הפקולטה למדעים מדוי יקים RAYMOND AND BEVERLY SACKLER "ע ש ריימונד ובברלי סאקלר FACULTY OF EXACT SCIENCES המחלקה לגיאופיזיקה ומדעים פלנטאריים GEOPHYSICS AND PLANETARY SCIENCES Thermal and Structural Evolution of Small Bodies in the Solar System Thesis submitted for the degree Doctor of Philosophy by Gal Sarid This work was carried out under the supervision of Prof. Dina Prialnik Dept. of Geophysics and Planetary Sciences Submitted to the Senate of Tel Aviv University October 2009 Thermal and Structural Evolution of Small Bodies in the Solar System By Gal Sarid October 2009 Abstract The designation ”small bodies” in Solar System studies refers to astronomical bodies smaller than planets, for which the Sun is the main gravitational attractor. The diversity in the dynamical properties of these bodies may be a result of the specific accretion locations of each class of bodies, or their subsequent orbital evolution, mainly due to gravitational perturbations by the planets. There are many dynamical classes of small bodies, but the ones that share a common dynamical evolution scheme, or a widely accepted chain of origin, are Comets, Centaurs and trans-Neptunian objects. In the work presented here we followed the thermo-chemical evolution of the relevant small bodies. This was done through the use of a sophisticated 1-D or quasi-3D numerical code, which solves the heat transfer and flow equations for a porous multi-component object, with full consideration of interior and exterior boundary conditions. We applied this general thermal evolution code to the modeling of several specific trans-Neptunian objects, which represent a sample of the various physical characteris- tics attributed to this population. Results indicate a general trend of more compacted, thermally-processed and volatile-depleted interiors, as the size and density of the object is larger. However, in the sub-surface layers, a mixed and intricate composition of volatile compounds can be found. An interesting and notable result is the viable and relatively robust occurrence of conditions for liquid water in the deep interior of the larger objects. Another application of the thermal evolution code is to the modeling of several specific Centaurs. We examined the effect of initial conditions on the thermal evolution on differ- ent unstable orbits. For this purpose, we set various scenarios for the succession of Centaur origin and emplacement – either as ’a chip of the old block’ of larger trans-Neptunian ob- jects, which scattered inwards, or as a ’rolling stone’ from out beyond Neptune, which leisurely diffused inwards. The end results of these sets of simulations should provide in- sight into the current internal state of Centaurs and the initial configurations of evolving Jupiter-family comet nuclei. The dynamical state of several specific Centaur objects is also examined, through N- body integrations. Since these objects are highly prone to gravitational interactions with the outer planets, their orbits are highly chaotic. As such, characterizing their orbital stability is a challenging task. We show some representative results of this effort here, and determine several global orbital evolution properties for each object. I We conclude with a detailed examination, through quasi-3D modeling, of two typical Jupiter-family comets. In the case of 9P/Tempel 1, we study the internal evolution, activity and impact modeling. In the case of 22P/Kopff, we study the dust activity and its connection to internal thermal evolution. The outstanding feature emerging from the quasi-3D thermal simulations is that even at relatively high cometocentric latitudes the nucleus will develop a complex pattern of volatile stratification with depth. We suggest that our detailed results of the thermal processing, which altered the internal configurations of trans-Neptunian objects, Centaurs and Jupiter-family comets, provide some insight into both the history of large ice-rock planetesimals and the prenatal constitution of cometary material. II Acknowledgments The work presented here is a product of the last six years of my main occupation – research in planetary science, and specifically into cometary science. Thus, it is only natural and obvious to acknowledge ones advisor. However, as it turns out, I went looking for an advisor and found one of the most special people I imagine I’ll ever meet (and one of the foremost experts in the field). Dina, for your kindness, your attention, your patience, your caring, your guidance and imparting of both ’know-how’ and ’know-when’, there is not enough I can say. So a simple ’thank you’ will have to suffice. Since this is the more personal part of the thesis, I will try to keep it simple. This is a necessary constrain, as this part is not meant to be the largest in bulk. So, i would just like to say, to all of you out there (you know who I mean), if you are reading this: I care for you and I appreciate you. Some more than others, naturally. I would like to mention by name, those closest to me, who helped me the