For decades, the dominant model of Earth’s volatile history asserted that our planet formed dry and hot, acquiring its water only later through a "late veneer" of water-rich asteroids and comets striking the cooling surface. Proponents of this external-delivery hypothesis pointed to the deuterium-to-hydrogen () ratios in Earth's oceans, which closely match those found in carbonaceous chondrite meteorites. However, recent isotopic analyses of deep-mantle plumes have challenged this consensus. Researchers discovered that water trapped in Earth's deep interior possesses a significantly lower ratio than surface water, aligning instead with the isotopic signature of the solar nebula itself. Skeptics of this nebula-absorption theory argue that hydrogen from the early solar nebula would have escaped Earth's gravitational pull during the energetic magma ocean phase before it could be sequestered. Yet, high-pressure laboratory simulations demonstrate that under early Earth conditions, iron droplets sinking through a magma ocean would dissolve hydrogen and transport it directly to the growing core, effectively shielding it from atmospheric escape.
According to the passage, skeptics of the nebula-absorption theory raise an objection concerning hydrogen escape. Which of the following best describes the function of this objection, and the author's response to it?
- AIt supports the late veneer hypothesis by establishing that carbonaceous chondrites are the sole source of deuterium in Earth's oceans.
- It presents a physical obstacle to the nebula-absorption model, which the author addresses by citing experimental evidence of a pathway that would protect the hydrogen.Cevap
- CIt details the primary mechanism by which the author believes the early Earth lost its initial water reserves to space.
- DIt refutes the deep-mantle plume isotopic data by proving that mantle plumes could not have survived the magma ocean phase.