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Astrochemistry Examples in the Classroom

Reggie L. Hudson Department of Chemistry, Eckerd College, St. Petersburg, FL 33711; [email protected]

For over 20 years I have taught undergraduate courses Table 1. Typical Characteristics in general and as well as in and of Cool Interstellar Nebulae . During that time I also have maintained an ac- Number Type of / Diameter/ Temp/ Pressure/ tive research collaboration with at NASA’s Goddard Density/ Nebula kg km K atm Space Flight Center and have been a member of both the cm᎑3 American Chemical Society and American Astronomical So- D0iffuse 1 32 1 x 1014 00500 805 x 1 ᎑19 ciety. These activities have placed me on the chemistry–as- Dense or tronomy interface and have given me insight and experience 1034 2 x 1014 50000 107 x 1 ᎑18 into how astronomical topics can be used in chemistry Molecular courses. In this article I will describe some of my own experi- their attendant planets, including those in our . ences with astrochemical examples in the classroom and of- The largest entity in the Milky Way, by volume, is the inter- fer suggestions for chemistry instructors. These suggestions stellar medium (ISM), the space between the . Far from are suitable for the first or second year in U.S. college chem- being empty, the ISM accounts for about 10% of the galaxy’s istry curricula, although some of them might fit into high- visible mass, mostly in the form of interstellar H at a school classes. A future article will cover applications and density of ∼1 H per cm3 (1, 2). examples for more-advanced chemistry students. The sugges- The ISM also contains a vast number of clouds, called tions made here are largely taken from my own research, and nebulae (singular is nebula). The physical and chemical con- all have been student-tested in chemistry classrooms. ditions within these clouds vary greatly, but their composi- tion is largely and (∼98%) and Some Background grains on the order of 0.1 µm in size. Although interstellar dust is known to have an amorphous component, Most of my students have studied chemistry in high grain chemistry and remain active areas of research school, but lack all but the most elementary knowledge of (3, 4). Characteristics of two types of interstellar nebulae of either astronomy or geology. These same students are largely interest to astrochemists (5) are given in Table 1. Note that from urban environments, and I suspect that that is true for entries in the last column follow from the . most students in the United States. Early in a general chem- Visual aids are useful in presenting this background ma- istry course I find it useful to say that while the Earth’s air terial to chemistry students, not to mention helpful in re- and are vital to our existence, the Earth’s total reser- moving the misconceptions that chemistry’s domain is only voir of water accounts for a trivial amount of our planet’s the microscopic and solely the terrestrial. A photograph of mass, and the height of our , compared to the the Andromeda Galaxy, which is thought to be similar to our Earth’s radius, is akin to the on an onion. At this point own Milky Way, is shown in Figure 1. The Andromeda Gal- I add that and chemistry are concerned with all of axy contains several hundred billion stars, is about 2.9 mil- and not just what we experience on the Earth’s sur- lion years (2.7 × 1019 km) away, and has a diameter of face. I also find it helpful to review some basic astronomical facts such as the following: • The is a . • All individual stars that can be seen with the unaided eye are part of a grand collection of stars called the Milky Way galaxy. • The Milky Way is one galaxy among billions. • The Milky Way’s shape is roughly that of a music or data CD with a diameter of about 1018 km. •To reach the Earth, a ray of light travels about 8 min- utes if it comes from the Sun, 4 years if coming from the next closest star, 50,000 years if coming from the center of the Milky Way, and about 2.9 million years if coming from the next nearest sizable galaxy. •For the overwhelming majority of its existence, over Figure 1. The unaided eye can see the Andromeda Galaxy on a 98% of a star’s mass is due to hydrogen and helium. clear, dark night but cannot resolve its component stars. This gal- axy is external to our own, as all other galaxies are, and has a All of this places the Earth in a context within the Milky diameter of about 170,000 light years (1.6 × 1018 km). (Courtesy Way, but what of our astronomical neighbors therein? The of Robert Gendler; http://www.robgendlerastropics.com, accessed Milky Way is home to about two hundred billion stars and June 2006).

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study, even in elementary courses, apply to a wide variety of environmental conditions. This brings us back to the title of this section. To astrochemists, natural products can form under conditions that are quite exotic by terrestrial standards. Nebulae scat- tered among the Milky Way galaxy can be found with densi- ties of only 1000 atoms and per cubic centimeter and on the order of 10 K. Chemical reactions under these conditions have to be powered by nonthermal sources, such as cosmic rays (mostly high-energy pro- tons) and UV light from stars. The products of such reac- tions include familiar molecules as well as species that are difficult or impossible to make in terrestrial laboratories, but they are all natural products to the astrochemist.

