THE STATE OF KNOWLEDGE OF ICE GIANT INTERIORS Brigette Hesman (UMD / NASA GSFC) with significant contributions from: Tristan Guillot (Observatoire de la Cote d’Azur) Nadine Nettelmann (UC Santa Cruz) Ravit Helled (Tel-Aviv University) Workshop on the Study of Ice Giant Planets 7/30/14 1 WHY INTERIORS? • What is the motivation for studying Ice Giant interiors? Two-fold: 1. Interiors of the planets are a historical record of the planetary formation process - we need to investigate the full continuum our solar system’s planetary interiors to build the architecture of the early solar system - these studies provide constraints on the physical and chemical properties of protoplanetary disks 2. Uranus and Neptune are the Super-Earth’s/Mini-Neptune’s of our solar system - a class of exo-planets similar in size to our Ice Giants is continuing to grow due to the highly successful Kepler mission - studying the interiors of our local Ice Giants provides the foundation for entangling the composition of extra-solar planets Workshop on the Study of Ice Giant Planets 7/30/14 2 THE “ICE GIANTS” • Uranus and Neptune were classified as the Ice Giants to distinguish them from the Gas Giants because of their smaller gaseous envelopes (10-20% of their mass as H2 and He) • Uranus and Neptune have historically been thought of as icy twins ➔ Very similar mass, radii, gravitational moments, rotation periods ➔ Assumed to be water rich based on formation theories Uranus: 14.5 M⊕ @ 19.2 AU Neptune: 17.1 M⊕ @ 30 AU THEREFORE, VERY SIMILAR INTERIORS … RIGHT? MAYBE / MAYBE NOT Workshop on the Study of Ice Giant Planets 7/30/14 3 DICHOTOMY URANUS NEPTUNE • Low internal heat flux • Strong internal heat flux • High obliquity (98°) • Saturn-like obliquity • Dense narrow rings • Extended dust disk with diffuse rings • 5 of the largest satellites on • 2 major satellites on irregular regular orbits orbits Different Formation Histories? Different Internal Structures? Workshop on the Study of Ice Giant Planets 7/30/14 4 DIFFERENT FORMATION HISTORIES? Standard Formation Model (Core Accretion): Three phases of formation: Phase 1: Initial core accumulates planetesimals (presumably rocks) by runaway accretion Phase 2: Low rate of accretion of gas and planetesimals Phase 3: Collapse of gas onto core of planet which provides hydrogen envelope • Uranus and Neptune probably never reached phase 3 – “failed giant planets” • Hydrogen and helium in the envelopes of these two planets was probably acquired in stage 2 … therefore, small envelopes BUT • Problems: formation timescales, critical core mass, getting a Uranus-like composition Multiple other models to solve issues: 1. Formation closer to the sun (Nice Model) 2. Disk physics/chemistry – disk evolution, enhancing the solids 3. High accretion rates 4. A combination … All formation models have trouble creating the dichotomy we see in Uranus and Neptune Workshop on the Study of Ice Giant Planets 7/30/14 5 DIFFERENT FORMATION HISTORIES? Standard Formation Model (Core Accretion): Three phases of formation: Phase 1: Initial core accumulates planetesimals (presumably rocks) by runaway accretion Phase 2: Low rate of accretion of gas and planetesimals Phase 3: Collapse of gas onto core of planet which provides hydrogen envelope Maybe Uranus and • Uranus and Neptune probably never reached phase 3 – “failed giant planets” Neptune formed in the • Hydrogen and helium in the envelopes of these two planets was probably acquired in stage 2 … therefore, small envelopes same method and did have similar structures BUT after formation. Then • Problems: formation timescales, critical core mass, getting a Uranus-like composition Multiple other models to solve issues: why the DICHOTOMY? 1. Formation closer to the sun (Nice Model) 2. Disk physics/chemistry – disk evolution, enhancing the solids 3. High accretion rates 4. A combination … All formation models have trouble creating the dichotomy we see in Uranus and Neptune Workshop on the Study of Ice Giant Planets 7/30/14 6 DIFFERENT INTERNAL STRUCTURE? The Possible Role of Giant Impacts Uranus: Oblique Collision Neptune: Radial Collision Angular momentum deposition causes: Energy of impact causes: 1. Inhibition of core convection 1. Core mixing resulting in an adiabatic and 2. Strong tilt of planet mixed interior 2. Efficient cooling A more thorough understanding of these planets interiors will reveal details about their formation and evolution in our solar system Workshop on the Study of Ice Giant Planets 7/30/14 7 DEFINING THE INTERIOR The gravitational field of a rotating planet: r, θ, φ are spherical polar coordinates M = total planetary mass a = equatorial radius (at the 1bar level) J2n = gravitational moments P2n = Legendre polynomials ω = angular velocity of rotation With M and Jn constrain the INTERIOR DENSITY, ρ(r) Workshop on the Study of Ice Giant Planets 7/30/14 8 The Astrophysical Journal,726:15(7pp),2011January1 Helled et al. Jupiter and Neptune that fit their measured gravitational