TERRESTRIAL PLANET FORMATION & THE FORMATION OF A WATER-RICH EARTH
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1 TERRESTRIAL PLANET FORMATION & THE FORMATION OF A WATER-RICH EARTH A. MorbidelliObservatoire de la Côte d’Azur
2 OUTLINEReasons to believe that water was accreted from more distant regions in the protoplanetary diskCometary or asteroidal sources?A model for the delivery of water from the asteroid beltAdvantageswater aboundancechronology of water accretionstochasticity and diversity of terrestrial planetsDrawbacks & ways outSiderophile elements in Earth’s mantleOxygen isotope compostion of Earth and Moon
3 The evident gradient of relative water content with heliocentric distance implies that water should have been accreted from distant material.
4 ASTEROIDS OR COMETS?The D/H ratio of Earth’s water rules out a dominant contribution of comets and suggests an asteroidal originNumerical 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
5 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.
6 `Classical’ model of terrestrail planet accretion: from embryos in the 0.5-2.0 AU zone.
7 What if the embryos existed also in the asteroid belt?
8 What if the embryos existed also in the asteroid belt?
9 This model explains quantitatively the main properties of the current belt Petit et al. (2001)
10 WATER FROM ASTEROIDSAccording 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)
12 WATER FROM EMBRYOSWe 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)
13 NOT a LATE VENEER scenario The accretion of the water rich embryo occurs DURING the formation of the Earth, although towards the end.NOT a LATE VENEER scenarioMorbidelli et al., 2000
14 ADVANTAGES We explain the accretion of a LARGE amount of water The accreted water has the correct D/H ratio, being of carbonaceous chondritic originThe water accretion occurs DURING the formation of the Earth, NOT in a late veneer phase, in agreement with geochemical modellingThis 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)
15 Stochasticity in the resulting water budget Raymond et al., 2004
16 A large ecc. of Jupiter inhibits the delivery of water to the inner S A large ecc. of Jupiter inhibits the delivery of water to the inner S.S.Chambers, 2001; Raymond et al., 2004
17 PROBLEMS (from Drake and Righter, Nature 416, 39) Siderophiles/water in the Upper mantleA carbonaceous embryo carrying enough water to the Earth would also carry too many siderophile elements with respect to what is retained in the upper mantlePossible 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
18 PROBLEMS (from Drake and Righter, Nature 416, 39) Oxygen isotope compositionThe 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…
19 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
20 Independently of the water origin problem, in the current Moon formation scenario, the similarity between Earth and Moon is a problemThe simulations show that terrestrial planet accretion is a heterogeneous processThis explains the diversity among the terrestrial planets…But the similarity between the Earth and the impactor would be due to chanceSemimajor axis(Chambers 2001)
21 A possible solution: EQUILIBRATION! from Stevenson, KITP meeting, march 15-19, 2004
22 CONCLUSIONSThe 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 planetsIt 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.