Independent

Chandrayaan-1, India's first-ever moon probe, was aimed at mapping the lunar surface and determining its mineral composition (the orbiter's mission ended 14 months prematurely in August after an abrupt malfunction). While the probe was still active, its NASA-built Moon Mineralogy Mapper (M3) detected wavelengths of light reflected off the surface that indicated the chemical bond between hydrogen and oxygen — the telltale sign of either water or hydroxyl.

Because M3 can only penetrate the top few millimeters of lunar regolith, the newly observed water seems to be at or near the lunar surface. M3's observations also showed that the water signal got stronger toward the polar regions. Pieters is the lead investigator for the M3 instrument on Chandrayaan-1.

Combined, the findings show that not only is the moon hydrated, the process that makes it so is a dynamic one that is driven by the daily changes in solar radiation hitting any given spot on the surface.

The sun might also have something to do with how the water got there.

There are potentially two types of water on the moon: that brought from outside sources, such as water-bearing comets striking the surface, or that that originates on the moon.

This second, endogenic, source is thought to possibly come from the interaction of the solar wind with moon rocks and soils.

The rocks and regolith that make up the lunar surface are about 45 percent oxygen (combined with other elements as mostly silicate minerals). The solar wind — the constant stream of charged particles emitted by the sun — are mostly protons, or positively charged hydrogen atoms.

NASA