Question

Origin of Earth's Water

The origin of Earth's water is a subject of ongoing debate among geochemists. Two scientists present their viewpoints on how Earth acquired its oceans.

Scientist 1
Earth's water was delivered primarily by carbonaceous chondrite meteorites during the Late Heavy Bombardment, approximately 3.9 billion years ago. The isotopic ratio of deuterium to hydrogen ($D/H$) in Earth's current oceans ($1.56 \times 10^{-4}$) closely matches the average $D/H$ ratio of carbonaceous chondrite meteorites found today. In contrast, comets have $D/H$ ratios that are significantly higher, and other classes of meteorites are almost entirely dry. Therefore, carbonaceous chondrites must have been the primary source of Earth's water.

Scientist 2
Earth's water was present from the beginning, trapped inside the mantle during Earth's initial accretion. High-pressure mantle minerals, such as ringwoodite, can store up to $1.5\%$ water by weight in the form of hydroxide ions ($OH^-$). As early Earth cooled, geological outgassing through volcanic eruptions released this water to the surface, creating the oceans. The similarity in $D/H$ ratios between Earth's oceans and carbonaceous chondrites is merely a reflection of the shared composition of the inner solar nebula during accretion, rather than evidence of late delivery.

Which of the following statements represents an underlying assumption of Scientist 1's hypothesis?