Question

Passage A

For decades, the dominant hypothesis regarding the origin of Earth’s water pointed toward the outer solar system as the primary source. According to this accretion model, early Earth was simply too hot and volatile to retain any water or gaseous elements during its initial formation, meaning the planet began its geological history completely dry. Instead, the planet’s vast oceans were delivered much later, during a period known as the Late Heavy Bombardment, by water-rich carbonaceous chondrite meteorites originating from beyond the asteroid belt. The primary evidence for this meteorite-delivery model lies in the deuterium-to-hydrogen ($D/H$) ratio. Hydrogen has two stable isotopes: protium ($^1H$, or simple hydrogen) and deuterium ($^2H$, or heavy hydrogen). When scientists measured the $D/H$ ratio of Earth’s oceans, they found it matched the $D/H$ ratio of carbonaceous chondrite meteorites almost perfectly—approximately $1.5 \times 10^{-4}$. In contrast, comets from the outer reaches of the solar system, such as Oort cloud comets, exhibit $D/H$ ratios that are twice as high as Earth’s ocean water, largely ruling them out as primary contributors to the early oceans. Thus, the geochemical consensus solidified around the idea that Earth was born dry and was subsequently decorated with water by asteroid impacts. Geochemical models suggested that as little as two percent of the late-accreting meteoritic mass could account for the entirety of Earth's surface water, making this a highly efficient and mathematically plausible explanation for the oceans we observe today. However, this model relies heavily on the assumption that the surface ocean's isotopic signature has remained unchanged since its deposition, representing a static geological record of these ancient impacts. This framework implies that surface measurements are sufficient for determining the origins of Earth's volatile elements, neglecting the possibility of significant interior contribution during accretion.

Passage B

While the meteorite-delivery model remains popular, recent advancements in deep-earth geochemistry and planetary accretion models have challenged its monopoly. Proponents of the "nebular ingassing" model argue that a substantial portion of Earth's water is primordial, originating from the solar nebula itself during the planet's initial formation. During the early stages of accretion, Earth was enveloped in a thick, hydrogen-rich envelope of solar nebula gas. As the planet’s surface was molten—a vast magma ocean—this atmospheric hydrogen dissolved directly into the silicate melt. Inside the Earth, this dissolved hydrogen reacted with iron oxides in the magma, synthesizing water ($H_2O$) that became trapped deep within the mantle. The key evidence for this model comes from measurements of deep-mantle rocks, particularly basaltic glasses from the oceanic island of Baffin Island. These rocks, which originate from plumes tapped deep near the core-mantle boundary, reveal a $D/H$ ratio that is roughly 25 percent lower than that of modern ocean water. This remarkably low ratio is characteristic of solar nebula gas, which has a very low $D/H$ ratio of approximately $2.1 \times 10^{-5}$. Because surface water has likely been contaminated and enriched in deuterium over billions of years through atmospheric escape (where lighter hydrogen escapes to space faster than deuterium), the ocean's current ratio does not represent Earth's original water. Instead, the deep mantle preserves the true relic of Earth's birth: water forged directly from the solar nebula. This means that surface-based geochemical comparisons are fundamentally flawed, as they ignore the planetary evolution processes that have altered surface signatures. Consequently, understanding Earth's water requires analyzing reservoirs that have remained isolated from atmospheric influence.

Based on Passage A and Passage B, which of the following statements best describes how the authors use the deuterium-to-hydrogen ($D/H$) ratio to support their respective theories?