Joseph A Nuth. American Scientist. Volume 89, Issue 3. May/Jun 2001.
In traditional Hindu mythology, the god Shiva is both creator and destroyer, bringing life and death to the world. It is an odd pairing of talents for a deity, and yet there may be a realworld counterpart in the heavens, an avatar if you will, in the form of comets. Some recent studies suggest that a rain of comets in the very early history of our planet, perhaps 4 billion years ago, may have seeded the young Earth with complex organic molecules from space-key ingredients necessary for life to arise. On the other hand, giant comet impacts may be responsible for some of the major extinction events in the history of life on Earth, including the demise of the dinosaurs 65 million years ago. Comets, it seems, may have been both creators and destroyers in our own history.
Yet the comets have a history of their own, and the more we find out about them the more enigmatic they seem. Although it has been said that “a comet is as close to being nothing as something can be”–in reference to the diffuse tail a comet emits near the sun–that bit of “something” holds important clues for the planetary scientist. As best as we can tell, comets are the most primitive bodies in the solar system. Some of the material inside a comet is preserved in nearly the same state it was in when the solar system was just taking shape, before the Sun and the planets were fully formed. Each comet is effectively a “grab bag” sample of the building blocks present in the nascent solar system-the solar nebula-at the time the comet was formed, about 4.5 billion years ago. Although the nebular material may have undergone considerable processing before it was incorporated into the comet, very little has been altered since. A comet is literally a little piece of the past.
Although we can’t snatch a comet from the sky and examine it in the laboratory, there are ways to get the next best thing. We can measure the spectral properties of a comet as it swings by the Earth, and we can examine some of the particulate remains of comets in the form of interplanetary dust particles collected by special, high-flying aircraft in the stratosphere. On the basis of these studies we can then make analogues in the laboratory, and so understand something about how a comet must be made. In this article I will review what such studies have told us about the comets, and what the comets tell us about the processes that must have taken place in the early history of our solar system.
A Snowball from Hell?
At a basic level, a comet is simply a collection of silicate dust and a smattering of organic molecules, coated with ices made primarily of water. Some of the ice-coated grains may have been present in the giant molecular cloud that partly collapsed to form the solar nebula, but others must have formed in the solar nebula itself. Part of the task of understanding comet chemistry is to determine when, where and how the dust, the organics and the ices came together.
In general, we believe that comets begin to form by an accreting “snowball” effect in which the icy dust grains stick together to form fractal-like aggregates. This process begins at some considerable distance from the center of the solar nebula, perhaps as far as 100 astronomical units (AU) away. (For a sense of scale, consider that the Earth is merely one AU from the Sun, whereas Pluto is 40 AU away.) At this stage, the movements of the dust grains and the small aggregates are coupled to the movements of the ambient nebular gas. Over time, however, as the aggregates accumulate into compact, boulder-sized snowballs, or cometesimals, they are slowed down by drag in the ambient gas, and they start to drift inward as their orbits decay As the cometesimals fall closer to the center of the solar nebula, they continue to grow by the accretion of ice and dust grains, as well as by merging with other aggregates in their path. In due course this pile of rubble becomes a comet, perhaps 10 to 20 kilometers across, which contains a collection of materials from a wide swath of its orbital radius.
Estimates of how long it takes to build a comet in this way depend partly on the size of the solar nebula in which the comet forms. In one model, Stuart Weidenschilling of the San Juan Capistrano Institute has shown that a good-sized comet could be made in about 100,000 years. Weidenschilling’s model assumes that the evolving solar nebula had merely the minimum mass needed to explain the composition of our solar system. A somewhat more massive solar nebula could assemble a comet much more quickly, perhaps in as little as 10,000 years, since the gasinduced drag and gravitational instabilities are greater in the larger nebula.
