Meteorite/Planetary Science Quiz

Question #7

When did the water on Earth arrive?

In one theory, referred to as the "late veneer scenario", all of the water presently on Earth came from the outer solar system, carried by comets, colliding with the Earth after its formation. However, recent astronomical and geochemical analysis suggest that Earth accreted water throughout its formation.

Summarizing a recent paper (Morbidelli, 2000): The Earth formed on a time scale of 10⁸ years by the high velocity collisions of planetary embryos. At the beginning of its growth, the Earth accreted only embryos originally formed in the Earth region, which should have been water-depleted both in standard nebula models, and in models in which condensation of water at 4-5 AU depletes the inner nebula of water vapor.

However, while the Earth's embryo was still embedded in the solar nebula (<10 Ma), hydrated 10 km asteroids came from the outer belt transported by gas drag. Shortly after that Jupiter accreted its envelope and reached its current mass (within ~10 Ma), the forming Earth experienced a shower of asteroids and comets attributable to the rapid depletion of the asteroid belt and of the Jupiter region. However this shower delivered to the Earth only a minor fraction of the present amount of water. The Earth was still small at that time (presumably less than half its present mass) and most of the accreted water was subsequently lost.

Towards the end of its formation, the Earth began to accrete planetary embryos originally formed in more distant regions. It is very likely that in this process the Earth also accreted a few embryos from the outer asteroid belt, which were heavily hydrated on the basis of meteorite analyses and modeling of the solar nebula. This could have brough to Earth as much as 10 times the present amount of water in the crust.

What fraction of this water could have survived up to the present time is an open question; however, the Earth being at the end of its accretion, it can be argued that this corresponds in fact to the bulk of the water that is presently on our planet. Finally, once its formation was completed, the Earth was bombarded by comets coming from the trans-Uranian region, and in particular from the Kuiper Belt. This is known as the "Late Veneer" phase.

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