Download When did Earth become habitable ? (which does not imply it was

When did Earth become habitable ?
(which does not imply it was inhabited)
Formation of solar system by contraction of the solar nebula
Contraction
volume
decrease
Proto-sun
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. ..
. .
12x106 K ignition
T-Tauri stage
.
. ..
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.
T
T
Evolution of solar
nebula ~ 4.6 Ga
More details
Elephant trunk nebula (Spitzer
telescope) = stellar nursery with
forming stars glowing. As the
solar nebula most likely did, it
contains Si-dust, H, He and
complex PAH’s
Planetary formation
from planetary embryos to the proto-Earth
Time frame 10 to 100 Myr
With Tº decreasing away from
the Sun, the refractory material
(metal, silicates) accumulate
near the center (rocky planets)
w h i l e vo l a t i l e s a n d i c e s
accumulate in the outer rings
(giant gas planets)
Fine dust particles floating in gas
collide into km-size planetesimals
that keep on growing through
continuous collisions. Open
questions: physics of collisions should
be disruptive ?, radial migration into
the sun by gas drag on meter-sized
particles ? unless multi-km objects
form < 1000 years.
Gradually proto-Earth develops,
gravity re-shapes it into a sphere,
and immediately differentiation into
core, mantle crust starts
Planetary formation
from planetary embryos to the proto-Earth
Time frame 10 to 100 Myr
With Tº decreasing away from
the Sun, the refractory material
(metal, silicates) accumulate
near the center (rocky planets)
w h i l e vo l a t i l e s a n d i c e s
accumulate in the outer rings
(giant gas planets)
Fine dust particles floating in gas
collide into km-size planetesimals
that keep on growing through
continuous collisions. Open
questions: physics of collisions should
be disruptive ?, radial migration into
the sun by gas drag on meter-sized
particles ? unless multi-km objects
form < 1000 years.
Gradually proto-Earth develops,
gravity re-shapes it into a sphere,
and immediately differentiation into
core, mantle crust starts
Solar system formation
1) The first million year: the stellar era: formation of the sun by accretion/contraction of material
circumstellar disk
2) The first 10 million years: the disk era: evolution of circumstellar disk to give birth to planets; oldest
objects in solar system formed and are still preserved in primitive meteorites
CAI & chondrules in CC first solid formed at Tº > 1800 K. U/Pb age of CAI =
4567.2±0.6 Ma, time zero t0.
Based on short-lived nuclide 26Al, chondrules are either 2-3 Myr younger or CAI
condensed closer to sun (reading)
3) The first 100 million years: the telluric era, the rocky planets formed and differentiated (layered
structure, atmosphere, ocean, crust etc)
Meteorite classification
a clue to the early Solar System
Almost 10 tons of meteorite fall on Earth / year (> cm size) coming from asteroid belt between
Mars and Jupiter, link to asteroid spectral classification and composition.
Chondrite composition
Except for volatile
elements
chondrite have the
same composition
as the Sun
Non-differentiated
meteorites
almost no
planetary evolution
since the origin of
solar system
Differentiated meteorites
Underwent planetary evolution
SiAlCaNa K
SiAlNaK
Silicates
Metal + silicates
SiMgFe
(CaAl)
FeNi
FeNi
Fe-Ni
Chronology
Formation of
first bodies
1st mineral phases
Preserved in primitive nondifferentiated meteorites
Allende CAI
Allende CC
Chondrule
Matrix
Age of 1st phases to condendate
= age of Earth
Considered as T0 for evolution of the Earth
CAI Efremovka meteorite 4.567 Ga (Amelin et al. 2002)
Absolute dating using radioactive decay
Parent isotope (P) to daughter isotope (D)
Law of radioactivity: dP/dt = -λ x P
P = P0 x e(-λ x t)
P0 - P = F - F0
Chronometers
Isotopes
Half life
40K - 40Ar
1.250 Ga
147Sm - 143Nd
1.060 Ga
176Lu - 176Hf
3.50 Ga
232Th - 208Pb
14.010 Ga
235U - 207Pb
0.703 Ga
238U - 206Pb
4.46 Ga
14C - 14N
5370 y
Mass spectrometry for isotopic measurements
Example 87Rb to 87Sr (λ = 1.42 x 10-11 years-1) or half
life of 48.8 x 109 years
Relative dating using extinct radioactivity
P is completely gone, very short half life only
daughter isotope remains
P0 - P = F - F0
Chronometers
Isotopes
Half life
41Ca - 40Ar
0.1 Ma
60Fe - 60Co
1.5 Ma
10Be - 10B
1.5 Ma
135Cs - 135Ba
2.3 Ma
53Mn - 53Cr
3.7 Ma
107Pd - 107Rh
6.5 Ma
182Hf - 182W
9.0 Ma
129I - 129 Xe
15.9 Ma
Example 26Al to 26Mg (λ = 9.1 x 10-7 years-1)
or half life of 0.7 x 106 years
Timing of the differentiation
•Recent data (182Hf to 182W, T1/2=9Myr chronometer) indicate bulk metal-silicate segregation
in < 30 Myr after begin of Solar System formation
•Core formed more rapidly in smaller bodies (Vesta, Moon, Earth) (reading)
•Moon forming impact took place as this differentiation was already going on within the
proto-Earth
•182Hf-182W of lunar basalt indicates a differentiation at 45 ± 5 Myr, which could mark the end
of the Lunar magma ocean
•Isotopic composition of Earth’s atmosphere differs from solar nebula, primitive meteorites
and comets, composition of the primary atmosphere is unknown
•Earth’ atmosphere was subject to several episodes of loss to space (large impacts or with
isotope fractionation such as thermal loss or pick up ion loss) during tens of Myr before
closure.
