Georgia Tech | EPICS Lab

Non-Thermal Generation of Prebiotic and Complex Organic Molecules Within Interstellar and Planetary Ices

One of the most important problems in astrobiology and astrochemistry is the lack of information regarding the production mechanisms, absolute cross sections, and overall yields of complex prebiotic organic molecules in interstellar media and on outer solar system bodies. Prebiotic molecules include potentially life-generating and biologically essential components such as amino acids, sugars and nucleic-acid bases. The formation of complex molecules undoubtedly occurs, but the low temperatures typical of the space environment preclude rapid build-up by simple thermally driven reaction schemes. Therefore, it is likely that non-thermal processes, brought about by radiation and energetic particles, dominate the chemistry leading to the formation of prebiotic molecules. We propose to conduct experimental studies that address fundamental questions regarding radiation processes relevant to astrobiology and astrochemistry. Specifically, we will investigate the production of prebiotic complex organics on and within surfaces and interfaces (i.e. molecular ices containing intercalated or adsorbed molecules such as H₂O, CH₄, NH₃, N₂, and CH₃OH) using both low-energy (1-100 eV) electrons and vacuum ultraviolet (6.2 - 10.5 eV) photons. We will also investigate the radiolytic decomposition of the products of these reactions since this limits the build up of organic materials. The experimental results, along with existing mission data, will be used to help determine which prebiotic species are produced at levels detectable by spacecraft, such as the Spitzer Space Telescope, the Hubble Space Telescope, or ground-based observations. The ultimate aim of this research is to determine the conditions under which potentially life-generating and life-sustaining molecules may accumulate in the harsh radiation environment of space. This information is essential for understanding the origins of life and its distribution throughout the universe.

Ices are abundant in interstellar space, particularly in cold dark dense clouds. Made of frozen molecules, not strictly water ice, but usually containing a large fraction of it, ices occur along with CO₂, CO and others. These small molecules condense onto any available surface in the cold depths of space, frequently nucleating onto microscopic silicate or carbonaceous dust grains. Space is also filled with a wide spectrum of radiation due from highly energetic sources such as stars undergoing nuclear fusion to supernova explosions and ultramassive stellar objects collapsing, for example. Radiation, in the form of high energy photons, fast ions and charged particles impinges upon these ice surfaces and causes reactions that are otherwise thermally prohibited. Chemical bonds can be broken, ionic fragments and free radicals can be produced that react readily and without an activation barrier to produce very complex molecules. This has been observed definitively to be occurring in space and demonstrated many times in laboratories. Most interestingly, the molecules are not only far more complex than expected, they are so complex they actually resemble the same molecules that life on Earth utilizes. Biomolecules are among the most complex known and exhibit a high degree of functional specificity. There are compelling arguments supporting the hypothesis that primordial compounds fell from space and stimulated the genesis of life 3.7 billion years ago.

Important information about product types, distributions and yields are being measured, and add considerably to the understanding of observations made by Earth and Space based telescopes. Our aim, as chemical physicists, is to break down the process of the irradiation of ices into elementary events to understand the exact chemical mechanisms involved.

Dense nebula are abundant with complex molecules and dust. These regions are shielded from light due to their opacity, and consequently very cold. Molecules like water, carbon monoxide and ammonia are relatively abundant, and sticky, so they freeze out into ices onto soot-like or silicate based dust grains. The accumulation of grains is the first step in the birth of solar systems, and dense clouds are referred to as "stellar nurseries." One way to determine the composition of such nebula is to compare the spectra of nearby stars to spectra of stars partially eclipsed by the cloud. These spectra carry chemical information about the cloud.

The three primary constituents that make up life on Earth are amino acids, carbohydrates and DNA. Amino acids, simple sugars and simple versions of the molecules that form DNA and RNA bases have all been detected in interstellar space.

Amino acids are the elementary components that assemble into proteins. All life on Earth utilize proteins for structure, enzymatic action, movement and food. Elementary amino acids have been detected in gaseous molecular clouds, but their mechanism of formation is elusive. Gas phase mechanisms are not realistically adequate to explain the growth of molecules with more that half a dozen atoms.

Glycolaldehyde, Methyl formate, and Acetic Acid are a set of three isomers (molecules with same empirical formula) and are the first isomer triplet found in interstellar clouds. Methyl formate, discovered in 1975 is the most abundant of these isomers, acetic acid was discovered in 1997 and is the least abundant, while glycolaldehyde, the simplest sugar, was discovered in May 2000 at the NRAO 12 Meter telescope at Kitt Peak, Arizona. (See press release)

Glycoaldehye and 1,3-dihydroxyacetone are carbohydrates known to be precursors to more complex biologically active sugars. These have more recently been reported in dense clouds toward our galactic center.

Interstellar space is a hostile environment in terms of radiation. We on Earth are fortunate to be protected by both our thick atmosphere, ozone layer and the van Allen belt. While dense molecular clouds can be opaque to visible and ultraviolet light, high energy charged particles (cosmic rays) and extremely high energy gamma rays, can penetrate. Ultimately there is an equalizing factor, however, since high energy radiation primarily decays by ionization of materials. This produces lower energy secondary electrons and photons (Prasad-Tarafdar photons) that happen to fall in the range of energies relevant to breaking chemical bonds.

Water ice is a condensed molecular solid that possesses an unusually strong intermolecular force - the hydrogen bond. The water molecule itself is relatively fragile, in that it may dissociate into H, H₂, O and OH (and their respective ions) with irradiation. The strong intermolecular coupling of ice makes multicenter excitation possible. This can produce more complex species, such as O₂, hydrogen peroxide and other metastable radicals. This coupling also results in a strong dielectric screening ability, so ions created or embedded in the ice can be stabilized, analogous to solvation in the liquid phase. Electronic excitations in condensed media are termed excitons, which can be transmitted between adjacent bonded molecules and travel. Traveling excitons can migrate to interfaces, defects and pores where impurity molecules are likely to accumulate. This brings the energy source for complex chemistry between ice and organic molecules in icy grain mantles in cold clouds where there is not enough heat to drive reactions thermally. The precise nature of the electronic states involved in the excitation dictates the types of radicals and ions produced by ice, as well as the nature of the intermolecular reaction dynamics with organic impurities.

We are in the preliminary stages of measuring the product distributions of mixed ices as a function of radiation type, excitation energy, phase, porosity, temperature of ice and its binary and trinary mixtures. We are developing a coherent model to account for the observed variety of products of ice reactivity with impurities and their abundances. We have already developed a consistent model for pure ice and this is our reference for expanding the theory.

References

Weaver, Widicus, Butler, Drouin, Petkie, Dyl, De Lucia, Blake. Astrophysical Journal, Supplement Series. 2005. 158(2), 188-192.
Weaver, Widicus, Blake. Astrophysical Journal. 2005. 624(1, Pt. 2), L33-L36.