Chemistry of Comets - C&EN Global Enterprise (ACS Publications)

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SPECIAL REPORT

Chemistry of |

Michael F. A'Hearn, University of Maryland

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Comet West 1975n, the most recent bright comet, was photographed on March 9,1976, by Henry Giclas at Lowell Observatory. Its ion tail is the straight, narrow one that appears to contain various "knots"; the dust tail is the much wider curving one. Most of the chemistry discussed in this article takes place in the coma, the more or less spherical region close to the nucleus, which appears in the lower right corner of the photograph. The nucleus itself is not visible 32

May 28, 1984 C&EN

From the earliest of times until the recent advent of extensive outdoor lighting, comets have drawn the attention of the public. A bright comet is undoubtedly second only to a total solar eclipse in presenting a spectacular and unusual variety in the otherwise very regular phenomena of the sky. Comets usually have been interpreted as portents of forthcoming catastrophes. In 69 AD, residents of Jerusalem observed a bright comet just before the destruction of their city by the Emperor Vespasian; a subsequent comet 10 years later heralded the death of Vespasian himself. As recently as the 1910 apparition of the famous Halley's comet, sheriffs deputies in Oklahoma stopped a group of people calling themselves Select Followers from sacrificing a virgin as a ritual for warding off potential disaster. Comets may, in fact, be true harbingers of disaster. On statistical grounds we expect that comets occasionally will collide with Earth. It has been suggested, although there is still much dispute, that a very small comet struck Siberia near the Tunguska River in 1908. Similarly, it has been suggested that the mass extinctions of dinosaurs and many other species at the Cretaceous-Tertiary boundary 65 million years ago, as well as a mass extinction in the late Eocene 34 million years ago, were caused by collision of a comet with Earth. Even without these impacts, comets are often spectacular objects. Although our present understanding of comets is far beyond that of 1910, we still find comets very mysterious objects. The imminent return of Halley's comet, which will again pass closest to the sun on Feb. 9,1986, should lead to quantum leaps in our understanding of comets. Four spacecraft (two Soviet, one European, and one Japanese) will intercept the comet and permit for the first time a reasonably complete set of in-situ measurements. We will learn a huge amount even from the more limited instrumentation on the third International Sun-Earth-Explorer spacecraft, which National Aeronautics & Space Administration has diverted to fly through comet Giacobini-Zinner in September 1985. One reason for our lack of knowledge about comets is common to all astronomical studies—astronomers usually must be totally passive observers rather than experimenters. The first astronomical experiment did not take place until World War II, when the development of radar made it possible to bounce.a signal off the

Comets surface of the moon. This was a primitive experiment, but the only possible one until the advent of the space program. The U.S. space program has allowed astrono­ mers and other scientists to carry out experiments on some of the planets, but the bulk of astronomy is still based on passive observation, albeit with increasingly sensitive and complex techniques. Comets are more difficult to study than most astro­ nomical objects because most of the bright comets arrive unpredictably, allowing very little time—typically only a few weeks—in which to plan observational programs. In fact, comet Halley is the only periodic (predictable) comet that is large enough and active enough to be seen regularly with the unaided eye. Despite the limitations on our means for studying comets, a moderate amount is known about them. The generally, although not universally, accepted paradigm was developed by Fred L. Whipple of the Smithsonian Astrophysical Observatory, Cambridge, Mass., in 1950. According to his dirty snowball model, a comet consists of a mixture of various ices (with frozen water probably predominating in most comets) and small particles of dirt, now known to include silicates. In a typical comet, this mixture is in a compact nucleus no more than a few kilometers in diameter. Some astronomers suggest that the nucleus also might contain an extensive core of more or less solid rock, but until recently little evidence existed either for or against this view. A few comets have completely disappeared, suggesting that they had no rocky core. On the other hand, comets Arend-Rigaux and Neujmin 1 recently have shown virtually no loss of gases. In 1977, ArendRigaux still did exhibit some outgassing, and observa­ tions of these comets both this year and in 1985 will search for gas. But the presence of a pointlike nuclear region with almost no gaseous emission suggests either a rocky core or a loosely bound agglomeration of small particles. The Infrared Astronomical Satellite (IRAS) recently discovered an asteroidal object known as 1983 TB that also appears to be a relic of solid material left from a comet that has lost its ice. This object has an orbit identical with those of the meteors in the Geminid me­ teor shower, and the association of most meteor showers with comets is well established. Therefore, reasonable evidence now exists that solid material remains long after a comet loses its ice, but its form—whether solid rock or loosely bound agglomer­

ation—is still not clear. Recent radio observations of comets, particularly of comet IRAS-Araki-Alcock 1983d (the fourth comet discovered in 1983), confirm the presence of extensive solid material and can be inter­ preted reasonably as showing, at least in part, reflection from a solid nucleus. Regardless of the exact nature of the nucleus, as a comet approaches the sun, its ices sublime. The resulting gases then expand into the vacuum of space while undergoing a variety of chemical processes; as they ex­ pand, they drag along the small particles of dirt, which with the gas form the coma or head of the comet. This dust generally is observed by means of reflected sunlight, and the gaseous species produce emission lines, ο

