Folie 1

Physical State of the Deep Interior of CoRoT-7b

F. W. Wagner

T. Rückriemen

F. Sohl

German Aerospace Center (DLR)

IAU Symposium 276 - 13 October 2010

Slide 2

What we knowIntroduction - Method - Results - Conclusions

Mass and radius only known for two out of ~ 30 exoplanets below < 15 M

Radius (1.58±0.10) R

(Bruntt, et al. 2010) The mass challenge

1-4 M Pont, et al. 2010

(4.8±0.8) M Queloz, et al. 2009

(5.2±0.8) M Bruntt, et al. 2010

(5.7±2.5) M Boisse, et al. 2010

(6.9±1.4) M Hatzes, et al. 2010

Mean density (7.2±1.8) Mg m-3 (Bruntt, et al. 2010)

rocky planet?

The CoRoT Family M-R Relations

CoRoT-7b

GJ 1214b

CoRoT-7b

Slide 3

Interior Structure ModelIntroduction - Method - Results - Conclusions

Mechanical Thermal

Spherical and fully differentiated Mechanical equilibrium and thermal steady state

Output: Rp, m(r), g(r), p(r), (r), q(r), T(r)

Input: Mp, composition, Psurf, Tsurf,

T(r)conv.

conv.

ᵋ

Slide 4

Mixing Length FormulationIntroduction - Method - Results - Conclusions

Heat flux

l

Effective thermal conductivity due to

thermal convection

T < Tref

T > Tref

Dynamic viscosity

RT

pV+Eη=η

*

ref

*

exp

Local Nusselt number

Slide 5

Internal Structure of CoRoT-7bIntroduction - Method - Results - Conclusions

Density Bulk composition Radius, R/R

Core mass fraction, wt.%

Ma

ss, M

/M

Density suggests rocky bulk composition

Earth-like

Iron-depleted

Slide 6

Present Thermal State of CoRoT-7bIntroduction - Method - Results - Conclusions

Pressure-induced sluggish convective regime in the lower mantle Substantial higher CMB temperatures in comparison to parameterized models Mantle pressures within stability field of post-perovskite (125 –1000 GPa)

5320K 5210K

6710K

7560K

Temperature Pressure

727GPa656GPa

1440GPa

1940GPa

PCM

(Valencia, et al. 2006)

Slide 7

Radiogenic HeatingIntroduction - Method - Results - Conclusions

Temperature CMB Specific heat production

Deep interior stays relatively hot despite decreasing radiogenic heat production

What is the role of accretional and tidal heating?

Age: 1.2 – 2.3 Gyr (Leger, et al. 2009)

Slide 8

Physical State of the CoreIntroduction - Method - Results - Conclusions

Temperature strongly depending on rheology Relatively high activation volume needed to initiate core melting Solid state of lower mantle and iron core due to high pressure

Activation volume, mantle Sulfur content, core

32.6 wt.% cmf

~3000K

~ 15 wt.% S

Melting pointreduction

Slide 9

ConclusionsIntroduction - Method - Results - Conclusions

The mean density of (7.2±1.8) Mg m-3 and high surface temperatures imply that CoRoT-7b is a dry and rocky planet.

Post-perovskite is expected to be the predominant mantle mineralogical phase.

Pressure-induced sluggish convection prevalent in the lower mantle.

Due to the large effect of pressure on melting, a pure iron core is expected to be solid.

But: A liquid core cannot completely be ruled out, depending strongly on mantle rheology and actual core composition.

Slide 10

Thank you for your attention!

Slide 11

Introduction - Method - Results - Conclusions

Comparison with 2D Convection Model

L. Noack

5M

Deep interior High pressure Highly sluggish layer No lateral temperature variation from

day-side to night-side

Upper mantle Convection pattern strongly influenced

by varying surface temperature

70 5,300K

Slide 12

On the Existence of a Magma OceanIntroduction - Method - Results - Conclusions

Temperature variation within the lithosphere less distinct Depth of a possible magma ocean depending on the predominant minerals

and actual surface temperatures

1810K

Slide 13

Introduction - Method - Results - Conclusions

Equation of State

Mao H., Hemley R.J., 2007: PNAS, 104, 9114-9115

Equation of State (EoS) relates

pressure, temperature, and

density

Generalized Rydberg EoS

(Stacey, 2005): Fit to high-

pressure experiments

Reciprocal K-primed EoS

(Stacey, 2000): Fit to PREM

Problem: Extrapolation

exoplanets