Questions? Contact us

Last Updated: 03/10/05

This is an unofficial version of the NASA version of the Astrochem lab site at http://web99.arc.nasa.gov/~astrochm/

The Production of Organic Molecules in Cometary and Interstellar Ice Analogs

Max P. Bernstein, Scott A. Sandford, Louis J. Allamandola, and Sherwood Chang

This is an excerpt from a talk Max Bernstein gave at the First International Conference on Habitable Zones here at NASA Ames, in 1994. For more information on interstellar (and thus cometary) ice click here.

Now that support is growing for the notion that comets may have brought organic materials crucial to the formation of life on earth, there is great interest in characterizing the chemicals of which comets are composed, and modeling the reactions that are expected to occur under astrophysical conditions. The identification of astrophysical organic molecules may profoundly affect our understanding of how life on this planet developed. The bulk of our knowledge about cometary (and interstellar) ices derives from the assignment of bands in astronomical spectra of distant objects, rather than direct measurement. So, laboratory simulations to determine what molecules may occur under conditions applicable to astronomical environments are essential for the identification organic species in comets and interstellar ice grains.

Interstellar ice grains reside in environments with such low temperatures (less than 40 K) that essentially anything hitting the grain, with the exception of He, sticks. As material accretes on the surface it is exposed to ambient ultra-violet (UV) radiation and cosmic rays. When this radiation impinges upon the ice, chemical bonds are broken, molecules are destroyed, and radicals and other molecular fragments are formed. A thermal event (such as the passage of a shock wave, grain collisions, or entry into a stellar system) then causes the ice to warm to the point at which molecular fragments begin to move, react, and form new molecular species. Thus the compounds found in interstellar and cometary ices are the products of radiation and heat. When the temperature exceeds ~200 K the ice sublimes away, leaving an organic residue. Until recently, the identity of the molecules that comprise these residues was a mystery.

The study of these organic residues has been the subject of a major research effort at NASA/Ames in the Astrochemistry Laboratory for several years. To synthesize an astronomical residue in the lab water, methanol, carbon monoxide and ammonia (simple materials known to comprise interstellar ices) have been frozen together, under vacuum, in proportions consistent with astronomical data, and exposed to UV radiation. These irradiated ices were then heated up to room temperature, and the resulting organic residue was then studied using infrared spectroscopy, nuclear magnetic resonance spectroscopy, and Gas Chromatography-Mass Spectrometry. Based on our lab simulations of interstellar and cometary ices we believe that Hexamethylenetetramine (HMT; C₆H₁₂N₄; an organic cage compound) may be one of the most abundant molecules in the organic crust on the surface of comets and ice grains.

The presence of significant amounts of HMT on the surface of comets and interstellar ice grains would be very significant for a number of reasons. Exposure of HMT to UV radiation produces cyanide (CN) compounds, including XCN, the mysterious interstellar cyanide compound that has been observed in dense molecular clouds in the direction of protostars. Exposure of cometary HMT to solar radiation could be a source of formaldehyde and cyanide seen in the comae of comets. Solutions of HMT in acidic water produce amino acids thus HMT may have been a source of amino acids delivered to the early earth by meteorites.

If you would like to get more technical please look at the talk I gave at the COSPAR meeting: click here and/or view the abstract of our paper on this subject in The Astrophysical Journal by clicking here