•Earth’s atmosphere formed from several volatile-rich components
•High dynamic state of the mantle during all of the Hadean (~ 0.5 Gyr)
Chronology of early planetary processes
4567
All complex planetary processes happen within first 100 Myr or less...
What is Earth made off ? Bulk composition
CI = solar system (except H, He), other meteorites
lost volatiles (H2O, K, Cl, S etc.) during T-tauri
phase and by subsequent evolution of parent body
Accretion is fast, gas planets formed within few Ma,
rocky ones a few 10 Ma. Lots of material exchange
in Solar System, as planetary embryos destabilized
by Jupiter. Primordial material does not remain in
orbit around Earth more than 100 Ma
Earth’s ∂17O values differs from other planetary
bodies. Earth-Moon fractionation line (slope 0.5)
indicate origin from similar orbits
Earth originally made of Enstatite chondrite-like
material
Origin of the Earth - Moon system
Moon: a very unusual satellite formed by mega-impact on the young Earth
The detailed Moon
forming-scenario
(Canup 2004)
A m o n g a l l s c e n a r i o ’s
proposed only the megaimpact explains all properties
of the Earth-Moon system
Such mega-collision is not
unusual during planetary
formation
Film illustrating the mega-impact and formation of the Earth-Moon system
Considered as the end of planetary accretion
After the mega-impact scenario reformation and
start of the geo-evolution of Earth: Hadean
Origin of water
Moceans= 1.4 x 1024 g
M⊕ = 5.97 x 1027 g
Moceans= 250 ppm⊕
H2Omantle = 5-10 x Moceans
MH2O
= 350-500 ppm⊕
Earth is relatively dry
Mwater-today = 1.6 x 1021 g (MARSIS)
Mearly ocean = 1.7 x 1022 g (volcanism)
Mearly ocean = 7.3 x 1022 g (MOLA)
M∅ = 6.42 x 1026 g
Moceans = 2.5-113 ppm∅
… just like the other rocky planets
Primitive meteorites
carbonaceous chondrites
H2O = 17-22 wt% (CI; Orgueil)
H2O = 3-11 wt% (CM2; Murchinson)
H2O < 2 wt% (CV; Allende)
in comparison to other planetary bodies
Micromteorites from Antarctica
(CM2): H2O = 2-8 wt%
Comets H2O ≤ 50 wt%
Hale-Bopp
Heliocentric distribution of water
Short & long period comets
H2O ≤ 50 wt%
Ordinary chondrites
Enstatite chondrites
H2O ≤ 1 wt% or anhydrous
Carbonaceous
chondrites 2-22 wt%
IDP-AMM
1-8 wt%
1 ua
2-5 ua (asteroid belt & Troyans)
30-50 ua (Kuiper belt)
How did water get to Earth ?
Heliocentric distribution of water D/H
Sun D/H = 25 x 10-6
Comets (Oort)
D/H = 300-330 x 10-6
Earth D/H = 153.7 x 10-6
Carbonaceous chondrites
D/H = 130-170 x 10-6
Interstellar clouds
HCN - D/H = 2000 x 10-6
Cold cores - D/H = 1260 x 10-6
Hot cores - D/H = 110 x 10-6
Temperature favors D loss and makes water lighter
HDOice + H → D + H2Ogas
Chondritic origin of water
Ocean water has an D/H
isotopic signature that is in the
range of that of chondritic
bodies (CO, CI, CM).
