Which combination explains how Earth's surface water is thought to have formed?

• A. Volcanism only
• B. Comets/asteroids only
• C. Volcanism AND comets/asteroids
• D. Water was always present during Earth's formation

Answer: (C)

Earth’s surface water most likely comes from a combination of internal and external sources. As the planet formed and differentiated, water trapped in minerals in the mantle was released by volcanic activity, adding water vapor to the atmosphere. As Earth cooled, this vapor condensed and formed oceans. At the same time, water-rich comets and hydrated asteroids bombarded the young Earth, delivering additional water and other volatile compounds. The evidence, including isotopic signatures (like the D/H ratio) in Earth's oceans that match a mix of meteorites and some comets, supports a mixed origin rather than a single source. Relying on volcanic outgassing alone would imply the mantle supplied all the water, which is unlikely given early high temperatures and the amount of water observed. Delivering all the water from comets/asteroids alone would struggle to explain both the total quantity and the isotopic mix. Water being present from the very beginning would also conflict with the extreme early conditions of a hot Earth. So the combination of volcanism and external delivery best explains how Earth's surface water formed.