Terra Aqua

Terra Aqua

Water and the Earth:

Where did it come from

and When did it Arrive?EPSC 666

Javier Herbas

Michael Patterson

Why Is Water Important?

• It drives all the processes on Earth!– Mantle melting– Continental Crust formation– Alteration, Volcanism, Metamorphism, Erosion

… – Climate– Ultimately …. Life!– Etc., etc…

Outline

A) Where from and When?1. Possible sources of Earth’s water

2. When did water arrived?

3. Issues

B) Where is the water?

C) What is it doing today?

A) Where from and When?1. Possible sources of Earth’s water

1. Primordial gas capture

2. Adsorption in the accretion disk

3. Comets

4. Asteriods

5. Inward migration of hydrous silicates

2. When did water arrived?

3. Controversy

Sources of Terrestrial Water

Where does water come from?

INTRODUCTION

4.55 Billion years ago, the sun and planets formed from the protosolar nebula

Rotating disk of gas and grains largelyMade of molecular hydrogen and helium, It’s believed to have had a homogeneous

Isotopic composition from its center to its edge

The hydrogen isotopic composition of water on Earth differs widely from that of the primitive Sun.

Bulk Earth D/H ratio: (149 +/- 3) x 10-6

Sun D/H ratio: (20 +/- 4) x 10-6 (deducted from solar wind implanted into lunar soils)

Where the water on Earth originated?


Where does water come from?

Water Chemical Compound Solar system

Identified in:

- Asteroids - Atmospheres- Comets - Rings and moons of giant planets- Mars - Poles or our moon and of Mercury

Possible Sources:

1. Primordial Gas Captured from solar Nebula2. Absorption of Water onto Grains in the Accretion disk3. Comets4. Asteroids5. Early Accretion of Water Inward Migration of Hydrous Silicates


Where does water come from?Sources of Terrestrial Water

1. Primordial Gas Captured from the Solar Nebula

A primordial atmosphere could not have been captured directly from the solar nebula

Earth’s D/H ratio in water and the noble gas abundances

Campis (2004)

Processes involved in planetary accretion, degassing, and

evolution of the hydrosphere and atmosphere

Complex and may have fractionated the chemical and

isotopic signatures of the sources of water

H

D

FractionatedResidual D/H ratio in the hydrosphere

may not reflect that of the original source


Accretion

These large impact events have the capacity to completely volatilize any oceans and blow off portions of earth’s atmosphere.

Fractionation of D/H and noble gas/water ratios may occur

Mask the signature of the original source material


2. Absorption of Water onto Grains in the Accretion Disk

- If thermodynamic equilibrium was attained there were 2 earth masses of water vapor in the accretion disk inside 3 AU

- Mass of the Earth = 5 x 1027 g- Mass of the Earth’s oceans = 1.4 x 1024 g

- According to Abe (2000), the extreme maximum amount of water in the earth is about 50 Earth oceans, with most estimates being 10 Earth oceans or less. For example, an estimate of minerals in the silicate earth is about 5 to 6 Earth oceans. Thus the mass of water vapor available in the region of the terrestrial planets exceeded the mass of water accreted.

Could water vapor be absorbed onto grains before the gas in the inner solar system was dissipated?


Stimpfl in 2004 examined the role of physisorption by modeling the absorption of water on to grains at:

- 1000 oK ¼ of an ocean of water could be absorbed

- 700 oK 1 Earth ocean could be absorbed

- 500 oK 3 Earth oceans could be absorbed

Monte Carlo simulation Exploded the Earth into 0.1 µm spheres of volume equal to Earth, recognizing that grains in the accretion disk are not spherical and would be fractal in nature.

So, if the surface area of the fractal grain was 100 times that of a sphere of corresponding volume, then:


There are also issues of retention of water as the grains collide and grow to make planets, although it is clear that planets are not completely outgassed even in planetary scale collisions.

In 2004 Stimpfl showed that the efficiency of absorption of water increases as temperature decreases, this means that the process should have been more efficient further from the sun that closer to it. So, it is likely that Mars, Earth, and Venus accreted some water by absorption, with Mars accreting the most.

