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.
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.