The structure of rocky planets

Transcription

1 he structure of rocky planets Christophe Sotin Iron planets (Mercury) errestrial planets Ocean / Icy planets Icy Moons Uranus and Neptune Giant planets References: Sotin et al. (21) errestrial Planet Interiors; in Exoplanets (Sara Seager, Ed), he Universy of Arizona Press, Leger et al. (24, 211); Valencia et al. (26, 27, ); O Neil and Lenardic (27), Sotin et al. (27); Grasset et al. (29); Charpinet et al. (211)

2 Outline Introduction Modeling the interior structure of terrestrial planets he Earth s case Equations of state Radius versus mass Application to water-rich planets Equation of state of water Conclusions Plate tectonics on terrestrial planets What has been found so far? Outstanding questions

3 Introduction Objective 1: Describe how one can obtain the relationship between mass and radius and then use that relationship to determine the class of exoplanet being discovered. Mass and radius are two of the easiest parameters to obtain. Objective 2: Address some outstanding issues such as the likelihood of plate tectonics on an exoplanet Plate tectonics is important because volatiles (H2O, CO2) can be recycled into the planet.

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5 Low mass exoplanets in March 211

6 Low mass exoplanets July 212

7 Modeling the interior structure of terrestrial planets he Earth s case Equations of state Radius versus mass

8 Internal structure of the Earth - composition EEH Earth model PUM LM Core O 3,28 44,76 43,8 1,61 Fe 33,39 5,89 12,69 8,25 Si 19,23 21,35 24,28 1,34 Mg 12,21 23,21 16,18 otal 95,11 95,21 96,95 92,2 Ni 2,2,25,71 4,99 Ca 1,1 2,32 1,2 Al,93 2,13 1,1 S,85,1,1 2,57 otal 99,92 99,92 99,97 99,76 O 3,28 44,76 43,8 1,61 Fe 35,41 6,14 13,4 85,24 Si 19,69 22,41 24,83 1,34 Mg 13,68 26,59 17,93 Core : Iron + light element (S, O, other). Mantle : (Mg,Fe) 2 Si 2 O 6, Ca(Mg,Fe)Si 2 O 6, (Mg,Fe) 2 SiO 4 and Al phase / (Mg,Fe)SiO 3, (Mg,Fe)O and Al phase

9 Input parameters EEH PUM LM Core 2/3LM+1/3Core Fe/Si,99,138,273 4,166,944 Mg/Si,83 1,372,835,,691 Fe/(Fe+Mg),531,92,246 1,,577 Solar values Mg,Fe,Si +Ni,Ca,Al,S' Fe/Si,977,986 Mg/Si 1,72 1,131 Fe/(Fe+Mg),477,466 Five parameters are required: 1) otal mass of the planet 2) Fe/Si (Stellar) 3) Mg/Si (Stellar) 4) Water mass fraction (Earth like / Ocean planet) 5) Mg# = Mg/(Mg+Fe) Large uncertainties on the composition of the Earth how does it influence the M(R) law

10 radius Internal structure of the Earth PREM density Mass = kg : 1/3 core and 2/3 mantle Upper and Lower mantle Subsolidus Convection in the mantle Core : Iron + light element (S, O, other). Mantle : (Mg,Fe) 2 Si 2 O 6, Ca(Mg,Fe)Si 2 O 6, (Mg,Fe) 2 SiO 4 and Al phase / (Mg,Fe)SiO 3, (Mg,Fe)O and Al phase 1

11 Modeling the mass - radius relationship. emperature d dp C P K S dp d : emperature P : Pressure : density (kg/m 3 ) : thermal expansion coefficient (K -1 ) C p : specific heat (J/kg/K) q

12 Relationship between radius and mass dp dr (r)g(r) r g( r) 4G r' ( r') dr' 2 r 2 P Cp S P th R M 4 r' ( r') dr' 2 K d Mass and radius are two of the few parameters. hey are related to each other through a simple equation in a 1D model. Density depends on composition (elementary and molecular), pressure, and temperature In the calculations, the main parameters are: Amount of volatiles (H 2 O) he amount of Fe Distribution of Fe between iron core and mantle We need an Equation of State (EoS) which relates density to pressure and temperature. Example of the Birch-Murnagham EoS : K 3 P 1 ( K' 4) 1 2 4

13 he 3 rd order Birch-Murnagham EoS 8 parameters known at ambient pressure: : the reference temperature : density K : bulk modulus K, P : pressure and temperature derivatives of bulk modulus a, b, c : thermal expansion coefficients 2, 3,, ' ',, 3 2, ', 3 5, 3 7,,.. exp , c b a d K K a K K K K P P Used for the upper mantle

14 he Mie-Grüneisen-Debye formulation hermal and static pressure are dissociated. 8 parameters : : the reference temperature : density K and K, : bulk modulus and its pressure derivative q D : reference Debye temperature n : number of atoms per chemical formula q and : scaling exponents Used for the lower mantle and core q D D t D D D th th D e dt t nr E E E V P K K P P P P ' ,, ,,, q q q q q q

15 Other formulations & comparisons Birch-Mürnhagan EOS Liquid layer Upper silicate mantle Mie-Grüneisen-Debye EOS Lower silicate mantle homas-fermi-dirac Icy mantle Metallic core (P> 1 Pa) Vinet EoS ANEOS (hompson, 199)

