I wonder if this water is a byproduct of the solar wind and radiation, rather than endogenous from the planet.
Cometary impact is considered the primary delivery method for water onto planets. In other words, this is hydrogen and oxygen that bonded together in the protoplanetary disk to form water; clumped together to form icy bodies (comets); eventually had its orbit disturbed sending it into the inner system; and finally, impacting a planet. It is also possible that the icy dust grains (water) may have been part of the planet as it formed, assuming the planet formed far enough out that water ice could exist. The planet would have then migrated inwards (which new models are showing is the likely origin for the close-in exoplanets).