13_plntry_frmtn
DESCRIPTION
GeologyTRANSCRIPT
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MIT OpenCourseWare http://ocw.mit.edu
12.001 Introduction to GeologySpring 2008
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.
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L. T. Elkins-Tanton
Mars Fundamental Research Program
The effects of magma ocean depth and initial
composition on planetary
differentiation
(Image courtesy of NASA/JPL.)
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Formation of new stars and planets
protoplanetary nebula = planetary nebula = preplanetary nebula, NEBULAS, not new SOLAR SYSTEMS
protoplanetary disk = proplyd ≈ accretionary disk (this really refers just to the star)
Horsehead nebula: NASA, NOAO, ESA and The Hubble Heritage Team (STScI/AURA)
Eagle nebula: Hester, Scowen (ASU), HST, NASA
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Rotating nebula begins to contract
Images removed due to copyright restrictions.
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Contraction and rotation of protoplanetary disk
Images removed due to copyright restrictions.
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Ca-Al inclusions in carbonaceous chondrites: 4567.2±0.6 million years old
Images removed due to copyright restrictions.
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Accretion simulations
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.
Images removed due to copyright restrictions.
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Size of the planets
Images courtesy of NASA.
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Young planets are hot
Factors lead to heating and melting early in a terrestrial planet’s history
Radioactive decay of elements
Accretion of large bodies (meters [planetesimals] to hundreds of kilometers [embryos])Converts kinetic energy to heat
Core formationConverts potential energy to heat
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Global Chemical Differentiation
Images removed due to copyright restrictions.
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Iron meteorites: The cores of failed planetesimals
Images removed due to copyright restrictions.
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Short-lived radioisotopes
Some short-lived radioactive elements produced enough heat to help melt planetesimals or embryos:
• 26Al (700,000 y) → 26Mg releases 4.8×10-13 J/atom; Mars may have had 1042 atomsWasserburg: 26Al was present in CAIs, meaning at most a few million years for CAI formation
• 60Fe (1,500,000 y)→ 60Co (5.2714 y) → 60Ni
Other short-lived radioisotopes date differentiation events:
• 182Hf (9,000,000y) → 182Ta (114.43 days) → 182W Core formation constraint (W into core, Hf into silicate mantle)
• 146Sm (103,000,000 y) → 142NdSilicate differentiation constraint (Sm slightly more into minerals, Nd into melt)
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Formation of the Moon(about 15 – 20 million years after CAIs)
Images removed due to copyright restrictions.
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Clearing out the inner solar system
Images removed due to copyright restrictions.
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Far side of the Moon
Image courtesy of NASA.
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OUTLINE: Solidification of a magma ocean
1. Theory and calculations2. Planetary radius and a heterogeneous mantle 3. Depth of initial magma ocean and resulting crustal compositions4. Production of a magnetic field5. Volatiles and plagioclase flotation
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Linked solidification and cooling processes
Images removed due to copyright restrictions.
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Volatiles partition among three reservoirs
Images removed due to copyright restrictions.
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Heat flux through atmosphere allows calculation of cooling rates
Images removed due to copyright restrictions.
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Structure and thermal evolution of an evolving magma ocean
Mineralogy and solidus based on data from Longhi et al. (1992)
Images removed due to copyright restrictions.
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Settling or entrainment: Calculation of liquid/crystal density inversion
Stolper et al. (1981) and Walker and Agee (1988): olivine buoyancy inversion between 7.5 and 9 GPa.
Elkins-Tanton et al. (2003)
Images removed due to copyright restrictions.
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Lunar magma ocean
Images removed due to copyright restrictions.
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Idealized overturn
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OUTLINE
1. Theory and calculations
2. Planetary radius and a heterogeneous mantle 3. Depth of initial magma ocean and resulting crustal compositions4. Production of a magnetic field5. Volatiles and plagioclase flotation
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
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Gravitational overturn: Nonmonotonic density gradients
Images removed due to copyright restrictions.
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Gravitational overturn: Numerical models in spherical coordinates
Monotonic density gradient Nonmonotonic density gradient
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Overturn creates a laterally-heterogeneous mantle
Axisymmetic models show: The majority of overturn complete in <2 Myr; small-scale heterogeneities last a long time
Before overturn After overturn
Images removed due to copyright restrictions.
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Depths of origin of lunar volcanic rocks
Images removed due to copyright restrictions.
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OUTLINE
1. Theory and calculations2. Planetary radius and a heterogeneous mantle
3. Depth of initial magma ocean and resulting crustal compositions4. Production of a magnetic field5. Volatiles and plagioclase flotation
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
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Volatile content considerations
• The Moon likely accreted dry, while Mars and the Earth had non-zero volatile contents
• 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
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Volatile contents of possible planetary building blocks
John A. Wood (2005) The chondrite types and their origins.
If planet up to ~1,300 km radius retains volatiles (Safronov), bulk Mars-size planet contains:
C H2O CO2 H2OC1 (CI) 3.5% 20% 0.06% 1.2%C2 (CM) 2.5 13CR 1.5 6 0.03 0.36C3 (CV, CO) 0.5 1 0.01 0.06OC (Type 3) 0.5 1
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Atmospheric growth and planetary solidification
Brain and Jakosky (1998): 95 to 99% of atmospheric has been lostCatling (2004): 99% of surface water has been lost
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Varying volatile mix in initial magma ocean
Mars-sized planet: Decreasing water, increasing carbon dioxide
Initial: 0.4 wt% H2O, 0.1 wt% CO2 0.3 wt% H2O, 0.2 wt% CO2 0.2 wt% H2O, 0.3 wt% CO2
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Depth of first plagioclase crystallization
Without a plagioclase flotation crust, planetary solidification is very fast:
90 vol% in less than 50,000 years
PLANETARY SURFACE
Images removed due to copyright restrictions.
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The Hadean Earth: Old Version
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Dynamic effects of volatiles in cumulates
• Even small amounts of water have a large effect on solid-state creep rates
• Volatiles are available for later degassing
Images removed due to copyright restrictions.
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Constraints on planetary formation from isotopic systems
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Liquid water
Liquid water may be stable on the surface within a few million years
Martian distributary channels
Courtesy of NASA.
Image removed due to copyright restrictions.