Comet

Conglomeration of frozen water and gases (methane, ammonia, CO₂) and silicates that that formed in the outer solar systemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. and orbits the SunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the. In recent years, the description of comets has shifted from dirty snowballs to snowy dirtballs with more dust than ice. However, the ratio is less than 10-to-1. Comets appear to contain a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it. High-temperature silicates (e.g., olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion) formed near the protosun and were ejected, perhaps by strong bipolar jets, to the outer parts of the solar systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with. Recent data indicate that comets probably have porosities of ~75% and perhaps no solid coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel.. It is likely that they consist of very loosely packed grains.

Comet orbits vary in eccentricityRatio of the distance between the foci and the major axis of an ellipse (planetary orbit). Eccentricity is: where ra = apoapsis distance and rp = periapsis distance. between an ellipse and a parabola and have known periods from 3 to 1000s of years. The names of such periodic comets are prefaced with “P/” (e.g., Comet P/HalleyComet with 76-year orbital period. Images returned by the Giotto Mission revealed its nucleus was a dark (albedo 0.04-0.05), peanut-shaped body, ~15 km long and 7-10 km wide; the measured density (0.3 g/cm) indicated a fluffy porous texture. The black surface is composed of organic compounds. Giotto imaged 7 jets).

Near the Sun, a comet’s materials are evaporated to produce a comaSpherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the vaporization of comet material. of gases and dust which may develop into a tail extending, in some cases, 80 million km. Comets have two types of tails, one comprised of dust and the other ionized gas, which point in different directions (white and blue, respectively, in the photograph; the red colors are of an emission nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive in the background).