The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites-over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

The infrared (IR) spectra of ultraviolet (UV) and thermally processed, methanol-containing interstellar/ cometary ice analogs at temperatures from 12 to 300 K are presented. Infrared spectroscopy, IH and _3C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry indicate that CO (carbon monoxide), CO2 (carbon dioxide), CH4 (methane), HCO (the formyl radical), H2CO (formaldehyde), CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(_O)NH 2 (acetamide), and R--C_-N (nitriles) are formed. In addition, the organic materials remaining after photolyzed ice analogs have been warmed to room temperature contain (in rough order of decreasing abundance),hexamethylenetetramine (HMT, C6HI2N4), (2) ethers, alcohols, and compounds related to polyoxymethylene {POM, (--CH20--),}, and (3) ketones {R--C(=O)--R'} and amides {H2NC(=O)--R}. Most of the carbon in these residues is thought to come from the methanol in the original ice. Deuterium and 13C isotopic labeling demonstrates that methanol is definitely the source of carbon in HMT. High concentrations of HMT in interstellar and cometary ices could have important astrophysical consequences.The ultraviolet photolysis of HMT frozen in H20 ice readily produces the "XCN" band observed in the spectra of protostellar objects and laboratory ices, as well as other nitriles. Thus, HMT may be a precursor of XCN and a source of CN in comets and the interstellar medium. Also, HMT is known to hydrolyze under acidic conditions to yield ammonia, formaldehyde, and amino acids. Thus, HMT may be a significant source of prebiogenic compounds on asteroidal parent bodies. A potential mechanism for the radiative formation of HMT in cosmic ices is outlined.

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4È13 km) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the di †use interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 km and the 4.27 km ice H 2 O C O 2 features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid and possibly HCOOH. Hydrocarbon dust in the di †use interstellar medium along the CH 4 , NH 3 , line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 km. Ground-based studies have identiÐed the 3.4 km feature with aliphatic hydrocarbons, and ISO has given us the Ðrst meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 km in the GCS 3 spectrum to the CwH stretch in aromatic hydrocarbons. This feature is not detected, and its CwC stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the di †use interstellar medium, analogous to aliphatic hydrocarbons.