It was previously assumed that only gas-phase ion-molecule reactions could lead to complex, biologically-relevant, organic molecules. However, the gas-phase formation of methyl formate, one of the most abundant organic interstellar molecules, is now known to be inefficient [1]. Additionally, organics have been observed in several interstellar environments where significant gas-phase chemical processing cannot occur. These findings have led to a revision of the traditional gas-phase chemical models to include the formation of organic molecules on interstellar grain surfaces [2]. The astrochemical pathway to complex organic molecules, including biologically-relevant molecules, is now thought to occur in the following steps:
I & II. Photolysis produces radicals from simple grain surface species. The ices on interstellar grains are known to contain mostly water, carbon monoxide, carbon dioxide, methanol, ammonia, and formaldehyde (confirmed by absorption spectra toward dark clouds).
III. As a new star forms, the dust and gas in the cloud is gradually warmed. The simple radicals become mobile on the grain surface and can react to form more complex organic molecules such as sugars and amino acids.
IV. Once the temperature in the cloud reaches ~100 K, most of the molecules are released into the gas phase via thermal desorption from the grain surface.
V. Gas-phase ion-molecule reactions can lead to even more chemical complexity. Reactions leading to amino acids are possible during this phase.
VI. The interstellar material is eventually incorporated into “parent bodies,” which include meteorites, comets, and planets. Aqueous-phase chemistry can occur in these objects, and so even more complex biological molecules can form. Impact of meteorites and comets is thought to be an important delivery mechanism of water and organic material to the early Earth, seeding the formation of life [3,4].
[1] Horn et al. (2004), ApJ 611, 605.
[2] Garrod, Widicus Weaver, and Herbst (2008), ApJ 682, 283.
[3] Oro (1961) Nature 190, 389.
[4] Raymond, Quinn, & Luine (2004) Icarus 168, 1.