The Interstellar Oxygen Crisis, or Where Have All the Oxygen Atoms Gone?
  The interstellar medium (ISM) seems to have a significant surplus of oxygen
which was dubbed as the "O crisis": independent of the adopted interstellar
reference abundance, the total number of O atoms depleted from the gas phase
far exceeds that tied up in solids by as much as ~160ppm of O/H. Recently, it
has been hypothesized that the missing O could be hidden in micrometer-sized
H2O ice grains. We examine this hypothesis by comparing the infrared (IR)
extinction and far-IR emission arising from these grains with that observed in
the Galactic diffuse ISM. We find that it is possible for the diffuse ISM to
accommodate ~160ppm of O/H in micron-sized H2O ice grains without violating the
observational constraints including the absence of the 3.1micron O-H absorption
feature. More specifically, H2O ice grains of radii ~4micron and O/H = 160 ppm
are capable of accounting for the observed flat extinction at ~ 3-8 micron and
produce no excessive emission in the far-IR. These grains could be present in
the diffuse ISM through rapid exchange of material between dense molecular
clouds where they form and diffuse clouds where they are destroyed by
photosputtering.
