . 69 6 Spectropolarimetry on Titan - Radiative transfer results 71 6.1 Reference atmospheric model . 71 6.2 Aerosol vertical profile . 73 6.3 Surface albedo . 75 6.4 Sun zenith angle . 75 6.5 Azimuthal angle . 80 6.6 Aerosol particles . 81 6.6.1 Aerosol size . 81 6.6.2 Aerosol inner structure: compact versus fluffy . 83 7 SPEX - Spectropolarimeter for Planetary EXploration 87 7.1 The classical method of polarization measurements . 87 7.2 Spectral modulation and optical design . 88 7.2.1 Retarders . 90 7.2.2 Polarizers . 93 7.2.3 Pre-optics . 93 7.2.4 Optical design . 95 7.3 Spectral modulation basic principle . 96 7.4 Retrieval algorithm . 100 7.5 SPEX end-to-end simulator . 102 8 SPEX on Titan - End-to-End simulator results 105 8.1 Simulated SPEX' observations . 105 8.2 Designing SPEX for a Titan balloon . 107 8.2.1 Viewing geometries . 108 8.2.2 Spectral resolution . 109 9 Conclusions and recommendations 113 9.1 Conclusions . 113 9.2 Recommendations . 115 Bibliography 117 Appendix A 125 Article presented at the IPPW-7 . 125 Appendix B 139 Titan facts and figures . 139 Appendix C 141 Cassini Huygens instrumentation . 141 Appendix D 143 Aerosols vertical mixing ratio profiles . 143 List of Figures 1.1 Titan's atmosphere temperature profile. .2 1.2 Liquid presence on Titan surface. .3 1.3 Titan haze layer. .7 2.1 Titan's interior. 10 2.2 Mass spectrum of the major constituents of Titan's atmosphere.