Interstellar Molecules

Figure 2. B68, a dark nebula within our own galaxy, The development of largely parallels the is about half a light year (5 × 1012 km) across. (Courtesy of the development of , and for good reason. Most as- European Southern Observatory; http://www.eso.org, accessed tronomical objects are so distant that only remote sensing is Jun 2006). feasible. Among the spectroscopic pioneers were Robert Bun- sen (1811–1899) and Gustav Kirchhoff (1824–1887), who are well known to most chemists, and William and Marie about 170,000 light years (1.6 × 1018 km). It is external to Huggins, of the late 19th century. They, their contemporar- the Milky Way galaxy. ies, and their successors demonstrated the utility of spectros- A much smaller and closer object, a dark nebula known copy for probing the chemistry and physics of extraterrestrial as B68 (Figure 2) is “only” about 500 light years (5 × 1015 objects (7). km) away and half a light year (5 × 1012 km) across. This The first century of astronomical spectroscopy was re- interstellar object is within the Milky Way. B68 looks dark stricted to observations with visible light, but other spectral because it contains cold (∼10 K) dust grains and gas, which regions became increasingly accessible and important in the block starlight behind the cloud from reaching us. Because 20th century. Observations have now been made with both the dust grains are cold, gas-phase atoms and molecules in Earth-based and orbiting and with light rang- the cloud can stick to them. Over time the grains acquire an ing from X-rays to radio wavelengths. icy coating of material that undergoes chemical reactions The molecules so far detected in interstellar space (8) from, for example, exposure to cosmic . If the reac- are summarized in Table 2. Students seeing this list for the tions are energetic enough, the product molecules can be first time are often surprised that supposedly empty space ejected back into the gas phase. Grain–grain collisions, sput- contains such an extensive and varied set of compounds. tering, photoejection, and shock waves from exploding stars Within this list are both organics and inorganics, ranging (supernovae) are other ways proposed to move material from from diatomics to long-chain unsaturated . When icy grain mantles to the surrounding gas phase (see, for ex- teaching introductory chemistry courses I have often found ample Bringa and Johnson (6) and references therein). that Table 2 can be used to raise students’ interest in such The remainder of this article focuses on chemistry class- tasks as nomenclature, assigning oxidation numbers, molecu- room examples, mainly taken from studies of the ISM. Since lar structure, Lewis dot , and chemical bonding. For the forming stars, and ultimately planets, comes from example, adding “These molecules have been discovered in the ISM, its chemistry can tell us much about the early Earth interstellar space” to a worksheet on Lewis structures can help and its environment. Remnants of the formation of our own motivate students while demonstrating the wide (literally!) solar system, and ancient ISM chemistry, are still with us in applicability of chemical concepts. Many examples of ͞ the form of cometary and meteoritic material. are found in Table 2, such as (CH3)2O CH3CH2OH, HCN͞HNC, and HCO+͞HOC+. The ability of one element What Is a Natural Product? to substitute for another, within the same family on the pe- ͞ ͞ riodic table, can be illustrated with H2O H2S, CH4 SiH4, ͞ ͞ The term “natural product” is often used by chemists to HNCO HNCS, and H2CO H2CS. Pairs such as ͞ ϭ ͞ ϭ identify an produced on the Earth with CH3CN NH2CN, H2C CH2 H2C NH, and ͞ minimal assistance from homo sapiens. Nature, however, goes CH3OH CH3NH2 can help demonstrate the concept of beyond 25 ЊC and 1 bar in her choice of environments. It is isoelectronicity. Table 2 also suggests some future interstellar important that chemistry students recognize this and be made molecular discoveries. Since SO2 is known, it is safe to as- aware that unconventional environments with extremes of sume that interstellar O3 might eventually be found, , radiation, pressure, salinity, and acidity, among having a much greater cosmic abundance than sulfur. Also, other factors, are all the subject of research by scientists. Fur- the existence of CH3CN and CH3NC, and HCN and HNC, ther, students should realize that the chemical concepts they suggests that other nitrile–isonitrile pairs await detection.

