  The need to characterize ices coating dust grains in dense interstellar
clouds arises from the importance of ice morphology in facilitating the
diffusion and storage of radicals and reaction products in ices, a well-known
place for the formation of complex molecules. Yet, there is considerable
uncertainty about the structure of ISM ices, their ability to store volatiles
and under what conditions. We measured the infrared absorption spectra of CO on
the pore surface of porous amorphous solid water (ASW), and quantified the
effective pore surface area of ASW. Additionally, we present results obtained
from a Monte Carlo model of ASW in which the morphology of the ice is directly
visualized and quantified. We found that 200 ML of ASW annealed to 20 K has a
total pore surface area that is equivalent to 46 ML. This surface area
decreases linearly with temperature to about 120 K. We also found that (1)
dangling OH bonds only exist on the surface of pores; (2) almost all of the
pores in the ASW are connected to the vacuum--ice interface, and are accessible
for adsorption of volatiles from the gas phase; there are few closed cavities
inside ASW at least up to a thickness of 200 ML; (3) the total pore surface
area is proportional to the total 3-coordinated water molecules in the ASW in
the temperature range 60--120 K. We also discuss the implications on the
structure of ASW and surface reactions in the ice mantle in dense clouds.
