Most interstellar complex organic molecules (COMs) are produced on and within the icy mantles of dust grains in the cold, dense interiors of molecular clouds and later become observable when those ices are warmed or shocked. Observational syntheses by Ewine van Dishoeck Leiden Observatory show that COMs concentrate in compact regions around young protostars and in shock zones where ices are released, indicating a two-stage origin: formation in cold ices followed by thermal or mechanical desorption.
Grain-surface chemistry and ice mantles
Laboratory experiments and theoretical work by Karin Öberg Harvard University demonstrate that simple atoms and molecules accrete on submicron dust grains at 10–20 kelvin and are converted to more complex species through hydrogenation, radical recombination, and energetic processing by ultraviolet photons and cosmic rays. Eric Herbst Ohio State University emphasizes through chemical models that these surface processes efficiently build complexity that is difficult to produce in the gas phase alone at low temperatures. This grain-surface pathway explains why cold dense cores, where visual extinction is high and gas-phase photodissociation is suppressed, are the initial sites of COM assembly.
Warm release: hot cores, corinos, and shocks
When a protostar forms and heats its surroundings, or when outflows and shocks perturb the cloud, the ice mantles sublimate or are sputtered, injecting complex organics into the gas. Observations with the Atacama Large Millimeter/submillimeter Array ALMA in the Atacama Desert reveal rich spectra of COMs from hot cores around high-mass protostars and hot corinos around low-mass protostars. Shock-dominated regions such as those in molecular outflows also show enhanced gas-phase COM abundances, underlining the role of mechanical desorption.
The relevance of this origin pathway extends to planet formation and prebiotic chemistry: molecules synthesized in ices can be incorporated into protoplanetary disks and ultimately into planetesimals, delivering complex organics to nascent planets. Environmental and territorial context matters for discovery: facilities like ALMA sited in Chile enable these measurements because of the dry high-altitude conditions, shaping where and how astronomers can test models. The combined laboratory, modeling, and observational evidence by researchers such as Karin Öberg Harvard University, Ewine van Dishoeck Leiden Observatory, and Eric Herbst Ohio State University establishes that most interstellar complex organics originate in the molecular cloud’s icy grain mantles and are later liberated into the gas in warm or shocked regions.