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Extraterrestrial –Base Chemistry are surprised that N2 or CO can serve as acceptors. As for what’s not in Table 3, although radio have + Discussions of acid–base chemistry in most introductory identified NH3, the corresponding conjugate acid, NH4 , has chemistry texts focus on aqueous solutions, but most of the not been found. This is likely due to the ’s known does not permit the existence of wa- lack of pure rotational transitions. The data in Tables 2 and ter. Nevertheless, acid–base concepts are still applicable in 3 suggest that other conjugate acid–base couples may await + extraterrestrial environments and cover molecules few stu- discovery. Aside from NH4 , one expects hardy anions such − − − dents would consider either acidic or alkaline. as CN , CH3COO , and OH to exist under the right con- − The most abundant interstellar is H2, as one ditions. An assignment of HCOO has been suggested for might expect from the high cosmic abundance of hydrogen. two solid-phase features in the spectra of several Cosmic radiation ionizes H2 in a dense nebula at a rate of protostellar sources (9). ᎑17 ᎑1 ᎑1 + about 10 molecule s , to give H2 . From there a proton- + transfer reaction produces H3 , the entire sequence being as Erlenmeyer’s Rule and Keto–Enol Tautomerism follows: Many of the molecules in Table 2 carry a story that can + − H2 H2 + e (1) be used to both capture student attention and illustrate a con- cept. Vinyl is one such molecule. As long ago as 1881, + + Emil Erlenmeyer (1825–1909) found that direct hydration H2 + H2 H + H3 (2) of gave an , as opposed to the enol tautomer Pathways similar to the above are thought responsible one might predict (10). This preference for the keto product for the formation of a variety of interstellar acid–base pairs. is still sometimes referred to as Erlenmeyer’s Rule. To take a The known interstellar conjugate and bases as of July specific example, the expected path for addition of H2O to 2006 (Table 3) were all detected spectroscopically. With the is exception of H O+, most of the conjugate acids in the table 3 H H H H probably are new to chemistry students. In using this table H2O fast HCC H CC H CC in my own lectures, I have found that even students who ~300 K (3) know the definition of a conjugate acid or conjugate base H OH H O texts often display such a reaction Table 2. Interstellar Molecules by Number of Atoms sequence, with comments about the instability of the expected No of enol compared to the observed aldehyde. The instability of Molecules Atoms , CH2CH(OH), relative to ,

2H2, CO, CSi, CP, CS, NO, NS, SO, HCl, NaCl, KCl, CH3C(O)H, is such that the alcohol’s preparation usually

AlCl, AlF, PN, SiN, SiO, SiS, NH, OH, C2, CN, HF, requires decomposition of a larger molecule (11–13) or syn- + + + FeO, LiH, CH, CH , CO , SO , SH, N2, O2 thesis from and a source of atomic oxygen, followed

3H2O, H2S, HCN, HNC, CO2, SO2, MgCN, MgNC, by trapping (14). Gas-phase IR (13) and microwave spectra NaCN, N2O, NH2, OCS, CH2, HCO, C3, C2H, C2O, (15) have been obtained for vinyl alcohol, while more com- C S, AlNC, HNO, SiCN, N H+, SiNC, c-SiC , HCO+, 2 2 2 plex variants, with somewhat greater stability, have been stud- HOC+, HCS+, H +, OCN− 3 ied with NMR and other methods (16). 4HN , H CO, H CS, C H , HNCO, HNCS, H O+, SiC , 3 2 2 2 2 3 3 Experiments in 1998 suggested that the Erlenmeyer’s rule C3S, H2CN, c-C3H, l-C3H, HCCN, CH3,C2CN, C3O, + + prediction of acetaldehyde from the addition of H O to acety- HCNH , HOCO 2 lene might be circumvented if the reaction took place at a 5HC 4, SiH4, CH2NH, NH2CN, CH2CO, HCOOH, + low temperature, such as in interstellar space (17). Subse- HCCCN, HCCNC, c-C3H2, l-C3H2, CH2CN, H2COH , C Si, C , HNCCC, C H quently, radio astronomers announced that vinyl alcohol had 4 5 4 been detected in a region of space near the center of our gal- 6HC 3OH, CH3SH, C2H4, H(CC)2H, CH3CN, CH3NC, + axy (18). Additional experiments soon showed that vinyl al- HC(O)NH2, HCCC(O)H, HC3NH , HC4N, C5N, C5H,

