Scientists have discovered that the most likely source of water on Moon is the constant stream of charged particles from the Sun known as the solar wind. 

The findings by researchers from the University of Michigan imply that ice inside permanently shadowed polar craters on the Moon, sometimes called cold traps, could contain hydrogen atoms ultimately derived from the solar wind.

Theoretical models of lunar water stability dating to the late 1970s suggest that hydrogen ions (protons) from the solar wind can combine with oxygen on the Moon's surface to form water and related compounds called hydroxyls.