The Cycle of Matter in Our Galaxy: from Clouds to Comets

Irvine and Lunine: From Clouds to Comets 25

The Cycle of Matter in Our Galaxy: From Clouds to Comets

William M. Irvine University of Massachusetts

Jonathan I. Lunine University of Arizona

The processing of grains from original interstellar material to that contained in comets occurred under a complex range of conditions extending from the diverse environments within molecular clouds to the protoplanetary disk itself. Grain surface chemistry at very low temperatures gives way to grain growth, heating and partial sublimation, recondensation, and then agglomeration into cometary-sized bodies and larger. While the overall direction of the evolution can be sketched, little in the way of direct observations is available once collapse occurs, so that it remains difficult to describe specifically the temperature-pressure-composition histories of grains that will eventually find their way into comets.

1. INTRODUCTION (Dutrey et al., 2004), models of the formation and evolution of solids from the grains to cometesimals (Weidenschil- All cometary matter was once, of course, interstellar. But ling, 2004), and the coupled physics and chemistry within unresolved is whether comets still contain demonstrable and the disk (Lunine and Gautier, 2004). This progression rep- significant signatures of the interstellar material from which resents a temporal sequence, but it also represents a dra- they formed, either in terms of preserved or partially altered matic reduction in the spatial scales of the evolution — and molecular constituents or in the isotopic ratios found in consequently a steep increase in the difficulty of the con- cometary molecules. This fundamental issue has been dis- straining observations. We know a lot about the broad na- cussed, e.g., by Ehrenfreund et al. (2002, 2004), Charnley ture of chemistry and physical processes in the vast sweep et al. (2002), Irvine and Bergin (2000), Irvine et al. (2000), of the interstellar clouds; we know much less in a direct Bockelée-Morvan et al. (2000), Crovisier and Encrenaz sense about the details of the chemical and physical pro- (2000), and Fegley (1999). One can imagine two extreme cesses affecting grains on spatial scales of astronomical scenarios in this regard: first, that comets are conglomer- units in planet-forming disks. But the chemical and isoto- ates of essentially unprocessed interstellar grains, as pro- pic evidence from small bodies in our solar system, includ- posed by Greenberg (1982, 1998); second, that the forma- ing comets, provides a detailed local perspective provided tion of the solar nebula was a sufficiently energetic process, we can correctly interpret it. In briefly recapitulating this even in its outer portions, that presolar molecules were com- cycle of matter from molecular clouds to comets, we will pletely destroyed and that the infalling material was homog- emphasize some of the major outstanding gaps in our cur- enized (Lewis, 1972). In the latter case the chemical com- rent knowledge. position of dense interstellar clouds is not directly relevant to the chemistry of comets; in the former situation, exactly the 2. THE INTERSTELLAR MEDIUM, opposite is true. As is usually the case in science, the truth INCLUDING MOLECULAR CLOUDS probably lies somewhere between these two extreme views. To understand the extent to which cometary material has Cometary dust begins with the nucleosynthetic produc- been reprocessed from its initial interstellar state requires tion of heavy elements in stars, both during their main se- that we understand the coupled physical and chemical pro- quence and the sometimes-explosive terminal phases of cesses at work in the various environments leading from the evolution. Novae, supernovae, and the winds of asymptotic interstellar medium to the modern Kuiper belt and Oort giant branch (AGB) stars (the state arrived at by stars less cloud — the main dynamical reservoirs of comets today. than 10 M when all the H and He is exhausted in the stel- The chapters in this section of the book consider each of lar core) enrich the interstellar medium in new elements. these environments in turn, from molecular clouds (Wooden Because the interstellar medium is mixed, and matter is et al., 2004), to the formation and evolution of planet-form- cycled through multiple generations of stars, the elements ing circumstellar disks (Boss, 2004), the observational prop- and the stable isotopes are a mixture of the products of stars erties of disks and their interactions with the central star of a variety of masses and stellar generations (the latter de-

