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

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

Formation of solar system by contraction of the solar nebula

. .... ..

....

..

T-Tauri stage

T T

Evolution of solar nebula ~ 4.6 Ga

12x106 K ignitionProto-sun

Contraction volume

decrease

More details

Planetary formationfrom planetary embryos to the proto-Earth

With Tº decreasing away from the Sun, the refractory material (metal, silicates) accumulate near the center (rocky planets) wh i l e vo l a t i l e s and i ces accumulate in the outer rings (giant gas planets)

Fine dust particles floating in gas collide into km-size planetesimals that keep on growing through con t i nuous co l l i s i on s . 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

Tim

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10 t

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0 M

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Planetary formationfrom planetary embryos to the proto-Earth

With Tº decreasing away from the Sun, the refractory material (metal, silicates) accumulate near the center (rocky planets) wh i l e vo l a t i l e s and i ces accumulate in the outer rings (giant gas planets)

Fine dust particles floating in gas collide into km-size planetesimals that keep on growing through con t i nuous co l l i s i on s . 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

Tim

e fr

ame

10 t

o 10

0 M

yr

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

3) The first 100 million years: the telluric era, the rocky planets formed and differentiated (layered structure, atmosphere, ocean, crust etc)

Based on short-lived nuclide 26Al, chondrules are either 2-3 Myr younger or CAI condensed closer to sun (reading)

CAI & chondrules in CC first solid formed at Tº > 1800 K. U/Pb age of CAI = 4567.2±0.6 Ma, time zero t0.

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.

Meteorite classificationa clue to the early Solar System

Chondrite composition

Except for volatile elements

chondrite have the same composition

as the Sun

Non-differentiated meteoritesalmost no

planetary evolution since the origin of

solar system

SiMgFe(CaAl)

SiAlCaNa K SiAlNaK

FeNi

FeNi

Underwent planetary evolution

Differentiated meteorites

Fe-Ni

Metal + silicates

Silicates

Chronology

Formation of first bodies

1st mineral phases Preserved in primitive non-differentiated meteorites

Allende CC

Chondrule

Allende CAI

Matrix

Age of 1st phases to condendate = age of Earth

CAI Efremovka meteorite 4.567 Ga (Amelin et al. 2002)

Considered as T0 for evolution of the Earth

Absolute dating using radioactive decayParent isotope (P) to daughter isotope (D)

Law of radioactivity: dP/dt = -λ x PP = P0 x e(-λ x t)

P0 - P = F - F0

Mass spectrometry for isotopic measurementsExample 87Rb to 87Sr (λ = 1.42 x 10-11 years-1) or half

life of 48.8 x 109 yearsChronometers

Isotopes Half life40K - 40Ar 1.250 Ga147Sm - 143Nd 1.060 Ga176Lu - 176Hf 3.50 Ga232Th - 208Pb 14.010 Ga235U - 207Pb 0.703 Ga238U - 206Pb 4.46 Ga14C - 14N 5370 y

Relative dating using extinct radioactivity

Example 26Al to 26Mg (λ = 9.1 x 10-7 years-1) or half life of 0.7 x 106 years

P is completely gone, very short half life only daughter isotope remains

P0 - P = F - F0

ChronometersIsotopes Half life41Ca - 40Ar 0.1 Ma 60Fe - 60Co 1.5 Ma10Be - 10B 1.5 Ma135Cs - 135Ba 2.3 Ma53Mn - 53Cr 3.7 Ma107Pd - 107Rh 6.5 Ma182Hf - 182W 9.0 Ma129I - 129 Xe 15.9 Ma

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

All complex planetary processes happen within first 100 Myr or less...

