KTS04-1 Structures of the Planets Jupiter and Saturn A Kerley Technical Services Research Report Gerald I. Kerley December 2004 P.O. Box 709 Appomattox, VA 24522-0709 Tel: (434) 352-8400 I Fax: (434) 352-4973 I E-mail: [email protected] KTS04-1 Structures of the Planets Jupiter and Saturn A Kerley Technical Services Research Report Gerald I. Kerley December 2004 ABSTRACT New equations of state (EOS) for hydrogen, helium, and compounds con- taining heavier elements are used to construct models for the structures of the planets Jupiter and Saturn. Good agreement with the gravitational mo- ments J2 and J4 is obtained with a model that uses a two-layer gas enve- lope, in which the inner region is denser than the outer one, together with a small, dense core. It is possible to match J2 with a homogeneous envelope, but an envelope with a denser inner region is needed to match both mo- ments. The two-layer envelope also gives good agreement with the global oscillation data for Jupiter. In Jupiter, the boundary between the inner and outer envelopes occurs at 319 GPa, with an 8% density increase. In Saturn, it occurs at 227 GPa, with a 69% density increase. The differences between the two planets show that the need for a density increase is not due to EOS errors. It is also shown that helium enrichment cannot be the cause of the density increase. The phenomenon can be explained as the result of enrich- ment of heavy elements from planetismals that were dispersed and mixed with the gases during planet formation. This conclusion is consistent with planetary formation models like that of Pollack, et al. P.O. Box 709 Appomattox, VA 24522-0709 Tel: (434) 352-8400 I Fax: (434) 352-4973 I E-mail: [email protected] CONTENTS CONTENTS FIGURES.............................................................................................................................4 TABLES ..............................................................................................................................5 1. INTRODUCTION .......................................................................................................6 1.1 Background........................................................................................................6 1.2 Elements of a Planetary Model.......................................................................6 1.3 Scope of this Investigation...............................................................................8 2. THE EOS MODEL.....................................................................................................10 2.1 Hydrogen EOS ................................................................................................10 2.2 Helium EOS .....................................................................................................12 2.3 Mixture Model for Envelope .........................................................................12 2.4 Model for Core ................................................................................................14 2.5 Calculation of Planetary Adiabats................................................................15 3. THE THEORY OF FIGURES ...................................................................................17 3.1 Basic Equations ...............................................................................................17 3.2 Calculated Quantities.....................................................................................20 3.3 Numerical Methods........................................................................................21 4. BASELINE MODELS (HOMOGENEOUS ENVELOPE).....................................22 4.1 Baseline Model for Jupiter .............................................................................23 4.2 Baseline Model for Saturn .............................................................................25 5. VARIATIONS USING HOMOGENEOUS ENVELOPE ......................................27 5.1 Density Scaling of Baseline Model ...............................................................27 5.2 Effect of Increasing the Element Abundances............................................28 5.3 Effect of a Radiative Region in the Adiabat ................................................29 5.4 Effect of Decreased Initial Temperature ......................................................30 6. HYDROGEN-HELIUM DEMIXING......................................................................31 7. TWO-LAYER ENVELOPE MODEL .......................................................................33 7.1 Two-Layer Models from Density Scaling of Baseline Models..................33 7.2 Possible Explanations for Two-Layer Model..............................................34 7.3 Two Layer Model as Result of Dispersed Planetismals ............................35 7.4 Two-Layer Model with Transition Region ..................................................40 2 Jupiter & Saturn CONTENTS 8. CONCLUSIONS........................................................................................................41 ACKNOWLEGEMENTS................................................................................................42 REFERENCES..................................................................................................................43 APPENDIX A: Rotational Corrections to the H 2 EOS .............................................47 Jupiter & Saturn 3 FIGURES FIGURES Fig. 1. Concentrations vs. pressure in planetary envelopes....................................14 Fig. 2. Temperature and density vs. pressure in atmosphere of Jupiter................24 Fig. 3. Planetary adiabats compared to He-H demix boundary.............................32 Fig. 4. Pressure vs. density on planetary adiabats for Jupiter and Saturn ............38 Fig. 5. Structural parameters vs. average radius for Jupiter ...................................38 Fig. 6. Structural parameters vs. average radius for Saturn....................................39 Fig. 7. Nuclear spin corrections to rotational energy and entropy for H2 ............49 4 Jupiter & Saturn TABLES TABLES Table 1 Measured Data for Jovian Planets .................................................................22 Table 2 Input Parameters for Baseline Models..........................................................23 Table 3 Calculated Results for Baseline Models........................................................25 Table 4 Results—Density Scaling of Baseline Models..............................................27 Table 5 Results—Baseline Models with Increased Helium Abundance................28 Table 6 Results—Baseline Models with Radiative Region ......................................30 Table 7 Results—Two-Layer Models Scaled from Baseline Models.......................34 Table 8 Results—Two-Layer Models Using Dissolved Planetismals.....................37 Table 9 Results—Two-Layer Models with Transition Regions ...............................40 Jupiter & Saturn 5 INTRODUCTION 1.INTRODUCTION 1.1 Background Speculation about the planets Jupiter and Saturn, what they are and how they came to be, has existed since ancient times. The invention of telescopes, which al- lowed observation of their beauty, and the discovery of the laws governing plane- tary motion, greatly aroused interest in these planets. Subsequent advances in observational methods, physical theories, and numerical computation not only intensified this curiosity but also allowed the development of quantitative plane- tary-structure models.1 In recent times, the Pioneer, Voyager, Galileo, and Cassini space missions, along with the impact of the Shoemaker-Levy 9 comet fragments into Jupiter, have stimulated the greatest level of interest thus far. An understand- ing of the structure of the Jovian planets could also help to answer questions about their formation and evolution, as well as that of planets recently discovered outside our solar system. My own interest in these problems originally developed from a desire to see if the planets could shed any light on the validity of equation of state (EOS) models for hydrogen and deuterium. While recent experimental and theoretical work has greatly improved understanding of the EOS for those materials, pressure-temper- ature states in the deep interiors of the Jovian planets still cannot be accessed in terrestrial laboratories at present. Of course, the planetary structure problem is much too complicated to permit definitive answers to questions about the validity of EOS models. However, the present study demonstrates that my new H2 EOS model [2] is at least consistent with the observational data currently available. It also offers new insights into the structures of Jupiter and Saturn that should be of interest to the astrophysical community. 1.2 Elements of a Planetary Model Standard models for Jovian planets involve four main features [3]-[7]: • All points in the planet’s interior are assumed to lie on a single pressure- density curve. This curve is usually taken to be an adiabat (isentrope) passing through a particular pressure-temperature state at the surface. Hence I will refer to this curve as the “planetary adiabat,” although the entropy can be discontinuous at points where the chemical composition changes. The surface state is determined from observational data. • The chemical composition (element abundances) along the adiabat is es- sentially a model parameter. It is relatively easy to show that the Jovian planets must be composed mostly of hydrogen and have