H2CCCC, c-C2H2C(O), H2CCNH 7HHC(O)C , c-C H O, CH CH(OH), CH CCH, CH NH , 3 2 4 2 3 3 2 Table 3. Gas-Phase Interstellar Acid–Base Pairs CH2CH(CN), H(CC)2CN, C6H Cdonjugate Base Conjugate Aci 8HC 3COOH, HC(O)OCH3, HOCH2C(O)H, CH3CCCN, + H2C6, H(CC)3H, C2H3C(O)H, C7H, H2CCCH(CN) H2 H3 + 9H(C 3)2O, CH3CH2OH, CH3CH2CN, CH3C4H, HC7N, N2 HN2 + + C8H, CH3C(O)NH2 COO HC /HOC + 1H0 (C 3)2CO, HOCH2CH2OH, C2H5C(O)H, CH3(CC)2CN CSS HC + 1C1 H 9N, CH3C6H CO2 HOCO + 1C2 6H6 H2OH3O + 1C3 H 11N HHCN HCN NOTE:fThe list is accurate as of July 1, 2006. See re 8 for methods of H CHO COH+ detection. Some molecules detected in circumstellar regions. c = cyclic 2 2 HC NCH NH+ and l = linear. 3 3

www.JCE.DivCHED.org • Vol. 83 No. 11 November 2006 • Journal of Chemical Education 1613 Chemistry for Everyone cohol can indeed form in interstellar space by the addition In 1999 the discovery of interstellar monomeric glycol- ϵ of H2O across the C C bond of acetylene and that the nec- aldehyde was reported (25). As of this writing, unambigu- essary energy can come from either UV or cosmic ous laboratory evidence for a feasible synthesis of this rays (19): molecule, under interstellar conditions, has not been pub- lished, but at least two possibilities suggest themselves. Since H H H H H2O slow the photochemical decomposition of yields form- HCC H ؉ CC H CC UV or p ~10 K (4) aldehyde as H OH H O H H Given the great difference in temperatures between cold, dark HCH HC nebulae (∼10 K) and typical lab-bench conditions, it is not (8) surprising that the tendency of vinyl alcohol to tautomerize OH O is altered in interstellar regions. then the decomposition of , a known extra- terrestrial molecule, should make as shown Interstellar Antifreeze below. Methyl alcohol was first identified as a gas-phase inter- H H H H stellar molecule in 1970 (20), and more recent observations HCC H HCC (9) indicate that it also exists frozen on the of cold in- OH OH OH O terstellar grains (21). The low temperatures of such grains prohibit thermally-initiated chemical reactions, and the opac- Alternatively, radiation- or photoinduced solid-phase - ity of the surrounding clouds hinders photo-initiated pro- ization of H2CO at interstellar temperatures could be a source cesses. However, as noted earlier, , such as of glycolaldehyde (26), perhaps roughly analogous to the cosmic rays, can penetrate such environments and initiate formose reaction for making sugars (27). chemistry on any icy material coating an interstellar grain. Chemists have long known (22) that room-temperature Carbonic Acid of liquid CH3OH yields ethylene glycol, HOCH2CH2OH, through formation and coupling of As a last astrochemical example, consider a molecule that • CH2OH radicals, but only recently have irradiations of solid- is not among the interstellar species of Table 2, namely car- phase methanol been done. Experiments in 2000 showed that bonic acid, H2CO3. Perhaps like some other readers of this cosmic-ray bombardment of frozen methanol will yield the Journal, I was first introduced to chemistry with a reaction glycol, suggesting that this molecule was a good candidate involving H2CO3, the combination of vinegar and baking for astronomical searches (9). Radio astronomers subsequently soda to make dioxide. Only later did I learn that the found ethylene glycol in interstellar space (23), as did astrono- CO2 could be explained by a double-replacement mers observing the gaseous products emanating from reaction followed by the decomposition of the unstable Hale-Bopp (24). H2CO3, as summarized in eqs 10 and 11: While the existence of extraterrestrial ethylene glycol is of inherent interest, its presumed synthetic path offers ex- CH3COOH + NaHCO3 H2CO3 + CH3COONa(10) amples of , proton transfer, and –radical re- actions for students. The sequence is given in eqs 5–7: H2CO3 H2O + CO2 (11)

