Comets

I. Composition: nucleus is dominatedby H2O, methane and ammonia are probably also present with silicates. Tails: type I is composed primarily of ions and appears to be structured by fine lines or streamers; type II are broad and diffuse and are composed of dust grains that scatter light. Many commets have both type I and type II tails.

II. Orbits: long-period (periods 100 years and greater) with nearly parabolic orbits; short-period comets with distinctly less eccentric orbits than long period orbits. Comets are concentrated in within a 50,000 AU radius Oort cloud with orbits having near random distribution with respect to the ecliptic plain of the solar system. At least 500,000 comets may be present in the Oort cloud, most of which are unobservable and never pass through the inner solar system

III. Heating and evaporation: rotation of comets affects heating and evaporation. Long period comets are relatively "younger" in the sense of experiencing less heating and evaporation by close encounters to the sun.

IV. Origin: most accepted theory is that comets were icy planetesimals originally formed in the outer solar system and ejected into an Oort cloud by encounters with the giant outer planets. Other, less favored, theories include condensation at great range near the dense portion of the Oort cloud or by ejection of volcanic gases from the planets within the solar system.

For images and more information see the JPL webpage on comets and asteroids, including missions

Meteors and Asteroids

I. Composition and classification: stony, iron, and stony-iron meteorites. Stony are the most common, include ones having little evidence of differntiation. These stony meteorites may include glassy silicate inclusions called chonrules; hence, stony meteorites are further classifed as either chondrites or achondrites. 80% of all meteorites reaching Earth are chondrites. Carbonaceous chondrites contain carbon and volatile elements and compounds. Chondrules may be formed by fast cooling. Iron meterorites have large iron crystals (Widmanstatten lines), indicating slow cooling in a large body. Radiometric dating gives ages on the order of the age of the solar system. Exposure ages of stony meteorites are much younger, indicating an origin due to collision in the asteroid belt. Some meteors and reaching earth may be devolatilized comet cores.

II. Orbits : point to origin in the asteroid belt. A puzzle is why so many meteroites reaching earth are chondrites, more indicative of a product of primary condensation and accretion as a small body rather than fragmentation of a large planet-sized object at the orbital distance of the asteroid belt. A mars size object may have disturbed the asteroid belt, scattering asteroids. Currently there is a mass deficiency in the asteroid belt.

III. Impacts: Tektites may be bits of silicates blasted out of Earth by meteor impact. Crater densities are useful for dating the surface age of planets.