Given that your average dense molecular cloud from which a Solar System will form has an average temperature of only tens of degrees above absolute zero, and the pressure is what most humans would call a good vacuum, it might seem like little chemistry could occur under such conditions. Indeed, one reviewer of one of our papers wrote that everyone knows that complex organic molecules cannot possibly form or survive in space. Actually, they can, and they do, and we know that they are out there thanks to astronomical observations, and the fact that some of these molecules come to Earth in meteorites and mircroscopic dust particles. UV (Ultraviolet) Radiation and cosmic rays probably cause significant photochemistry within the mixed-molecular ices found in space. We simulate this process in our laboratory using some high-tech gadgetry. When we simulate the photochemistry of interstellar and cometary ices in the lab we make a host of organic compounds (i.e. compounds composed primarily of carbon, the kinds of molecules from which we and all living things are made). We believe this process may be responsible for the richness of the organics seen in the Diffuse and Dense Interstellar Medium, comets, and meteorites.

Organic Compounds

Since comets and interstellar ices are composed primarily of water we started with ices of water and other simple compounds known to be in space like methanol (CH₃OH), carbon monoxide (CO) and ammonia (NH₃). From such simple starting materials we were able to make things like methane (CH₄), carbon dioxide (CO₂), ethanol (CH₃CH₂OH), formamide, acetamide, ketones, and alcohols. Such molecules were either known to exist at that time or have since been observed in these environments thanks to ISO and ground based IR observations. After these sun-burnt ices are warmed up and the volatiles sublime away we were able to detect some larger compounds like polyoxymethylene - based polymers (POMs), and hexamethylene tetramine (HMT). This molecule may have importance for the understanding of comet chemistry. For example, its been suggested that HMT (C₆H₁₂N₄) is a potential extended source of CN and HCN in the coma of comets. See this PDF version of a paper by Cottin et al for details. Follow these links to see how we identified HMT in our residues and how we think it formed. Since we published this work in 95 really excellent papers have been published on the formation this molecule in space by H. Cottin and also by G. Munoz Caro.

We also have seen other compounds that have potential implications for the origin of life on the Earth. For example, in a paper in the journal Science in 2002, we demonstrated that amino acids (the basic components of proteins) could form from interstellar ice photochemistry. for more information on this subject follow this link to pop science articles, or read our paper "Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues." Nature 416, 401Ð403, which can be downloaded as a PDF file at our publications page

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More Complex Organic Compounds

Even though HMT and POM have molecular weights over 100 amu these are just the smaller compounds. There are much larger organic materials produced when we perform our interstellar/cometary ice simulations. Gas Chromatograph Mass Spectrometry (GCMS) and Laser Desorption Mass Spectrometry analyses (Richard Zare's Lab) of our residues show the presence a hundreds of different compounds, most of which have yet to be identified. Many of these compounds have properties that are potentially of interest to issues associated with the origin of life. For example, some of these compounds fluoresce, others spontaneously form membranes in solution, and so on. To learn a little more about these compounds, click here.

PAH-related photoproducts (alcohols, ketones, and H_n-PAHs, etc.)

Polycyclic aromatic hydrocarbons (PAHs) represent one of the most abundant forms of carbon in the interstellar medium, and many variations on these molecules are seen in meteorites and asteroidal and cometary dust. While it is understood that basic PAHs should form in the outflows of carbon rich stars, it was not understood how PAHs bearing side groups would form.

Since most PAHs are relatively non-volatile compounds, in dense interstellar clouds they are expected to feeze out into the ice. Indeed, astronomers (such as Kris Sellgren of Ohio State and Jean Chiar of NASA Ames) have directly observed PAHs frozen into the ices. Since we know that PAHs will be photoprocessed like all the other molecules in the ices we studies the photolysis of PAHs in H₂O-rich ices and this resulted in a few papers (see below). In 1999 (Science 283, 1135-1138.) we showed that photoprocessing of PAHs in water ice leads to the production of a number of new compounds including aromatic ethers, alcohols, and ketones as well as PAHs that contain excess peripheral H atoms (H_n-PAHs). These kinds of compounds are all seen in carbon-rich meteorites and we believe that ice photochemistry is the source of these compounds. Furthermore, H_n-PAHs explain the infrared emssion at 3.4 microns seen towards energetic environments such as the Orion Bar (Bernstein, Sandford, & Allamandola, ApJ see below)

Since then we have shown that exposure of naphthalene to UV photolysis in water ice at low temperature forms 1,4- naphthoquinone (see Bernstein et al 2001 below). The production of naphtoquinones is of particular interest since this class of compounds includes important biomolecules such as the K vitamins which play a key role in electron transport in living systems, and the late Wynn-Williams proposed that naphthoquinones were the first chromophores employed by the first organisms to protect them from UV radiation back before there was an ozone layer.

