The first 50 Ma: Planetary formation and the role of water

1. Eagle nebula: Hester, Scowen (ASU), HST, NASA The first 50 Ma: Planetary formation and the role of water L. T. Elkins-Tanton, E.M. Parmentier Mars Fundamental Research Program ((NASA/JPL)

2. The Hadean Earth: Old Version

3. Outline • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Time to clement conditions

4. Planetary disk NASA/JPL

5. Ca-Al inclusions in carbonaceous chondrites: 4567.2±0.6 million years old

6. Iron meteorites: The cores of failed planetesimals

7. Planetary accretion simulation from Raymond et al. (2006), using 1054 initial planetesimals from 1 to 10 km radius. Earthlike planets are formed, but their orbital eccentricities are too high. Accretion simulations: Planets form from differentiated material

8. Outline • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Time to clement conditions

9. Young planets are hot Factors lead to heating and melting early in a terrestrial planet’s history • Accretion of large bodies (meters [planetesimals] to hundreds of kilometers [embryos]) • Converts kinetic energy to heat • Radioactive decay of elements • Core formation • Converts potential energy to heat • 26Al (700,000 y) → 26Mg • 60Fe (1,500,000 y)→ 60Co (5.2714 y) → 60Ni

10. NASA/Lunar Orbiter 4 Far side of the Moon

11. MOON: Lunar anorthositic highlands led to the model of a deep initial magma ocean Extreme age of the anorthositic highlands Their positive Eu anomalies and the corresponding negative anomalies in the mare basalts Experimental evidence that plagioclase is not present on the liquidus of the mare basalts (Wood et al., 1970; Wood, 1972; Töksoz and Solomon, 1973; Taylor and Jakes, 1974; Wood, 1975) (Nyquist, 1977; Nyquist and Shih, 1992) (e.g., Morse, 1982; Ryder, 1982; Haskin et al., 1981) (Green et al., 1971a, b) NASA/JPL/Galileo Do terrestrial planets experience magma oceans? MOON

12. MARS: SNC meteorites: Extreme eNd values and presence of 142Nd anomalies Magnetized crustal provinces are evidence for very ancient crustal formation Connerney et al., GRL (2001) (Jones, 1986; Breuer et al., 1993; Harper et al., 1995; Borg et al., 1997) (Acuňa et al., 1999; Purucker et al., 2000) Do terrestrial planets experience magma oceans? MARS

13. EARTH: Difference between 142Nd/ 144Nd anomalies between Earth and chondrites may require an unsampled reservoir in mantle and may indicate silicate differentiation >4.53 Ga (Boyet and Carlson, 2005) BUT isotopic heterogeneity may be expected (Ranen and Jacobsen, 2006) Do terrestrial planets experience magma oceans? EARTH

14. The core formation constraint Sinking of iron-rich liquids by porous flow through a silicate mantle requires interconnectivity of pore space. [Stevenson, 1990]. In an iron-rich Martian mantle high dihedral angles would prevent percolation of iron-rich core material at pressures above 3 GPa. [Terasakiet al., 2005] . The timing of core formation is therefore an upper bound on the completion of accretion, melting, and core formation [e.g.Jacobsen, 2005].

15. Outline • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Solidification • Overturn • Cooling before convection • Time to clement conditions

16. Elkins-Tanton Linked solidification and cooling processes

17. Elkins-Tanton Volatiles partition among three reservoirs

18. Process after Abe and Matsui (1985); Solomatov (2000) Emissivity parameterizations from Pujol and North (2003), Hodges (2002) Elkins-Tanton Heat flux through atmosphere allows calculation of cooling rates

19. Elkins-Tanton et al. (2003) Mineralogy and solidus based on data from Longhi et al. (1992) Structure and thermal evolution of an evolving magma ocean

20. EARTH, MARS 2.1 GPa MOON 3.6 GPa Stolper et al. (1981) and Walker and Agee (1988): olivine buoyancy inversion between 7.5 and 9 GPa. Elkins-Tanton et al. (2003) Settling or entrainment: Calculation of liquid/crystal density inversion

