Physical conditions in the primitive solar nebula

Physical conditions for model of primitive solar nebul

Photographic networks for fireballs

Photographic networks in United States and Czechoslovakia for bright meteor observatio

Forming Planetesimals in Solar and Extrasolar Nebulae

Planets are built from planetesimals: solids larger than a kilometer which grow by colliding pairwise. Planetesimals themselves are unlikely to form by two-body collisions; sub-km objects have gravitational fields individually too weak, and electrostatic attraction is too feeble for growth beyond a few cm. We review the possibility that planetesimals form when self-gravity brings together vast ensembles of small particles. Even when self-gravity is weak, aerodynamic processes can accumulate solids relative to gas, paving the way for gravitational collapse. Particles pile up as they drift radially inward. Gas turbulence stirs particles, but can also seed collapse by clumping them. While the feedback of solids on gas triggers vertical shear instabilities that obstruct self-gravity, this same feedback triggers streaming instabilities that strongly concentrate particles. Numerical simulations find that solids 10-100 cm in size gravitationally collapse in turbulent disks. We outline areas for progress, including the possibility that still smaller objects self-gravitate.Comment: To appear in Annual Reviews. This review is intended to be both current and pedagogical. Incorporates suggestions from the community; further comments welcome. v2: Single-space

THERMAL HISTORY, CHEMICAL-COMPOSITION AND RELATIONSHIP OF COMETS TO THE ORIGIN OF LIFE

It is generally believed that a comet consists basically of a loose conglomeration of frozen gases with embedded material similar to that found in the carbonaceous chondritic meteorites, and consequently that comets may be nearly pristine samples of the original solar nebula1−5. We show here that thermal processing within comets could have played an important part in determining their present state; in particular, we find that liquid water might have been available in some comets over geologically and biologically significant spans of time. It follows that a cometary origin is not excluded for some thermally metamorphosed meteorites and asteroids, that comets may contain quite complex organic molecules, and that comets may have played a role in the origin and conceivably even in the subsequent evolution of terrestrial life