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This objective prism spectrogram of comet West was taken by mounting a 30° prism in front of a standard camera lens. The spectrum of the coma is the series of bright, elliptical regions extending across the figure about a quarter of the way up from the bottom. ( The many continuous spectra are those of stars in the field of view of the camera.) Each of the bright, elliptical spots is an image of the coma in the light of a particular emission line or band. The image at the far left (actually violet, although it appears blue in the photograph) is due to CN emitting at a wavelength of 388 nm. The weaker images to the right of it are attributable to C3 and another emission band of CN. The other bright blue image is due to Ci as are the green images. The yellow and red images are actually blends of several images due to Na, C^ NH2, and O. The spectrum of the tail can be seen as the very diffuse spectrum extending upwards and slightly to the right from the spectrum of the coma. The spectrum of the tail is superimposed on many stellar spectra and is caused primarily by sunlight reflected by dust grains, although a contribution from emission by ions is also present May 28, 1984 C&EN

33

Special Report Comets: clues to the birth of the solar system In addition to the natural desire to understand these spectacular, occasional visitors, there is a fundamental scientific reason for studying the composition of comets. Comets generally are thought to preserve a unique record of conditions in the early solar system, at the time of formation of the planets. Comets preserve this record much better than do the planets themselves. They are much smaller than the planets, so that internally generated processes are negligible. And they have spent most of their life far from the sun, making solar-induced processes also negligible. In the same year (1950) in which Fred L. Whipple of the Smithsonian Astrophysical Observatory first propounded the dirty snowball model of comets, Jan H. Oort of Leiden Observatory in the Netherlands took an earlier suggestion by Ernst J. Opik (who recently celebrated his 90th birthday at the Armagh Observatory in Northern Ireland) and proposed the existence of a large, spherical cloud (known as the Oort cloud) of comets surrounding the solar system and extending to 30,000 or

40,000 astronomical units. This distance is roughly 1000 times farther than the distance to Pluto, the outermost known planet, and is nearly half the distance to the sun's closest stellar neighbor, Alpha Centaur i. Most comets in the Oort cloud never come close enough to the sun or Earth to be seen, but occasionally a passing star will change their orbits just enough that a few comets will have new perihelion distances that make them observable. (Depending on its proximity, the passing star can trigger sudden, pronounced showers yielding hundreds of comets a year instead of the current 10 to 30 per year.) Such comets will have orbital periods on the order of a million years, but they do not stay in these orbits. Gravitational perturbations by the planets, primarily Jupiter and the outer planets, rapidly transform the orbit either into an orbit of shorter period or into a hyperbolic orbit in which the comet, after circling the sun, leaves the solar system, never to return. The periodic comets are thought to lose all their ice eventually because of the continual

primarily by fluorescence in the solar radiation field. The coma typically is observed to be 100,000 km in radius, but photographs in the ultraviolet light of the Lyman alpha line of hydrogen show that it extends even further. The density in the coma is quite low—of the order 10 12 molecules per cc near the surface of the nucleus and decreasing rapidly with distance. The dust particles in the coma frequently are small enough that the effect of solar radiation pressure is not negligible compared to the sun's gravity. These small particles, therefore, feel an effective gravity that is somewhat less than that felt by larger particles and by the nucleus itself. As a result, they take up a new orbit somewhat farther from the sun than that of the nucleus, forming the sometimes spectacular long, curved dust tails seen in all the classical pictures of comets. Neutral gas molecules in the coma also are affected by radiation pressure, primarily through absorption and re-emission in resonance lines, but the effect is usually much smaller than that for dust particles. Consequently, the neutral molecules do not form a long tail. Some species, however, become ionized and are driven back very rapidly into a tail by a poorly understood interaction with the solar wind and the magnetic field that it carries. Tails formed by ionized gases usually are much narrower than dust tails and, on the whole, they are straight rather than curved. Occasionally, though, they exhibit sharp kinks and a variety of fine detail. 34

May 28, 1984 C&EN

sublimation with each perihelion passage. Their lifetimes are estimated to be on the order of 1000 orbital periods. Comets cannot form today in the Oort cloud because the typical density of interstellar matter is much too low. Therefore, comets generally are assumed to have formed in the outer solar system, perhaps in the vicinity of Uranus and Neptune, at the same time those planets were forming. Shortly thereafter, they were ejected by gravitational effects to form the Oort cloud. Although the details of these processes are very controversial, the important thing is that most comets have been residing in the Oort cloud, far from any processes that might change their internal structure, for most of the 4.5 billion years since they and the rest of the solar system formed. Although some astronomers argue that the comets in the Oort cloud did not originate at the same time as the rest of the solar system, most accept that they did. This is the reason that a study of comets may be able to tell us much about the primordial cloud out of which the entire solar system formed.