The solar nebula itself had a limited lifetime, from the moment it started to collapse from the molecular cloud to the point where the last of the gas had dissipated and the Sun and the planets had formed. Current estimates for its duration range from about 100,000 years to tens of millions of years. It was during this period that the comets and most of their organic constituents must have been made. The relative time it takes to build a comet and the duration of the solar nebula have consequences for the composition of the comets. If the time scales are comparable, then all comets should be fairly similar. If, however, the lifetime of the solar nebula was much greater than the time it takes to assemble a comet, then we could expect some diversity among the comets. Since the chemical composition of the nebula changes with age, comets assembled early on should be quite different from those built late in the nebula’s life.
Attempts to model the composition of the cometary volatiles-the ices and the organics-have met with mixed success. Bruce Fegley of Washington University in St. Louis has been trying to match the spectral properties of these enigmatic bodies by resorting to a seemingly arbitrary mixture of interstellar ices, volatiles from the solar nebula, plus some volatile components that must be synthesized at relatively high temperatures and pressures. Such a concoction is difficult to explain with Weidenschilling’s model, in which the cometary components are all formed far from the high temperatures and pressures near the center of the solar nebula.
To get around this problem, Fegley has suggested that the more complex organics formed in the giant gaseous subnebulae that have been proposed as the first stage in the formation of the giant planets. These subnebulae would have much higher temperatures and pressures than other parts of the outer solar nebula. As the giant, gaseous protoplanets migrated inward to their present locations, some of the gas from the subnebulae escaped, providing a source for the high-temperature, highpressure volatiles. In this view, variations among comets are due to different proportions of materials arising from various regions of the outer nebula.
Some recent work has further complicated scientists’ efforts to explain the formation of comets. Observations of infrared spectra from Comet Halley by Humberto Campins, now at the University of Arizona, and Eileen Ryan, now at New Mexico Highlands University, indicate that some of its silicate grains must consist of crystalline olivine. This has been confirmed more recently by the Infrared Space Observatory, which found that Comets Hyakutake and Hale-Bopp both contain magnesium-rich, crystalline olivine. The troubling aspect of these observations is that crystalline olivine has never been found in the general interstellar medium or within the giant molecular clouds that ultimately collapse to form new stars. It stands to reason that the olivine crystals must be a product of processes that took place as the solar nebula was evolving.
What does it take to make a grain of crystalline olivine? To answer this question, my colleagues and I have been attempting to create analogues in the laboratory with properties much like those of cometary grains. By burning silane (SiH^sub 4^) and magnesium-metal vapor in a stream of hydrogen gas at temperatures near 800 kelvins, and then heating (annealing) the resulting “smokes” at higher temperatures in a vacuum, we have effectively been able to “cook up” grains of crystalline olivine that bear a close resemblance to those formed in the solar nebula. The trick, it turns out, is to anneal the ingredients at the right temperature for the right amount of time.
Starting with amorphous silicate grains, much like those that would have been present in the interstellar medium, we found that crystalline olivine could be produced in a matter of months at a temperature of about 1,000 kelvins. Raise the temperature a notch to about 1,100 kelvins and the same task can be accomplished within a matter of minutes. However, if you raise the temperature as high as 1,600 kelvins, the grains are vaporized. On the other hand, lowering the temperature to below 850 kelvins would require more than one billion years to produce a crystal of olivine from our smokes!
These experiments place a strong constraint on the way in which the comets must have been made. Because no one believes the solar nebula could have existed for a billion years, the crystalline olivine must have been annealed at temperatures close to 1,000 kelvins. But since such temperatures would have destroyed the icy mantles that cover the silicate grains, we can conclude that the “hot” and “cold” components were made in separate regions of the nebula, and then later mixed together. This means that the standard scenario of comet formation, involving a “oneway trip” of agglutinating cometesimals, cannot be the full story.
A Complex Solar Nebula
Scientists who study meteorites have known for decades that a certain amount of mixing must have taken place in the primitive solar nebula. Some meteorites contain highly processed materials that are inexplicably embedded within a matrix of very primitive materials. The processed materials include the CAIs (calcium-aluminum inclusions), which required temperatures peaking near 2,200 kelvins for their manufacture, and the chondrules, which contain less heat-resistant minerals (such as olivine and plagioclase) that saw temperatures no higher than 1,700 kelvins. The CAIs and the chondrules are often embedded in a matrix containing highly fragile carbonbased components (diamond, graphite and silicon carbide grains), some of which are only a few nanometers across, and would be destroyed at temperatures as low as 600 kelvins.