25 26 Comets II termined by the heavy element abundance at the time of group represent the third most abundant molecule after the star’s formation). molecular H and CO (Wooden et al., 2004). They are the Grains may form in a variety of environments, but the most plausible candidate species for producing the diffuse circumstellar envelopes of AGB stars are particularly im- interstellar bands, whose origin has long been a mystery portant for forming both silicate and carbonaceous grains (e.g., Herbig, 1995). However, no single PAH has defini- as the C/O ratio varies over time in the star’s envelope. Once tively been identified in the interstellar medium, and the formed, grains do not simply persist until their incorpora- precise mix of species and the (presumably environment- tion in comets; rather, they are cycled many times from gase- dependent) ratio of neutral to ionized molecules remain ous to condensed phases. Grains formed in the diffuse inter- unknown. Polycyclic aromatic hydrocarbons crop up in a stellar medium may be modified or evaporated by ultraviolet multitude of environments, from barbeque grills to martian photolysis powered by the intense radiation from newly meteorites to the interstellar medium; they should be present formed stars. Shocks generated by supernova explosions — as well in cometary matter. Because of their ubiquity and the end product of massive star formation — may destroy ease of formation, they are rather ambiguous signposts to grains at large distances from the original site of the explo- the details of C chemistry, and in the diffuse interstellar sion, with grains reforming both in the wakes of the shocks medium are only the most abundant of a menagerie of C and in the much denser environments of molecular clouds. forms such as chains, fullerenes, and (rarely) diamonds. A detailed characterization of the chemical nature and The cold, dense (n > 200 cm–3) clouds of the interstel- physical structure of grains has not been achieved. The na- lar medium are sites of star formation, most likely of low ture of the circumstellar and interstellar silicates, including mass stars, and are characterized by extremely low tempera- their composition and crystal structure, is complex and is tures around 10–20 K. External ultraviolet radiation is ex- relevant to the origin of cometary silicates (see Wooden et cluded, but penetrating cosmic rays provide a population al., 2004). For the more volatile elements, it seems unavoid- of ions and electrons that provide what little heating there able that the grains are the primary reservoir for C and a is. Under such conditions, very tiny (0.1-µm radius) grains major reservoir for O, and very likely that there is a continu- and hence extensive surface chemistry dominate, and it is ous size range from large carbonaceous molecules (PAHs; here that some of the very-low-temperature features of com- see below) to particulate grains (cf. Whittet, 2003; van Di- etary matter, such as the para-ortho ratio in water, may be shoeck and Blake, 1998). Given the difficulty of characteriz- locked in (Mumma, 1997). The dominant C form in the gas ing terrestrial kerogen, the lack of consensus on the nature is CO, and observations of nearby clouds such as TMC-1 of the organic component of interstellar grains is perhaps and the perhaps more typical L134N indicate a rich suite not surprising. of hydrocarbons, nitriles, and acids — foreshadowing a com- Mixing processes in the galaxy must be fairly efficient, plexity of C-based cometary chemistry that has yet to be for the vast majority of material in interplanetary dust par- fully explored (Wooden et al., 2004). Unlike the major res- ticles (IDPs) — perhaps the best samples of the non-ice ervoir for C in the grains, the bulk of the N in these clouds is phases of comets available — is isotopically of so-called probably in the gas phase as N2, although direct observa- “solar” composition. Although isotopic anomalies are dra- tional confirmation is limited. matic and informative of the details of solar system forma- Water is present in the dense clouds, but is frozen out on tion (Cameron, 2002), the material exhibiting these typically grains where it is the dominant ice component. This water represent only 1% of the material present (see Wooden et ice may be the principal O reservoir in these regions, al- al., 2004). though quantitative assessment is difficult (cf. Bergin et al., Chemistry in the interstellar medium is complex because 2000). Water ice forms predominately on more refractory of the enormous range of environments contained therein. grains through H-atom additions to O atoms, rather than In the diffuse interstellar medium (H density 1–100 cm–3, direct condensation in what is still an extremely tenuous temperature ~100 K), gas phase chemistry is dominated by medium. Mechanisms for returning molecules to the gas the abundant ultraviolet radiation pumped out by massive from cold (10 K) grains are highly uncertain, although a O and B stars and largely unimpeded over stellar distances. variety of means have been proposed. Thermal and cosmic- The size of the molecules formed is generally limited be- ray-powered sublimation and chemical cycling of water cause of the instability of long chains and other complex likely occur in portions of the cloud. Thermal processing structures against UV dissociation. There