Many gases in the coma have been identified spectroscopically, at ultraviolet, optical, infrared, and radio wavelengths. Most species, however, were first identified in the optical region. A few species that have been identified only at radio wavelengths are in dispute because of poor signal-to-noise ratios in the observations. A significant fraction of the species are free radicals, species presumably not present in the solid nucleus. Thus any statements about the composition of the nucleus must be indirect and based on arguments regarding the mechanism by which such species are produced. With the exception of sodium, metallic atoms are found only in comets that pass very close to the sun, presumably because these species are contained primarily in refractory grains that vaporize only when very close to the sun. Even sodium is rarely seen as far from the sun as 1 astronomical unit (AU, equal to the mean distance from Earth to the sun). Other species are seen to distances of several astronomical units from the sun, although the relative intensity of the spectral features often varies systematically with the distance. Generally, observers have interpreted their data on the basis of models that are considerably oversimplified. More elaborate theoretical modeling, however, has been carried out primarily by groups headed by Walter Huebner at Los Alamos National Laboratory and by Wesley Huntress at the Jet Propulsion Laboratory. Both groups have developed elaborate computer codes that

Cometary orbits Short-period comets

Long-period and new comets

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The two diagrams here show schematic orbits of comets in the solar sys­ tem. That on the left depicts orbits for two short-period comets: comet Encke, which has the smallest cometary orbit, and comet Halley, which has one of the largest orbits for a short-period comet. The scale is approximate but the orien­ tations are not correct. The system of

10,000 AU Dm 35 AU

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planets and short-period comets is nearly flat, and comet Halley is unusual; it is one of only three short-period comets, of the more than 100 known, which travel in a direction opposite that of all the planets. On the right, at a totally different scale, are shown typical comets in the Oort cloud, as well as a "new" comet

start with an assumed chemical composition for the vaporizing ices and then theoretically follow the com­ plete chemical evolution through thousands of reactions while allowing for the expansion and consequent cool­ ing of the gas in the coma. The first step in interpreting the observations, how­ ever, is to understand the emission mechanism, so that the brightness of spectral features can be related to the abundance of the relevant species. In most cases, the emission mechanism is either known or assumed to be fluorescence in the solar radiation field. For several species, this has been shown unambigu­ ously by the existence of the Swings effect, named for Pol Swings, then at Lick Observatory in California, who pointed out the effect in 1941. In general, cometary gas is so cold (about 200 Κ or less) that most species are in their ground electronic states. Electrons in a typical molecule can reach an upper electronic state only by absorption of sunlight in a few discrete transitions. When one of these transitions happens to coincide with a strong absorption line in the solar spectrum, the com­ etary emission lines originating from that upper level will be absent or weak in the observed spectrum. The Swings effect occurs as the radial velocity of the comet changes, causing Doppler shifts to move certain transitions on and off strong absorption features in the solar spectrum. As a result, certain emission lines in the cometary spectrum will appear and disappear strictly as

Typical comets in Oort cloud

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Encke

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making its first approach to the inner solar system. These orbits in the Oort 1 cloud are randomly oriented in all three dimensions, yielding a distribution that is nearly spherically symmetric. Comets in the Oort cloud are too far from the sun to have tails. Drawn to scale the entire planetary system would be contained within the symbol for the sun.

a function of the comet's radial velocity. These effects are widely observed. In fact, a number of groups, in­ cluding mine at the University of Maryland, have made detailed calculations of synthetic spectra for several species, most notably the radicals OH and CN, and have reproduced the entire emission spectrum reasonably well. In these cases of pure fluorescence, observations of the emission intensity can be related readily to the abundances of various species. Allowance must be made carefully, of course, for the Swings effect, since the total efficiency of fluorescent emission (that is, the relation between brightness and column density) can vary—by a factor of seven, for example, for OH. The emission mechanism cannot be fluorescence in at least one case: the forbidden lines of neutral oxygen, both the green line at 557.7 nm and the red doublet at 630.0 and 636.4 nm. Cometary densities are too low for collisions to excite these upper levels and the lines have been interpreted to be a result of formation of the oxygen atoms in excited states by the dissociation of parent molecules. The strength of the emission line in this case yields directly the rate of formation of oxygen atoms and hence the rate of dissociation of the parent. Probably none of the transitions observed at radio wavelengths are due directly to fluorescence either. In some cases, collisional excitation may play a role. In other cases, other special pumping mechanisms must be considered. For example, the observed transition of May 28, 1984 C&EN

35

Special Report A diverse array of chemical species has been observed on comets Species