Most recently we have been concentrating on adding other functional groups to the side of PAHs under interstellar ice conditions. The formation of these molecules in space has important implications for the search for life in the Solar System. Substitued aromatics have been invoked by McKay et al as potential biomarkers in a martian meteorite, but if such molecules can form non-biotically in ice, then they could be false biomarkers i.e. they could confuse things when we land a robot on the surface of Mars or Europa and start looking for chemical signs of life (the most likely manner in which life will be detected in the Solar System).

For more detailed information and reviews on our laboratory work on interstellar and cometary ice analogs, see:

Bernstein, Moore, Elsila, Sandford, Allamandola, and Zare, (2003). Side Group Addition to the Polycyclic Aromatic Hydrocarbon Coronene by Proton Irradiation in Cosmic Ice Analogs ApJLett., 582, L25-L29.

Bernstein, M. P., Elsila, J. E., Dworkin, J. P., Sandford, S. A., Allamandola, L. J. & Zare, R. N. (2002). Side Group Addition to The Polycyclic Aromatic Hydrocarbon Coroneneby Ultraviolet Photolysis in Cosmic Ice AnalogsAstrophys. J.576, 1115Ð1120.

Bernstein, M. P., Dworkin, J. P., Sandford, S. A., & Allamandola, L. J. (2001). Ultraviolet Irradiation of Naphthalene in H2O Ice: Implications for Meteorites and BiogenesisMeteoritics and Planetary Science, 36, 351-358.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Gillette, J. S., Clemett, S. J., & Zare, R. N. (1999). UV Irradiation of Polycyclic Aromatic Hydrocarbons in Ices: Production of Alcohols, Quinones, and Ethers. Science 283, 1135-1138.

Bernstein, M. P., Sandford, S. A., & Allamandola, L. J. (1996). Hydrogenated Polycyclic Aromatic Hydrocarbons (H_n-PAHs) and the 2940 and 2850 Wavenumber (3.40 and 3.51 Micron) Infrared Emission Features. Astrophys. J. 472, L127-L130.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Chang, S., & Scharberg, M. A. (1995). Organic Compounds Produced by Photolysis of Realistic Interstellar and Cometary Ice Analogs Containing Methanol. Astrophys. J. 454, 327-344.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., & Chang, S. (1994). Infrared Spectrum of Matrix-Isolated Hexamethylenetetramine in Ar and H2O at Cryogenic Temperatures. J. Phys. Chem. 98, 12206-12210.

Schutte, W. A., Allamandola, L. J., & Sandford, S. A. (1993). Organic Molecule Production in Cometary Nuclei and Interstellar Ices by Thermal Formaldehyde Reactions. Icarus 104, 118-137.

Schutte, W. A., Allamandola, L. J., & Sandford, S. A. (1993). Formaldehyde and Organic Molecule Production in Astrophysical Ices at Cryogenic Temperatures. Science 259, 1143-1145.

Allamandola, L. J., Sandford, S. A., & Valero, G. (1988). Photochemical and thermal evolution of interstellar/pre-cometary ice analogs. Icarus 76, 225-252.

Sandford, S. A. (1998). Organic Chemistry: From the Interstellar Medium to the Solar System. In ORIGINS, Astron. Soc. Pacific Conf. Series, Vol. 148, Proceedings of the International Conference, Estes Park, Colorado, 19-23 May, 1997, C. E. Woodward, J. M. Shull, & H. A. Thronson, Jr. (eds.), (ASP: San Francisco), pp. 392-414.

Sandford, S. A., Allamandola, L. J., & Bernstein, M. P. (1997). The Composition and Ultraviolet and Thermal Processing of Interstellar Ices. In From Star Dust to Planetesimals, Astron. Soc. Pac. Conf. Ser., Vol. 122, Y. J. Pendleton & A. G. G. M. Tielens (eds.), (ASP: San Francisco), pp. 201-213.

Bernstein, M. P., Allamandola, L. J., & Sandford, S. A. (1997). Complex Organics in Laboratory Simulations of Interstellar/Cometary Ices. In Complex Organics in Space, 31st COSPAR Scientific Assembly, July 1996, Birmingham, UK, Advances in Space Research 19, #7, 991-998.

Allamandola, L. J., Bernstein, M. P., & Sandford, S. A. (1997). Photochemical evolution of interstellar/precometary organic material. In Astronomical and Biochemical Origins and the Search for Life in the Universe, C.B. Cosmovici, S. Bowyer, & D. Werthimer (eds.), Proc. 5th International Conf. on Bioastronomy, IAU Coll. #161