+ − Many introductory chemistry texts include carbonic acid CH3OH CH3OH + e (5) in discussions of buffers and acid–base reactions in the blood- stream and in all cases mention the instability of H2CO3 rela- CH OH+ + CH OH CH OH + CH OH + (6) 3 3 2 3 2 tive to H2O and CO2. Although H2CO3 does exist in aqueous solution, with well-known Ka values, its low con- ᎑8 Њ 2 CH2OH HOCH2CH2OH (7) centration (< 10 M at 25 C and 1 bar) makes its direct study difficult. • Other routes to CH2OH are possible, such as the initial for- In contrast to its behavior under typical laboratory con- • mation of CH3O followed by . ditions, if H2CO3 is formed at the temperature of many ex- traterrestrial environments, its dissociation might be hindered. Carbohydrate Astrochemistry This was the reasoning behind the first isolation and IR spec- tra reported for carbonic acid (28). A 1:1 H2O:CO2 mixture An important lesson from these examples is that differ- was frozen at 20 K in a vacuum chamber and then bombarded ences in terrestrial and extraterrestrial conditions can change with 700 keV , from a Van de Graaff accelerator, to what one regards as a stable molecule. Glycolaldehyde pro- simulate exposure to cosmic rays. Infrared (IR) spectroscopy vides yet another illustration. As purchased from commer- was used to examine both the initial reaction mixture and cial suppliers, crystalline glycolaldehyde is actually a cyclic the final products. After proton irradiation, the sample had dimer, which can be heated to about 80 ЊC under vacuum an IR spectrum showing unreacted starting material, some to produce the , HC(O)CH2OH. Given the expected products (CO, O3, and CO3) and IR absorbances monomer’s empirical of C2(H2O)2, and its structure, from an unknown carrier. Subsequent warming from 20 K glycolaldehyde is sometimes considered an elementary sugar. to 250 K removed the starting materials, CO, O3, and CO3,

1614 Journal of Chemical Education • Vol. 83 No. 11 November 2006 • www.JCE.DivCHED.org Chemistry for Everyone and left only a distinct set of IR features that soon were as- signed to H2CO3. Warming H2CO3 above 250 K in the vacuum chamber resulted in the molecule’s rapid sublima- tion. To confirm the synthesis of H2CO3, isotopic substitu- tion (28, 29) and independent syntheses (30, 31) were used. Later work included ab initio calculations of the IR spectrum and potential energy of H2CO3 (32) and searches for H2CO3 in Martian spectra (33). Gerakines et al. (34) showed that H2CO3 could be destroyed photolytically and used this observation to measure the molecule’s intrinsic IR band strengths. More recently, both experimental (35) and - retical (36) work have demonstrated that the presence of H2O molecules is a critical factor in carbonic acid’s decomposi- tion into H2O and CO2. One of the first IR spectra of pure H CO (28), along Figure 3. An early infrared spectrum of solid H2CO3 at 250 K. 2 3 The lowest-energy structure is shown. with the molecule’s lowest-energy conformation (32), is shown in Figure 3. With the molecule drawn this way, stu- dents can see that it is merely a special type of carboxylic acid. Undergraduates who have been introduced to IR spec- troscopy may recognize a few features in Figure 3, such as are free and online at the Harvard ADS abstract service (42). the CϭO stretch near 1700 cm᎑1 and the network of OH Also, the NASA Cosmic Laboratory Web site (43) has stretches in the 3000 cm᎑1 region, probably broadened by links to some active astrochemistry research groups and to hydrogen bonding. It is