The current differences in the apparent water abundances among the terrestrial planets are probably the result of:

1. Different initial inventories

2. Subsequent geologic and atmospheric processing


3. Comets

Long considered the leading candidate for the origin of water in the terrestrial planets.This hypothesis was attractive because of the 2 following reasons:

1. It is widely believed that the inner solar system was too hot for hydrous phases to be thermodynamically stable (Boss 1998). Thus an exogenous source of water was needed.

2. The Earth and other terrestrial planets underwent one or more magma ocean events that some authors believed would effectively degas the planets of any existing water.

Fred Burger (Used with permission), NASA/JPL Archive. Hyakutake, a long-period comet from the Oort Cloud Comet Halley’s tail, with colors indicating varying

levels of brightness and type-I ion tail visible below A type-II, dust tail (from Lowell Observatory)


The D/H ratios in H2O in three comets, meteorites, Earth, prosolar H2, and Mars. CC = carbonaceous chondrites, LL3-IW = interstellar water in Semarkona, LL3-PS = protostellar water in Semarkona. After Drake and Righter (2002).

This Figure compares the isotopic composition of hydrogen in Earth, Mars, 3 Oort Cloud comets, and various early solar system estimates.

It show that it is clear that 100% of Earth’s water did not come from Oort Cloud comets with D/H ratios like the 3 comets measured so far.

D/H ratios in Martian meteorites do agree with the 4 cometary values shown in this figure.

What Limits the cometary contribution to Earth’s water?


Assumption - Earth Accreted some Hydrous phases or absorbed water- Some amount of additional water came from comets

- Indigenous Earth Water could have had D/H ratios representative of the inner solar system (i.e., low values because of relatively high nebular temperatures, perhaps like protosolar hydrogen: 2 – 3 x 10 -5), in which case, a cometary contribution of up to 50% is possible.

- Alternatively, Indigenous Earth water could have had D/H ratios representative of a protosolar water component identified in meteorites: ~ 9 x 10-5, in which case, there could be as little as a 10-15% cometary contribution.

Caveats:

1. Most probably, the Oort Cloud Comets studied so far (Halley, Hale-Bopp, and Hyakutake) are not representative of all comets.

2. The D/H measurements available are not of the solid nucleus, but of gases emitted during sublimation, because the differential diffusion and sublimation of HDO and H2O may make such measurements unrepresentative of the bulk comet.

3. The D/H ratio of organics and hydrated silicates in comets are unknown.D/H ratios up to 50 times higher than a standard have been measured in some chondritic porous interplanetary dust particles which may have cometary origins.


ADDITIONAL INFO: A Taste for comet Water

Comet Linear – 2000 broke apart as it passed near the Sun.

Now Isotopic Composition is the same as water in earth

According to Michael Mumma of NASA’s Goddard Space Flight Center, this comet carried 3.3 billion kilograms of water Fill a small lake

So this comet has enough water, but, was it the same type of water found here on Earth?

A team of astronomers led by Hal Weaver, used the Hubble Space TelescopeTo capture this image of comet Linear breaking up in August 2000.

Comets

Earth

Heavy Water (HDO)

Hydrogen

Oxygen

Deuterium


4. Asteroids

Plausible source of water based on dynamical arguments.

Morbidelli (2000), up to 15% of themass of the earth could be accreted late in Earth’s growth by collision of one or a few asteroids

Os isotopic composition of the “late veneer”, the material that mayHave contributed the highly siderophile elements (HSEs) that are Present to within 4% of chondritic proportions at about 0.03 ofChondritic absolute abundances

- PUM has a significantly higher 187Os/188Os ratio than carbonaceous chondrites- The PUM 187Os/188Os ratio overlaps anhydrous ordinary chondrites and is distinctly higher thanAnhydrous enstatite chondrites.


5. Early Accretion of Water from Inward Migration of Hydrous Silicates

- Solar nebula models suggest that the growth of zones of the terrestrial planets were too hot for hydrous minerals to form.

- Ciesla and Lauretta (2005) hydrous minerals were formed in the outer asteroid belt region of the solar nebula hotter regions of the nebula by gas drag incorporated into the planetesimalsthat formed there.

