Transcript 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?
Sources of Terrestrial Water
A) Where from and When?
1. Possible sources of Earth’s water
1.
2.
3.
4.
5.
Primordial gas capture
Adsorption in the accretion disk
Comets
Asteriods
Inward migration of hydrous silicates
2. When did water arrived?
3. Controversy
Where does water come from?
Sources of Terrestrial Water
INTRODUCTION
4.55 Billion years ago, the sun and planets formed from the protosolar nebula
Rotating disk of gas and grains largely
Made 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?
Sources of Terrestrial Water
Water  Chemical Compound  Solar system
Identified in:
-
Asteroids
Comets
Mars
- Atmospheres
- Rings and moons of giant planets
- Poles or our moon and of Mercury
Possible Sources:
1.
2.
3.
4.
5.
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
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
Processes involved in planetary
accretion, degassing, and
evolution of the hydrosphere and
atmosphere
Campis (2004)
Earth’s D/H ratio in water and the
noble gas abundances
Complex and may have
fractionated the chemical and
isotopic signatures of the
sources of water
H
Fractionated
D
Residual D/H ratio in the hydrosphere
may not reflect that of the original source
Sources of Terrestrial Water
Where does water come from?
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
Sources of Terrestrial Water
Where does water come from?
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?
Sources of Terrestrial Water
Where does water come from?
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:
Sources of Terrestrial Water
Where does water come from?
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
Sources of Terrestrial Water
Where does water come from?
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)
Sources of Terrestrial Water
Where does water come from?
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.
The D/H ratios in H2O in three comets, meteorites, Earth, prosolar H2, and Mars.
CC = carbonaceous chondrites, LL3-IW = interstellar water in Semarkona, LL3PS = protostellar water in Semarkona. After Drake and Righter (2002).
What Limits the cometary contribution to Earth’s water?
Where does water come from?
Sources of Terrestrial 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.
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 NASA’s 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
Comets
Heavy Water (HDO)
Earth
Oxygen
Deuterium
Sources of Terrestrial Water
Where does water come from?
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 Earth’s 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.
Sources of Terrestrial Water
Where does water come from?
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.
Timing of Terrestrial Water
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!
Timing of Terrestrial Water
Compston & Pideon, 1986
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
• 4,404±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)
Timing of Terrestrial Water
Wilde et al., 2001
Harrison, 2005
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)
• Isn’t this all very interesting?
• But what does this have to do with water?
Timing of Terrestrial 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
Timing of Terrestrial Water
Peck et al., 2001
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 4,567.2±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
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/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
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 4.44±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!
Timing of Terrestrial Water
Benchmark Events in Early Earth History
4.65
4.60
123 Million Years!
4.50
4.45
4.40
4.35
Jack Zircons (Oceans
Possible)
Moon / Final Earth
Accretion
Chondrules
CAI's
4.30
Nebula Condensation?
Time (Ga)
4.55
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