What could have brought these materials together? Some meteoriticists suspected that lightning or magnetic reconnection events might have provided localized regions of high temperature to form the chondrules while preserving the more fragile materials in the cooler, adjacent nebular regions. This, however, doesn’t explain the CAls, which were formed at much higher temperatures and required a much longer cooking time than is possible in a transient event such as lightning. Suitable environments could be found closer to the protosun, and some scientists suggested that turbulent convection might have lofted the CAls out to distances of a few AU, where the asteroids are currently found. (Asteroids are generally considered to be the parent bodies of the meteorites.)
Pieces of the puzzle began to come together in the mid-1990s when astronomers studying a superficially unrelated problem came up with a viable mechanism for mixing materials in the solar nebula. Frank Shu and his colleagues at the University of California, Berkeley, were trying to understand the dynamic interactions between growing protostars and their nebular accretion disks. According to their calculations, interactions between the disk and the protostar could produce a powerful wind that could account for the bipolar outflows observed around many young stars. Soon after proposing this “extraordinary wind” (or X-wind) model, Shu’s team realized that the same violent interactions might be responsible for producing both the CAIs and the chondrules in the solar nebula. The interface between the surface of the protostar and the inner edge of the accretion disk was just the right temperature to produce these meteoritic inclusions. Moreover, these winds could then toss the finished products out to about 3 to 10 AU, where they would be incorporated into accreting planetesimals and become part of some planet or asteroid.
The X-wind model of the solar nebula makes explicit predictions about the temperatures, pressures and travel time for materials ejected along specific trajectories from the protosun. To date, tests have generally validated the model. Kevin McKeegan of the University of California, Los Angeles, and his colleagues have shown that the measured isotopic ratios of beryllium and boron in CAIs from the Allende meteorite are consistent with radiation fluxes expected from the X-wind model. They also noted that these same exposure histories would produce the observed isotopic ratios of calcium-41 and manganese-53 seen in the meteorites. It now appears likely that some fraction of the solids falling into the protosun might have been ejected back into the accretion disk after a period of high-temperature processing.
Although the X-wind model does well in explaining the composition of meteorites, it does not provide an easy mechanism for annealing amorphous silicates to produce the crystalline grains seen in comets. Individual grains exposed to the 1,600 to 2,200 kelvin temperatures of the X-wind near the inner edge of the accretion disk would be vaporized rather than crystallized. When the vapors later cooled and recondensed, it is likely that they would form amorphous silicates (such as those observed in circumstellar outflows around other stars), rather than the crystalline grains seen in comets.
All of this suggests that the theoreticians need to add yet another level of complexity to the dynamic models of the solar nebula. There must be a mechanism that is capable of transporting grains that have been annealed at temperatures near 1,000 kelvins, out to regions where the ices of water and hydrocarbons are stable and the cometesimals begin to accrete. One possibility is the presence of large-scale convective cells near the inner regions of the disk that interact with material in the X-wind in such a way that some of the dust and gas becomes entrained and transported outward.
Alternatively, Ronald Prinn of the Massachusetts Institute of Technology may have suggested an appropriate mechanism nearly a decade ago. He noted that most models of the solar nebula simplified the equations used to calculate the transfer of angular momentum in the system by dropping some higher-order terms. It is ostensibly a harmless act that greatly reduces the computational complexities involved. Prinn suggested, however, that if the missing terms were included in the computations, they would generate outflowing vortices that could mix some of the gas and dust processed near the inner nebula out to significant distances. To date, no one has followed up on Prinn’s suggestion.
Clues from Other Stars
Fortunately, we needn’t rely solely on theoretical models to understand the processes that took place in the solar nebula. At this very moment there are countless numbers of protostars in various stages of birth within our Galaxy. Although protostellar systems comparable in mass to the ancient solar nebula are too dim to observe, the behavior of more massive protostars-Herbig Ae and Be stars–can be studied to a certain extent. Despite being more than twice the mass of our Sun, these protostars are generally believed to behave and evolve in a fashion similar to the protosun.
