Accretion and Differentiation of Earth. Dave Stevenson Caltech Neutrino Sciences 2007 Deep Ocean Anti-Neutrino Observatory Workshop Honolulu, Hawaii March 23-25, 2007. Definitions. Accretion means the assembly of Earth from smaller bits
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Neutrino Sciences 2007 Deep Ocean Anti-Neutrino Observatory Workshop Honolulu, Hawaii March 23-25, 2007
Some multidimensional space
Some multidimensional space
Focus of this talk
Astronomy, geochemistry, physical modeling
Geochemistry, geology, geobiology
The (logarithmic) way one should think about time if you want to understand processes and their outcome
Nucleosynthesis in massive stars (supernovae for the heaviest elements)
Sun & planets
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
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)
Gas density enhancement
Mars mass embryo -hot & differentiated
This predicts only modest ingassing (even assuming the embryo has an accessible magma ocean)
(but misleading because of infrequent large impacts and steam atmosphere)
Other kinds of differentiation (ocean & atmosphere formation, continental crust) are not conceptually that different although the details differ a lot.
182Hf 182W 1/2 ~ 9 Ma
Excess 182W observed
CORE MERGING EVENT (Hf-W timescale planet formation timescale)
EMULSIFICATION DURING IMPACT (Hf-W timescale planet formation timescale provided emulsification is sufficiently small scale)
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?)
Adiabat of core alloy
Present mantle and core
Frozen (but very hot!)
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?
Kellogg et al, 1999
*4.4 to 3.8Ga
(but not the beginning of the end)
Sometimes initial conditions don’t matter much….e.g., heat flow Tn with n > 2 or 3
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)
2. How is the starting state expressed in the mantle and core composition and layering?
Bulge could arise from melt migration in transition zone
May (or may not) become well mixed after freezing & RT instabilities?
But this all depends on the (as yet unknown) phase diagram!
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?
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 planetFundamental Principles of Magma Oceans
Liquid (magma ocean)
Most of Earth history
Contributing regions of last equilibration
Magma ocean base
Approximate conditions in present Earth
Wood et al, 2006
Cold slab sinks under the action of gravity
Canup & Asphaug
Has ~0.8 before processing
Liquid silicate disk
Core is isolated
Silicate vapor atmosphere
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