As part of its nuclear fusion processes, the Sun produces a particularly interesting isotope of helium, known as helium-3. Some of this helium-3 escapes the Sun's gravitational pull as part of the solar wind, a stream of charged particles that emanates out across the Solar System.
Over billions of years, this wind has bombarded every body in the vicinity of the Sun, including Earth's Moon. Lacking a magnetic field or atmosphere, the Moon's surface has no barrier to prevent the constant flow of this wind.
This means that helium-3 has been striking the lunar soil, or regolith, for a long time. These helium ions penetrate only a little way into the individual grains of this soil, but periodically meteorites have struck the lunar surface to churn the soil, mixing some of this helium-3 a little below the surface.
DW News 