21. Outline Magma oceans would solidify from the bottom up Volatiles partition between cumulates and atmosphere Garnet may sink into a near-monominerallic layer Shallowest cumulates are denser than deeper cumulates • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Solidification • Overturn • Cooling before convection • Time to clement conditions

22. Elkins-Tanton et al. (2003, 2005) Idealized overturn

23. Elkins-Tanton et al. (2005) Gravitational overturn: Nonmonotonic density gradients

24. Before overturn Contours of initial depth (proxy for composition) After overturn Contours of density Axisymmetic models show: The majority of overturn complete in <2 Myr; small-scale heterogeneities last a long time Models by Sarah Zaranek Overturn creates a laterally-heterogeneous mantle

25. Gravitational overturn: Numerical models in spherical coordinates View movie m3H of numerical experiment of solid-state overturn of gravitationally unstable mantle cumulates: Cooled surface produces shorter-wavelength downwellings Numerical experiment by Elkins-Tanton

26. Gravitational overturn: Numerical models in spherical coordinates View movie m3L of numerical experiment of solid-state overturn of gravitationally unstable mantle cumulates: Insulated surface produces longer-wavelength downwellings Numerical experiment by Elkins-Tanton

27. Elkins-Tanton et al. (in prep, 2007) Mantle heterogeneity after gravitational overturn

28. Elkins-Tanton et al. (in prep, 2007) Depths of origin of lunar volcanic rocks

29. Elkins-Tanton et al. (2005) Melting from overturn

30. Outline Cumulate overturn is fast and efficient Following early differentiation there will be no simple composition-depth relationships Following early differentiation the mantle will have a stable density stratification • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Solidification • Overturn • Cooling before convection • Time to clement conditions

31. Volatile content considerations • The Moon likely accreted dry, unlike Mars and the Earth • Volatiles form an insulating atmosphere and partition into nominally anhydrous mantle cumulates • No mafic quench crust is likely to form on a magma ocean, because of density and temperature considerations • Water in the silicate liquid inhibits the formation of plagioclase • If plagioclase forms and floats, it may significantly slow planetary cooling

32. If planet up to ~1,300 km radius retains volatiles (Safronov), bulk Mars-size planet contains: • C H2O CO2 H2O • C1 (CI) 3.5% 20% 0.06% 1.2% • C2 (CM) 2.5 13 • CR 1.5 6 0.03 0.36 • C3 (CV, CO) 0.5 1 0.01 0.06 • OC (Type 3) 0.5 1 John A. Wood (2005) The chondrite types and their origins. Volatile contents of possible planetary building blocks

33. 500-km-deep magma ocean on Earth Elkins-Tanton et al. (in prep, 2007) Atmospheric growth and planetary solidification: Earth

34. Parmentier and Elkins-Tanton (in prep, 2007) Magma ocean solidification

35. Elkins-Tanton et al. (in prep, 2007) Evolving magma ocean liquids

36. PLANETARY SURFACE EARTH (30 km, 0.005r) Without a plagioclase flotation crust, planetary solidification is very fast: 90 vol% in less than 200,000 years MARS (80 km, 0.02r) MOON (240 km, 0.14r) Elkins-Tanton et al. (in prep, 2007) Depth of first plagioclase crystallization

37. MARS • Even small amounts of water have a large effect on solid-state creep rates • Volatiles are available for later degassing Elkins-Tanton et al. (in prep, 2007) Dynamic effects of volatiles in cumulates

38. Elkins-Tanton et al. (in prep, 2007) Constraints on planetary formation from isotopic systems

39. Outline Formation of a plagioclase flotation crust is controlled by volatile content and planetary size A plagioclase flotation crust may be the only way to slow solidification In the absence of plate tectonics, mantle volatile content should be controlled by processes of early differentiation • Planetary accretion • Likelihood of a molten planet • Processes of solidification • Time to clement conditions

40. Elkins-Tanton et al. (in prep, 2007) MARS: Liquid water within ~15-20 Ma of a magma ocean

41. Elkins-Tanton et al. (in prep, 2007) EARTH: Liquid water within ~35-40 Ma of a magma ocean

42. Solidification is fast unless there is a conductive lid An initially molten planet creates a mantle slow to convect There were likely multiple magma ocean events Valley et al. (2002) Conclusions