Interstellar ice consists of grains of volatiles in the ice phase that form in the interstellar medium. Ice and dust grains form the primary material out of which the Solar System was formed. Grains of ice are found in the dense regions of molecular clouds, where new stars are formed. Temperatures in these regions can be as low as 10 K, allowing molecules that collide with grains to form an icy mantle. Thereafter, atoms undergo thermal motion across the surface, eventually forming bonds with other atoms. This results in the formation of water and methanol. Indeed, the ices are dominated by water and methanol, as well as ammonia, carbon monoxide and carbon dioxide. Frozen formaldehyde and molecular hydrogen may also be present. Found in lower abundances are nitriles, ketones, esters and carbonyl sulfide. The mantles of interstellar ice grains are generally amorphous, only becoming crystalline in the presence of a star.
The composition of interstellar ice can be determined through their infrared spectrum. As starlight passes through a molecular cloud containing ice, molecules in the cloud absorb energy. This adsorption occurs at the characteristic frequencies of vibration of the gas and dust. Ice features in the cloud are relatively prominently in this spectra, and the composition of the ice can be determined by comparison with samples of ice materials on Earth. In the sites directly observable from Earth, around 60–70% of the interstellar ice consists of water, which displays a strong emission at 3.05 μm from stretching of the O–H bond.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."
- Gibb, E. L. et al. (March 2004), "eInterstellar Ice: The Infrared Space Observatory Legacy", The Astrophysical Journal Supplement Series 151 (1): 35–73, Bibcode:2004ApJS..151...35G, doi:10.1086/381182
- Allamandola, Louis J.; Bernstein, Max P.; Sandford, Scott A.; Walker, Robert L. (October 1999). "Evolution of Interstellar Ices". Space Science Reviews 90 (1/2): 219–232. Bibcode:1999SSRv...90..219A. doi:10.1023/A:1005210417396.
- Greenberg, J. Mayo (1991). "Interstellar Dust-Gas Relationships". In Maurice Mandel Shapiro, Rein Silberberg, J. P. Wefel. Cosmic rays, supernovae, and the interstellar medium. NATO ASI series: Mathematical and physical sciences (337). Springer. p. 58. ISBN 0-7923-1278-3.
- Pirronello, Valerio; Krełowski, Jacek; Manicò, Giulio; North Atlantic Treaty Organization. Scientific Affairs Division (2003). Solid state astrochemistry. NATO science series: Mathematics, physics, and chemistry 120. Springer. p. 288. ISBN 1-4020-1559-3.
- Staff (September 20, 2012). "NASA Cooks Up Icy Organics to Mimic Life's Origins". Space.com. Retrieved September 22, 2012.
- Gudipati, Murthy S.; Yang, Rui (September 1, 2012). "In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies". The Astrophysical Journal Letters 756 (1). Bibcode:2012ApJ...756L..24G. doi:10.1088/2041-8205/756/1/L24. Retrieved September 22, 2012.