worth telling students that if one original articles. melted the solid sample from which this IR spectrum was Finally, while the suggestions described here have mostly taken, the resulting pure liquid H2CO3 would have a con- been drawn from recent work on interstellar chemistry, the centration near 13 M, nine orders of magnitude above that astrochemistry of the solar system also provides many inter- ᎑8 of H2CO3 in natural rainwater, ∼10 M. esting case-histories for classroom use. For example, organic The H2CO3 story is valuable in itself since students find molecules, including amino acids, have been found in mete- it interesting that a three-element, six- molecule com- orites (44), H2SO4 clouds exist on Venus (45), H2O acts like • monly found in soft drinks has so far eluded most attempts dry ice on Mars (46), and the CH3 radical has been discov- at isolation and direct detection. Significantly, it has been ered in the of the planets Saturn (47) and Nep- shown that H2CO3 can be sublimed under vacuum and then tune (48). To borrow from Descartes, these case-histories have recondensed without dissociation (37). This gives cause to been “intentionally omitted so as to leave to others the plea- hope that gas-phase interstellar H2CO3 may one day be de- sure of discovery” (49). tected, adding this elusive species to the other interstellar molecules of Table 2. Along these lines, it is useful to remind Acknowledgments students that there are many exciting problems, such as the astronomical detection of H2CO3, awaiting the next genera- Colleagues at Eckerd College and NASA’s Goddard tion of scientists. Space Flight Center are thanked, as are the many students I have taught since 1978. Financial support has come from Other Examples and Material Eckerd College’s leave program, U.S. government IPAs, and these NASA programs: Space Research and For additional background material, the review in this Analysis, Planetary Atmospheres, and and Journal by Carbó and Ginebreda (38) is still useful as an in- . Recent support through the NASA Astrobi- troduction, although it lacks coverage of laboratory-related ology Institute is appreciated. work, newer molecular detections, and developments in theo- retical models. Other earlier articles in this Journal have Literature Cited treated somewhat more-specialized astronomy-related topics such as and subatomic (39), ab 1. Hartquist, Thomas W.; Williams, David A. The Chemically initio calculations (40), and a lab assignment for “advanced Controlled Cosmos; Cambridge University Press: Cambridge, physical chemistry” students (41). The present article has em- 1995. phasized astrochemical examples of molecular properties such 2. Wynn-Williams, Gareth. The Fullness of Space; Cambridge as structure and , two concerns of essentially all who University Press: Cambridge, 1992. teach and study beginning chemistry. 3. Nuth, Joseph A., III. Amer. Sci. 2001, 89, 228–235. Many of the citations in this article are to papers and 4. Evans, Aneurin. The Dusty Universe; Ellis Horwood: New York, books that are easily located, but some of the periodicals may 1993. be unfamiliar and unavailable to chemistry instructors. For- 5. Dyson, J. E.; Williams, D. A. Physics of the Interstellar Me- tunately, many of the best astronomical journals allow free dium; Halsted Press: New York, 1980; pp 166–173. online access to back issues older than about five years. 6. Bringa, Eduardo M.; Johnson, Robert E. Astrophys. J. 2004, Searches covering almost all of the astrochemical literature 603, 159–164.

