Accretion and Differentiation of Earth

1. Accretion and Differentiation of Earth Dave Stevenson Caltech Neutrino Sciences 2007 Deep Ocean Anti-Neutrino Observatory Workshop Honolulu, Hawaii March 23-25, 2007

2. Definitions • Accretion means the assembly of Earth from smaller bits • Differentiation means the separation of components within Earth during or after assembly - in this talk it will be primarily the “initial” differentiation (~4.4Ga or earlier).

3. The Big Questions • What is the radiogenic heat production inside Earth both now and in the past? • How is this related to other reservoirs we know about (Sun & meteorites)? • How is that heat production distributed spatially now and in the past? • How is heat production related to heat output now and in the past? • Are there any important unconventional heat sources (radiogenic or otherwise)? • What was the initial condition?

4. EARTH HISTORY This Some multidimensional space Initial condition Evolutionary path Present state That

5. EARTH HISTORY This Some multidimensional space Initial condition Focus of this talk Astronomy, geochemistry, physical modeling Geophysics Evolutionary path Geochemistry, geology, geobiology Present state That

6. How to think about a Planet (e.g., Earth)? • Could discuss provenance- the properties of an apple depend on the environment in which the tree grows • Or could discuss it as a machine (cf. Hero[n], 1st century AD) • Need to do both

7. The (logarithmic) way one should think about time if you want to understand processes and their outcome Phanerozoic 106 yr 107 108 109 1010 yr Earth accretion

8. Nucleosynthesis in massive stars (supernovae for the heaviest elements) Interstellar medium Solar nebula Sun & planets

9. Interstellar medium contains gas & dust that undergoes gravitational collapse A “solar nebula” forms: A disk of gas and dust from which solid material can aggregate

10. Terrestrial Planet formation • Rapid collapse from ISM; recondensation of dust; high energy processing • Small (km) bodies form quickly (<106yr)[observation]. Some of these bodies differentiate ( 26Al heating) • Moon & Mars sized bodies may also form as quickly[theory] -will also therefore differentiate (perhaps imperfectly) • Orbit crossing limits growth of big bodies: Time ~ 107- 108 yr. • Last stages in absence of solar nebula [astronomical obs.] • Mixing across ~1AU likely (chemical disequilibrium?)

11. In current terrestrial accretion models, the material that goes into making Earth comes from many different regions Volatile depletion in the terrestrial planet forming materials (affects potassium; not U & Th) Zonation of composition in terrestrial zone is unlikely Results from Chambers, 2003 (Similar results from Morbidelli)

12. Solar nebula Gas density enhancement ~exp[GM/Rc2] Mars mass embryo -hot & differentiated This predicts only modest ingassing (even assuming the embryo has an accessible magma ocean)

13. The Importance of Giant Impacts • Simulations indicate that Mars-sized bodies probably impacted Earth during it’s accumulation. • These events are extraordinary… for a thousand years after one, Earth will radiate like a low-mass star! • A large oblique impact places material in Earth orbit: Origin of the Moon

14. Formation of the Moon • Impact “splashes” material into Earth orbit • The Moon forms from a disk in perhaps a few 100 years • One Moon, nearly equatorial orbit, near Roche limit- tidally evolves outward

15. Some Important Numbers • GM/RCp~ 4 x 104Kwhere M is Earth mass, R is Earth radius, Cp is specific heat • GM/RL ~1where L is the latent heat of vaporization of rock • Equilibrium temp. to eliminate accretional heat ~400K (but misleading because of infrequent large impacts and steam atmosphere) • Egrav~10 Eradiowhere Egrav is the energy released by Earth formation and Eradio is the total radioactive heat release over geologic time

16. What Memory does Earth have of Accretion? • Overall composition (almost a closed system) • Isotopic • Bulk chemistry (partitioning; provided reservoirs are not fully equilibrated) • Thermal if layered

17. Core Formation requires… • Immiscible components (iron & silicate) • Macrosegregation of components: At least one was mostly molten • Substantial Difference in density Other kinds of differentiation (ocean & atmosphere formation, continental crust) are not conceptually that different although the details differ a lot.