is a relatively high of the water aided by exothermic chemical reactions could abundance of atomic (ionized or neutral) C, N, S, H, etc. sublimate the ice, which then would reattach to grains by However, the molecular phase in the diffuse clouds is domi- adsorption. In any case the ice would be expected to be nated by polycyclic aromatic hydrocarbons (PAHs), which amorphous. What other types of chemistry might occur, and are relatively stable against UV dissociation and may be a the isotopic fractionation achievable (for example, in H, C, very important, if not dominant, agent in determining the N), depend on the details of very-low-temperature chemi- thermal balance of the interstellar medium. Astoundingly, cal processes on grains that are extremely challenging to they may contain up to 10% of the C in the galaxy (although replicate under normal laboratory conditions. Wooden et al. some estimates are an order of magnitude lower), and as a (2004) claim that clathrate formation has been inferred to Irvine and Lunine: From Clouds to Comets 27

occur in dense clouds, although the crystalline form — what The survival of interstellar grains during the infall is is normally called a clathrate — is likely kinetically inhib- assured only for those that fall into the outermost parts of ited at 20 K in favor of direct adsorption. The abundance the disk, where the shock is weak and infall velocities low. of ammonia (NH3) in these ice mantles is also somewhat Grain material that is sublimated may readsorb or condense uncertain. on the grain residues in the relatively cool and high-den- Low-mass and high-mass star formation lead to warmer sity gas of the disk midplane, but the resulting distribution clouds and “hot cores” respectively. The hot cores provide a of various molecular species in the recondensation may wealth of information on condensed species in colder en- differ from that of the original grain. Inward, progressively vironments because such species are sublimated at the high more refractory species are sublimated during infall; in the hot-core temperatures, while still protected against the free- inner regions of the disk it is likely that all but the most radical chemistry abundant in high-ultraviolet environments. refractory material is returned to the gas phase. In the Orion hot core, for example, H2O, H3N, H2S, CH3OH, Given that interstellar (as opposed to circumstellar) sili- and HCN are all more abundant than in cold clouds. To- cates appear to be amorphous, the nature of cometary sili- gether with CO and HCHO, this mix of species is “familiar” cates, which include both crystalline and amorphous ma- to us from the cometary point of view — the hot-core sub- terial, presents a challenge. The prevailing view is that the limated mix contains the species seen in cometary comae, amorphous component probably represents surviving inter- if not quite in the same relative abundances. Regardless of stellar material, presumably similar to the GEMS (glass whatever other processing occurs in the protoplanetary disk, imbedded with metal and sulfides) found in IDPs, while the the existence of some familiar species emphasizes the link crystalline silicates are probably produced or at least an- between cometary and interstellar (molecular cloud) grains nealed in the solar nebula. The issue is discussed in detail (cf. Irvine and Bergin, 2000; Tielens, 2001). This link is in Ehrenfreund et al. (2004). even more evident in the large isotopic fractionation mea- As the protostar grows, ignites fusion reactions, and sured for H in cometary H2O and HCN, which reflects that develops a strong bipolar outflow roughly perpendicular to seen in molecular species in interstellar clouds, although not the disk, other chemistry begins to affect the grains. Ultra- always to the same extent (see Ehrenfreund et al., 2004; violet irradiation again becomes an issue for gas and grains Irvine et al., 2000). While the environments of low-mass that end up at large distances from the disk center, where star formation typically do not exhibit the same relative they may be wafted upward periodically to the surface of abundances, this is surely because temperatures are not high the flared disk and hence be exposed to the protostellar enough to sublimate the material except in the observa- emission by virtue of their altitude above the midplane. The tionally inaccessible, optically thick, inner portions of the bipolar flow encounters the surrounding molecular cloud collapsing clumps. Some recent high-spatial-resolution ob- in the form of a wind-cloud shock (Wooden et al., 2004), servations do reveal hot-core-type molecules in the hot gas where additional heating and energetic chemistry may occur. close to the low-mass protostar IRAS 16293-2422 (Cazaux There are, however, major uncertainties concerning the et al., 2003). physical nature of the disk. As Boss (2004) points out, the detailed thermal, density, and turbulent structure cannot yet 3. FROM COLLAPSING CORES TO