In 2009, NASA’s Lunar Crater Observation and Sensing satellite, known as LCROSS, slammed into a permanently shadowed lunar crater and ejected a plume of material that was surprisingly rich in water ice. Water and related compounds have also been detected in the lunar regolith, the layer of fine powder and rock fragments that coats the lunar surface.

But the origin of lunar surface water has remained unclear. Is it mainly the result of impacts from water-bearing comets and other chunks of space debris, or could there be other sources?