Complex Organics in Laboratory Simulations of Interstellar/Cometary Ices
Max P. Bernstein, Louis J. Allamandola, and Scott A. Sandford.
This talk was presented at the COSPAR meeting in July 1996
ABSTRACT
We present the photochemical and thermal evolution of both non-polar and polar ices representative of interstellar and pre-cometary grains. Ultraviolet photolysis of the non-polar ices comprised of O2, N2, and CO produces CO2, N2O, O3, CO3, HCO, H2CO, and possibly NO and NO2. When polar ice analogs (comprised of H2O, CH3OH, CO, and NH3) are exposed to UV radiation, simple molecules are formed including:
H2, H2CO, CO2, CO, CH3, and HCO. (the formyl radical). Warming produces moderately complex species such as CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN and/or R-NC (nitriles and/or isonitriles). Several of these are already known to be in the interstellar medium, and their presence indicates the importance of grain processing. Infrared spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry demonstrate that after warming to room temperature what remains is an organic residue composed primarily of hexamethylenetetramine (HMT, C6H12N4) and other complex organics including the amides above and polyoxymethylene (POM) and its derivatives. The formation of these organic species from simple starting mixtures under conditions germane to astrochemistry may have important implications for the organic chemistry of interstellar ice grains, comets and the origins of life.
1. INTRODUCTION
Understanding the chemical and physical history of matter in the dense interstellar medium (ISM) is important since it is these materials that are the ultimate building blocks of new stars, planetary systems, and life. Our Solar System formed only some 4.55 billion years ago in a Universe perhaps 15 billion years of age, so it is clear that most interstellar materials have a long and complex history before ever becoming part of a solar system. Understanding the extraterrestrial evolution of chemical complexity is crucial to understanding both the early chemical state of our own solar system and the frequency with which similar or related conditions exist elsewhere in the universe. During the past two decades ground, air, and space-based infrared astronomy, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust. Most interstellar material is concentrated in large molecular clouds where simple molecules are formed by gas-grain and gas phase reactions. Gaseous species striking the cold (10 K) dust will stick, forming an icy grain mantle (Sandford and Allamandola 1993).
The composition of interstellar ices is revealed by obtaining the infrared spectra of stars situated in or behind clouds. As the radiation from these infrared sources passes through the cloud, the intervening molecules absorb at their characteristic vibrational frequencies. The interstellar spectra and the spectra of with relevant materials prepared in the laboratory can then be compared directly in order to identify ice composition and determine molecular abundances. This approach has provided evidence for the presence of both the polar and non-polar ices in dense clouds.Gas phase species readily condense onto grains in dense molecular clouds forming ice mantles (Sandford and Allamandola 1993). Thus, interstellar ices should contain many of the same species seen in the gas phase (see van Dishoeck et al. 1993 for a review of identified interstellar gas phase species).
Mantle compositions do not simply reflect gas phase abundances, however, since new compounds are formed when reactive gaseous species condense on the grain surfaces and the ices are energetically processed by UV radiation and cosmic rays (cf. Tielens and Hagen 1982, d'Hendecourt et al. 1985, Allamandola et al. 1988, Brown and Charnley 1990, Hasegawa et al. 1992, Bernstein et al. 1995, Moore et al. 1996). Since hydrogen is 3 to 4 of orders of magnitude more abundant than the next most abundant reactive heavier elements, grain surface chemistry is largely moderated by the local H/H2 ratio. In regions where this ratio is large, H atom addition (hydrogenation) dominates and species such as CH4, NH3, and H2O are expected to be prominent. If the H/H2 ratio is substantially less than one, however, reactive species such as O and N are free to interact with one another forming molecules such as CO, CO2, O2 and N2.
Thus, two qualitatively different types of ice mantle may be produced by grain surface reactions, one dominated by polar, reduced molecules and the other dominated by non-polar, or only slightly polar, oxidized molecules.Of course, the separation of molecules into nicely defined polar and non-polar ices is a gross over simplification. In reality, the composition and distribution of these two types of interstellar ices are likely to vary in a more graduated way and will depend on the history and current environment of the material. However, there is currently enough information to make some general conclusions about the relative distributions of these two classes of interstellar ice.
The evidence for the presence of ices dominated by non-polar components comes largely from the study of the CO (carbon monoxide) ice feature that falls near 2140 wavenumber (4.67 micron). The position, width, and profile of this feature are sensitive to the ice matrix in which the CO is frozen (Sandford et al. 1988). Many of the lines-of-sight that contain H2O ice also contain CO ice and the relative strengths of the H2O and CO bands indicate CO/H2O ratios ranging from 0.0 to as much as 0.3. Although a few of the CO bands have positions and profiles consistent with much of the CO frozen in H2O-rich matrices, most lines-of-sight exhibit profiles indicative of CO frozen in non-polar matrices, not H2O (Sandford et al. 1988, Tielens et al. 1991, Chiar et al. 1995). Our recent laboratory studies of the CO feature indicate that these non-polar ices are most likely dominated by O2, N2 and CO2 (Figure 1 from Elsila et al. 1997), the sorts of molecular species that one would expect where H/H2 < 1. Additional interstellar ice constituents consistent with this environment include CO2 and OCS or CO3.
