Comets: clues to the early history of the solar system

Abstract

Based on current observations of star-forming regions in the galaxy and the presence of short-lived radioactive nuclides in meteorites, it is highly unlikely that the solar system formed in isolation. The picture that emerges for the protosolar environment 4.6 Gy. ago is a clumped dense molecular cloud complex consisting of hundreds of protostars in various stages of evolution. Since comets formed at the periphery of the solar accretion disk (10 ≲ r ≲ 10,000AU), their formation processes may have been significantly influenced by the inter-protostar conditions rather than solely by the protosun. Comets are probably relicts of the early history of the primitive solar nebula, preserved in a relatively pristine state. Of the estimated 10 11-10 14 comet nuclei which presently exist, approximately 10 pass through the inner solar system ( r ≲ 5AUeach year where they become observable with earth-based instrumentation. When the comet nuclei (ice-dust conglomerates∼ 1–50 km in radius) are heated above the water ice sublimation threshold,T≳ 180K, thermal and dynamical effects from solar radiation together with the magnetohydrodynamical effects of the solar wind give rise to comae (− 10 4km in extent) and tails (∼ 10 6–10 7km) which have characterized comets for centuries. Two dynamical groups of comets are recognized having short (P ≲ 200y.) and long (P ≳ 200y.) orbital periods, arising from two distinct source regions. The reservoir for the long-period comets is the Oort cloud, originally consisting of comet nuclei which were gravitationally scattered by the giant planets from their formation region30 ≲ r ≲ 40AU out to distances10,000 ≲ r ≲ 50,000AU from the Sun. The short-period comets probably arise from diffusion of perihelia inward directly from the outermost region of the comet formation zone,100 ≲ r ≲ 1000AU, the Kuiper belt. Due to gravitational perturbations by giant molecular clouds, passing stars and the galactic gravitational potential the comet nuclei continue to gradually diffuse into orbits of smaller or larger perihelion distances, with the Oort cloud reservoir replenished on timescales of∼ 3 Gy. After formation comet nuclei apparently develop mantles of refractory material due to irradiation by high-energy particles (cosmic rays, solar and stellar winds) and, to a less extent, ultraviolet flux from the sun and neighboring protostars. As the short-period comets deplete their volatiles, they may in addition retain significant amounts of rubble which contributes to the surface mantle, possibly eventually choking off the vaporization process, giving rise to extinct comet nuclei some of which may now occupy the inner solar system, in earth-crossing orbits producing recoverable meteorites. The observation that most comet nuclei are water-dominated may be in large part attributable to rapid diffusion of highly volatile gases (e.g. CO and CO 2) through a porous nucleus. Thus as a result of successive solar passages a comet may evolve into a water-dominated state. For this reason the thermal history of a comet nucleus may be required to relate the relative abundances of volatiles in comet comae to models of the primitive solar nebula. Isotope abundance ratios, on the other hand, may be the key diagnostics for models of the early solar nebula. Spacecraft in-situ analyses of dust in the coma of comet Halley indicate chondritic compositions and heterogeneities on scales of submicrons. Inhomogeneities in meteorites on similar size scales have recently been found in isotope abundances representing ≲ 1% of the bulk material. The few interplanetary dust particles which have been analyzed also show large isotope variations on submicron scales. These meteoritic isotope anomalies have been attributed to individual interstellar grains which have survived processing in the solar nebula since presolar times. Due to the relatively unprocessed state of comet nuclei, it is highly likely that they have also incorporated interstellar grains which have survived intact to present epochs. Because of the different thermal histories of meteorites (T ≲ 2000K) and comet nuclei (T ≲ 100K), a substantial fraction of the cometary bulk material is probably of presolar origin, consisting of preserved, relict interstellar matter, predating the time of formation of the solar system 4.6 Gy. ago.