### The Origin of Earth's Water
Earth is unique among the terrestrial planets in having vast surface oceans of liquid water. Scientists have proposed different models to explain the source of Earth's water.
#### Model 1 (Late Veneer Model)
During the early stage of Earth's formation, the temperature in the inner Solar System was too high for volatile substances like water to condense. As a result, the protoplanet that became Earth formed dry. Approximately to million years after Earth's initial accretion, a period of heavy bombardment occurred. Water-rich carbonaceous chondrites (asteroids from the outer asteroid belt) and comets containing high concentrations of ice collided with the cooled Earth. These impactors delivered water, which vaporized upon impact and later condensed to form the oceans. This model suggests that Earth's water is entirely of extraterrestrial origin, arriving after the planet had reached its final size.
#### Model 2 (Endogenous Source Model)
Earth's water was present from the very beginning of the planet's formation. The planetary building blocks, specifically enstatite chondrite-like materials, contained significant amounts of hydrogen and mineral-bound water, even at high inner-system temperatures. As these materials accreted to form the early Earth, the water was trapped deep within the mantle. Over hundreds of millions of years, intense volcanic activity and planetary degassing released this trapped water as steam into the early atmosphere. As the surface cooled, this atmospheric water vapor condensed and fell as rain, filling the ocean basins. In this model, the source of Earth's water is internal and coeval with the formation of the planet itself.
Based on Model 1 and Model 2, which of the following statements best describes how the two models differ regarding the timing of when water became associated with the planetary material that formed Earth?
- AModel 1 proposes that water was present in Earth's building blocks during accretion, whereas Model 2 proposes that water was delivered by comets after accretion.
- Model 1 proposes that water arrived after Earth had accreted, whereas Model 2 proposes that water was present in the building blocks of Earth from the beginning of its formation.Cevap
- CModel 1 proposes that water was released from the mantle during accretion, whereas Model 2 proposes that water was vaporized by impacts before accretion.
- DModel 1 proposes that water was present from the beginning of Earth's formation, whereas Model 2 proposes that water was delivered by carbonaceous chondrites after the surface cooled.