Alteration of an Apollo 12 sample by adsorption of water vapor

The adsorption of water vapor caused radical changes in the surface characteristics of an Apollo 12 lunar fines sample. In the initial state, the sample had a relatively low specific surface area and no inherent porosity as deduced from the adsorption isotherms of nitrogen and argon. However, the sample interacted strongly with water vapor, particularly at high relative pressures, and irreversibly retained, in vacuum, a quantity of water much larger than can be explained by a simple chemisorption process. The interaction with water vapor more than doubled the specific surface area and created a pore system. The suggested explanation is that water penetrates the severely radiation damaged, amorphous surface layers of the particles and creates pores by leaching out channels or forcing apart adherent particles. Lunar fines from the Apollo 11, 12 and 14 missions have markedly different reactivities toward water vapor, an observation which remains unexplained at the present time.