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Phd. Compositional Variation of Small Bodies Across the Solar System

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Observatoire de Paris Ecole´ Doctorale Astronomie et Astrophysique d'^Ile-de-France THESE` DE DOCTORAT pr´esent´eepour obtenir le grade de DOCTEUR DE L'OBSERVATOIRE DE PARIS Sp´ecialit´e:Astronomie & Astrophysique par Francesca E. DeMeo La variation compositionnelle des petits corps `atravers le syt`emesolaire soutenue le 16 juin 2010 devant le jury: Dr. Bruno Sicardy Pr´esident Dr. Hermann Boehnhardt Rapporteur Dr. Alberto Cellino Rapporteur Dr. Humberto Campins Examinateur Dr. Beth Clark Examinateur Dr. Daniel Hestroffer Examinateur Dr. M. Antonietta Barucci Co-Directrice de th`ese Dr. Richard P. Binzel Co-Directeur de th`ese LESIA, Observatoire de Paris-Meudon [email protected] The Paris Observatory Doctoral School of Astronomy and Astrophysics of ^Ile-de-France DOCTORAL THESIS presented to obtain the degree of DOCTOR OF THE PARIS OBSERVATORY Specialty: Astronomy & Astrophysics by Francesca E. DeMeo The compositional variation of small bodies across the Solar System defended the 16th of June 2010 before the jury: Dr. Bruno Sicardy President Dr. Hermann Boehnhardt Reviewer Dr. Alberto Cellino Reviewer Dr. Humberto Campins Examiner Dr. Beth Clark Examiner Dr. Daniel Hestroffer Examiner Dr. M. Antonietta Barucci Co-Advisor Dr. Richard P. Binzel Co-Advisor LESIA, Observatoire de Paris-Meudon [email protected] Abstract Small bodies hold keys to our understanding of the Solar System. By studying these populations we seek the information on the conditions and structure of the primordial and current Solar System, its evolution, and the formation process of the planets. Constraining the surface composition of small bodies provides us with the ingredients and proportions for this cosmic recipe. This thesis, comprised of studies of inner and outer Solar System small bodies, is dedicated to understanding the compositional gradient across the Solar System through spectroscopic and photometric measurements. I present a taxonomy of visible and near-infrared spectral data based on 371 asteroid spectra. The taxonomy consists of 24 classes that best categorize the spectral variation seen among inner Solar System small bodies. From the creation of this taxonomy we learn that with only visible wavelength data there is uncertainty in shape of the 1-µm band. While near-infrared wavelength range is excellent for interpreting data containing diagnostic 1- and 2-µm bands, the more subtly featured C- and X-complexes appear to be largely degenerate in this wavelength regime. I analyze the photometric colors of 23 Transneptunian Objects and Centaurs, nine of which have never been previously observed, and assign them taxonomic classifications. I discuss objects that either have changed classes from previous data or have significant changes in absolute magnitude. Furthermore, I interpret the surface composition of three outer Solar System small bodies, Jupiter-coupled object (52872) Okyrhoe, and TNOs (90482) Orcus and (73480) 2002 PN34, by modeling spectroscopic measurements in the visible and near-infrared wavelength ranges. The spectra reveal varying amounts of H2O ice among these bodies. For Orcus I provide rough constraints for the presence of materials more volatile than water ice. I present a search for solid ethane, C2H6, on the surfaces of Pluto and Triton, based on near-infrared spectral observations. I model each surface using a radiative transfer model based on Hapke theory (Hapke, 1993) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. While the presence of less than a few percent of ethane cannot be excluded on both bodies, there is no strong detection on either. Finally, I review the current knowledge of the compositional distribution of material in our Solar System, providing the global view of small bodies. I particularly focus on the presence of water in all its phases which is especially pertinent our understanding of our own planet, Earth, and the life on it. I briefly compare the general structure of our Solar System to other imaged debris disks to put into perspective the detailed, though narrow, view of our own Solar System with the broad, low resolution view of others. Keywords: Planetology, Asteroids, Transneptunian Objects, Centaurs, Observations, Spectroscopy, Photometry R´esum´e Les petits corps sont des cl´espour comprendre notre syst`emesolaire. L'´etudede cette population nous donne en effet acc`esaux informations sur l'´etatet sur la structure du syst`emesolaire primordial et du syst`emesolaire actuel, ainsi que sur son ´evolution et sur les processus de formation des plan`etes. Conna^ıtrela composition de surface des petits corps nous fournit des ingr´edients et des proportions pour cette recette cosmique. Cette th`ese,qui inclut l'´etudedes