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The Photoproduction of Organic Residues in Laboratory Interstellar Ice Analogs
UV (Ultraviolet) radiation probably causes 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.
Many people believe that organic compounds from comets and asteriods helped to make life on Earth possible, so some of these compounds have potential implications for the origin of life on the Earth.
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. 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). A number of these compounds have potential implications for the origin of life on the Earth. For example, hexamethylenetetramine (HMT; C6H12N4) is a potential source of formaldehyde and ammonia and the acid hydrolysis of HMT leads to the production of amino acids. Follow these links to see how we identified HMT and how we think it formed.
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.
Polycyclic aromatic hydrocarbons (PAHs) represent one of the most abundant forms of carbon in the interstellar medium. 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 Science paper (see below). These experiments 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 (Hn-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, Hn-PAHs explain the infrared emssion at 3.4 microns seen towards energetic environments such as the Orion Bar (Bernstein, Sandford, & Allamandola, ApJ see below) The production of quinones 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.
For more detailed information and reviews on our laboratory work on interstellar and cometary ice analogs, see:
Bernstein, M. P., Sandford, S. A., & Allamandola, L. J. (1999). Molecules from Space and the Origin of Life. Scientific American, Cover story, July.
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 (Hn-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, Capri, 1-5 July 1996, (Editrice Compositori: Bologna), pp. 23-47.
Sandford, S. A. (1993). Interstellar Ices, Their Processing, and Their Relationship to Refractory Dust. In Astronomical Infrared Spectroscopy: Future Observational Directions, Astron. Soc. of the Pacific Conf. Series, Vol. 41, ed., S. Kwok, (Astron. Soc. Pacific: San Francisco), pp. 181-188.
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