DISKS be predicted, and there is a serious lack of observational data (Dutrey et al., 2004), reflecting the need for higher- The loss of magnetic support as cloud clumps become angular-resolution studies that will only become possible denser and the neutral molecules “slip through” the much- with future facilities such as the Atacama Large Millimeter smaller ion population is thought to initiate the collapse Array (ALMA) telescope and the James Webb Space Tele- toward formation of a low-mass protostar and, depending scope (JWST). There is, for example, no agreement on the on the clump’s angular momentum, a protoplanetary disk mechanisms for transfer of mass and angular momentum [outflows from massive stars may also trigger clump col- in the disk, a problem related to uncertainties in the char- lapse; see Boss (2004) for a comparison of the two pro- acter of the “viscosity” that is assumed to exist and is mod- cesses]. Once initiated, the collapse leads to elevated tem- eled through the parameter α (hence, the so-called α-disk peratures in the center eventually sufficient to trigger ther- models). Moreover, the process of giant planet formation monuclear fusion, and the protostar feeds on the gaseous is still controversial. All this is relevant to comets, since it and solid material falling inward over a period on the or- describes the environment in which volatile species con- der of 1 million years. If a disk forms (Boss, 2004), then dense/sublimate, material is transferred radially, etc. the interaction of the disk with the surrounding collapsing Furthermore, an even greater uncertainty exists as to cloud is complex. Infalling grains, subjected to increasing whether low-mass stars like the Sun might in fact form in density, grow in size well past that of the tiny interstellar clusters where massive stars form, such as the Orion mo- material. However, heating of the grains by direct exposure lecular cloud, which would be suffused with ultraviolet to accretion shocks and drag (Lunine et al., 1991; Chick and radiation from surrounding young protostars, shocks, and Cassen, 1997) occurs, and can modify the grain composi- infusion of material from supernovae (Boss, 2004), lead- tion or destroy it entirely by sublimation. ing to a much more active chemistry than in the more tra- 28 Comets II ditional picture of relatively isolated star formation. Even trapped volatiles for comets: water ice that survived infall if they formed in a less-energetic environment that might into the disk and water ice formed in the disk by conden- characterize more isolated low-mass star formation, Dutrey sation. The mechanism of volatile trapping and the total et al. (2004) point out that many T Tauri stars are mem- volatile content are likely to be very different for the two bers of binary or multiple systems, complicating the tradi- sources, but both may be relevant since the source region tional approach to the physics of the disks. Although the for some of the Oort cloud comets could be the Jupiter- physics of some of these processes has been modeled, little Saturn feeding zone, which was not far from the snowline. attention has been paid to the connection to the resulting The Galileo probe measurements of supersolar abun- grain chemistry and isotopic reequilibration at various dis- dances in the jovian atmosphere do not rule out a prima- tances from the protostar. Furthermore, direct observation rily solar nebula condensed (“native”) vs. interstellar (Owen of the material in the collapsing core is exceedingly difficult et al., 1999) origin for the water ice that carried these as spatial scales shrink from parsecs to tens or hundreds of volatiles into growing Jupiter, because we do not yet know astronomical units and densities rise by many orders of the O (hence H2O) abundance of the bulk planet (Gautier magnitude relative to the average (103–105 cm–3) in the mo- et al., 2001). It is highly likely that these two kinds of water lecular cloud itself. ice both existed in the protoplanetary disk, because it is hard to avoid sublimation and recondensation of some of the 4. EVOLUTION OF GRAINS WITHIN THE water, but equally difficult to argue that all the infalling PROTOPLANETARY DISK grains were heated sufficiently to completely sublimate the water ice phases. Cometary tests of just how much native Assuming that the protoplanetary disk is not destroyed vs. interstellar water ice is present must rely on the isoto- on very short timescales by the external molecular cloud pic composition of the water ice and trapped volatiles, and environment, gas phase processes drive chemistry in the perhaps the ortho-para ratio of the H in the water. Unfor- disk over timescales on the order of 1 million years or more, tunately, definitive predictions of these properties tied to based on astronomical observation of gas in disks (Dutrey the disk evolution are difficult to make. et al., 2004). Gravitational and magnetic torques presum- Within protoplanetary disks, the chemistry and