FEATURE ARTICLE
How Were the Comets Made?
Explaining the composition of these 4.5 billion-year-old relics may require scientists to revise their models of the primitive solar nebula
Joseph A. Nuth III
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 (Figure 2). 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 gas-induced 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, high-pressure 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 (Figure 4). 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 (Figure 5). By burning silane (SiH4) 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 "one-way trip" of agglutinating cometesimals, cannot be the full story.
» Post Comment