Meteorites and the Formation of the Early Solar System

As mentioned, scientists study meteorites for insights into the events that took place surrounding the birth and early evolution of the solar system. They know from astronomical observations that all stars form by gravitational collapse of dense regions in interstellar molecular clouds. This is almost certainly how the solar nebula formed, and the presence of preserved circumstellar and interstellar material in meteorites is consistent with this idea. Less clear is what precipitated the gravitational collapse of the region of the molecular cloud that became the solar system.

Gravitational collapse can occur spontaneously—i.e., through random fluctuations of density. Another possibility, however, is suggested by the finding in meteorites (particularly in their refractory inclusions) of short-lived radionuclides that were present at formation (as opposed to the later production of radionuclides by recent cosmic-ray irradiation). The shortest-lived of the radionuclides found to date, calcium-41, has a half-life of only about 100,000 years. This radionuclide must have been made and incorporated into refractory inclusions within just a few half-lives (less than a million years), or its abundance would have been too low to detect. This is remarkably short by astronomical standards. Because the other short-lived radionuclides have longer half-lives, they do not put such stringent time constraints on the interval between their synthesis and formation of refractory inclusions. Nevertheless, the absolute and relative abundances of the short-lived radionu-clides can be compared with the values predicted for likely sources of the radionuclides.

One potential source of radionuclides is nucleosynthesis in stars ending their lives in catastrophic explosions called supernovas and in a class of dying stars known as asymptotic giant branch (AGB) stars. Both supernovas and AGB stars produce massive, fast-moving winds (flows of matter) that are rich in shortlived radionuclides. Numerical simulations show that under some conditions, when these winds hit an interstellar molecular cloud that cannot collapse spontaneously, they compress it to the point that it becomes gravitationally unstable and collapses. The simulations also show that some of the wind material with its complement of short-lived radio-nuclides is mixed into the collapsing cloud. Thus, in this scenario, the radionu-clides are fingerprints of the stellar wind responsible for triggering the collapse of the molecular cloud that evolved into the Sun and planets.

An alternative explanation for the short-lived radionuclides in meteorites has not been ruled out—their synthesis in the solar nebula by intense radiation from an early active Sun. This concept has proved somewhat less successful than the stellar-wind idea at explaining the absolute and relative abundances of the short-lived radionuclides. Nevertheless, no model incorporating either of these explanations has been completely successful in this regard.

After collapse was initiated, the first solids known to have formed in the solar nebula were the refractory inclusions, which apparently were made in relatively short-lived heating events about 4,567,000,000 years ago. The gradation of planetary compositions from dry, rocky, metal-rich Mercury to gas-rich Jupiter and its icy moons suggests that there was a temperature gradient in the inner solar system. Astrophysical models predict such gradients, although the absolute value of the gradient varies with the conditions assumed for a given model. One of many ideas for producing the refractory inclusions is that they formed in convection currents circulating at the edge of the hottest region of the inner nebula.

The ambient environment in the asteroid belt at the time the asteroids were being assembled must have been thermally a rather tranquil one. The fact that presolar material is preserved in meteorites argues against widespread heating of the asteroidal region, as do the presence of water-bearing minerals and the relatively high content of volatile elements in many chondrites. Again, this is consistent with most current astrophysi-cal models. Despite the evidence for an overall low temperature in this region of the solar system, the abundance of chon-drules in all chondritic meteorites except the CI chondrites attest to local, transient episodes of very high temperatures.

If chondrules were relatively rare in meteorites, their formation could be regarded as of secondary importance in the early solar system. Chondrules and their fragments, however, make up most of the mass of the most abundant class of meteorites, the ordinary chondrites, and a major portion of other chondrites, which indicates that their formation must have been of central importance. Even if the parent bodies of ordinary chondrites formed only within a restricted region of the asteroid belt adjacent to the major resonance that is thought to put chon-dritic material into Earth-crossing orbits, this region still represents about 10 percent of the asteroid belt. It also is likely that the parent asteroids of other chon-drule-bearing meteorites formed outside this region, even though they may be in that region today.

Ideas abound for how chondrules formed—e.g., electrical discharges, shock waves, collisions between molten asteroids, and outflows associated with the early active Sun—but none has gained general acceptance. The ages of chon-drules are crucial to distinguishing between some of these ideas. If they really formed over a period of 1-10 million years after refractory inclusions, this would be problematic for certain models. Fewer problems would arise if it turned out that the measured ages of most chondrules reflect when they were reheated or altered in their parent body.

Asteroidal bodies began to form perhaps as early as one million years after refractory inclusions. Certainly, within 5-10 million years they were being heated, aqueously altered, and melted. Volcanic activity on some asteroidal bodies, presumably the larger ones, continued for as long as about 170 million years. The process responsible for this heating remains to be clearly identified. The short-lived radioactive isotopes aluminum-26 and iron-60 appear to be the most likely heat sources, but heat from electric currents induced by early solar activity and from the release of gravitational potential energy as asteroids formed also may have contributed.

Asteroid-sized bodies presumably were forming not just in the asteroid belt but everywhere in the solar system. Concurrently, they would have begun aggregating into larger bodies in a process that eventually produced the rocky inner planets. This process was remarkably rapid. The Moon probably formed by an impact of a Mars-sized body with the growing Earth. The oldest Moon rocks that have been dated are about 4.44 billion years old, but there is evidence that the Moon actually formed within 30 million years of the refractory inclusions. Similarly, the oldest meteoritic material from Mars is about 4.5 billion years old, but there is evidence that Mars itself formed about 13 million years after the refractory inclusions. Thus, within as little as 30 million years of the appearance of the first solids, the aggregation process that started with tiny particles had produced the rocky inner planets.

In order for asteroids to have formed and developed at all on the timescale of a few million years, theoretical calculations suggest that the density of matter required was more like that in the regions occupied by the giant planets. The quantity of material observed in the asteroid belt today, however, is quite small, perhaps as little as 1/10,000 of that originally present. Some natural process must have removed almost all the material in this region of the solar system after the formation of the asteroidal bodies.

Although the details are not yet fully understood, it seems most likely that the formation of the giant planets, particularly Jupiter, quickly resulted in the evacuation of most of the matter from this region of the solar system.

The foregoing scenario of early solar system evolution is likely to be wrong in some, and perhaps many, of the details. Nevertheless, without the samples of asteroids and primitive solar system materials provided by meteorites, there would be little observational basis at all for formulating models of this kind. For good reason, meteorites have been dubbed "poor man's space probes." Until spacecraft missions bring back a variety of samples from asteroids and comets, the most precise and detailed data for the evolution of the solar system will come from meteorites.

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