Let us now consider polar ices. Figure 2 shows a comparison between the spectrum of W33A, a protostar deeply embedded within a molecular cloud (Capps et al. 1978) with the laboratory spectra of various interstellar ice analogs. Other than the strong silicate absorption near 1000 wavenumber (10 microns) (Willner et al. 1982), and the weak absorption near 2880 wavenumber (3.47 micron), from micro diamonds (Allamandola et al. 1992), all other absorptions are caused by ices. Comparison of interstellar and laboratory spectra like those shown in Figure 2 have led to the characterization of polar ices dominated by H2O and containing CO, CO2, and CH3OH (1-10% relative to H2O), NH3, H2CO, (1-4% ) and small amounts of CH4, H2, and more complex organic materials. A recent review on the composition of polar interstellar ices can be found in Sandford (1996).
The fact that H2O, a highly polar molecule, is the most abundant molecule in the ices along all lines-of-sight, but that the CO is mostly in non-polar ice provides clear evidence for the existence of multiple chemical environments along lines of sight where both are seen. Available data indicates that the polar ices are more common in lines of sight towards embedded protostars, while the non-polar ices tend to dominate towards field (background) stars (Elsila et al. 1997). This suggests that the non-polar ices are most abundant in the quiescent portions of dense clouds. The dearth of non-polar ices toward embedded protostars may be the result of ice sublimation in the warmer environments surrounding these objects, since ices dominated by N2, O2, CO2, and CO will sublime at much lower temperatures leaving behind H2O, CH3OH, and NH3 (Sandford and Allamandola 1993).
2.0 Lab Studies of Interstellar Ice Analogs
We expect UV radiation to impinge on interstellar ices and this will modify or destroy some species, but create others. Any molecules formed in this manner may return to the gas phase and take part in other chemistry or become incorporated into forming solar systems. In any event we need to know the affects of radiation if we are to understand astrochemistry.
2.1 Non-Polar Ices
UV photolysis experiments of CO in mixtures containing various amounts of N2, O2, and CO2 (Figure 3) indicate that most of this photochemistry is driven by hot O atoms liberated by photolysis of O2, or CO. The principle species produced from these O atom reactions are CO2, N2O, O3, and CO3. CO2 is detected in all cases, and N2O in most. Generally, as the concentration of CO decreases, N2O production is favored over CO2 production. Ozone appears as a photoproduct in all the mixtures, although the amount produced is highly variable. Relatively little O3 is produced in N2:CO and N2:H2O:CO ices while N2:O2:CO and N2:O2:CO2:CO ices show the strongest ozone production. Photolysis of O2-containing ices also produced carbon trioxide (CO3). While there is some tentative evidence for the production of minor amounts of NO2 and NO in these mixtures, the laboratory results indicate that few photoproducts containing a single nitrogen atom are formed, implying that N2 is relatively immune to photolytic destruction under these conditions. For example, while "XCN" is easily made in the photolysis of polar interstellar ice analogs, (see below) it is not made when N2-rich, non-polar ices are exposed to UV radiation. The UV irradiation of N2-CO mixtures where small amounts of H2O have been added, so the ice is non-polar in character, produce all of the same molecular species seen in the H2O-free ices. In addition, the liberation of H from the H2O produces the formyl radical, HCO., and formaldehyde, H2CO (Elsila et al. 1997). Since formaldehyde requires the addition of one more hydrogen atom than the formyl radical, it appears only when the H2O concentration is high enough to be comparable to the other, non-polar components in the ice. Several other hydrogenated forms of CO and CO2, such as HOCO, are also seen. The non-polar ices did not leave a noticeable organic residue at room temperature, in contrast to the polar ice analogs discussed below.
2.2 Lab Studies of Polar Ices
Given the very different compositions of the polar and non-polar ices, it is not surprising that they yield very different photoproducts. The ultraviolet irradiation of polar ices dominated by H2O and CH3OH has far richer photochemistry than that described above. Infrared spectroscopy demonstrates that when polar ice analogs comprised of H2O, CH3OH, CO and NH3 at 10 K (Figure 4a) are exposed to UV radiation this results in the destruction of methanol and the creation of several species including H2CO, CO, CO2, (Figure 4b) and, not visible in this figure, H2, CH4, and HCO. (the formyl radical), see Bernstein et. al 1995. Irradiated ices made from 13C and D labeled starting materials (not shown) confirm that none of the photoproducts are the result of contamination and all but the formyl radical have been identified in interstellar ices, which implies that radiation processing is responsible for their production.
With warming, the peaks corresponding to less stable species, such as H2CO and the methyl and formyl radicals, vanish as they react, and the bands of volatile compounds such as H2O, CO, CH4, and CO2 diminish as they evaporate. New peaks appear during warming as new bonds are formed by reactive species that become mobilized as the temperature increases.