16 Results : Validation of the model - Earth Model : Fe/Si =.987 Mg/Si = Mg# =.9 H 2 O:.1 wt % M=M Earth R=6414 km (.6%)

17 Results : Validation of the model Solar system R R M Earth M Earth.274 Earth-like Ocean/Icy A planet with 5% water is 26% larger than a planet without water (for the same total mass) he points Uranus and Neptune have 1 Earth radius of atmosphere removed. GJ1214 has more than 5% ice in it. It fits well with Uranus and Neptune without their H2/He atmosphere

18 What do we know about extra-solar planets? Composition: Data from Beirao et al. (26) and Gilli et al. (26) Empty square is solar composition. Filled square is the enstatite end-member composition for the Earth s mantle. Empty circle is barycenter of all the stellar compositions. he large cross is typical uncertainties Lines are values of Mg# Areas give mass fraction of the core

19 Radius versus composition (1 M E < M < 1 M E ) otal radius does not vary significantly with on the composition he amount of Fe plays a significant role for the radius of the core

20 Application to water-rich planets How much water to add Equation of state of water/ice Radius versus mass for water-rich planets

21 How much water to add? Four models: Solar and Enstatite and two different Mg#.

22 Internal structure of large icy satellites: a model for icy exoplanets (ocean exoplanets) Outer ice layer liquid layer High-pressure phases of ice Silicate layer Iron core

23 Internal structure of large icy satellites (2/2)

24 Results : Extrapolation to larger planets E E E E M M M M M M R R log log log i i i X w Reference Case : Fe/Si = 1.1 * Mg/Si = 1.25 * Mg# =.8 H 2 O:.1 wt % * Averaged from Gilli et al., A&A, Earth M Earth M R R Each coefficient depends on the amount of water (X)

25 Uranus and Neptune / Earth Uranus Neptune Mass (1 24 kg) (14.5) (17.1) Volumetric radius (km) 25,364 (3.98) 24,625 (3.87) Mean density (kg/m 3 ) 1,27 (2) 1,638(4) Albedo.3(49).29(67) Absorbed power (x 1 15 W) 5.26(37) 2.4(19) Emitted power (x 1 15 W) 5.6(11) 5.34(29) Intrinsic power (x 1 15 W).34(38) 3.3(35) Intrinsic flux (W/m 2 ).42(47).433(36) Black-body temperature (K) bar temperature b (K) 76 (2) 72 (2) J 2, (x 1-6 ) 3,516(3) 3,539 (1) J 4, (x1-6 ) (4.1) -28(22) Q=w 2 R 3 /GM.2951 (5).269(26) Moment of inertia (I/MR2)

26 Uranus and Neptune / Earth

27 Results : Validation of the model Solar system R R M Earth M Earth.274 Earth-like Ocean/Icy A planet with 5% water is 26% larger than a planet without water (for the same total mass) he points Uranus and Neptune have 1 Earth radius of atmosphere removed. GJ1214 has more than 5% ice in it. It fits well with Uranus and Neptune without their H2/He atmosphere

28 Conclusions Plate tectonics on terrestrial planets What has been found so far? Outstanding questions

29 Plate tectonics on large Earths Plate tectonics provides a recycle of volatiles on geological timescales that may be important for the development of life wo papers came out at the same time with two different conclusions Valencia et al., ApJ, 27 O Neill and Lenardic, GRL, 27 Valencia et al. :. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. hese effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-earths, even if dry, will exhibit plate tectonic behavior. O Neil and Lenardic : mantle convection simulations have been carried out to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses, and thus super-sized Earths are likely to be in an episodic or stagnant lid regime.

30 Calculation of lithosphere thickness and stress he deviatoric horizontal normal stress (s) responsible for causing failure on the plate is (to first order) balanced by the shear stress () applied over the base of the plate. L s d he thickness of the lithosphere or boundary layer (d) depends on the Rayleigh number (Ra) a parameter governing convection. Ra 4 gd q k d D u u Ra. 5 D D Ra Ra c S With S=-1/4 L u d 2 q is heat flux (radiogenic and cooling rate) is the deviatoric stress is viscosity (Pa.s) U is velocity D is thickness of the convective layer

31 Results u D Ra g 4 gd q k M Earth M Earth g M Earth M Earth D D M Earth M Earth q is proportional to M d M.45 and s M 1. Given that Earth s convective state leads to plate tectonics, the more favorable conditions experienced by super-earths will inevitably lead to plate tectonics.

32 Geological consequences of super-sized Earths by C. O Neill and A. Lenardic mantle convection simulations have been carried out to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses, and thus supersized Earths are likely to be in an episodic or stagnant lid regime. s

33 s Geological consequences of super-sized Earths L d u D by C. O Neill and A. Lenardic M.27 u M 1.19 Why is there no plate tectonics on Mars and Venus? Water?

34 Low mass exoplanets

35 CoRo Exo-7b Leger et al., 29, 211

36 Conclusions Models give very good prediction of radii Amount of water is a first order parameter Radius is 26 % larger for an Ocean planet with 5 %wt of ices emperature is a second order parameters. Composition and Mg# control the size of the core. If Mass and Radius are perfectly known, the amount of water can be known at ± 4.4 % If 1% uncertainty of mass and radius, then the amount of water can be known at ± 2 % Number of terrestrial planets is increasing YES, super-earths and mini-neptunes can be distinguished BU [Super-Earths with H2/He atm] give same values than mini-neptunes