Neptune's World: the Fascinating Origin of Earth's
Oceans of Water
Robert
Malcuit (Denison University, Granville, Ohio, USA)
21 September 2010
Albarede
(2009) - Nature - he’s a French geochemist who published in last fall’s Nature
an article that inspired this study.
Earth’s water: ~97% is
in the oceans, ~2% is in glaciers & ice caps, ~1% is in other places
(atmosphere, lakes, streams).
Earth’s special features:
-
liquid water at the surface
-
free oxygen in the atmosphere
-
highly developed biological systems
-
strong magnetic field (for the Earth)
-
very large moon (mass ratio 1:81)
-
only planet with “true granite”
-
only planet with continental crust (granitic rocks - true granite + others)
-
only planet with plate tectonics (continents moving around)
The
free atmospheric oxygen, biologic systems, and strong magnetic field are
related to the presence of the large moon.
The
true granite, continental crust, and plate tectonics features are all
associated with water.
Earth
is the Water Planet. But traditionally, there’s been no reasonable
explanation for the presence of a large volume of water.
Ocean
water origin models: 1) simplest source; 2) far-out source; 3) intermediate
source
1)
The simplest source model refers to outgassing from the mantle. This
model assumes that the water was originally associated with meteoritic debris
that accreted to form the Earth. This is a commonly held view, frequently
heard in geology teaching.
2)
The far-out source model was championed by Carl Sagan. Water was
delivered to Earth by water-rich comets from the Kuiper Belt/Oort Cloud.
1st problem - the deuterium/hydrogen ratio (D/H ratio) of Earth’s
ocean water is not the same as the cometary water D/H ratio. 2nd
problem - lots of far-out comets can’t get to Earth with Jupiter’s
gravitational well in the way.
3)
The intermediate source model holds that Earth’s ocean water was delivered from
the impact of water-rich carbonaceous chondrites from the outer Asteroid
Belt. The D/H ratio of Earth’s ocean water is similar to the D/H
ratio in water-rich carbonaceous chondrites.
D/H Ratios
Earth’s oceans -
149 ±3 x 10-6
Comets
- 310 ±40 x 10-6
Carbonaceous
chondrites - 140 ±2 x 10-6 (ranges from 120 to 280 x 10-6)
(they come from ~2.5 AU)
Martian mantle -
300 x 10-6 (perhaps a cometary source)
The
D/H ratio of Earth’s ocean is similar to that of carbonaceous chondrites.
Looking
at the K/U ratio of the Moon, Earth, Mars, and meteorites. K is a
volatile element. U is a refractory element. The K/U ratio is a
volatility index.
Moon -
K/U = ~3000 The Moon lacks ~95% of
the nebular K inventory.
Earth
- K/U = ~10,000 Earth lacks ~85% of the nebular K
inventory.
Mars
- K/U = ~20,000 Mars lacks ~70% of the
nebular K inventory.
Earth
is also depleted, from 92 to 98%, of the nebular inventory of Zn, Ag, Sb, As,
Sn, Pb, S.
Early
water would’ve been removed from Earth along with all of these volatile
elements.
K
index - K is depleted in Mars (SNC meteorites), Earth, Vesta
(eucrite meteorites), Moon,
and the parent body of angrite
meteorites. Vesta and the Moon probably originally came from within
the orbit of Mercury.
Using
Pb-206-Pb-204 dating, Earth is 100 m.y. younger than it should be. Lead
is a volatile element - it apparently was removed and reintroduced - that
accounts for the anomalously young date using this method.
How
much water is needed to be introduced to Earth?
Earth’s
oceans have 1.664 billion cubic kilometers of water.
If
you add mantle water, then double this value. Most water-rich asteroids
we know have 10% water. So, need an asteroidal body 10x the size of this
double value (= 3.328 billion cubic kilometers) introduced to Earth.
Albarede
(2009) - Nature 10/29/2009 - talks about the volatile accretion history
of the terrestrial planets & dynamic implications.
See
also Nature 464 (29 April 2010) - two groups of astronomers found a
water-loaded asteroid (24 Themis). This discovery supports the prediction
of Albarede (2009).
Alberede
model - Malcuit suggests there’s no need for a lunar giant impact, and change
the T-Tauri phase to an X-Wind phase.
Earth’s
oceans originated from smearing of water-rich asteroids during close
encounters.
Beginning
of the solar system - got a bipolar outflow. Dehydration took place in
the accretion disc.
Iron Line - at 0.4 AU
& at 1100° C - iron & nickel condense here. This is at the orbit
of Mercury. Mercury
is very heavy (lots of iron & nickel) - it’s at the Iron Line - that’s why
Mercury is heavy.
Snow Line - at 5 AU
& below 0° C.
Get
spherical bodies with clay minerals interlayered with water.
Jupiter
controls our lives. It occurs at 5.2 AU.
Mars
- at 1.52 AU. Zone of asteroids - 2.0 to 4.1 AU. Three asteroid
belts in this zone. Themis is at ~3.2 AU.
Hypothetical
water-rich planetoid - here named Aquarius.
Ceres
is the largest body in the Asteroid Belt. Ceres is ~1/4 the diameter of
the Moon & has ~1/4 of the mass of the Asteroid Belt.
Vesta,
according to the K index, should’ve formed inside the orbit of Mercury.
Seeing
evidence that the early solar system had reorganization events involving
shifting close things out and shifting asteroidal bodies in.
The
zonation seen in the Asteroid Belt is the result of orbital resonances with
Jupiter.
The
time scale for resonance escape (perturbation events; “cracks”) is 100,000 to
150,000 years - can get escapes on short time scales.
Gravitational
perturbation sequence - if Aquarius was located in the Asteroid Belt, and its
orbit elongated to the point of crossing Earth’s orbit, Jupiter would grab it
at that eccentricity.
If
Aquarius had a more Earth-like orbit, it had to have smeared around Earth to
get water transfer, otherwise a big impact results in water vaporization and no
water stays on Earth.
Malcuit
modeled prograde encounters between Aquarius and Earth (prograde =
counterclockwise orbit, as seen looking down from the North Pole). Models
indicate grazing collisions/encounters with the water-rich asteroid.
These grazing encounters are important for getting the late veneer of Alberede
(2009) model.
Tidal
Capture of Moon - at 3.95 Ga.
X-Wind
Model - accounts for the origin of CAIs (= calcium-aluminum inclusions in
chondrite meteorites).
The
T-Tauri episode remelts the magma ocean region of the Moon & aids in the
magnetization of lunar anorthosite crust. The T-Tauri episode also
remelted Mercury’s surface.
The
D/H ratio is the key factor that needs to be explained for the origin of
Earth’s oceans.
There’s
a special brand of water-rich asteroids, here named Aquarioids, that
need to have grazed Earth & Venus to get their waters. Remember that Venus
had water once.
The
Martian atmosphere has a much higher D/H compared with the Martian mantle
(known from SNC meteorites).
The
Moon was captured about 600 m.y. after time zero (as defined by the oldest
CAIs).
Then,
we’ve got the cool early Earth (evidence from 4.404
b.y. zircon crystals) from ~100 m.y. on to 3.95 Ga.
Retrograde
encounters (clockwise approach when viewed from North Pole) - get a lot of
collisions on models.
T-Tauri
episode - a much lower energy event than the X-Wind event.
When you have a glass of water this
afternoon, think about where it came from.