18. Wood et al, 2006 Core Formation Stevenson, 1989

19. Core Formation with Giant Impacts • Imperfect equilibration no simple connection between the timing of core formation and the timing of last equilibration • No simple connection between composition and a particular T and P. Molten mantle Unequilibrated blob Core

20. The Importance of Hf-W 182Hf  182W 1/2 ~ 9 Ma Core-loving Excess 182W observed “early” No excess “late”

21. Early differentiation event in Moon sized bodies collision CORE MERGING EVENT (Hf-W timescale  planet formation timescale)

22. Early differentiation event in Moon sized bodies collision EMULSIFICATION DURING IMPACT (Hf-W timescale  planet formation timescale provided emulsification is sufficiently small scale)

23. Quantitative Interpretation of  CHUR Chondritic reference (=0) Very Early core formation  >>1 Late core formation  ~0 Earth observation is =1.9 Many combinations of events can give this value.. but the likely inference is that the last major core forming events occurred ~50 Ma (last giant impact?)

24. Core Superheat Early core Core Superheat • This is the excess entropy of the core relative to the entropy of the same liquid material at melting point & and 1 bar. • Corresponds to about 1000K for present Earth, may have been as much as 2000K for early Earth. • It is diagnostic of core formation process...it argues against percolation and small diapirs. T Adiabat of core alloy Present mantle and core depth

25. The “Inevitability” of a Magma Ocean Steam atmosphere • Burial of accretional energy prevents immediate re-radiation - a chill crust can form. • In presence of sufficient atmosphere (e.g., steam), the magma ocean is protected. • Lower mantle can easily freeze because of pressure - this limits magma ocean depth surface Magma ocean ~500km Frozen (but very hot!)

26. Differentiation in the Mantle? Dense suspension, vigorously convecting. May be well mixed Solomatov & Stevenson(1993) Much higher viscosity, melt percolative regime. Melt/solid differentiation? High density material may accumulate at the base.Iron-rich melt may descend? CORE

27. A Layered Mantle? • Unlikely to arise in the magma ocean (suspended crystal stage) • Could arise from percolative redistribution (melt migration near the solidus) after magma ocean phase • Might (or might not) be eliminated by RT instabilities & thermal convection • Could be relevant to D”, or to a thicker layer. • Growing evidence for its existence Kellogg et al, 1999

28. Cooling times …to decrease mean T by ~1000K • From a silicate vapor atmosphere: 103yr • From a deep magma ocean/steam atmosphere: 106 yr • Capped magma ocean: Up to 108 yr [cold surface!] • Hot subsolidus convection : Few x108 yr • At current rate: >1010 yr

29. Early Earth* Environment? *4.4 to 3.8Ga • Ocean and atmosphere in place. • Ocean may not have been very different in volume from now. Might be ice-capped. • Atmosphere was surely very different… driven to higher CO2 by volcanism, but the recycling is poorly known. When did plate tectonics begin? • Uncertain impact flux but consequences of impacts are short lived.

30. Conclusions • Timing of Earth formation still uncertain but compatible with a few x 107 yr duration. Hf-W constrains but does not clearly provide this timing. • High energy origin of Earth extensive melting and magma ocean • Legacy expressed in core superheat & composition (siderophiles in the mantle, light elements in the core) -but not yet understood. Maybe also in primordial mantle differentiation. • Rapid cooling at surface but a “Hadean” world. Impacts may affect onset time of sustained life.

31. Responses to the Big Questions • What is the radiogenic heat production inside Earth both now and in the past?Determined by U, Th and K in the source material… maybe some K is lost. • How is this related to other reservoirs we know about (Sun & meteorites)?Closely related (U, Th) ; K depleted; but some uncertainty • How is that heat production distributed spatially now and in the past? Core formation: Any U, Th or K in the core? Primordial mantle differentiation? • How is heat production related to heat output now and in the past? Later speakers • Are there any important unconventional heat sources (radiogenic or otherwise)? No compelling evidence or good candidates • What was the initial condition? Very hot!

