Soru

The following passage explores the scientific debate surrounding the origins of Earth's water.

For decades, planetary scientists widely accepted the 'late veneer' hypothesis to explain the origin of Earth’s oceans. According to this model, early Earth was entirely dry due to the high-temperature conditions of the inner solar nebula, which would have vaporized any volatile compounds. Water, therefore, must have been delivered to the surface after the planet had fully accreted and cooled, presumably during the Late Heavy Bombardment around $3.9$ billion years ago. The primary candidates for this delivery were comets—icy remnants of the early solar system originating from the outer reaches, where water could easily freeze and accumulate.

However, this comet-delivery model faced a critical setback with the advent of direct space-probe measurements. In $1986$, the Giotto mission measured the deuterium-to-hydrogen ($D/H$) ratio in the water of Halley's Comet. Scientists discovered that Halley’s water had a $D/H$ ratio roughly twice that of Earth’s oceans. Subsequent missions to other comets, including the Rosetta mission to Comet 67P in $2014$, confirmed this discrepancy: comet water is isotopically heavier than terrestrial water. Because the isotopic signature of water does not change over geological time, comets could not have been the primary source of Earth’s oceans.

This geochemical mismatch prompted a reevaluation of carbonaceous chondrite meteorites as the true carriers of terrestrial water. Originating from the outer asteroid belt, these meteorites contain water bound in hydrated clay minerals. Isotopic analyses of carbonaceous chondrites revealed that their $D/H$ ratios match Earth’s ocean water almost perfectly. This suggested that water delivery occurred earlier than previously thought, during the main phase of Earth’s accretion, rather than as a late addition.

This chondritic origin has been further reinforced by recent analyses of deep mantle rocks. Geochemists studying volcanic glass from Baffin Island discovered hydrogen isotopic signatures in Earth's deep mantle that are even lower than those of chondrites. This evidence suggests that a significant portion of Earth's water was present in the dust cloud from which the planet formed, trapped within the Earth since its accretion. Thus, the narrative has shifted from late cometary bombardment to early, endogenous water accumulation.

Based on the passage, arrange the phases of the scientific debate regarding the origin of Earth's water in the order they are presented, tracing the progression from the initial hypothesis to the most recent findings.