Possible sources of water on the Moon

Abstract

In a previous paper we calculated the upper limit of H2O density in the lunar atmosphere using the Explorer 35 data on the lunar perturbation of the solar wind. In this paper we search for various H2O sources on the moon and find all possible sources to be incapable of producing H2O density equal to the upper limit except possibly degassing from the lunar interior. The degassing rate is inversely proportional to the depth at which an H2O source can exist. Thus the upper limit of the atmospheric H2O density puts a lower limit on the depth (or provides a minimum depth) of a uniform H2O source in the lunar interior. We give a table of minimum depths for various subsurface H2O source temperatures and soil permeabilities. These depths are greater than the expected depths of lunar regolith; we therefore conclude that the minimum depth at which an H2O source can exist is at some depth in the bedrock. If we accept the lunar subsurface temperatures measured with the Apollo 15 heat flow experiment package and extrapolate these measured data to greater depths, it appears possible that H2O of the source, which may exist at the minimum or greater depths, is in the form of liquid.