Dust tail

Figure 6.16 Structure of the comet showing the nucleus, the coma and two tails - an ion tail and a dust tail...

Low dust (tail-end) plant, 10 101 Loweite, 5 785t Low emittance coatings, 23 16 Low energy dyes, 9 195, 416 Low energy electron beam irradiation, silicone network preparation via, 22 567... 

The activity that characterizes a comet is driven by solar heating. As the comet approaches the Sun, jets of dust and gas erupt from active areas (Fig. 12.1), as they periodically rotate into the sunlight. The nucleus becomes surrounded by a spherical coma formed by the emitted gas and dust. Emitted gas becomes ionized due to interaction with solar ultraviolet radiation, and the ions are swept outward by the solar wind to form the comet s ion tail. A separate dust tail commonly has a different orientation, reflecting variations in the velocities of solid particles and ions. A popular term describing comets is dirty snowballs , although that description probably understates the proportion of rock and dust relative to ices. 

Fig. 1. Comet Mrkos 1957 V with straight gas tail and curved, diffuse dust tail on August 27, 1957. Photogr. Mt. Wilson and Palomar Observatories...

A review of the processes in dust tails has been given by SekaninaThe smaller particles are accelerated to higher velocities, and a considerable percentage can leave the solar system on hyperbolic orbits, especially if the comet is nearly parabolic. 

Information on the chemical composition of active comets is available from two different "reservoirs , viz. solid debris and sublimated volatile species. The former is extracted by IR spectroscopy of the dust tail and dust trails in the comet s orbit, from collected IDPs, possibly a fraction of CI carbonaceous meteorites, and from meteor data. Information on species in the comet coma and plasma tail is obtained by UV-VIS-IR spectroscopy and radio astronomy. 

The second of the two tails, the dust tail, consists of tiny particles of matter expelled from the coma and nucleus by the Sun s radiative pressure. This tail is visible only because the particles of which it is composed reflect sunlight. The composition of cometary dust is thought to be similar to that of interstellar dust, but much remains to be learned about this portion of the comet s structure. In April 2003, NASA launched an experiment to obtain better data about the composition and structure of cometary dust. High-altitude research airplanes were flown through the upper atmosphere toward the tail of Comet Grigg-Skjellerup. Special collectors in the planes scooped up samples of the cometary dust. Analysis may take years. 

Besides the planets and their satellites, the Solar System harbors a large number of smaller objects, ranging from hundreds of kilometers in size down to dust particles. If they consist of solid material and have at least the size of small boulders, they are called asteroids. If they enter the atmosphere of Earth and reach the surface, they are named meteorites. If they are very small and bum up on entry, they are referred to as meteors. If the bodies contain a substantial fraction of ices and develop tails as they come closer to the Sun, they qualify as comets. However, the classifications are not very consistent for example, remnants of the dust tails of comets cause meteor showers, and older comets, once they have expended most of their volatile matter during many passes near the Sun, may not be distinguishable from asteroids. This section is devoted to comets and the next section (7.3) deals with asteroids. 

Tail When comets get closer to the Sun, a tail forms which always points in the direction opposite to the Sun because of the influence of solar wind and solar radiation pressure. There are two types of tails that point to slightly different directions. The ion tail consists of gas (plasma) points directly away from the Sun, because it is more strongly affected by the solar wind (and magnetic field since the particles are charged) than dust that is affected by the radiation pressure. The ion tail has a blueish appearance, the dust tail is brighter and curved (because of the Keplerian motion of particles). The cometary tails might become very spectacular and extend more than 1 AU (150 million km) (see Fig. 5.4). 

Beneficiation faciUties require air and water pollution control systems, including efficient control of dust emissions, treatment of process water, and proper disposal of tailings (see AiRPOLLUTlON CONTROLMETHODS). In handling finished fluorspar, operators must avoid breathing fluorspar dust and contacting fluorspar with acids. Proper disposal of spills and the use of respirators and other personnel protective equipment must be observed. Contact with fluorspar may irritate the skin and eyes. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

Where water cover cannot be maintained, keep the tailings wet or revegetate to reduce dust. (Note, however, that the revegetation process may increase the rate of radon emissions.)... 

PVC has been shown to have a head-to-tail structure. Typical experimental evidence for this is that when dissolved in dioxan and treated with zinc dust, it undergoes a Wurtz-type reaction to yield a product containing a small amount of chlorine and no detectable unsaturation. The alternative possible structure, the head-to-head arrangement, would yield unsaturated sites where adjacent chlorine atoms had been removed (Reaction 1.4). 

The interstellar dust was shown to contain quinone derivatives as well as oxygen-rich condensed aromatic compounds the quinones were present in both hydrated and carboxylated form. Very little nitrogen was present in the compounds detected. The cometary material, however, contained condensed nitrogen heterocycles. Hardly any oxygen was detected in the solid phase of the cometary dust it possibly evaporates from the tail of the comet in the form of water or oxidized carbon compounds. The authors assume that these analytical results could lead to a reconsideration of the current biogenesis models (Kissel et al 2004 Brownlee, 2004). 

A member of the New World Hemorrhagic fever viruses normally found in Venezuela. The natural reservoirs are the short-tailed cane mouse (Zygodontomys brevicauda) and the cotton rat (Sigmodon alstoni), and the virus is shed in their urine. Infection occurs after inhalation of dust contaminated with excreta from infected mice or from aerosol of animal blood or fluids. Does not produce disease in animals. This is a biosafety level 4 agent. 

In many mountain-industrial areas there are 3 1 landscape-functional zones with different extents of the anthropogenic transformation of natural environments. As a rule, the first zone is the spatial complex joining mines, pits and tails site area with almost whole degradation of soil and vegetation cover and high metal concentrations in dust, technogenic depositions, waters and plants. 

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