Monoatomlc5 C H 0 S

Spectral regions where observed

OH Ultraviolet Ultraviolet, optical Ultraviolet, optical Ultraviolet

Diatomics

c2 CH CN CO CS NH

Species

Ultraviolet, optical Optical, radio Optical, infrared Ultraviolet, optical Ultraviolet Optical

s2 Polyatomics C3 CH3CN NH 2 HCN H20 NH3 HCO Silicates8

Spectral regions where observed

Ultraviolet, optical, radio Ultraviolet Optical Radio Optical Radio Radio Radio Optical Infrared

Species

Ions C+ Ca + CH+ CN + CO+ N2+ OH + C02+ H 2 0+ H2S+

Spectral regions where observed

Optical Optical Optical Optical Ultraviolet, optical Optical Optical Ultraviolet, optical Optical Optical

Spectral regions Species where observed

Metalsb Na Κ Ca V Mn Fe Co Mo Cu

Optical Optical Optical Optical Optical Optical Optical Optical Optical

Note: Ultraviolet observations are made from rockets or spacecraft, optical observations are ground-based, infrared observations are at wavelengths greater than 1 μνη. a Silicates are in the solid phase and their exact chemical structure is unknown, b Metals except sodium have been seen only in comets very close to the sun. Source: Adapted from Armand H. Delsemme, University of Toledo

water corresponds to a highly excited state and the pumping mechanism is not at all understood. The radio transition of the OH radical near 18 cm wavelength is also pumped nonthermally and, in fact, is the result of maser action. This transition, which ac­ tually consists of four components, results from transi­ tions between the Λ-doubled levels of the ground state. If collisions dominate these transitions, half of the OH radicals will be in each of the two Λ-doubled levels, but, in fact, the ultraviolet bands of OH (mentioned above as varying in intensity because of the Swings effect) can either invert or anti-invert these populations of radicals, depending on the comet's radial velocity. The comet then acts as either a maser or an antimaser (that is, it shows an absorption line) in the nearly isotropic radia­ tion field of the 3 Κ black-body left over from the socalled big bang at the birth of the universe. The mecha­ nism, however, is reasonably well understood so that abundances can be derived. For many well-observed species, considerable infor­ mation exists regarding their spatial distribution within the comet. To interpret their data, observers commonly assume that comets are spherically symmetric. Although many comets are observed to be highly asymmetric and there are reasons to expect asymmetry in most comets (vaporization occurs preferentially on the side facing the sun), some comets do exhibit a close approximation to circular symmetry when seen in projection on the sky. By assuming spherical symmetry, the observed distri­ bution of surface density can be converted into a volume density as a function of distance from the nucleus. For many species, the spatial distribution can be de­ scribed mathematically by the symmetric ejection of parent molecules from the nucleus followed by disso­ ciation of the parent into the observed species (plus other fragments). This, in turn, is followed by the disappear­ ance of the observed species, for example by ionization or further dissociation. The combination of a lifetime and an abundance for an observed species yields directly a production rate which, one hopes, can be related to the vaporization rate of one or more parent species. 36

May 28, 1984 C&EN

Two classic cases are those of CN and OH, with the presumed parents being HCN and H2O. In both cases, the distance from the nucleus at which the observed species is formed is consistent with estimates of the ex­ pansion velocity of the gas and with estimated rates of photodissociation for the parent. The distance at which the species disappears, in turn, is consistent with pho­ todestruction rates for the observed species. More elab­ orate calculations of the spatial distribution allowing for the redistribution of velocities of the fragments during photodissociation change the numerical details but do not affect the general argument. In the case of OH, which typically is produced nearly two orders of magnitude faster than any other radical, nearly all the daughter products of H 2 0 can be observed. Observations of the brightness of Lyman alpha and of the oxygen resonance lines are consistent with the total production rates of H and Ο that would be expected from H 2 0 , whereas the strength of the forbidden red lines of oxygen in a number of comets is consistent with the expected 10% dissociation of H 2 0 into H 2 plus O^D). Similarly, the abundance of H 2 0 + , although very poorly known, is consistent with ionization of a small to moderate fraction of H 2 0 . Only the H 2 itself is not seen, because of a lack of observable fluorescence bands. It therefore seems that H 2 0 dominates the vaporiza­ tion in most comets by about two orders of magnitude. This result is also consistent with the argument that H 2 0 ices control the vaporization of the nucleus. This argu­ ment is based on the observation that many comets brighten steeply (their vaporization rates increase sharply) as they come within about 2 AU of the sun. This is just the distance at which the nucleus should get warm enough to start rapid sublimation of H 2 0 ice. Similarly, the jet action of vaporization from only the sunward side of the nucleus produces a perturbation of the orbit which generally is negligible beyond about 2 AU. Other simple ices, such as those of NH 3 , CH 4 , C0 2 , and CO, should begin to sublime rapidly when far from the sun, a phenomenon observed in some new comets but

