CHEM291D Seminar: SJSU, October 15, 1998

Duncan Hall 250: 430 to 520 PM

NASA Ames Research Center, Moffett Field, CA

Comets have been in the science news lately, and it is no wonder that this was the case with the appearances of three spectacular comets, Shoemaker-Levy 9, Hyakutake, and Hale-Bopp all in the same decade. A tremendous amount of data and excitement came out of these apparitions and many missions (the first sample returns since Apollo!) to comets are planned for the near future. The current list includes:

But there is another very important reason why we should be interested in comets; they may have been crucial for the formation of life on Earth. Although the exact role of non-terrestrial materials in the development of life on this planet continues to be debated, there is a growing consensus in the science community that molecules from space must have, at least in part, made our planet the pleasant place that it is today. We believe that not only did comets bring the water and gases that became the oceans and atmosphere, but they probably brought complex organic molecules that were essential for the development and survival of the earliest life on this planet. I will report the results of laboratory simulations of the UV photo-chemistry of ices representative of interstellar grain mantles, and comets, including ices containing Polycyclic Aromatics (PAHs). For example, IR and GC-MS and NMR demonstrate that exposing simple, common, interstellar molecules to radiation at low temperatures results in the formation of some fairly complex molecules including hexamethylenetetramine (HMT, C6H12N4), formamide {HC(=O)NH2}, acetamide, CH3C(=O)NH2} and compounds related to polyoxymethylene {POM, (-CH2O-)n} (ref #1). IR and laser desorption mass spectrometry show that when PAHs are exposed to interstellar ice conditions results in both oxidation and reduction of the PAH edge carbon atoms, sometimes both in the same molecule. By far the most common functional groups formed are ketones (C=O) and aliphatic domains (-CH2-CH2-), although intramolecular ether links are also observed under certain circumstances. Furthermore, isotopic labeling experiments demonstrate that the aromatic C-H bonds exchange readily with hydrogens from the water ice matrix at low temperature under UV radiation. The products detected are consistent with chemical analyses of carbonaceous chondrites such as The Murchison Meteorite, and may provide some insight into some enigmas of molecular astrophysics including: The role of extraterrestrial organics in the origins of life, the mechanism by which cometary and meteoritic organics are deuterium enriched, the cause and spatial variation of the 3.4 micron emission from The Orion Bar and other UV intense sources (ref# 2), and the nature of the organic material that holds meteoritic noble gases. In addition, PAHs are ubiquitous terrestrial pollutants and often more carcinogenic when partially oxidized so understanding the photo-chemistry of PAHs may well be of interest to the environmental community.

ref# 1 "The Spectroscopy of Methanol-Containing Interstellar and Cometary IceAnalogs", Max P. Bernstein, Scott A. Sandford, Louis J. Allamandola, Sherwood, Chang and Maureen A. Scharberg, The Astrophysical Journal 454, 327 (1995).

ref# 2 "Hn-PAHs and the 2940-2850 cm-1 (3.4-3.51 mm) Infrared Emission Features", Max P. Bernstein, Scott A. Sandford, and Louis J. Allamandola, The Astrophysical Journal 472, L127 (1996).

Max Bernstein <mbernstein@mail.arc.nasa.gov>