CASSINI FINAL MISSION REPORT 2019 1 COMPOSITE INFRARED SPECTROMETER The Composite Infrared Spectrometer (CIRS) was an infrared Fourier Transform Spectrometer that measured infrared radiation from 7-1,000 µm that was emitted by Saturn, its rings and its moons. The science objectives of CIRS were to measure infrared emissions from atmospheres, rings and surfaces in the Saturn system to determine their composition, temperatures and thermal properties; to map the atmospheres of Saturn and Titan in three dimensions to determine temperature and pressure profiles with altitude, gas composition, and the distribution of aerosols and clouds. Molecular composition is determined in the infrared by the emitted or absorbed energy at specific wavelengths. The infrared spectrum identifies the gases or solids present and their relative amounts. CIRS consisted of two interferometers that operated in the far-infrared and mid-infrared. They shared a common telescope and scan mechanism. CIRS included three sets of infrared detectors (Focal planes 1, 3, and 4) and a passive radiative cooler plate. 2 VOLUME 1: MISSION OVERVIEW & SCIENCE OBJECTIVES AND RESULTS CONTENTS COMPOSITE INFRARED SPECTROMETER .................................................................................................................. 1 Executive Summary................................................................................................................................................ 6 Key Science Results .................................................................................................................................... 6 Saturn ................................................................................................................................................ 6 Rings ................................................................................................................................................. 6 Titan .................................................................................................................................................. 7 Icy satellites ....................................................................................................................................... 7 Jupiter ................................................................................................................................................ 8 CIRS Summary ....................................................................................................................................................... 8 Key Objectives for CIRS ......................................................................................................................................... 9 Key Science Objectives for Saturn .............................................................................................................. 9 Key Science Objectives for the Rings .......................................................................................................... 9 Key Science Objectives for Titan ............................................................................................................... 10 Key Science Objectives for the Icy Satellites ............................................................................................. 11 CIRS Science Assessment ................................................................................................................................... 11 Saturn ........................................................................................................................................................ 11 Rings .......................................................................................................................................................... 13 Titan ........................................................................................................................................................... 14 Titan AO objectives ......................................................................................................................... 14 Titan Cassini Solstice Mission (CSM) traceability matrix objectives................................................ 14 Icy Satellites ............................................................................................................................................... 16 CIRS Saturn System Science Results .................................................................................................................. 17 Saturn ........................................................................................................................................................ 17 Formation, evolution, and internal structure .................................................................................... 17 Atmospheric gas composition ......................................................................................................... 20 Clouds and aerosols ........................................................................................................................ 21 Atmospheric structure and circulation ............................................................................................. 22 Temporal behavior .......................................................................................................................... 23 Rings .......................................................................................................................................................... 26 Introduction ...................................................................................................................................... 27 Ring thermal structure ..................................................................................................................... 29 Ring optical depth at mid-IR wavelengths ....................................................................................... 35 Ring thermal models ....................................................................................................................... 36 Particle scale properties .................................................................................................................. 39 Ring-scale properties ...................................................................................................................... 44 Regolith properties deduced from CIRS emissivity roll-off .............................................................. 47 Conclusions and future work ........................................................................................................... 50 Titan ........................................................................................................................................................... 53 Introduction ...................................................................................................................................... 53 Spectral modeling and inversion ..................................................................................................... 55 Titan’s surface ................................................................................................................................. 59 Atmospheric dynamical state .......................................................................................................... 62 Gas composition and chemistry ...................................................................................................... 69 Hazes and condensates .................................................................................................................. 83 Conclusions and further work .......................................................................................................... 89 Icy Satellites ............................................................................................................................................... 89 CASSINI FINAL MISSION REPORT 2019 3 Changes on Enceladus (IC1a) ........................................................................................................ 89 Enceladus’ interior (IN1a) ................................................................................................................ 92 Compare Saturn’s mid-sized satellites (IN1b) ............................................................................... 101 Activity on Dione (IN1c), Howett et al. [2018] ................................................................................ 111 Rhea’s ring (IN2a) ......................................................................................................................... 112 Tethys and the E-ring (IN2b) ......................................................................................................... 113 Characterize the surfaces of Saturn’s small satellites (IN2d) ........................................................ 113 Open Questions ....................................................................................................................................... 114 Saturn ............................................................................................................................................ 114 Rings ............................................................................................................................................. 115 Titan .............................................................................................................................................. 115