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 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 carbon-based 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 CAIs, 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 CAIs 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 (Figure 7).
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.
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