Question

### The Origin of Earth's Water

Liquid water covers approximately $71\%$ of Earth's surface, yet the source of this water remains a subject of ongoing debate among geologists and planetary scientists. Three scientists present competing hypotheses regarding the origin of Earth's oceans.

Scientist 1
During Earth's formation $4.5$ billion years ago, water was trapped inside the hot planetary mantle within hydrous (water-bearing) minerals. As the early planet differentiated, mantle convection and intense volcanic activity released this water as steam and volcanic gases into the primordial atmosphere. Once Earth's surface cooled below $100^\circ\text{C}$, the atmospheric water vapor condensed and fell as torrential rain, filling the oceanic basins. This volcanic outgassing was the primary source of Earth's oceans. The deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans matches the $D/H$ ratio of ancient mantle-derived rocks, confirming that the water originated from deep within the planet rather than from space.

Scientist 2
Early Earth was extremely hot and dry due to energetic collisions during accretion, which vaporized any primordial water and blew it into space. Earth's water must have been delivered after the planet had cooled, during the Late Heavy Bombardment approximately $3.9$ billion years ago. The primary source was carbonaceous chondrite asteroids from the outer asteroid belt. These asteroids, which contain up to $20\%$ water by weight, impacted the cooled Earth, and their water condensed to form the oceans. The $D/H$ ratio of Earth's ocean water matches the ratio found in these carbonaceous chondrites, whereas the $D/H$ ratio of comets is far too high, and mantle outgassing was insufficient to form oceans.

Scientist 3
Early Earth was dry, and asteroid impacts alone could not have delivered the vast volume of water found in the oceans today. Instead, Earth's water was delivered primarily by comets from the outer solar system. Comets, composed largely of water ice, migrated inward due to gravitational interactions with the gas giant planets. These comets collided with Earth after its crust had solidified and cooled below $100^\circ\text{C}$, releasing water vapor that quickly condensed into liquid oceans. Although some comets have high $D/H$ ratios, recent measurements of Kuiper Belt comets show a wide range of ratios, some of which match Earth's oceans, confirming comets as the major source.

Based on the viewpoints presented, all three scientists would agree with which of the following statements regarding the formation of Earth's oceans?