Currently, these bodies occur in
the main asteroid belt between
Mars and Jupiter or among the
Troyans (Jupiter orbit)
Micrometeorites that have a
chemistr y and mineralogy
similar to CM2 could be
another transport agent to
Earth
Yokochi & Marty (2007); Robert (2003); Aléon et al., (2005); Engrand et al.
(1999))
Another (less likely) possibility
0,012
CI
0,011
Solar
Earth
CM
0,010
R = 0.001
0,009
0,008
13
C/ 12 C
Ocean water could result from
mixing heavy cometary water
with light H from proto-solar
nebula, that was later oxidized
in H2O
0,007
However C & N isotopes do
not agree with this hypothesis
0,006
R = 1000
Cometary CN
0,005
0,004
0,003
0,0E+00
R = (H/12C)solar/(H/12C)cometary
1,0E-04
2,0E-04
3,0E-04
4,0E-04
D/H
Yokochi & Marty (2007); Hashizume et al. (2000). Figure Yokochi & Marty
(2007) modifiée.
A chondritic Earth
1,E+00
1,E-02
Mantle+atm+hydr
Mantle
Terrestrial abundance, mol/g
1,E-04
H2O
1,E-06
1,E-08
C
N
Terrestrial=chondritic
A chondritic input
between 0.3 to 2% of the
Earth mass explains the
whole volatile budget on
Earth
Terrestrial=0.02 x chondritic
1,E-10
Terrestrial=0.003 x chondritic
1,E-12
1,E-14
22
Ne
1,E-16
36
Ar
84
Kr
130
Xe
1,E-18
1,E-18 1,E-16 1,E-14 1,E-12 1,E-10 1,E-08 1,E-06 1,E-04 1,E-02 1,E+00
Chondritic abundance, mol/g
This agrees with the 0.5 to
1% mass necessary to
explain high siderophile
elements after core
formation
Late Veneer hypothesis
sprinkel material on early Earth
When did water arrive on Earth?
100
Initial upper limit = 50 x oceans
RASSC = 109today FAMM = 5x1010 g/yr; H2O = 8 wt%
10
RASSC = 109today FAMM = 1010 g/yr; H2O = 2 wt%
MH2O
1-10x Moceans
1
AMM could produce oceans if
flux 109 x higher than today
(10-50 x 109 g/y)
Model for a lower flux (1000x)
only 4% of the water
0,1
RASSC = 106today FAMM = 5x1010 g/yr; H2O = 8 wt%
0,01
RASSC = 106today FAMM = 1010 g/yr; H2O = 2 wt%
0,001
Zircons : a cool early earth
Moon formation
Earth accretion
0,0001
0
40
80
Time since ASSC, Ma
120
160
If water comes with CI-CM
bodies assume 10x current mass
of asteroid belt
+ small late cometary
contribution (~%)
First cooling of magma ocean
Alteration of basalt to produce serpentinite
crust ~ as today on seafloor
146Sm
→ 142 Nd (T1/2= 103 Ma) silicate/silicate fractionation before total decay
of 146Sm < 150 Ma, done early Hadean
Habitable ocean:
Abe, 1993.
Habitable ocean: Propiano model
Asteroidal water delivery
AMM water delivery
LHB
Zircons: evolved crust
Primordial crust
Moon-forming impact
Core formation
Earth accretion
4600
4400
4200
4000
Time before present (Ma)
3800
Early ocean
Warm based on O & Si isotopes in chert
Only 2 x more saline than today, fluid inclusions in chert
Na-Ca-Cl chemistry
Major hydrothermal input, trace elements & noble gas
More acid and anoxic
Chondritic origin
Early (during) after end of accretion, direct after Moon
impact
The Hadean: Building an habitable planet
There is no real rock record of the first 700 Myr (4.5 to 3.8 Ga) because of
resurfacing of Earth
Analogy with other planets that preserved their oldest terranes (Moon, Mars),
basaltic meteorites (Eucrites)
Jack Hills zircons, core at 4.4 Ga, recycled in younger sequences
Oldest rock Acasta Gneiss (Canada) at 4.1 Ga metamorphosed
Isua oldest rock sequence (Greenland) at 3.8 Ga