### Origin of Earth's Water
Scientists have proposed two competing models to explain the source and timing of the accumulation of Earth's water.
Model 1 (Late Veneer Delivery)
Earth accreted in a region of the solar nebula that was too hot for volatile compounds, such as water, to condense. Consequently, the proto-Earth was dry. After Earth’s core formed, water-rich carbonaceous chondrites (asteroids) from the outer solar system impacted Earth, delivering water and volatile elements. This model is supported by the concentrations of highly siderophile (iron-loving) elements (HSEs) in Earth's mantle, which are found in chondritic proportions. Since core formation would have stripped primordial HSEs from the mantle, these elements must have arrived via a "late veneer" of asteroid impacts after core formation. The deuterium-to-hydrogen () ratio of Earth's oceans matches that of carbonaceous chondrites ().
Model 2 (Endogenous Wet Accretion)
Earth accreted from material that already contained water-bearing minerals. Primordial dust grains and chondrites in Earth's accretion zone contained adsorbed water or hydrous silicates that survived the high temperatures. As Earth grew, this water was incorporated directly into the mantle and dissolved in the early magma ocean. High-pressure mineral phases, such as ringwoodite in the transition zone, stored vast reservoirs of water. Over time, volcanic activity outgassed water vapor to form the oceans. This model is supported by isotopic analyses showing that deep mantle reservoirs have a ratio of , which is significantly lower than surface oceans but matches enstatite chondrites, the primary isotopic match for Earth’s bulk rock composition.
Based on the models, which of the following statements best describes a major difference between Model 1 and Model 2 regarding the timing of Earth's core formation relative to the arrival of Earth's water?
- Model 1 asserts that water was delivered to Earth after core formation, whereas Model 2 asserts that water was present in Earth's accretionary material before and during core formation.Answer
- BModel 1 asserts that water was incorporated into Earth during core formation, whereas Model 2 asserts that water arrived via asteroid impacts after core formation.
- CModel 1 asserts that core formation occurred after water delivery, whereas Model 2 asserts that core formation did not play a role in Earth's development.
- DBoth models agree that Earth's water was fully accumulated prior to core formation, but they disagree on whether the core was formed by chondritic impacts.