Subsurface chemistry of mantles of interstellar dust grains in dark
  molecular cores
  Context. The abundances of many observed compounds in interstellar molecular
clouds still lack an explanation, despite extensive research that includes both
gas and solid (dust-grain surface) phase reactions. Aims. We aim to
qualitatively prove the idea that a hydrogen-poor subsurface chemistry on
interstellar grains is responsible for at least some of these chemical
"anomalies". This chemistry develops in the icy mantles when photodissociation
reactions in the mantle release free hydrogen, which escapes the mantle via
diffusion. This results in serious alterations of the chemical composition of
the mantle because pores in the mantle provide surfaces for reactions in the
new, hydrogen-poor environment. Methods. We present a simple kinetic model,
using existing astrochemical reaction databases. Gas phase, surface and
subsurface pore reactions are included, as are physical transformations of
molecules. Results. Our model produces significantly higher abundances for
various oxidized species than most other models. We also obtain quite good
results for some individual species that have adequate reaction network. Thus,
we consider that the hydrogen-poor mantle chemistry may indeed play a role in
the chemical evolution of molecular clouds. Conclusions. The significance of
outward hydrogen diffusion has to be proved by further research. A huge number
of solid phase reactions between many oxidized species is essential to obtain
good, quantitative modeling results for a comparison with observations. We
speculate that a variety of unobservable hydrogen-poor sulfur oxoacid
derivatives may be responsible for the "disappearance" of sulfur in dark cloud
cores.