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7. Hearnshaw, John B. The Analysis of Starlight: One Hundred 29. DelloRusso, N. K.; Khanna, Raj K.; Moore, M. H. J. Geophys. and Fifty Years of Astronomical Spectroscopy; Cambridge Uni- Res. 1993, 98, 5505–5510. versity Press: Cambridge, 1986. 30. Hage, Wolfgang; Hallbrucker, Andreas; Mayer, Erwin J. Chem. 8. Lovas, Frank J. J. Phys. Chem. Ref. Data 2004, 33, 177–355; Soc. Faraday Trans. 1995, 91, 2823–2826. see also references therein. 31. Brucato, John R.; Palumbo, Maria E.; Strazzulla, Giovanni 9. Hudson, Reggie L.; Moore, Marla H. Icarus 2000, 145, 661– Icarus 1997, 125, 135–144. 663. 32. Wight, Charles; Boldyrev, Alexander I. J. Phys. Chem. 1995, 10. Erlenmeyer, Emil. Chem. Berichte 1881, 14, 320–323. 99, 12125–12130. 11. Rodler, Martin; Blom, C. E.; Bauder, A. J. Am. Chem. Soc. 33. Strazzulla, Giovanni; Brucato, John R.; Cimino, G.; Palumbo, 1984, 106, 4029–4035. Maria E. Planet. Space Sci. 1996, 44, 1447–1450. 12. Kunttu, Henrik; Dahlqvist, Matrtti; Murto, Juhani; Rasanen, 34. Gerakines, Perry G.; Moore, Marla H.; Hudson, Reggie L. Markku J. Phys. Chem. 1988, 92, 1495–1502. Astron. Astrophys. 2000, 375, 793–800. 13. Joo, Duck-Lae; Merer, A. J.; Clouthier, Dennis J. J. Molec. 35. Al-Hosney, Hashim A.; Grassian, Vicki H. J. Amer. Chem. Soc. Spectros. 1999, 197, 68–75. 2004, 12, 8068–8069. 14. Hawkins, Michael; Andrews, Lester. J. Am. Chem. Soc. 1983, 36. Loerting, T.; Tautermann, C.; Kroemer, R. T.; Kohl, I. I.; 105, 2523–2530. Hallbrucker, A.; Mayer, E.; Liedl, K. R. Angew. Chem., Int. 15. Rodler, Martin; Bauder, A. J. Am. Chem. Soc. 1984, 106, Ed. Engl. 2000, 39, 891–894. 4025–2028. 37. Hage, Wolfgang; Liedl, Klaus R.; Hallbrucker, Andreas; Mayer, 16. Capon, B.; Guo, B. Z.; Kwok, F. C.; Siddhanta, A. K.; Zucco, Erwin. 1998, 279, 1332–1335. C. Acc. Chem. Res. 1988, 21, 135–145. 38. Carbó, R.; Ginebreda, A. J. Chem. Educ. 1985, 62, 832–836. 17. Moore, Marla, H.; Hudson, Reggie L. Icarus 1998, 135, 518– 39. Atwood, Charles H. J. Chem. Educ. 1990, 67, 73–735. 527. Huebner, Jay S.; Vergenz, Robert A.; Smith, Terry L. J. Chem. 18. Turner, Barry E.; Apponi, Aldo J. Astrophys. J. Lett. 2001, 561, Educ. 1996, 73, 1073–1076. L207–L210. 40. Arnau, A.; Tunon, I.; Silla, E.; Andres, J. M. J. Chem. Educ. 19. Hudson, Reggie L.; Moore, Marla, H. Astrophys. J. 2003, 586, 1990, 67, 905–906. L107–L110. 41. Sorkhabi, Osman; Jackson, William M.; Daizadeh, Iraj. J. 20. Ball, John A.; Gottlieb, Carl A.; Lilley, A. E.; Radford, H. E. Chem. Educ. 1998, 75, 1472–1476. Astrophys. J. Lett. 1970, 162, L203–L210. 42. NASA Astrophysics Data System Home Page. http:// 21. Pontoppidan, Klaus M.; Dartois, Emmanuel; van Dishoeck, adswww.harvard.edu/ (accessed Jun 2006). Ewine F.; Thi, W.-F.; d’Hendecourt, Louis. Astron. Astrophys. 43. NASA Cosmic Ice Laboratory Home Page. http://www- 2003, 404, L17–L20. 691.gsfc..gov/cosmic.ice.lab (accessed Jun 2006). 22. Meshitsuka, G.; Burton, Milton. Rad. Res. 1958, 8, 285–297. 44. Glavin, Daniel P.; Dworkin, Jason P.; Aubrey, Andrew; Botta, 23. Hollis, Jan M.; Lovas, Frank J.; Jewell, Philip R.; Coudert, L. Oliver; Doty, James H., III; Martins, Zita; Bada, Jeffrey L. H. Astrophys. J. Lett. 2002, 571, L59–L62. Meteroritics and Planetary Sci. 2006, 41, 889–902. 24. Crovisier, Jacques; Bockelée-Morvan, Dominique; Biver, 45. Cattermole, Peter. Venus, the Geological Story; Johns Hopkins Nicolas; Colom, Pierre; Despois, Didier; Lis, Darek C. Astron. University Press: Baltimore, MA, 1994; pp 38–39; see also ref- Astrophys. 2004, 418, L35–L38. erences therein. 25. Hollis, Jan M.; Lovas, Frank J.; Jewell, Philip R. Astrophys. J. 46. Hartmann, William K. A Traveler’s Guide to Mars; Workman: Lett. 2000, 540, L107–L110. New York, 2003; p 113. 26. Sodeau, John R.; Lee, Edward K. C. Chem. Phys. Lett. 1978, 47. Bézard, B.; Romani, P. N.; Feuchtgruber, H.; Encrenaz, T. 57, 71–74. Astrophys. J. 1999, 515, 868–872. 27. Mason, Stephen F. Chemical Evolution; Clarendon Press: Ox- 48. Bezard, B.; Feuchtgruber, H.; Moses, J. I.; Encrenaz, T. Astron. ford, 1991; pp 239–241. Astrophys. 1998, 334, L41–L44. 28. Moore, Marla, H.; Khanna, Raj K. Spectrochim. Acta, Part A 49. Descartes, René. La Géométrie; Dover Publishing: Mineola, NY, 1991, 47, 255–262. 1954; p 240; translated by David E. Smith and Marcia L. Latham.

This article is featured on the cover of this issue. See page 1571 of the table of contents for a description of the cover.

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