fields are 4 derived using Monte Carlo searches (Marley et al. 1995;Podolak J6 et al. 2000). This approach is free of preconceived notions about planetary structure and composition and is not limited by the 3 J4 EOSs of assumed materials. Once the density profiles that fit the gravitational coefficients are found, conclusions regarding their nctions u J2 possible compositions can be inferred using theoretical EOSs 2 tion f (Marley et al. 1995). u J0 In this paper, we apply the method previously used in our models of Saturn (Anderson & Schubert 2007; Helled et al. Contrib 1 2009a)toderivecontinuousradialdensityandpressureprofiles core CORE “SAMPLING” that fit the mass, radius, and gravitational moments of Uranus and Neptune. The use of a smooth function for the density 0 0.0 0.2 0.4 0.6 0.8 1.0 with no discontinuities allows us to test whether Uranus and The Astrophysical Journal,726:15(7pp),2011January1 Helled et al. Helled et al. (2011) Normalized Mean Radius β Neptune could have interiors with no density (and composition) Jupiter and Neptune that fit theirNeptu measuredne gravitational fields are discontinuities. Section 2 summarizes the models and results. In 4 4derived using Monte Carlo searches (Marley et al. 1995;Podolak Section 3,weusephysicalequationofstatetablestoinferwhat J6 et al. 2000). This approach is free of preconceived notions about these density distributions imply about the internal composition J planetary structure and composition and is not6 limited by the of Uranus and Neptune. Conclusions are discussed in Section 4. 3 J4 3 EOSs of assumed materials. Once the densityJ4 profiles that fit the gravitational coefficients are found, conclusions regarding their 2. INTERIOR MODEL: FINDING RADIAL PROFILES OF nctions nctions u DENSITY AND PRESSURE u J2 possible compositions can be inferred using theoretical EOSs 2 2 J2 tion f tion f (Marley et al. 1995). J0 The procedure used to derive the interior model is described in u u J0 In this paper, we apply the method previously used in our detail in Anderson & Schubert (2007)andHelledetal.(2009a). models of Saturn (Anderson & Schubert 2007; Helled et al. Contrib Contrib 1 1 The method is briefly summarized below. 2009a )toderivecontinuousradialdensityandpressureprofiles core core The gravitational field of a rotating planet is given by that fit the mass, radius, and gravitational moments of Uranus 2n and Neptune. The use of a smooth function for the density GM ∞ a 1 2 2 2 0 0 U 1 J2nP2n (cos θ) + ω r sin θ, 0.0 0.2 0.4 0.6 0.8 1.0 0.0with no discontinuities 0.2 0.4 allows us 0.6 to test whether 0.8 Uranus 1.0 and = r − r 2 ! n 1 % Normalized Mean Radius β Neptune could haveNormalized interiors Mean with Radi nous β density (and composition) "= # $ (1) Neptune Figurediscontinuities. 1. Normalized integrands Section of2 thesummarizes gravitational the moments models (contribution and results. In 4 functions)Section of Jupiter3,weusephysicalequationofstatetablestoinferwhat (top) and Neptune (bottom). The values are normalized to where (r, θ, φ) are spherical polar coordinates, G is the gravita- Ice Giants have advantage of gravitational moments sampling the core! make the area under each curve equal unity. J is equivalent to the planetary these density distributions imply0 about the internal composition tional constant, M is the total planetary mass, and ω is the angular J6 mass. The range of possible core sizes is indicated. Here, core designates a regionof of Uranus heavy elements and Neptune. below the Conclusions H/He envelope. are It is discussed clear that Neptune’s in Section 4. velocity of rotation. We assume that the planets rotate as solid 3 (Uranus’) interior is better sampled by the gravitational harmonics compared to J4 bodies with Voyager rotation periods (Table 1), although this Jupiter (Saturn).2. INTERIOR MODEL: FINDING RADIAL PROFILES OF assumption is a simplification since the interior rotation profiles nctions u (A color version of this figureDENSITY is available AND in the online PRESSURE journal.) of Uranus and Neptune are actually poorly known and could 2 J2 tion f J0 The procedure used to derive the interior model is described in be more complex (Helled et al. 2010). The potential U is rep- u uses physicaldetail in equationsAnderson of & state Schubert (EOSs) (2007 of the)andHelledetal.( assumed materials2009a). resented as an expansion in even Legendre polynomials, P2n Workshop on the Study of Ice Giant Planets 7/30/14 9 Contrib 1 to deriveThe method a density is briefly(and associated summarized pressure below. and temperature) (Kaula 1968;Zharkov&Trubitsyn1978). The planet is defined core profile thatThe best gravitational fits the measured field of a gravitational rotating planet coefficients. is given by The by its total mass, equatorial radius a at the 1 bar pressure level, physical parameters of the planets, such as mass and equatorial and harmonic coefficients J2n,whichareinferredfromDoppler 2n radius, areGM used as additional∞ a constraints.