Observations of the Icy Universe
  Freeze-out of the gas phase elements onto cold grains in dense interstellar
and circumstellar media builds up ice mantles consisting of molecules that are
mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review
summarizes the detected infrared spectroscopic ice features and compares the
abundances across Galactic, extragalactic, and solar system environments. A
tremendous amount of information is contained in the ice band profiles.
Laboratory experiments play a critical role in the analysis of the
observations. Strong evidence is found for distinct ice formation stages,
separated by CO freeze out at high densities. The ice bands have proven to be
excellent probes of the thermal history of their environment. The evidence for
the long-held idea that processing of ices by energetic photons and cosmic rays
produces complex molecules is weak. Recent state of the art observations show
promise for much progress in this area with planned infrared facilities.