Carol Grady of NASA’s Goddard Space Flight Center and her colleagues have shown that at least one Herbig system (HD 163296) exhibits a collimated bipolar wind that is consistent with the X-wind model, as well as an uncollimated outflow that may lie parallel to the disk plane. This uncollimated outflow may be the key to understanding how the dust grains annealed near the protostar make their way to the outer reaches of the nebula.
Unfortunately, because the closest of these stars is about 300 light-years away, detailed observations of these objects are extremely difficult to make, even using the state-of-the-art instruments aboard the Hubble Space Telescope. It is not yet possible to examine these objects with sufficient resolution to see the inner portions of the disk or even its geometry (the angle of the plane of the disk with respect to the Earth). Without the geometry of the system it is impossible to gauge the strength and extent of the winds that might be emanating from the inner regions of these systems. Nevertheless, Grady’s observations are tantalizing. If some form of uncollimated wind recycles even a fraction of the material in the accretion disk, it would have enormous consequences for the chemistry of the nebula.
Herbig stellar systems are interesting for another reason as well. Infrared spectra of older Herbig Ae and Be stars indicate that much of the dust surrounding the stars comes from infalling comets, which shed the particles on their inward voyage. Collectively, the infrared spectra of many Herbig systems reveal a distinct trend: The dust around the youngest observable stars is amorphous, but it becomes increasingly crystalline as the age of the nebula increases. Dust grains are highly unstable over the period of the nebula’s lifetime. Small grains are blown out of the system by photon pressure, whereas larger grains spiral into the protostar, where they are destroyed. In order for us to see them in the older systems, the dust grains must be released over time from cometesimals and comets; otherwise they would have been destroyed.
These observations provide further evidence that processes inside protostellar nebulae are responsible for producing the crystalline grains. They also suggest that the crystalline silicates are not created by a process such as the impact shock associated with amorphous dust falling onto the stellar accretion disk, otherwise even the youngest stars would have crystalline silicates. But it also suggests that as the nebula ages, there is a steady accumulation of the crystalline grains produced in the solar nebula that must be transported to the region where the comets are made.
The trend toward increasing crystallinity in the older nebulae has important implications for future studies of the chemical evolution of the solar nebula. Comets that contain only amorphous silicates must have formed early in nebular history, whereas those containing a large fraction of crystalline grains formed much later, perhaps as the last gases of the nebula were dispersing into space. We might expect other chemical components in comets to show similar evolutionary trends with age. For example, as the solar nebula evolved, primitive species (such as CO, CO^sub 2^, N^sub 2^) from the parent molecular cloud would be increasingly converted into complex organic molecules, such as hydrocarbons, alcohols and amines. In this scenario, the fraction of complex organic molecules in a comet should be positively correlated with the fraction of crystalline silicate dust it contains.
The possibility of placing comets into a chronological order opens up some interesting possibilities. We are far from understanding how the simple organic ices found in the cores of molecular clouds combined to form the complex organic molecules present in a comet. Modeling the chemistry of a comet on the basis of telescopic observations of its dusty and gaseous emanations (in its coma, or “head”) is a very complex affair. Molecules evaporate at various rates from a surface that is far from homogeneous, and many of the complex chemical species are destroyed by ultraviolet light from the sun. More important, some of the most interesting compounds-high-molecular-weight amino acids, sugars and proteins-would not be present in sufficient quantities in the coma to be observed. The only feasible way to detect these materials is by means of a sample-return mission to a passing comet.
However, in order to provide a complete record of the processes in the solar nebula we would want to select comets that were formed at different stages of nebular evolution. This is where the spectral differences between amorphous silicate grains and crystalline grains should come in handy. If our proposed comet chronology is correct, we may be able to assess the relative age of a comet while it is still some distance from the Earth merely by observing its infrared spectrum. In the near future we could thus target specific comets for sample return missions and so study various stages in the chemical evolution of the solar nebula. We would then begin to understand the origin of the molecules that gave a “jump-start” to life on Earth.