Meteor stream

Comets are generally considered to be weakly consolidated, and active comets are commonly observed to split into fragments. This is sometimes due to the tidal forces of a close planetary encounter, such as affected comet Shoemaker-Levy when it passed close to Jupiter in 1992 and broke into 21 pieces. More commonly, a comet spontaneously fragments multiple times over its orbit period, without any obvious cause. Disintegrating comets leave trails of small particles in their wakes. These trails are known as meteor streams, and when the Earth passes through such a meteor stream, as it does several times a year, a meteor shower occurs. Meter-sized rocks are known to occur within cometary meteor streams. 

The radiant applies to meteors of a particular meteor stream, and is the point in infinity in the sky from which the meteors appear to come. In the case of the Leonid meteor shower that occurs annually in mid November (see Table 1), the radiant coincides with the constellation, Leo. 

Particles found in highly correlated orbits are called stream components and those found in random orbits are called sporadic components. It is thought that most meteor streams are formed by the decay of a comet nucleus and consequently are spread around the original orbit of the comet. When Earth s orbit intersects a meteor stream, the meteor rate is increased and a meteor shower results. Typically, a meteor shower will be active for several days. A particularly intense meteor shower... 

Close-up images of Mars by the Mariner 9 probe indicated networks of valleys that looked like the streambeds on Earth. These images also suggested that Mars once had an atmosphere that was thick enough to trap the sun s heat. If this is true, something happened to Mars billions of years ago that stripped away the planet s atmosphere. Thus, a large meteor must have crashed into Mars s atmosphere billions of years ago and thinned its atmosphere so that those streams evaporated. 

Meteoric water. Water derived from rain, snow, streams, and other bodies of surface water that percolates in rocks and displaces interstitial water that may have been connate, meteoric, or of any other origin. Meteoric water in sedimentary basins is generally recharged at higher elevations along the margins of the basin. The time of last contact with the atmosphere is intentionally omitted from this definition, but may be specified to further define meteoric water. Thus, Recent meteoric water, Pleistocene meteoric water, or Tertiary meteoric water, would indicate the time of last contact with the atmosphere (Kharaka and Carothers, 1986). 

In sum, there are four major sources of soluble salts in river basins (i) meteoric salts (ii) salts derived from water-rock interaction (e.g., dissolution of evaporitic rocks) (iii) salts derived from remnants of formation water entrapped in the basin and (iv) anthropogenic salts (e.g., waste-water effluents). Meteoric salts are concentrated via in-stream net evaporation and evapotransprra-tion along the river flow. In addition, meteoric salts can be recycled through irrigation in the watershed and development of saline agricultural drainage water that flows to the river. 

Each of these sources is characterized by identifiably different chemical and isotopic compositions. Hence, the different sahnization processes induce different chemical and isotopic changes in the salinized river. By using conservative tracers of Br/Cl, 6 Owater. and C1/C1 ratios, it is possible to discriminate between these processes (Figure 18). Recycling of meteoric salts by in-stream river salinization results in... 

Astronomers have found that the visually observed meteors are derived from two meteoroid populations a continuously active, but sporadic, background and a number of specific sources called meteoroid streams. 

Meteor showers occur when Earth passes through the tube-like structure of meteoroids left in the wake of a comet. Such meteoroid tubes, or as they are more commonly called meteoroid streams, are formed after a comet has made many repeated passages by the Sun. Meteoroid streams are composed of silicate (i.e. rocky) grains that were once embedded in the surface ices of a parent comet. Grains are released from a cometary nucleus whenever solar heating causes the surface ices to sublimate. New grains are injected into the meteoroid stream each time the comet passes close by the Sun. 

The individual dust grains (technically meteoroids once they have left the comet) move along orbits that are similar to that of the parent comet. Gradually, over the course of several hundreds of years, the meteoroids form a diffuse shell of material around the whole orbit of the parent comet. Provided that the stream meteoroids are distributed in a reasonably uniform manner, a meteor shower will be seen each year when the Earth passes through the stream (Eig. 1). The shower occurs at the same time each year because the position at which the meteoroid stream intersects Earth s orbit does not vary much from one year to the next. There are long-term variations, however, and the days during which a shower is active will change eventually. 

Probably the best known meteor shower is the one known as the Perseid shower. This shower reaches its peak on the night of August 12th each year, but meteors can be observed from the stream for several weeks on either side of the maximum. The shower s radiant first appears in the constellation of Andromedia in mid-July, and by late August it has moved into the constellation of Camelopardalis. The radiant is in the constellation of Perseus on the night of the shower maximum. 

Any gas that dissolves in groundwater could, given the appropriate conditions, migrate by streaming. Groundwater is most likely to be saturated in gases dissolved in meteoric water, i.e., N2, O2, Ar, CO2. These then are the gases from which bubble streams may form. 

A third issue is that even though the soil is one source of stream humic substances, it is not necessary that soil and stream humic substances have the same composition. If they were of the same composition, then stream humic substances would be primarily humic acids, because the humic cidjo J fulvic acid ratio in soil is approximately 3 1. However, as previously dis- -cussed in this chapter, stream humic substances are approximately 90% fulvic acids. One may say that fulvic acids are leached from soils in preference to humic acids. This may be true, but no one has shown water leachates of soil to contain fulvic acid of the same composition as in the bulk soil. Beck et al. (1974) state that meteoric waters percolating through soil will selectively mobilize nonrepresentative fractions of the soil organic matter. It should be emphasized that even if stream humic substances are the same as soil humic substances, one can not infer that one is the source of the other, but that the same precursors and humification process are probably operable in both soil and stream environments. 

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