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Special Report

Photolytic reactions on comets involving water can be complex These two reaction paths show part of the chemistry involving water, which is thought to be the most abundant species vaporizing from the nuclei of comets. The lower left diagram shows the purely photolytic processes that involve more than 1 % of the reactants during the photolytic destruction of water. The energy thresholds differ for the various reactions, as do the cross sections. The branching ratios indicated take both these effects into account and assume that all the ultraviolet flux responsible for the dissociation is from the sun. All of the destruction products shown have been detected in comets with the ex­ ception of H 2 , H 2 + , and e". The relative amounts of H, 0( 1 D), 0( 3 P), OH, and H 2 0 + observed in comets are all more or less consistent with these branching

ratios, which are based on pure photo­ chemical reactions. The lower right diagram shows all of the chemical reactions expected to contribute in an important way to pro­ ducing or destroying H 2 0 + , just one of the breakdown products of water. It is a species, however, that might be ex­ pected to be well behaved because of the predominance of water in comets. Theoretically, production is expected to be dominated by pure photodissociation everywhere but where the densities are very high, say within 1000 km of the nucleus. If the C 0 2 abundance in the nucleus is as postulated, then reaction of H 2 0 with C 0 2 is nearly as important in producing H 2 0 + as the photoprocess. Similarly, the dominant reaction de­ stroying H 2 0 + is collision with a neutral

H 2 0 molecule where the densities are high but collision with an electron where the densities are low. Of the various species shown, C0 2 , H 2 CO + , H 3 0 + , H 2 , and e~ have never been directly observed in comets and H 2 CO has only recently and tentatively been detected in comet 1983d. Although all the other species have been ob­ served, relative abundances of CO + , C 0 2 + , H 2 0 + , and CO are very poorly known. The species whose abundances are reasonably well known, such as OH, H, and 0 , are produced primarily by processes other than the ones shown, so virtually no tests of this theoretical chemistry have been made. Data for these diagrams were taken from pub­ lished work by Walter Huebner's group at Los Alamos National Laboratory.

Important near nucleus

(H 2 O+co 2 \^>(^^) Dominates everywhere

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Dominates far from nucleus

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not in most comets or in any periodic comets. One thus might infer that cometary chemistry consists primarily of photodissociative processes, but this may not be the case even for these simple species. The proposed parent molecules, HCN and H 2 0 , can be observed only at radio wavelengths, and extensive searches for them have been conducted. Unfortunately, the relatively low emission rate at radio wavelengths, coupled with the sensitivity of existing radio receivers, makes detection of these, and nearly all other, cometary species very difficult. The close approach to Earth of comet IRAS-ArakiAlcock 1983d provided an unprecedented opportunity for radio observations. A group headed by Wilhelm I. Altenhoff of Max Planck Institute in Bonn, West Ger­ many, was able to detect both H 2 0 and HCN. Both 38

May 28, 1984 C&EN

species had been reported before, but these detections had been questioned. Because the emission mechanism of H 2 0 is not understood, rates for production of H 2 0 from the nucleus by vaporization could not be derived. For HCN, however, a straightforward interpretation of the results leads to a production rate by vaporization of about 10% that observed for CN, a value many standard deviations below the expected 100%. This, therefore, strongly suggests that CN is not produced directly from HCN and that the chemistry might be quite different from that previously assumed. We therefore have a batting average, at best, of 500 on two supposedly wellunderstood species with simple chemistry. Another likely case for a simple photochemical treatment is that of the observed NH 2 , which can be produced by the direct photodissociation of NH3, a

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Special Report

Abundance correlation provides evidence of conditions before the sun was born This graph shows the correlation of L.KJKi ^ 0 ^MNJ/ 5» euunu production rates for two species, OH and 27 # CN, from one comet to another. It is • Sr based on numerous measurements of abundances in various fields of view for many comets. All abundances have 26 been corrected for the limited field of • • view, converted to lifetimes, and cor­ rected to unit distance from the sun. These corrections lead to systematic 25 errors that may shift all points system­ ^· atically in one direction or another. If OH is destroyed primarily by photodisso­ Jί ciation, the lifetime is a function of the 24 27 28 29 30 radial velocity of the comet. This effect Log Q0[OH]/second has not been taken into account and therefore artificially increases the magnitude from one comet to another, scatter in the points. Each point repre­ only two comets exhibit a ratio deviating sents a different comet. from the straight line fit by more than a Although the absolute amount of gas factor of two. Similar results are found produced varies over three orders of for other species, including 0 , CS, C2,

: f*-

species found in the atmospheres of the outer planets and widely hypothesized to exist in comets as well. Re­ cent work by Armand H. Delsemme of the University of Toledo, Ohio, however, has shown that the spatial distribution of NH2 is not consistent with expected photodissociation rates for NH 3 . On the other hand, Altenhoffs group detected NH3 in comet 1983d in fairly high abundance, although again there is some question of whether the emission is fully understood. In any of the several estimates thus far, the production of NH3 is substantially less than that of H2O. But it is not yet clear whether it is comparable to that of NH2.

Chemical modeling For most other species, the production mechanisms are not yet understood. In general, it has been impossible to find suitable parent molecules that would photodissociate rapidly enough to produce the observed abun­ dances and spatial distributions of species such as C2 or C3. The source of these species can be determined only by chemical modeling using elaborate computer codes. As noted above, these calculations usually are done by assuming a chemical composition for the nuclear ices and calculating the vaporization rates, assuming that a single, simple species, such as H 2 0 or CO2, controls the vaporization in equilibrium with incident sunlight. The gases then are assumed to expand, which leads to rapid cooling, and the gas-phase reactions are followed. Current models include photodissociation, two- and three-body neutral-neutral reactions, ion-molecule re­ actions, dissociative recombinations, photodissociative ionizations, and even attenuation of the sunlight by the cometary material. One problem in these models, then, is to obtain reasonable values for all the reaction rates. 40

May 28, 1984 C&EN

and C 3 . This suggests that the cloud of gas and dust in which the comets formed was very homogeneous. The data are from both comets entering the inner solar system for the first time and also short-period comets that already must have lost hundreds of meters of ice on previous passages near the sun. Con­ sequently, the interior of the nucleus must have basically the same compo­ sition as the outer layers, except per­ haps for the outermost meter or two, which has not been observed. This means either that conditions in the nebula prior to formation of the sun did not change much with time or that all the comets formed quickly and simulta­ neously. Similarly, the portion of the protosolar nebula in which comets formed must have been spatially ho­ mogeneous.

Another is to sort out the important processes from the thousands of processes calculated when only a dozen or fewer species are observed quantitatively for a typical comet. One conclusion that seems to emerge from the calcu­ lations of the groups at Los Alamos and the Jet Propul­ sion Laboratory (as well as others) is that the cometary nucleus does not consist primarily of the simple mole­ cules expected from chemical equilibrium of the atomic species present. A variety of more complicated species—those typically observed in dark interstellar clouds—also must be present. This is not surprising, because the solar system formed from an interstellar cloud, but the detailed chemistry of such clouds is not much better understood than the chemistry of comets. For example, it turns out in these models that over much of the cometary coma the C2 is produced ulti­ mately from acetylene, C2H2, the final reactions being typically photodissociation of C2H and dissociative re­ combination of C2H2+. Some C2 is also produced by photodissociation of C3 which, in turn, is produced from other carbon-chain molecules. As another example, the CN is produced by a variety of pathways and in one group's models the important final steps are photodis­ sociation of CH3CN and HCN with lesser contributions from photodissociation of HNC and dissociative re­ combination of K^CN4". Ultimately, these chemical models should tightly constrain the conditions in the protosolar nebula, the cloud of gas and dust out of which the solar system formed. Although comets are notoriously difficult to observe, reliable spatial distributions can be derived for each observed species, something that cannot be done readily for interstellar clouds because of the unknown variation of abundances along the line of sight. Even

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Iodine Value % Transmission, 440 nm., min.

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The Enhanced IBM A computer system with higher levels computers in its price range. System 9000 family Left: The IBM 9001 Benchtop Computer with optional integral printer/plotter and diskette drive. Center: The IBM 9002 Desktop computer with optional diskette drive. Right: The IBM 9002 Desktop computer with processor in a separate location to minimize use of desk-top space. (Diskette dnve optional.)

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System 9000 Computer of function and performance than other ajor enhancements to the IBM System 9000 give it substantially more capability. A new optional operat­ M ing system and upgrading of the standard system, com­

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Upgraded standard CSOS operating system

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Multi-tasking capability has been extended from six to fif­ teen tasks. New operating system extensions include a full-screen editor, a new version of the assembler, a new link editor and numerous utilities. Improved FORTRAN and Pascal compilers produce more compact code that compiles and executes faster. IBM 3101 terminal emulation allows the IBM 9000 to be linked to a host computer or included in a communica­ tions network, providing better access to scientific libraries. A new scientific subroutine library includes 66 individual subroutines in statistics, eigensystems analysis, quadra­ ture/curve fitting and linear algebra.

Disk storage improvements The capacity of 5 W diskettes has been doubled to 640 Κ bytes and the IBM 9000 now provides up to 40 M bytes of hard disk storage (not shown).

New configuration options The IBM System 9000 is available in two models, the familiar 9001 Benchtop Computer and the new 9002 Desktop Computer. The 9002 provides features and functions which are more suited to a professional desk-top environment. For in­ stance, the 9002 processor requires substantially less desk area than the 9001 since the processor can be placed nearby. The 9002 Desktop Computer supports the IBM 5182 Color Printer. Other printers or plotters can be attached.

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Find out how friendly and versatile NMR can be The QE-300 spectrometer was specifically developed for high effi­ ciency, high-throughput carbon and hydrogen NMR It provides a complete range of routine and high resolution capabilities with a 300 MHz (7 Telsa) superconducting mag­ net, synthesizer based frequency control, quadrature phase detection, high-speed pulse programming, and a dual 13 €/Ή switchable probe. To make operation dramatically simple, the QE-300 also features auto-locking, auto-shimming, and auto-spectral phasing plus an incredible menu driven software package called CHARM. And now, to make the QE-300 even more versatile, we've added a number of accessories to make your work faster and more efficient than ever before.

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Special Report without a detailed knowledge of the production mechanisms, however, some conclusions about the protosolar nebula can be reached from the observations. Although only in a few comets have a large number of species been observed, a few species have been widely observed, primarily those with strong emissions in the optical region of the spectrum. Some general conclusions can be made regarding the number of species and the distance of the comet from the sun, both statistically for a large group of comets and, in a few cases, for a single comet observed over a range of distances. For example, comets far from the sun, say more than 2 AU, tend to exhibit less C2 than do those closer to the sun, say less than 1.5 AU. This perhaps is because, in part, production of C2 is more dependent on the density in the coma than is production of CN, which is caused largely by photo processes. Whatever the cause, the effect is seen even in a single comet as it changes its distance from the sun. This indicates that the effect is not connected with the chemical composition of the nucleus but rather with the actual method of production of the species. It is reasonable, therefore, to correct for these effects when trying to infer something about abundances in the nucleus. Although the data, after correction for systematic effects, suggest a great deal of homogeneity in the chemical composition of comets, other data suggest large variations in relative abundance of CO and CO2. In the first place, production of CO + is known to vary by orders of magnitude from comet to comet, some comets showing prominent ion tails and others almost none. This could be caused by differences in ionization mechanisms, however, rather than by differences in composition. Another piece of evidence is that some comets change in brightness very slowly with changes in distance from the sun, even when several astronomical units from the sun. This implies that vaporization of the nucleus is controlled by a substance more volatile than H 2 0 , with CO and CO2 being the most widely suggested candidates. Observations of the forbidden lines of atomic oxygen also suggest the presence of significant amounts of CO or CO2 in some, but not all, comets. In a few comets, the red doublet arising from the lD state is much stronger than would be expected from the observed production of OH and H. Furthermore, the green line arising from the *S state is prominent in some comets. Photodissociation of H2O leads, with 10% probability, to 0( X D), which produces the red doublet. 0( 1 S) can be produced only by photodissociation of CO, which is the source of both the green line and the red doublet. Estimates of the relative abundance of CO (or CO2) from this technique suggest production rates of up to 50% that of H2O in a few comets. Unfortunately, direct observation of CO is very difficult; it has a very small dipole moment, so that the oscillator strengths for all its transitions are very small, leading to very weak fluorescence. Strong ultraviolet bands of CO were observed in comet West 1975n, which also is a comet for which the CO production is estimated to be relatively high, based on the oxygen lines. Comets such as West seem to be relatively rare, so that relevant observations are quite sparse. Nevertheless, there are

these indications that one class of comets may have much more CO or CO2 than most other comets. This could result from a mixing, in the Oort cloud (a large spherical cloud of comets beyond the solar system), of comets derived from two distinct regions of origin, a very interesting possibility having significant implications for the origin of the solar system.

Nuclear chemistry Another interesting type of chemistry in comets is that which occurs in the solid ices. J. Mayo Greenberg of the University of Leiden, the Netherlands, has shown that ultraviolet photolysis of simple, equilibrium mixtures of ices, such as mixtures of H 2 Ô, CO2, and NH3, produces both highly volatile species and refractory polymers. More recent experiments by Bertram Donn and Maria Moore at Goddard Space Flight Center show that irradiation of similar simple mixtures of ices by protons leads to qualitatively similar results.

This photo ofHalley's comet was taken from Lowell Observatory approximately three weeks after perihelion, May 13,1910. The large circular image is a greatly overexposed image of the planet Venus; the long streaks in the lower right corner are the city lights of Flagstaff, Ariz., trailed out by the motion of the camera during the exposure. The sharp kink in the tail of the comet is due to inhomogeneities in the solar wind that was passing the comet at that time May 28, 1984 C&EN

45

Special Report I 1

In May 1983, comet IRAS-Araki-Alcock 1983d came within 3 million miles of Earth, closer than any comet in the past 200 years. The comet was first seen at the end of April as a moving infrared object by the Infrared Astronomical Satellite, but it was then discovered independency as a visible comet some 10 days later by amateur astronomers Genichi Araki in Japan and George ΑΙcock in England. Its closest approach to Earth occurred on May 12, and during several days near this date astronomers all over the world mobilized a variety of telescopes to study the region near its nucleus. Radar astronomers bounced radar signals off the comet, radio astronomers searched for elusive parent molecules near the nucleus, and all astronomers searched for hitherto unseen details near the nucleus, details that could be resolved only because of the comet's close approach. A radio astronomical group at Max Planck Institute in Bonn, West Germany, made the most convincing observations yet of molecular emissions from species other than OH. They observed, for the first time, an emission line of NH 3 . And they convincingly detected a line of H 2 0 that had been reported once previously but which had left many astronomers unconvinced. Cristiano Cosmovici, also

