Soru

Passage

The origin of Earth's volatile elements, particularly water, remains a central question in planetary science. Two scientists present competing hypotheses regarding the source and evolution of Earth's water.

Scientist 1
Earth accreted inside the "frost line"—the radial distance in the solar nebula where temperatures were cool enough for volatile compounds like water ice to condense. Consequently, the primordial materials that formed the proto-Earth were completely dry. Earth’s water was delivered during the late veneer phase, after core differentiation was complete, by carbonaceous chondrite meteorites originating from the outer solar system. The deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans has remained constant at approximately $1.5 \times 10^{-4}$ since this delivery. Because this value matches the $D/H$ ratio of carbonaceous chondrites, it serves as a pristine chemical signature of the late-accreting outer solar system material.

Scientist 2
Earth's water is primordial and was accreted directly from enstatite chondrite-like planetesimals in the inner solar system. Although temperatures were too high for water ice to condense, hydrogen was incorporated directly into the iron and silicate mineral lattices of the accreting planetesimals. The $D/H$ ratio of Earth's surface water has not remained constant. Initially, Earth's primordial water had a $D/H$ ratio of $1.0 \times 10^{-4}$, identical to enstatite chondrites. Over billions of years, solar ultraviolet radiation photolyzed atmospheric water vapor, and the lighter protium ($^1H$) isotope preferentially escaped Earth’s gravity compared to the heavier deuterium ($^2H$). This selective escape of protium acted as the primary driver that gradually elevated the surface $D/H$ ratio to its current value of $1.5 \times 10^{-4}$.

Based on the passage, Scientist 1 and Scientist 2 differ in their views regarding which of the following?