most, mentally and otherwise, during this long period. And the nominees are: Ofer, David, Sharon, Elinor, Shachar and Yonit. Friends, the above statement refers to you especially! It is a pleasure to thank several faculty members who helped me along the way with a kind word, a logistic nudge, a rewarding grade, or some professional advice. Thank you Eyal, Morris, Attay, Peter and Shay. Almost last, but certainly not least is my family. My parents, who gave me the freedom to pursue my ambitions and desires and the confidence and insight to know how to use it. Oh, and also endless love to make all the freedom and confidence worth it. My sisters, who supported my endevours, sometimes quietly and sometimes with a bang, but always with appreciation and love. Don’t think it went unnoticed. I love you, mom. I love you, dad. I love you, Hadar and Adva. To my grandparents, passed and living, thank you for nurturing me and making me feel like a unique grandson. If I forgot someone, and most chances are that I did, don’t be offended. As you know me, I’m doing this at the last moment and I am probably preoccupied. Also, I wanted to keep it short. And then there is, of course, Yael. What can I say to avoid a cliche? You are ny sunshine, my only sunshine, you make me happy when skies are gray. I am only ecstatically joyful that we spend our time together. III I love you, buba! IV Contents Abstract I Acknowledgments III List of Figures XI List of Tables XIII 1 Introduction 1 1.1 SketchofThisWork .............................. 1 1.2 TheClassesOfSmallBodies.......................... 2 1.2.1 TheCometaryPopulation ....................... 2 1.2.2 TheCentaurPopulation ........................ 3 1.2.3 TheTrans-NeptunianPopulation . 7 1.3 TheDynamicalSchemeOfSmallBodies . 10 1.3.1 CometaryTaxonomy .......................... 10 1.3.2 The Link Between TNOs And JFCs . 11 1.3.3 The Structure And History Of The Trans-Neptunian Region . 12 1.3.4 Number Estimates For The Dynamical Classes . 15 1.3.5 The Size-Distribution Of The TNOs Population . 16 2 Methodology 17 2.1 GoverningEquations .............................. 17 2.2 NumericalScheme ............................... 21 2.3 PorousStructureandComposition . 21 2.4 Pore-SizeDistribution ............................. 24 2.5 RadioactiveHeatSources............................ 27 2.6 SelfGravity ................................... 29 2.7 EquationofState................................ 33 2.7.1 Birch-MurnaghanEOS . .. .. 33 2.7.2 VinetEOS................................ 36 2.7.3 Stability Analysis . 37 2.8 A“Thermal”Impact .............................. 40 V CONTENTS 3 Evolution of Trans-Neptunian Objects 43 3.1 The Inner-Workings of Trans-Neptunian Objects . ..... 43 3.1.1 Introduction............................... 43 3.1.2 Model assumptions and choice of key parameters . 44 3.1.3 ResultsofInternalEvolution. 46 3.1.4 Re-distribution of Pore Sizes . 56 3.1.5 EffectoftheEquationofState. 58 3.1.6 Effect of Various Radioactive Species . 61 3.1.7 Possible Liquid Water . 63 3.2 End-StatesofSomeEvolvedObjects . 67 3.2.1 (50000)Quaoar-ExtendedEvolution. 67 3.2.2 1992 QB1 and 1998 WW31 –SmallerandColderObjects . 72 4 Evolution of Centaurs 79 4.1 Dynamics Within the Centaur Population . 79 4.1.1 Introduction............................... 79 4.1.2 IntegrationScheme ........................... 83 4.1.3 InitialConfiguration .......................... 85 4.1.4 SummaryofResults .......................... 90 4.2 Thermal Evolution In the Region of the Outer Planets . 103 4.2.1 Introduction...............................103 4.2.2 InitialParameters. .. .. .. .103 4.2.3 Initial State Scenarios – The Fragment and Immigrant Cases . 107 4.2.4 ThermalEvolutionAsATNOFragment . 108 4.2.5 ThermalEvolutionAsATNOImmigrant . 114 5 Evolution of Jupiter-Family Comets 119 5.1 Thermal Evolution and Activity of Comet 9P/Tempel 1 and Simulation of aDeepImpact .................................119 5.1.1 Introduction...............................119 5.1.2 SomeGround-BasedObservations . 120 5.1.3 Numerical Model and Initial Parameters . 122 5.1.4 Results of Evolutionary Calculations . 125 VI CONTENTS 5.1.5 SimulationofaThermalImpact. 131 5.2 Internally-Driven Dust Activity of Comet 22P/Kopff . 135 5.2.1 Introduction...............................135 5.2.2 Dust-Dynamical Modeling Results . 136 5.2.3 Numerical Model and Initial Parameters . 138 5.2.4 Thermal Evolution Modeling Results . 141 6 Summary and Conclusions 147 Bibliography 153 VII List of Figures 1.1 Distribution of JFCs, HFCs and Centaurs in the orbital phase space of a vs.e........................................ 5 1.2 Distribution of JFCs, HFCs and Centaurs in the orbital phase space of e vs.i........................................ 6 1.3 Distribution of TNOs in the orbital phase space of a vs. e. ... 8 1.4 Distribution of TNOs in the orbital phase space of e vs. i. .. 9 1.5 Distribution of KBOs in the orbital phase space of a vs. e. ... 13 2.1 Relation between ρ, Ψ and Xdust/Xice. .................... 23 2.2 Correction factor for the SV R parameter.