Terrestrial planet formation the formation of a water rich earth
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TERRESTRIAL PLANET FORMATION & THE FORMATION OF A WATER-RICH EARTH. A. Morbidelli Observatoire de la C ô te d’Azur. OUTLINE. Reasons to believe that water was accreted from more distant regions in the protoplanetary disk Cometary or asteroidal sources?

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TERRESTRIAL PLANET FORMATION & THE FORMATION OF A WATER-RICH EARTH

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Terrestrial planet formation the formation of a water rich earth

TERRESTRIAL PLANET FORMATION& THE FORMATION OF A WATER-RICH EARTH

A. Morbidelli

Observatoire de la Côte d’Azur


Terrestrial planet formation the formation of a water rich earth

OUTLINE

  • Reasons to believe that water was accreted from more distant regions in the protoplanetary disk

  • Cometary or asteroidal sources?

  • A model for the delivery of water from the asteroid belt

  • Advantages

  • water aboundance

  • chronology of water accretion

  • stochasticity and diversity of terrestrial planets

  • Drawbacks & ways out

    • Siderophile elements in Earth’s mantle

    • Oxygen isotope compostion of Earth and Moon


Terrestrial planet formation the formation of a water rich earth

The evident gradient of relative water content with heliocentric distance implies that water should have been accreted from distant material.


Terrestrial planet formation the formation of a water rich earth

ASTEROIDS OR COMETS?

The D/H ratio of Earth’s water rules out a dominant contribution of comets and suggests an asteroidal origin

Numerical integrations also show that comets could have contributed at most 10% of the current water on Earth Levison et al. (2000); Morbidelli et al. (2000)

Courtesy of F. Robert


Terrestrial planet formation the formation of a water rich earth

To understand the delivery of water from asteroids we need a model for the primordial depletion and dynamical excitation of the main belt.

We believe that the best model to date is the one proposed by Wetherill 1992, and simulated in detail by Petit, Morbidelli and Chambers 2001.

This model `fits’ the framework of Terrestrial planet accretion.


Terrestrial planet formation the formation of a water rich earth

`Classical’ model of terrestrail planet accretion: from embryos in the 0.5-2.0 AU zone.


Terrestrial planet formation the formation of a water rich earth

What if the embryos existed also in the asteroid belt?


Terrestrial planet formation the formation of a water rich earth

What if the embryos existed also in the asteroid belt?


Terrestrial planet formation the formation of a water rich earth

This model explains quantitatively the main properties of the current belt

Petit et al. (2001)


Terrestrial planet formation the formation of a water rich earth

WATER FROM ASTEROIDS

According to our asteroid belt sculpting scenario, only 0.1% of the “primitive” asteroids would have been accreted by the Earth. Assuming 1 Earth mass of material and 10% water content this amounts to only 20% of the water currently on Earth. Moreover it arrived “early” in the Earth formation history.

Morbidelli et al. (2000)


Terrestrial planet formation the formation of a water rich earth

WATER FROM EMBRYOS

Raymond et al., 2004


Terrestrial planet formation the formation of a water rich earth

WATER FROM EMBRYOS

We did 11 simulations of terrestrial planet accretion, forming in total 24 planets.

In 2/3 of the cases a terrestrial planet accretes at least one embryo coming from the outer asteroid belt.

We expect that 5 to 20 times the current amount of Earth’s water could be accreted in this way.

Morbidelli et al. (2000)


Terrestrial planet formation the formation of a water rich earth

The accretion of the water rich embryo occurs DURING the formation of the Earth, although towards the end.

NOT a LATE VENEER scenario

Morbidelli et al., 2000


Advantages

ADVANTAGES

  • We explain the accretion of a LARGE amount of water

  • The accreted water has the correct D/H ratio, being of carbonaceous chondritic origin

  • The water accretion occurs DURING the formation of the Earth, NOT in a late veneer phase, in agreement with geochemical modelling

  • This mechanism for water accretion seems generic: confirmed by Levison and Agnor (2003) and Raymond, Quinn, Lunine (2004)

  • The accretion of the water is a stochastic event, and therefore explains why not all terrestrial planets had an identical primitive water budget (e.g.Mars)


Terrestrial planet formation the formation of a water rich earth

Stochasticity in the resulting water budget

Raymond et al., 2004


Terrestrial planet formation the formation of a water rich earth

A large ecc. of Jupiter inhibits the delivery of water to the inner S.S.

Chambers, 2001; Raymond et al., 2004


Problems from drake and righter nature 416 39

PROBLEMS (from Drake and Righter, Nature 416, 39)

A carbonaceous embryo carrying enough water to the Earth would also carry too many siderophile elements with respect to what is retained in the upper mantle

Possible solutions:

Siderophile elements are distributed in the whole mantle; no significant water losses occurred after the delivery; the mantle of the water carrying embryo was depleted in siderophiles due to partial differentiation

  • Siderophiles/water in the Upper mantle


Problems from drake and righter nature 416 391

PROBLEMS (from Drake and Righter, Nature 416, 39)

The Earth and the Moon are at the same position on the same fractionation line. It should be different if the Earth got the water from a carbonaceous embryo.

However, this is a general problem, not specifically related to the origin of water…

  • Oxygen isotope composition


Terrestrial planet formation the formation of a water rich earth

The Moon-forming impact.

SPH simulations by Canup and Asphaugh

~80% of the Moon forming material comes from the impactor.

The similarity between Earth and Moon therefore implies the similarity between the Earth and the impactor


Terrestrial planet formation the formation of a water rich earth

Independently of the water origin problem, in the current Moon formation scenario, the similarity between Earth and Moon is a problem

The simulations show that terrestrial planet accretion is a heterogeneous process

This explains the diversity among the terrestrial planets…

But the similarity between the Earth and the impactor would be due to chance

0.5 1.0 1.5 2.0

Semimajor axis

(Chambers 2001)


Terrestrial planet formation the formation of a water rich earth

A possible solution: EQUILIBRATION!

from Stevenson, KITP meeting, march 15-19, 2004


Terrestrial planet formation the formation of a water rich earth

CONCLUSIONS

  • The delivery of water from the asteroid belt is a natural outcome of terrestrial accretion and asteroid belt primordial sculpting.

  • It is a stochastic process, which may explains the large differences existing among the terrestrial planets

  • It is a robust process, that works in a variety of Solar System configurations, and is inhibited mainly by the large eccentricities of the giant planets.

  • A number of issues concerning the chemistry of the Earth are still open.

  • In particular, more work is needed in order to understand the puzzling similarities between the Earth and the Moon.


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