4567

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 systemMoon: a very unusual satellite formed by mega-impact on the young Earth

The deta i led Moon f o r m i n g - s c e n a r i o (Canup 2004)

A m o n g a l l s c e n a r i o ’ s proposed only the mega-impact explains all properties of the Earth-Moon system

Such mega-collision is not unusual during planetary formation

After the mega-impact scenario reformation and start of the geo-evolution of Earth: Hadean

Film illustrating the mega-impact and formation of the Earth-Moon system

Considered as the end of planetary accretion

Origin of water

Earth is relatively dry

Moceans= 1.4 x 1024 gM⊕ = 5.97 x 1027 g

Moceans= 250 ppm⊕

H2Omantle = 5-10 x Moceans

MH2O = 350-500 ppm⊕

… just like the other rocky planets

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∅

in comparison to other planetary bodies

Primitive meteoritescarbonaceous chondrites

H2O = 17-22 wt% (CI; Orgueil)H2O = 3-11 wt% (CM2; Murchinson)

H2O < 2 wt% (CV; Allende)

Micromteorites from Antarctica (CM2): H2O = 2-8 wt%

Comets H2O ≤ 50 wt%

Hale-Bopp

1 ua

2-5 ua (asteroid belt & Troyans)30-50 ua (Kuiper belt)

Heliocentric distribution of water

Ordinary chondritesEnstatite chondrites

H2O ≤ 1 wt% or anhydrousCarbonaceous

chondrites 2-22 wt%

Short & long period comets H2O ≤ 50 wt%

IDP-AMM1-8 wt%

How did water get to Earth ?

Heliocentric distribution of water D/H

Earth D/H = 153.7 x 10-6Carbonaceous chondrites

D/H = 130-170 x 10-6

Comets (Oort)D/H = 300-330 x 10-6

Temperature favors D loss and makes water lighterHDOice + H → D + H2Ogas

Sun D/H = 25 x 10-6

Interstellar cloudsHCN - D/H = 2000 x 10-6

Cold cores - D/H = 1260 x 10-6

Hot cores - D/H = 110 x 10-6

Yokochi & Marty (2007); Robert (2003); Aléon et al., (2005); Engrand et al. (1999))

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 chemistry and mineralogy similar to CM2 could be another transport agent to Earth

Yokochi & Marty (2007); Hashizume et al. (2000). Figure Yokochi & Marty (2007) modifiée.

Another (less likely) possibility

Ocean water could result from mixing heavy cometary water with light H from proto-solar nebula, that was later oxidized in H2O

However C & N isotopes do not agree with this hypothesis

0,003

0,004

0,005

0,006

0,007

0,008

0,009

0,010

0,011

0,012

0,0E+00 1,0E-04 2,0E-04 3,0E-04 4,0E-04

D/H

13C/12C

Solar

Cometary CN

Earth

CI

CM

R = 1000

R = 0.001

R = (H/12C)solar/(H/12C)cometary

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

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

Terr

estri

al a

bund

ance

, mol

/g

Mantle+atm+hydrMantle

N

CH2O

36Ar22Ne

84Kr130Xe

Terrestrial=chondritic

Terrestrial=0.003 x chondritic

Terrestrial=0.02 x chondritic

A c h o n d r i t i c i n p u t between 0.3 to 2% of the Earth mass explains the whole volatile budget on Earth

This agrees with the 0.5 to 1% mass necessary to explain high siderophile e l e m e n t s a f t e r c o re formation

A chondritic Earth

 Late Veneer hypothesissprinkel material on early Earth

0,0001

0,001

0,01

0,1

1

10

100

0 40 80 120 160

Time since ASSC, Ma

MH

2O

1-10x Moceans

Initial upper limit = 50 x oceans

RASSC = 106today FAMM = 1010 g/yr; H2O = 2 wt%

RASSC = 106today FAMM = 5x1010 g/yr; H2O = 8 wt%

Earth accretionMoon formation

Zircons : a cool early earth

RASSC = 109today FAMM = 1010 g/yr; H2O = 2 wt%

RASSC = 109today FAMM = 5x1010 g/yr; H2O = 8 wt%

When did water arrive on Earth?

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

If water comes with CI-CM bodies assume 10x current mass of asteroid belt

+ s m a l l l a t e c o m e t a r y contribution (~%)

First cooling of magma ocean

146Sm → 142 Nd (T1/2= 103 Ma) silicate/silicate fractionation before total decay of 146Sm ⇒ < 150 Ma, done early Hadean

Alteration of basalt to produce serpentinite crust ~ as today on seafloor

Moon-forming impact

38004000420044004600

Earth accretion

Core formation

Primordial crust

Zircons: evolved crust

LHB

AMM water delivery

Asteroidal water delivery

Habitable ocean: Propiano model

Habitable ocean:Abe, 1993.

Time before present (Ma)

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