Cometary Formation and the Oort Cloud

Any modern theory about cometary origins has to consider the origin of the Oort cloud. None of the comets observed today left the Oort cloud more than three or four million years ago. The Oort cloud is, however, gravitationally bound to the solar system, which it follows in its orbit around the Milky Way Galaxy. Therefore, it is likely that the Oort cloud has existed for a long time. The most probable hypothesis is that it was formed at the same time as the giant planets by the very process that accreted them. The Soviet astronomer Viktor S. Safronov developed this accretionary theory of the planetary system mathematically in 1972. According to his model, the planets originated from a disk or a ring of dust around the Sun, and cometary nuclei are nothing more than primordial planetesimals that accreted first and became the building blocks of the planets. From the accreted mass of the giant planets, Safronov predicted the correct order of magnitude of the mass of the Oort cloud, which was built up by those planetesimals that missed colliding with the planetary embryos and were thrust far away by their perturbations. In effect, the Oort cloud in this theory becomes the necessary consequence and the natural by-product of the accretion of the giant planets.

Later, in the 1970s, the American astronomer A.G.W. Cameron developed a much more massive model of the protostar nebula, in which the comets accreted in a circular ring at some 1,000 AU from the Sun, which is far beyond the present limits of the planetary system. The primeval circular orbits were then transformed into the elongated ellipses present in the Oort cloud by mass loss of the primitive solar nebula. Both the Cameron and Safronov models put the origin of comets together with that of the solar system some 4.6 billion years ago. Plausibility is given to the general idea of accretion from dust disks by the existence of such disks around many young stars— a fact established by infrared observations in the 1980s and confirmed visually in at least one case (^ Pictoris). Further support is found in clues derived from meteorites.

Since the early 1980s, new ideas have been explored to determine whether the Oort cloud could be much younger than the solar system or at least periodically replenished. The role of the massive and dense molecular clouds that exist in interstellar space has been reexamined in different ways. Could comets have accreted in these clouds directly from interstellar grains? Mechanisms for later capturing them into the Oort cloud cannot be very effective, but the efficiency is not capital, and some possibilities have been proposed. Since the solar system itself was probably formed from the gravitational collapse of such a molecular cloud, it seems more likely that either comets or the interstellar grains that were going to accrete into comets followed suit during gaseous collapse and were put into the Oort cloud at the same time that the planets were being formed. Elemental isotopic ratios deduced from the Comet Halley flyby have not brought about any conspicuous anomalies that could be attributed to matter coming from outside the solar system. So far, observational clues all favour the idea of cometary matter deriving from the same primeval reservoir as the stuff of the solar system, but it must be recognized that the evidence remains weak.

Possible Pre-Solar-System Origin of Comets

Telltales based on the chemical constitution of cometary nuclei as well as on the evolution of their orbits suggest that the origin of comets goes back beyond that of the planets and their satellites. Two scenarios are among the likeliest possibilities. In the first, comets had already accreted in all dense molecular clouds of the Milky Way Galaxy by the agglomeration of interstellar grains covered by a frost of organic molecules that cemented them together. Later, such a cloud collapsed to form the solar system. In the second scenario, dense molecular clouds were not able to accrete their frosty interstellar grains into larger bodies. When one of these molecular clouds collapsed to form the future solar system, however, the interstellar grains did likewise and eventually formed a dusty disk around the central star—the proto-Sun. Accretion into objects of 10-kilometre (6.2 mile) diameter is more likely in dusty disks of this type. The outer grains of the disk had not lost their frost, and some of them were ejected into the Oort cloud during the accretion of plane-tesimals into giant planets after some very moderate processing by heat. It is hoped that one day, space probes will secure data that will make it possible to determine whether frosty interstellar grains have lost their identity or can still be recognized as pristine and unaltered objects in cometary dust.

Comets seem to be the most pristine objects of the solar system, containing intact the material from which they were formed. Included are the hydrogen, carbon, oxygen, nitrogen, and sulfur atoms needed to build the volatile molecules present in the terrestrial biosphere (including the oceans and the atmosphere). Comets also seem to be the link between interstellar molecules and the most primitive meteorites known—the carbonaceous chondrites. The molecules required to initiate prebiotic chemistry (e.g., hydrogen cyanide, methyl cyanide, water, and formaldehyde) are present in interstellar space just as they are in comets; larger prebiotic chemistry molecules (e.g., amino acids, purines, and pyrimi-dines) occur in some chondrites and possibly in comets. An early cometary bombardment of Earth, predicted in some accretion models of the solar system, may have brought the oceans and the atmosphere, as well as a veneer of the molecules needed for life to develop on Earth. Comets could well be the link between interstellar chemistry and life.

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