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47

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Special Report Abundance of hydrogen and carbon seems relatively low in comets Elements

Cosmic Abundance8

Abundance in comets'

H C Ν Ο S Si

26,600 11.70 2.31 18.40 0.50 1.00

24.30 3.73 1.51 22.30 0.55 1.00

a Normalized to Si = 1. Source: Adapted from Armand H. Delsemme, University of Toledo

of comet Halley. This comet orbits the sun with a period that varies (because of planetary perturbations) but which is currently about 76 years. It has been seen ap­ proaching the sun since October 1982. Although the four spacecraft being sent to probe it will encounter the comet at flyby speeds of nearly 70 km per second, they will be able to make unique measurements. The European Space Agency's Giotto spacecraft (named for the Italian painter who incorporated Halley's comet into his fresco of the Adoration of the Magi in Padua) and both Soviet Vega spacecraft (an acronym for Venera-Galleya, the Russian names of Venus and Halley, since these spacecraft will make measurements at Venus on the way to Halley) will carry a variety of mass spec­ trometers. Although a number of cameras and spec­ trometers will be able to determine spatial distributions of unprecedented resolution, it is the mass spectrometers that will detect those species that do not have sufficient emission to be detected from Earth. The limited instru­ mentation on the Japanese mission, that nation's first interplanetary spacecraft, will measure only the total hydrogen content and the magnetic field of the comet. The two Soviet spacecraft will encounter the comet about three days apart to study temporal changes. Although not firm, present plans call for Giotto to pass 500 km from the nucleus and Vega to pass about 10,000 km from it. To identify likely parent molecules, the neutral mass spectrometers will have ranges from 1 atomic mass unit up to 130 amu, with resolutions better than 1 amu. Combinations of the different spectrometers will allow separation of cometary gas from residual outgassing of the spacecraft and measure the masses of both the original molecules and of the total spectrum of atoms making up the molecules. These spectrometers should detect many hitherto undetected parent molecules, as well as map the radial profiles of CO and CO2 to indicate how these two species are related. An ion mass spectrometer on Giotto also will identify new species of ions, such as Η 3 θ + , which is widely predicted, as well as map the distribution of ions as an indication of their ionization mechanisms and their mode of interaction with the solar wind. In addition, dust mass spectrometers on all three spacecraft will de­ termine the composition of individual refractory parti­ cles from the comet. A variety of other instruments will provide a great deal of other information about the

comet; measurements by the mass spectrometers, how­ ever, are most relevant to its chemical composition. Since the spacecraft will provide cometary data at only four instants of time, it is critical to know, also, as much as possible, the state of the comet at those particular times relative to its average condition. Is it in the midst of an outburst? Is it at the peak of its vaporization curve? And, perhaps most important, are its chemical properties representative of other comets? To answer these ques­ tions, the International Halley Watch will organize a systematic program of observations from Earth and from Earth-orbital space. These observations will monitor changes over time in comet Halley in a way directly comparable to observations of other comets. This combination of spacecraft measurements, Earth-based observations, orbital observations, and theoretical interpretation should bring a better under­ standing of conditions in the early solar system. D Additional reading "Comets," edited by Laurel Wilkening (University of Arizona Press, Tucson, 1982), contains a series of invited review papers, each of which covers one aspect of cometary studies. "Introduction to Comets," by J. Brandt and R. Chapman (Cam­ bridge University Press, 1981), is relatively recent and up to date, although the chemistry of comets is treated only mini­ mally. "Cometary Exploration," edited by T. I. Gombosi (Hungarian Academy of Sciences, 1983), contains both invited reviews and contributed papers. It includes a large number of papers on the planned space missions to Halley.

Michael F. A'Hearn, 44, is professor of astronomy at the University of Maryland. He earned a B.S. in physics at Boston College in 1961 and a Ph.D. in astronomy at the University of Wisconsin in 1966 and joined the faculty at Maryland the same year. He has written extensively on cometary chemistry. At present, he also is serving as discipline specialist for pho­ tometry and polarimetry for the International Halley Watch observational program. A'Hearn's current research activities involve observational programs for determining chemical abundances on many comets and modeling fluorescence theory for comets. When not peering at the heavens, he enjoys sailing on Chesapeake Bay and along the East Coast. Reprints of this C&EN special report will be available at $3.00 per copy. For 10 or more copies, $1.75 per copy. Send requests to: Distribution, Room 210, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036. On orders of $20 or less, please send check or money order with request.

May 28, 1984 C&EN

49