proper- ably launch waves in the gas and dust, compressing and ties of grains will be altered in the immediate environment heating material and perhaps in some disks forming giant of formation of giant planets. Depending upon the details gaseous planets that may or may not be analogs of Jupiter of such planet formation, the balance of oxidized vs. re- and Saturn. These torques could be strong enough to melt duced C and N species will be altered in the gas very close solids in certain parts of the disk, one explanation offered to the giant planets (Mousis et al., 2002; Irvine et al., 2000). for the chondrules seen in meteorites; strong gas dynamical More reduced species may find their way into solids form- heating of material falling into the inner portion of the disk ing near the planets, these solids then are transported out- could also lead to such melting. Turbulent viscosity within ward through the main disk, or grains may be destroyed the disk heats the midplane even in the absence of stellar entirely by catastrophically rapid giant planet formation radiation blocked by the optically thick conditions. Excur- (Mayer et al., 2002). In our own solar system, the latter sions in the accretion rate may lead to cooling and then seems unlikely, because abundant solid material remained to sudden heating events akin to the observed “FU Orionis” populate the Kuiper belt and the Oort cloud. Nonetheless, phenomenon. The star itself, in blowing a bipolar flow ver- the gaps that giant planets open during their formation re- tically, also clears out the innermost part of the disk, sub- duce the gas density and perturb the orbits of larger solid jecting adjacent material to extreme heating. bodies, while inward and outward of the gap higher densi- The strong radial temperature gradient imposed on the ties might have perturbed the volatile content of the icy protoplanetary disk by turbulent dissipation ensures a ra- planetesimals. dial gradation in the condensation and stabilization in grains The growth of comet-sized planetesimals, their dynami- of species of differing volatility. Hence the most refractory cal histories as they were ejected from the region of giant silicates (e.g., corundum) condense closest to the protostar, planet formation and portions of the Kuiper belt, and the followed by the less-refractory Mg silicates and Fe, then a resulting compositional distinctions between comets resid- complex series of S compounds, followed by water ice at ing in different dynamical reservoirs were rather complex several astronomical units or beyond. The condensation (Weidenschilling, 2004). This evolution occurred as the disk front of water ice — sometimes called the snowline — is transitioned from gas-rich to gas-poor, a process that was crucial because it represents a steep outward increase in the complex and may differ in terms of timescale from one surface density of solids in the disk, with potential impli- protoplanetary disk to another. The growth from centime- cation for the formation timescale of giant planets if these ter- to kilometer-sized bodies is particularly poorly under- were seeded by initial formation of solid cores. But from stood, as it is unclear theoretically in which circumstances the point of view of cometary grains and their composition, collisions among the objects will lead to a net gain in the the condensation of water ice adds complexity to the his- mass of the larger partner (Weidenschilling, 2004). The tory. We are forced to consider two sources of water ice and vertical as well as the radial structure of the disk may be Irvine and Lunine: From Clouds to Comets 29 important in particle growth, but no three-dimensional simu- this time, comets existed much as they exist today — in lations have yet been carried out. different dynamical reservoirs, some perturbed inward to- During the gas-rich phase, interactions of comet-sized ward the Sun to sublimate, possibly break up, and generate small bodies with each other and with the giant planets was wonder in the skies of Earth for humans who would arise strongly affected by the presence of the gas, both from a 4.55 billion years after the solar system’s formation. dynamical point of view and with respect to the incorporation of additional volatiles in the water ice of the small bodies 6. EXPULSION OF COMETARY MATTER (Lunine and Gautier, 2004). As the gas dissipated, additional AT THE END OF THE SUN’S volatile incorporation ceased and the dynamical interactions MAIN-SEQUENCE EVOLUTION of the cometary bodies and the planets simplified. The Sun is approximately halfway through its main-se- 5. COMETS IN THE REMNANT DISK quence life, and at the end of this time will expand to become a red giant, with an outer envelope at the orbit of the The disk remaining after much of the gas is removed — Earth. The initiation and then termination of He fusion in a process that might take 107 yr or more, although the mas- the red giant’s core will trigger a second collapse and expan- sive gas phase