petits corps du syst`emesolaire interne et externe, est d´edi´ee`ala compr´ehensionde la tendance compositionnelle des corps `atravers le syst`eme solaire en utilisant des mesures photom´etriqueset spectroscopiques. Je pr´esente une classification (taxonomie) dans les longueurs d'ondes du visible et du proche infrarouge (de 0.4 2.4 µm), bas´eesur les donnes spectrales de 371 ast´ero¨ıdes.Cette taxonomie comprend 24 classes qui chacune caract´eriseau mieux les variations spectrales observ´eesparmi les petits corps du syst`eme solaire interne. De part la cr´eationde cette taxonomie, nous apprenons qu'en analysant les donn´eesdans les longueurs d'ondes du visible uniquement, il reste des incertitudes sur la forme de la bande d'absorption `a1 micron. Bien que la gamme de longueur d'onde du proche infrarouge soit excellente pour interpr´eter les donn´eesincluant les bandes diagnostiques `a1 et 2 microns, les complexes C et X des spectres sans fortes bandes paraissent plut^otd´eg´en´er´es dans ce r´egime. J'analyse les couleurs photom´etriquesdes 23 objets trans-neptuniens (OTN) et Centaures, parmi lesquels neuf n'avaient jamais ´et´eobserv´espr´ec´edemment, et je leur assigne une classe taxonomique. Je discute des objets qui ont soit chang´ede classe depuis les donn´eespr´ealablessoit chang´econsid´erablement de magnitude absolue. De plus, j'interpr`etela composition de surfaces de trois petits corps du syst`eme solaire externe, l'objet coupl´eavec Jupiter (52872) Okyrhoe et les OTNs (90482) Orcus et (73480) 2002 PN34, en mod´elisant des mesures spectroscopiques dans les gammes du visible et du proche infrarouge. Les spectres r´ev`elent des variations de quantit´ede glace d'eau `ala surface de ces corps. Pour Orcus j'apporte des contraintes approximatives sur la pr´esence de mat´eriauxplus volatiles que la glace d'eau. Ensuite, je pr´esente une recherche de l'´ethanesolide, C2H6, sur les surfaces de Pluton et de Triton. Celle-ci est bas´eesur les observations spectrales dans les longueurs d'ondes du proche infrarouge. Je mod´elisechaque surface en utilisant un mod`elede transfert radiatif fond´esur la th´eoriede Hapke (Hapke, 1993) de trois mani`eres: sans ´ethane,avec de l'´ethanepur, et avec de l'´ethanedilu´edans de l'azote. La pr´esencede moins de quelques pourcents d'´ethanesur chaque corps ne permet pas d'exclure ce composant de Triton et Pluton, cependant il n'y a pas non plus de forte d´etection. Finalement, je reconsid`erela connaissance actuelle de la distribution compositionnelle des mat´eriaux de notre syst`emesolaire en fournissant une vue globale des petits corps. Je me concentre particuli`erement sur la pr´esencede l'eau dans toutes ses phases qui est pertinente surtout pour notre propre plan`ete,la Terre, et la vie. Je compare bri`evement la structure g´en´eralede notre syst`emesolaire aux autres disques d'accr´etion,afin de mettre en perspective la vue d´etaill´eemais cependant ´etroitede notre syst`emesolaire avec celle, plus large mais `abasse r´esolution,des autres syst`emesplan´etaires. Mots-cl´es: Plan´etologie,Ast´ero¨ıdes, Objets Trans n´eptuniens,Centaures, Observations, Spectro- scopie, Photom´etrie Contents Abstract 2 I Introduction and Background9 Introduction 11 1 Background 13 1.1 The Current Structure of the Solar System .......................... 14 1.1.1 Planets .......................................... 14 1.1.2 Dwarf Planets ...................................... 14 1.1.3 Comets .......................................... 14 1.1.4 Asteroids ......................................... 15 1.1.5 Centaurs and TNOs ................................... 17 1.2 Solar System Evolution ..................................... 18 1.2.1 Solar System Formation................................. 19 1.2.2 Planet Migration: The Nice Model........................... 20 1.2.3 Passing Star, Companion Star, and Rogue Planet Theories ............. 21 1.2.4 The Late Heavy Bombardment............................. 22 1.2.5 Effects currently shaping the Solar System....................... 22 1.3 The surfaces of small bodies................................... 23 1.3.1 Composition ....................................... 23 1.3.2 Surface Evolution..................................... 25 2 Observational Data 30 2.1 Methods of investigating surface composition ......................... 31 2.1.1 Photometry........................................ 31 2.1.2 Spectroscopy ....................................... 32 2.2 Telescopes and Instruments................................... 32 2.2.1 IRTF ........................................... 32 2.2.2 VLT............................................ 33 2.3 Data Reduction.......................................... 33 2.3.1 Calibration files...................................... 35 2.3.2 Photometry Reduction.................................
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