For example, at 200 K moderately complex species such as CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), ketones, and nitriles including the "XCN" species (Figure 4c) are detected. Several of these are already known to be in the interstellar medium, and their presence indicates the importance of grain processing. (Many suggestions have been made as to the identity of "XCN" (Lacy et al. 1984, d'Hendecourt et al. 1986, Nuth & Moore 1988, and Grim and Greenberg 1987), but none have been embraced by the community) After warming to room temperature an organic residue remains on the window (Figure 4d). Infrared spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry demonstrate that this organic residue is a mixture of non-volatile organic compounds comprised of hexamethylenetetramine (HMT, C6H12N4, polyoxymethylene {POM, (-CH2O-)n} derivatives, amides,and ketones (Bernstein et al. 1996).
HMT formation depends on the presence of ammonia and formaldehyde which is, in these ices, a UV photoproduct of CH3OH. Ices that do not contain methanol produce very little HMT (Agarwal et al. 1985, d'Hendecourt et al. 1986). Without UV radiation HMT does not form, even in ices of ammonia and formaldehyde, POM is made instead (Schutte, Allamandola, & Sandford 1993a,b). Perhaps UV radiation stimulates formation of the imine of formaldehyde, the first intermediate on the path to HMT.
H2C=O + NH3 -----> H2C=NH (imine of formaldehyde) -----> HMT
If Equation 1 applies to interstellar ice then one would certainly expect to see the imine of formaldehyde, and indeed it has recently been observed (Irvine 1996). For descriptions of the UV photolytic evolution of a variety of interstellar ice analogs, see Hagen et al. (1979), Agarwal et al. (1985), d'Hendecourt et al. (1986), Allamandola et al. (1988), and Bernstein et al. (1995).
3.0 Implications
"XCN", the putative nitrile or isonitrile mentioned above and seen near 2165 wavenumber in Figures 4c and 5 is the strongest evidence of radiation processing within dense clouds (Lacy et al. 1984, Tegler et al. 1993). There is an indication that this feature is conspicuous in the spectra of embedded protostellar objects but not in that of background stars (Tegler et al. 1995). In laboratory experiments where the feature is made by the radiative processing of interstellar ices analogs, it is most prominent after warming (Bernstein et al. 1995) consistent with the notion that it is the heat from the protostar, acting on photo-processed ice that makes "XCN" obvious towards embedded objects. Although other nitriles are known to exist in the gas phase, (see Mann and Williams 1980; Irvine et al. 1987; van Dishoeck et al. 1993 for reviews) none have been measured in ice by infrared astronomy because telluric CO2 blocks the relevant wavelengths. This may be changing thanks to ISO, and recent laboratory investigations of nitriles under planetary (Russo and Khanna 1996) and interstellar (Bernstein et al. 1997) conditions are in press.
Our detection of CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), ketones {R-C(=O)-R'}, nitriles, HMT, and compounds related to POM in the photolysis residues of polar ices, as well as the identification of similar compounds in other ice analog experiments (Agarwal et al. 1985; Briggs et al. 1992; Schutte et al. 1993a,b) suggests that these species may be present in interstellar and cometary ices and thus have seeded the early Earth. In addition to the possible prebiotic activity of such molecules themselves, the products that derive from them should also be considered. For example, HMT photolysis produces nitriles and oxides of carbon, making it a good candidate for a common extended source of CN, H2CO, and CO in the tail of comets (Bernstein et al. 1994). In addition HMT hydrolysis under acidic conditions yields ammonia, formaldehyde and amino acids (Wolman et al. 1971).
Finally, these organic residues from photolysis of interstellar ice analogs show lipid-like behavior when they are dissolved rapidly in water or methanol (Allamandola et al. 1997). The result is the formation of insoluble droplets, some of which selectively trap photoluminescent material from the residue. This behavior is reminiscent of simple 'membranes.' All of these findings indicate that the organic material produced by the photolysis of ices in the interstellar medium may have had a profound effect on the formation of life in our solar system.
4. CONCLUSIONS
Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust. Interstellar ices consist of both non-polar and polar components. The non-polar ices are dominated by species like O2, N2, CO, and CO2 whereas polar ices contain H2O, CH3OH, CO, CO2, H2, and probably some NH3 and H2CO. Based on laboratory studies of realistic ice analogs, we suggest that ultraviolet photolysis of the non-polar ices produces CO2, N2O, O3, CO3, HCO, H2CO, and possibly NO and NO2.
Photolysis of the polar, H2O-rich ices yields H2, H2CO, CO2, CO, CH4, HCO, and warming reveals moderately complex organic molecules: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), "XCN" (nitriles), amides, ketones, HMT, and more complex molecules including polyoxymethylene and related species (POMs). The ultraviolet photolysis of HMT frozen in H2O ice produces the "XCN" band observed in the spectra of protostellar objects and laboratory ices, as well as carbon oxides and other nitriles.
The ready formation of complex organic species from simple starting mixtures, under conditions germane to interstellar and cometary ices suggests that these materials may well be common in forming solar systems. The observation that the more complex refractory photoproducts show lipid-like behavior and readily form micelles upon exposure to liquid water suggest that the processing of interstellar ices may have played an important role in the origin of life.
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