Terra Aqua

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Terra Aqua

• Water and the Earth
• Where did it come from
• and When did it Arrive?
• EPSC 666
• Javier Herbas
• Michael Patterson

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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

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Outline

• Where from and When?
• Possible sources of Earths water
• When did water arrived?
• Issues
• Where is the water?
• What is it doing today?

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Sources of Terrestrial Water

• Where from and When?
• Possible sources of Earths water
• Primordial gas capture
• Adsorption in the accretion disk
• Comets
• Asteriods
• Inward migration of hydrous silicates
• When did water arrived?
• Controversy

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Where does water come from?
Sources of Terrestrial Water
INTRODUCTION
4.55 Billion years ago, the sun and planets
formed from the protosolar nebula
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?
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Where does water come from?
Sources of Terrestrial Water

• 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
• Primordial Gas Captured from solar Nebula
• Absorption of Water onto Grains in the Accretion
  disk
• Comets
• Asteroids
• Early Accretion of Water Inward Migration of
  Hydrous Silicates

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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
Earths D/H ratio in water and the noble gas
abundances
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Where does water come from?
Sources of Terrestrial Water
Accretion
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Where does water come from?
Sources of Terrestrial Water
2. Absorption of Water onto Grains in the
Accretion Disk
there were 2 earth masses of water vapor in the
accretion disk inside 3 AU
- If thermodynamic equilibrium was attained ?

• Mass of the Earth 5 x 1027 g
• Mass of the Earths 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?

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Where does water come from?
Sources of Terrestrial Water

• 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

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Where does water come from?
Sources of Terrestrial Water

• 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
• Different initial inventories
• Subsequent geologic and atmospheric processing

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Where does water come from?
Sources of Terrestrial Water
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
• 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.
• 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 Halleys tail, with colors indicating
varying levels of brightness and type-I ion tail
visible below A type-II, dust tail (from Lowell
Observatory)
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Where does water come from?
Sources of Terrestrial Water
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 Earths 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.
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).
What Limits the cometary contribution to Earths
water?
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Where does water come from?
Sources of Terrestrial Water

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

Assumption

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

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Where does water come from?
Sources of Terrestrial Water
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
NASAs Goddard Space Flight Center, this comet
carried 3.3 billion kilograms of water ? Fill
a small lake
A team of astronomers led by Hal Weaver, used the
Hubble Space Telescope To capture this image of
comet Linear breaking up in August 2000.
So this comet has enough water, but, was it the
same type of water found here on Earth?
Hydrogen
Heavy Water (HDO)
Oxygen
Deuterium
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Where does water come from?
Sources of Terrestrial Water
4. Asteroids

• Plausible source of water based on
• dynamical arguments.
• Morbidelli (2000), up to 15 of the
• mass of the earth could be accreted
• late in Earths growth by collision
• of one or a few asteroids
• Os isotopic composition of the late veneer, the
  material that may
• Have contributed the highly siderophile elements
  (HSEs) that are
• Present to within 4 of chondritic proportions at
  about 0.03 of
• Chondritic 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 than
• Anhydrous enstatite chondrites.

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Where does water come from?
Sources of Terrestrial Water
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 planetesimals
• that 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.

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Timing of Terrestrial Water

• A Where from and When?
• Possible sources of Earths water
• When did water arrive?
• During Accretion
• Early bombardment
• Late veneer
• Controversy

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Bounding Constraints
Timing of Terrestrial Water

• 1 Evidence for the youngest age constraint
• Jack Hills zircons 4,2766 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!

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Timing of Terrestrial Water
Compston Pideon, 1986
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Timing of Terrestrial Water

• 4.3 Ga age supported by hafnium isotopic studies
  (Amelin et al. 1999)
• Suggested integrated Lu/Hf and U/Pb study
• 4,4048 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)

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Timing of Terrestrial Water
Wilde et al., 2001
Harrison, 2005
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Timing of Terrestrial Water

• 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)
• Isnt this all very interesting?
• But what does this have to do with water?

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Timing of Terrestrial Water

• d18O of this zircon population indicate liquid
  water interaction and the possible existence of
  an ocean! (Peck et al., 2001)
• Preservation of d18O is demonstrated by
• Distinct oxygen isotope ratios for each
  population
• Grains are not in isotopic equilibrium with host
  quartz
• Zircon should have d18O of 5.30.3 if they are
  in equilibrium with mantle
• All five age populations show elevated d18O (up
  to 7.8 )
• This is consistent with zircon that has
  interacted with hydrothermal circulation

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Timing of Terrestrial Water
Peck et al., 2001
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Timing of Terrestrial Water

• 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 4,567.20.6 Ma from
  calcium-aluminum-rich inclusions in chondrites
  (Amelin et al., 2002)
• Chondrules ages are slightly younger at
  4,564.70.6 Ma (Amelin et al., 2002)
• This scenario requires that the water arrived
  during accretion

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Timing of Terrestrial Water

• We may be able to constrain this further with the
  age of the final accretion of the Earth
• Chondrites are aggregates of chondrules/CAIs!
• Earth likely accreted from meteorites
• Accretion age should be younger than
  chondrules/CAIs
• Final stage of accretion considered to the impact
  from which the Moon formed

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Timing of Terrestrial Water

• 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 4.440.02 Ga (Carlson Lugmair,
  1988)
• This gives a 36 Million year window for the
  arrival of water
• Hf/W age of 4.5270.01 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!

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Timing of Terrestrial Water
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Timing of Terrestrial Water

• 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