Question

### Source of Lunar Water

Water ice has been detected in permanently shadowed regions (PSRs) of craters near the Moon's poles. Three scientists present opposing hypotheses regarding the origin and distribution of this water.

Scientist 1
Lunar water is primarily exogenous, delivered by cometary impacts over the last $3$ billion years. Comets are rich in water ice and volatile organic compounds. When a comet impacts the lunar surface, a temporary vapor atmosphere is created. Most water molecules escape into space, but a significant fraction ($15\%$) migrates to the cold, polar PSRs where temperatures remain below $100\text{ K}$, trapping the water molecules indefinitely. Spacecraft data showing high concentrations of hydrogen and associated volatile organic molecules in polar craters support this cometary origin, as solar wind and volcanic outgassing would not deliver these organic co-volatiles.

Scientist 2
The primary source of lunar water is endogenous, originating from volcanic outgassing during the Moon's early history, specifically between $3.8$ and $3.0$ billion years ago. During this period of intense mare volcanism, eruptions of basaltic lavas released significant amounts of water vapor from the lunar mantle. Calculations show that these volcanic eruptions could have produced a transient atmosphere containing up to $10^{16}\text{ kg}$ of water. While most of this water escaped, approximately $1.5\%$ was cold-trapped in polar craters, yielding ice deposits that are now buried beneath meters of regolith. This explains why the water ice is deeply buried and not just present as surface frost.

Scientist 3
Lunar water is continuously produced via interaction with the solar wind, which implants protons ($H^+$ ions) into the lunar regolith. These protons react with oxygen atoms within the silicate minerals of the regolith to form hydroxyl ($OH$) and water ($H_2O$) molecules. Micro-meteorite bombardment provides the necessary thermal energy to mobilize these molecules, allowing them to migrate through the exosphere and accumulate in polar cold traps. Because solar wind implantation is an ongoing process, the water ice in PSRs should be thin, widespread, and concentrated in the uppermost millimeters of the surface regolith, rather than deep layers.

Based on the passage, which of the following statements best represents the core hypothesis of Scientist 2 regarding the origin and physical distribution of lunar water?