Transcript Early Differentiation of Terrestrial Planets: The Relative

Early Differentiation of Terrestrial
Planets: The Relative Importance
of Big Impactors and Small
Impactors
Dave Stevenson
Caltech
Prague Goldschmidt
Conference, August
15, 2011
The Main Points
• Giant impacts are an essential part of Earth
formation. Not just the lunar forming impact.
• Small impactors are also essential &
contemporaneous.
• These two classes have different geochemical
& geophysical consequences even if they have
same timing and composition
• Implications for the core & mantle , isotopic
signatures and chronology.
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
What should you Believe about
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)
Current State of Play
• Nice model (and other similar work) shows
how dynamical properties of the terrestrial
planets are linked to both giant impacts and
small bodies (dynamical friction).
• In these models, the giant impacts are an
essential ingredient. But the small bodies are
also an essential ingredient.
Giant Impacts
Small Impactors
Thermal effect
Resets the magma ocean to Helps maintain magma
great depth & pressure
ocean to ~ transition zone
Equilibration
Poor for the Fe core of the
projectile
Good for all components
T, P for any equilibration
Extends up to core-mantle
boundary P and very high T
(plausibly 6000K)
Modest T (2500K) and
P(maybe 30GPa)
Implications for Hf/W
Could mislead chronology
(but some disequilibrium is
OK)
Can give good guide to
chronology
Implications for
siderophiles
Not known since we don’t
know the partition
coefficients at extreme T
Can be estimated; some
evidence for agreement
with observation
Origin of the moon is
the most striking
consequence of
giant impacts, but it
is singular only in
being the last event
of its kind. (Earlier
moons could have
existed)
Giant Impact
Formation of the
Moon
• Impact “splashes” material
into Earth orbit. Mostly from
projectile! But subsequent
turbulent mixing.
• The Moon forms from a disk
in perhaps ~100 to 1000
years
• One Moon, nearly equatorial
orbit, near Roche limit
(beyond corotation)- tidally
evolves outward
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”
• May be a consequence
of mixing between Earth
& lunar forming disk.
Phase boundary temperature
Phase boundary Temperature (K)
Defined as vapor-liquid equilibrium
r (m)
Entropy Distribution
• Post giant impact, the disk is of almost
uniform entropy
• But the outermost earth is of higher entropy:
higher than both the disk an the deeper earth.
This suppresses mixing of the mantle initially
and also suppresses core-mantle equilibration.
Does this mean the deep earth has different
oxygen (and other isotopic) signature than the
rest of Earth? Open question.
Core-Mantle Equilibration
• If there is a magma ocean right at the CMB then
the thickness of mantle that can chemically
equilibrate with core is ~
(Dt)1/2.(T/t) where D is diffusivity, t is the
convective instability time, T is total elapsed time.
This is 100km for t ~104 sec, T ~1011 sec, D ~10-4
cm2/sec.
This probably means that the lowermost mantle is
isolated from the rest of the mantle over geologic
time.
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. May be
relevant to 142Nd
CORE
Core Superheat
Early core
• 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
Core
Superheat
Adiabat of core alloy
Present mantle
and core
depth
Popular Cartoons of
Core Formation
Stevenson, 1989
Wood et al,
2006
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
Turbulent mixing of fluids
Rayleigh-Taylor Instability
dense
Dalziel et al.
J. Fluid Mech. 1999
light
light
dense
Turner
J. Fluid. Mech 1986
Smyth et al.
J. Phys Ocean. 2001
light
dense
Kelvin-Helmholtz Instability
Heterogeneous Accretion?
• The Nice model predicts some heterogeneous
accretion, both for the large and small bodies.
• Bottkeet al (2010) :Late impacts of lunar sized
bodies (not as big as the moon forming impact)
may occur.
• Giant impact story is (still) falling short in
geochemical explanation or prediction, partly
because the physics is imperfectly understood
“Doubt may be uncomfortable but certainty is absurd” Voltaire
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!]..but cap may be broken for Earth
• Hot subsolidus convection : Few x108 yr
• At current rate: >1010 yr
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 icecapped.
• 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.
The Main Points
• Giant impacts are an essential part of Earth
formation. Not just the lunar forming impact.
• Small impactors are also essential &
contemporaneous.
• These two classes have different geochemical
& geophysical consequences even if they have
same timing and composition
• Implications for the core & mantle , isotopic
signatures and chronology.