is likely to be a factor of 10 shorter — con- sion, this time to a red supergiant with an outer envelope sists of solids ranging from dust to giant planets. Planet at the orbit of Mars. A subsequent series of “flashes” and formation is not complete, however, because the formation thermal pulses will eject half the mass of the Sun to space, of the terrestrial planets in our system evidently took sev- leaving behind a white dwarf and stripping the atmosphere eral tens of millions of years, based on isotopic evidence of Saturn’s moon Titan (Lorenz et al., 1997). The luminos- (Yin et al., 2002; Kleine et al., 2002) and dynamical simula- ity excursions during this time will sublimate the ices from tions (Chambers and Wetherill, 1998). The process during Kuiper belt comets, and the loss of half the Sun will free this time was the pumping up of the eccentricities of an much of the Oort cloud from the gravitational bonds of the initially quiescent disk, inward of Jupiter, dominated in mass solar system — an echo of the postformation ejection of by bodies from Mercury- to Mars-sized. As the orbits of cometary bodies from the region of the giant planets. Com- these perturbed “planets” crossed, collisions and net growth etary material, in the gaseous and solid phases, will travel occurred. Meanwhile the giant planets were perturbing the freely through the interstellar medium, and the cycling of orbits of more distant icy bodies — the massive host of what matter that 10 billion years before became the Sun and solar we would today call comets — leading both to outward system will have been closed. ejection and movement inward on eccentric orbits (Morbi- delli et al., 2000). 7. FUTURE OBSERVATIONS Many of these comets struck the growing terrestrial planets over millions of years, supplying organic compounds It is neither unkind nor disrespectful to refer to the above and water, but comets were not the dominant supplier of as a kind of fairy tale grounded in a spotty tapestry of ob- water to Earth if the isotopic evidence is taken at face value. servations glued together by dynamical and chemical mod- The D-to-H ratio of measured long-period comets is twice eling. Seminal spacecraft observations, such as the ESA that of the Earth’s oceans (Meier et al., 1998). Furthermore, Giotto compositional studies, have been supplemented by the dynamical simulations themselves suggest that large increasingly powerful groundbased studies. Nonetheless, it bodies in an asteroid belt much more heavily populated than is exceedingly difficult to penetrate collapsing cores or to today were the principal contributors to the ocean of the tease apart the details of planet-forming disks on scales of Earth (Morbidelli et al., 2000), although the same conclu- astronomical units. Remnant disks are easier, but spatial sion need not hold for the waters of Mars. Nonetheless, the resolution is still a problem. Future cometary measurements accretion of cometary material by the Earth was a direct re- from, most spectacularly, Rosetta, are discussed elsewhere sult of the process by which the giant planets gravitationally in this book, but upcoming ground- and space-based facili- cleared much of the outer solar system of small icy bodies, ties for examining astronomical structures involved in planet relegating these to the Oort cloud. formation are equally exciting (Dutrey et al., 2004). ALMA At the same time, modest migration of the giant plan- will see details in planet-forming disks on scales of a few ets — particularly Neptune — sculpted the orbits of comets astronomical units, and will be able to sensitively probe the remaining beyond 30 AU into what are now the classical and gas composition and grain properties. The Space Infrared resonant disks of the Kuiper belt, and also ejected comets in- Telescope Facility (SIRTF) will track the colder dust in rem- ward into the so-called scattered Kuiper belt disk (Luu and nant and planet-forming disks with sufficient sensitivity to Jewitt, 2002). These gravitational interactions with the giant perhaps indicate the effect of massive planets on the dust planets — and for the Oort cloud comets, with the surround- populations, while the JWST will apply more powerful an- ing environment of young stars formed in the same epoch gular resolution and midinfrared sensitivity to probe the as the Sun — represent another set of perturbations on the compositional gradations in the grains of remnant disks. thermal and radiation environment of the comets. Beyond These and other facilities will severely test our ideas about 30 Comets II the process of planet formation, and how interstellar grains Gautier D., Hersant F., Mousis O., and Lunine J. I. (2001) Enrich- become — through many different evolutionary path- ments in volatiles in Jupiter: A new interpretation of the Gali- ways — the stuff of cometary nuclei. leo measurements. Astrophys. J. Lett., 550, L227–L230 (erratum 559, L183). Acknowledgments. 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