- Drake (2005) seems unlikely that hydrous silicates could be decoupled from other minerals and transported into the inner solar system.

The proposed radial migration of hydrous minerals would be subject to the same objection involving Os isotopes, unless the hydrous silicates arrived prior to the differentiation of Earth.

A Where from and When?1 Possible sources of Earth’s water

2 When did water arrive?1 During Accretion

2 Early bombardment

3 Late veneer

3 Controversy

Timing of Terrestrial Water

Bounding Constraints

1 Evidence for the youngest age constraint– Jack Hills zircons – 4,276±6 Ma (Compston & Pideon,

1986)

• Source region is a Metasedimentary Belt• Concentrate from a chert pebble conglomerate• High greenschist metamorphic grade• Possible lead loss at ~3,500 Ma• 17 grains analysed – only one with above age,

remaining grains give average age of ~4,180 Ma!• Considered minimum age – therefore could be

older!


Compston & Pideon, 1986


• 4.3 Ga age supported by hafnium isotopic studies (Amelin et al. 1999)

• Suggested integrated Lu/Hf and U/Pb study

• 4,404±8 Ma age confirmed minimum age postulation (Wilde et al., 2001)

• 4.4 to 4.5 Ga model ages were indicated by the Lu/Hf and U/Pb study! (Harrison et al., 2005)


Wilde et al., 2001


Harrison, 2005

• 4.3 Ga age supported by hafnium isotopic studies (Amelin et al. 1999)

• Suggested integrated Lu/Hf and U/Pb study• 4.4 to 4.5 Ga model ages were suggested by just

such a study! (Harrison et al., 2005)

• Isn’t this all very interesting?

• But what does this have to do with water?


• δ18O of this zircon population indicate liquid water interaction and the possible existence of an ocean! (Peck et al., 2001)

– Preservation of δ18O is demonstrated by;• Distinct oxygen isotope ratios for each population• Grains are not in isotopic equilibrium with host

quartz

– Zircon should have δ18O of 5.3±0.3‰ if they are in equilibrium with mantle

• All five age populations show elevated δ18O (up to 7.8 ‰)

– This is consistent with zircon that has interacted with hydrothermal circulation


Peck et al., 2001


2 Evidence for the oldest age constraint– More difficult to resolve and has implication

on the source of said water.– First order assumption is that this constraint

must be younger than the Solar System age• A minimum estimate is 4,567.2±0.6 Ma from

calcium-aluminum-rich inclusions in chondrites (Amelin et al., 2002)

• Chondrules ages are slightly younger at 4,564.7±0.6 Ma (Amelin et al., 2002)

– This scenario requires that the water arrived during accretion


• We may be able to constrain this further with the age of the final accretion of the Earth– Chondrites are aggregates of

chondrules/CAI’s!– Earth likely accreted from meteorites– Accretion age should be younger than

chondrules/CAI’s– Final stage of accretion considered to the

impact from which the Moon formed


• Could terrestrial water survive the Lunar forming impact?– Possible, but not likely!– If not then water had to arrive after the age of

the Moon• Sm/Nd age of 4.44±0.02 Ga (Carlson & Lugmair, 1988)

– This gives a 36 Million year window for the arrival of water

• Hf/W age of 4.527±0.01 Ga (Kleine et al., 2005)

– This increases the window to 123 Million years

– The fact that the Moon is bone dry is simple, but strong evidence that the Earth was also dry before the impact!


Benchmark Events in Early Earth History

4.30

4.35

4.40

4.45

4.50

4.55

4.60

4.65

Neb

ula

Con

dens

atio

n?

CA

I's

Cho

ndru

les

Moo

n /

Fin

al E

arth

Acc

retio

n

Jack

Zirc

ons

(Oce

ans

Pos

sibl

e)

Tim

e (G

a)

123 Million Years!


• It appears that water had a short period in which it could have arrived– This constraint is in agreement with a single

or low number of cometary impacts delivering water to the Earth

– The simple observation of a dry Moon limits the ability of other solar materials as being the delivery source


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