32. The End….of the beginning (but not the beginning of the end)

33. Geology, 2002

35. Sometimes initial conditions don’t matter much….e.g., heat flow  Tn with n > 2 or 3 T(t=0)=Ti T(t=) depends only weakly on Ti if T, Ti differ significantly Sometimes initial conditions matter a lot; e.g., layered system with compositional differences comparable or larger than T Some history is preserved in the compositional layering (through imperfect mixing or through heat storage)

36. Some Specific issues with Earth • How hot was it? (And does any of that “signature” remain?) 2. How is the starting state expressed in the mantle and core composition and layering? • How does this depend on our (imperfect) understanding of planetary accumulation. • What do we learn from the Moon, & from other planets. • What were conditions like on early Earth? What is the origin of atmosphere and ocean. • What about life?

37. Rayleigh-Taylor Instabilities & Convective Stirring? Height Height Bulge could arise from melt migration in transition zone May (or may not) become well mixed after freezing & RT instabilities? Uncompressed Density Uncompressed Density But this all depends on the (as yet unknown) phase diagram!

38. Core-Mantle Equilibration Significant (perhaps unexpected) success in explaining mantle siderophiles through equilibrium at a particular P,T representative of the base of the magma ocean Problem: Lack of knowledge at higher P,T.. Could still fit the data with a mixing line that includes higher P,T?

39. Melting curve steeper than the adiabat (at most depths) Freezing of most of the deeper part of the ocean is fast (~1000yrs). Processes deep down involve solid silicates. Freezing of shallow part can be slow (up to 100Ma). T vs. P in a planet Fundamental Principles of Magma Oceans melting curve T Adiabat (convective) Liquid (magma ocean) solid P Rheological boundary

40. T (K) Realistic Consequence Most of Earth history 6000 Contributing regions of last equilibration Magma ocean base 4000 Approximate conditions in present Earth Precursor bodies 2000 0.01 0.1 1 P(Mbar)

41. Halliday, 2003

42. Core Formation; Mantle Oxidation State • General idea may still work even with giant impacts Wood et al, 2006

43. Core-Forming Processes • Rainfall & ponding • Percolation • Diapirism (Rayleigh-Taylor)includes l=1 and self-heating as special cases • Cracks

44. Earth’s Engine • Plate tectonics is not at all obvious! But once in motion, it is a heat engine. • But why do plates happen? Mantle convection does not require plates! Cold slab sinks under the action of gravity

45. Plate Tectonics & the Role of Water • Water lubricates the asthenosphere • Water defines the plates • Maintenance of water in the mantle depends on subduction; this may not have been possible except on Earth

46. What Happens During a Giant Impact? • Most of the material is melted; part is vaporized. • Much of the Core of projectile is often intact and crashes into Earth, plunging to the core on a free fall time. • Severe distortion (sheets, plumes; not spheres). But SPH does not indicate much direct mixing. Canup & Asphaug

47. Oxygen Isotopes • Fundamental origin of the differences between Earth, Mars and meteorites is not understood • Still, the “identity” of Earth & Moon is often taken to imply same “source”

48. 0.1  Has ~0.8 before processing Liquid silicate disk Core is isolated Silicate vapor atmosphere IN BETWEEN A disk exists for 102 103 years. Radiates at ~2500K. Vapor pressure ~10 to 100 bars. Timescale for exchange between vapor & atmosphere ~10c/(G) ~ week. Aided by “foam”. Convective timescale in disk or Earth mantle ~week Convective timescale in atmosphere ~days

49. Volcanism & Volatile Release • Earth’s atmosphere & ocean came in part through outgassing • But volatiles are recycled on Earth- the inside of Earth is “wet”

50. Some Conclusions • SPH or other large scale codes do not tell you the extent of mixing. • There is the possibility of incomplete mixing (i.e., preservation of Hf-W from an earlier core separation event). But the importance of this is not deterministic. Most likely when the iron is in large quasi-spherical blobs. • Roughly speaking, this applies to planets independent of size (except that small bodies may suffer higher energy impacts where vimpact >> vescape, which enhances mixing. • There is no straightforward connection between the measured W and the timing of Earth core formation