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.

Polycyclic aromatic hydrocarbons (PAHs) in water ice were exposed to ultraviolet (UV) radiation under astrophysical conditions, and the products were analyzed by infrared spectroscopy and mass spectrometry. Peripheral carbon atoms were oxidized, producing aromatic alcohols, ketones, and ethers, and reduced, producing partially hydrogenated aromatic hydrocarbons, molecules that account for the interstellar 3.4-micrometer emission feature. These classes of compounds are all present in carbonaceous meteorites. Hydrogen and deuterium atoms exchange readily between the PAHs and the ice, which may explain the deuterium enrichments found in certain meteoritic molecules. This work has important implications for extraterrestrial organics in biogenesis.

Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous species. Some of these compounds are amphiphilic, having polar and nonpolar groups on the same molecule. Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. The goal of future investigations will be to fabricate artificial cells as models of the origin of life.

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The largest non-cyclic molecules detected in the interstellar medium (ISM) are organic with a straight-chain carbon backbone. We report an interstellar detection of a branched alkyl molecule, iso-propyl cyanide (i-C 3 H 7 CN), with an abundance 0.4 times that of its straight-chain structural isomer. This detection suggests that branched carbon-chain molecules may be generally abundant in the ISM. Our astrochemical model indicates that both isomers are produced within or upon dust grain ice mantles through the addition of molecular radicals, albeit via differing reaction pathways. The production of iso-propyl cyanide appears to require the addition of a functional group to a non-terminal carbon in the chain. Its detection therefore bodes well for the presence in the ISM of amino acids, for which such side-chain structure is a key characteristic.Around 180 molecules have been detected so far in the interstellar medium (ISM) (1). Apart from the fullerenes that are cyclic, the largest of these interstellar molecules are organic with a straight-chain carbon backbone. However, more complex molecules have been identified in 1 meteorites found on Earth, including more than 80 amino acids (2) -the building blocks of proteins. The composition of these meteoritic amino acids suggests that they or their direct precursors have an interstellar origin (3). Chemistry at work in the ISM may thus be capable of producing organic molecules more complex than those detected so far and thus of great importance to astrobiology. However, the degree of complexity that may be reached in the ISMis still an open question, as well as how widespread these complex organic molecules are in our galaxy.Our previous observations of the star-forming region Sagittarius B2(N) -hereafter Sgr B2(N) -yielded the first interstellar detection of the straight-chain organic molecule normal-propyl cyanide (n-C 3 H 7 CN), the largest molecule yet detected in this source (4). This spectral line survey, conducted using the 30-m single-dish radio telescope of the Institut de Radioastronomie Millimétrique (IRAM), provided continuous spectral coverage throughout the 3-mm transmission window of Earth's atmosphere (5). It allowed the identification of other complex organic species such as ethyl formate (4) and aminoacetonitrile (6), a possible precursor of the amino acid glycine. On the basis of the success of this single-dish line survey, we performed a survey in the same waveband by using the Atacama Large Millimeter/submillimeter Array (ALMA), resulting in an increase of more than one order of magnitude in both sensitivity and angular resolution. This interferometric project, called EMoCA (Exploring Molecular Complexity with ALMA), aims to decipher the molecular content of Sgr B2(N) in order to test the predictions of astrochemical numerical simulations and to gain insight into the chemical processes at work in the ISM.Sgr B2 is the most massive star-forming region in our galaxy. It is located close to the Galactic Center, which is 8.34 ± 0.16 kpc from the Sun (7)...

We present a measurement of the mean density profile of Ca ii gas around galaxies out to ∼200 kpc, traced by Fraunhofer's H & K absorption lines. The measurement is based on cross-correlating the positions of about one million foreground galaxies at z ∼ 0.1 and the flux decrements induced in the spectra of about 10 5 background quasars from the Sloan Digital Sky Survey. This technique allows us to trace the total amount of Ca ii absorption induced by the circumgalactic medium, including absorbers too weak to be detected in individual spectra. We can statistically measure Ca ii rest equivalent widths down to several mÅ, corresponding to column densities of about 5 × 10 10 cm −2 . We find that the Ca ii column density distribution follows N Ca ii ∼ r −1.4 p and the mean Ca ii mass in the halo within 200 kpc is ∼5 × 10 3 M , averaged over the foreground galaxy sample with median mass ∼10 10.3 M . This is about an order-of-magnitude larger than the Ca ii mass in the interstellar medium of the Milky Way, suggesting that more than 90% of Ca ii in the universe is in the circum-and inter-galactic environments. Our measurements indicate that the amount of Ca ii in halos is larger for galaxies with higher stellar mass and higher star formation rate. For edge-on galaxies we find Ca ii to be more concentrated along the minor axis, i.e., in the polar direction. This suggests that bipolar outflows induced by star formation must have played a significant role in producing Ca ii in galaxy halos.