Comet dust
Encyclopedia
Comet dust refers to cosmic dust
that originates from a comet
. Comet dust can provide clues to comets' origin.
The models for the origin of comets are: 1) the interstellar model, 2) the solar system model, 3) primordial rubble piles 4) aggregation of planetesimals in the dust disk around the Uranus-Neptune region 5) cold shells of material swept out by the protostellar wind. Bulk properties of the comet dust such as density as well as the chemical composition can distinguish between the models. For example the isotope ratios of comet and of interstellar dust are very similar, indicating a common origin.
The 1) interstellar model says that ices formed on dust grains in the dense cloud that preceded the Sun
. The mix of ice and dust then aggregated into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in 1986.
In the 2) solar system model, the ices that formed in the interstellar cloud first vaporized as part of the accretion disk of gas and dust around the protosun. The vaporized ices later resolidified and assembled into comets. So the comets in this model would have a different composition than those comets that were made directly from interstellar ice.
The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter
was forming.
Stardust
's discovery of crystalline silicates in the dust of comet Wild 2 implies that the dust formed above glass temperature (>1000K) in the inner disk region around a hot young star, and was radially mixed in the solar nebula from the inner regions a larger distance from the star or the dust particle condensed in the outflow of evolved red giants or supergiants. The composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars.
A comet and its dust allow investigation of the solar system beyond the main planetary orbits. Comets are distinguished by their orbits; long period comets have long elliptical orbits, randomly inclined to the plane of our solar system, and with periods greater than 200 years. Short period comets are usually inclined less than 30 degrees to the plane of our solar system, revolve around the Sun in the same counterclockwise direction as the planets orbit, and have periods less than 200 years.
A comet will experience a range of diverse conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the comet nucleus, beginning the formation of a dust layer. Near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely.
A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses of order meters are thought to have accumulated on the surfaces of short period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid.
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
that originates from a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
. Comet dust can provide clues to comets' origin.
Dust and comet origin
The 1) interstellar model says that ices formed on dust grains in the dense cloud that preceded the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. The mix of ice and dust then aggregated into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in 1986.
In the 2) solar system model, the ices that formed in the interstellar cloud first vaporized as part of the accretion disk of gas and dust around the protosun. The vaporized ices later resolidified and assembled into comets. So the comets in this model would have a different composition than those comets that were made directly from interstellar ice.
The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
was forming.
Stardust
Stardust
Stardust may refer to:* A type of cosmic dust composed of particles in space-Music:* Stardust * "Stardust" , by Galneryus* "Stardust" , by Hoagy Carmichael* "Stardust" , by David Essex* Stardust...
's discovery of crystalline silicates in the dust of comet Wild 2 implies that the dust formed above glass temperature (>1000K) in the inner disk region around a hot young star, and was radially mixed in the solar nebula from the inner regions a larger distance from the star or the dust particle condensed in the outflow of evolved red giants or supergiants. The composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars.
A comet and its dust allow investigation of the solar system beyond the main planetary orbits. Comets are distinguished by their orbits; long period comets have long elliptical orbits, randomly inclined to the plane of our solar system, and with periods greater than 200 years. Short period comets are usually inclined less than 30 degrees to the plane of our solar system, revolve around the Sun in the same counterclockwise direction as the planets orbit, and have periods less than 200 years.
A comet will experience a range of diverse conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the comet nucleus, beginning the formation of a dust layer. Near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely.
A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses of order meters are thought to have accumulated on the surfaces of short period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid.