WATER ON EARTH

Alessandro Morbidelli

CNRS, Observatoire de la Cote d’Azur, Nice

HOW MUCH WATER IS ON EARTH?

•Hydrosphere:2.8x10-4 Earth masses – fairly well constrained

•Mantle: 0.8-8x10-4 Earth masses (Lecuyer et al. 1998) –poorly constrained

New estimate from Marty (2011):•new K abundances (Arevalo et al., 2009) and 40K->40Ar suggests that 75% of 40Ar is trapped at depth•From N/ 40Ar (Marty and Dauphas, 2003), C/N (Marty and Zimmermann, 1999), H/C (Hirschmann and Dasgupta, 2009) derives H2O/40Ar•Obtains 2.7(+/-1.3)x10-3 Earth masses of water

Could the Earth have lost most of its volatiles?

NO! Albarede, 2009

Water content in Earth and other bodies

Isotopic composition of Earth water

?

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

Local planetesimals were water-rich, because water could be absorbed by grains even inside the snowline (Muralidharan, Drake et al., 2008)

Would water be lost when grains accrete into planetesimals?Why are the parent bodies of enstatite and ordinary chondrites so dry?

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

Dust & small icy/hydrated planetesimals drifting inwards from beyond the snowline due to gas drag could have brought water to the terrestrial planet region (Lauretta and Ciesla, 2005)

This mechanism was invoked by Cyr et al. (1999) to explain the hydration of C-type asteroids

The deficiency of water in S/E type asteroids suggests that this mechanism was not effective inside 2.5-3 AU

Local condensation of volatile-rich grains as the temperature was dropping in the disk suffers the same problem: why didn’t S/E asteroids accrete such grains?

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

Primitive atmospheres of H could have been captured by planetary embryos from the solar nebula; the reaction of H with the silicate could have hydrated the embryos (Genda and Ikoma, 2008)

This could explain why embryos were hydrated even if planetesimals were not

The water produced by this mechanism would have a solar D/H composition.

Necessity for a fractionation mechanism. Similarity with D/H ratio in carbonaceous chondrites would be a coincidence.

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

Cometary bombardment (Delsemme)

We do expect a cometary bombardment in the Nice model

Not enough to supply all the water to Earth (not Nice-model dependent):

•Earth-collision probability per comet: 10-6

•Fraction of water in comet: ~0.5

•Total mass in the cometary disk: ~50 ME

•Water supplied: 2.5x10-5 ME~10% Ocean mass

Water on Earth predates the LHB (see zircons)

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

Water from the asteroid belt (Morbidelli et al., 2000; O’brien et al., 2006; Raymond et al., 2006)

It works best if the giant planets are on circular orbits

However, Mars is always too big in this scenario.

SCENARIOS FOR THE ORIGIN OF EARTH WATER (from the most unlikely to the most likely)

The Grand Tack scenario (Walsh et al., 2011)

Water delivery in GT• Planets > 0.5 Earth mass accrete median value of

~1% Earth mass of C-type material (2-3% is not rare)

• Assuming 10% water by mass (consistent with carbonaceous chondrites), this gives ~1x10-3 Earth masses of water– Earth has ~5-20x10-4 Earth masses of water

Murchison (CV meteorite)

Additional water may be delivered through more massive embryos that were not included in the simulations

Timing of water/volatile accretion

Water arrived kind of late….

Rubie et al., 2011; see also Wood et al., 2008

Run 152 Planet 6

Run 151 Planet 4

Timing of water/volatile accretion

…this is consistent with the Grand Tack scenario

Timing of water/volatile accretion

Calibration of Late Veneer

Timing of water/volatile accretion

Late but not in a late veneer (Mann et al., 2009)

Ga & Mn are moderately volatile elements, in chondritic proportion in the mantle, depleted relative to CI but much more abundant than HSE. So, they must have “seen” the core formation at large pressures, i.e. in the late stages.

Timing of water/volatile accretion

Late but not in a late veneer (Wood et al., 2010)

The abundance of elements with same condensation temperature is clearly dependent on affinity with iron (red=HSE, black=MSE, white=lithophile)

Timing of water/volatile accretion

Marty, 2011

Late Veneer

A Late Veneer of 3x10-3 ME would give 1.5-3 x 10-4 ME of water… a bit short (Drake and Righter, 2002)

Timing of water/volatile accretion

The Earth and the Moon have indistinguishable oxygen isotope composition.

All carbonaceous meteorites (with the exception of CI) have clearly different Oxygen isotope composition

The delivery of water AFTER the Moon forming event would have made the Earth and the Moon distinguishable!

CONCLUSIONS

• Water (and volatile elements) argue for an heterogeneous accretion of the Earth

• They have been delivered towards the end of the Earth accretion, but not in a Late Veneer fashion

• All this is consistent with the latest dynamical models, provided that the Moon-forming event is fortuitously late.