Comets neutral molecules

Medicina radiotelescope (Fig. 5.6). They detected water in this comet when it was only 0.23 AU from the Sun. Moreover, the lines observed were split and showed velocities varying from 22 to 44 km/s. Normally, the outflow velocity of neutral molecules from cometary nuclei is expected at about 1 km/s. These high values of the velocities observed may be explained by the strong solar coronal activity which could have accelerated icy grains. 

Kaiser, R.I. Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. Suits, A.G. A combined experimental and theoretical study on the formation of interstellar C3H isomers. Science 1996, 274, 1508-1511. Kaiser, R.L Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. Neutral-neutral reactions in the interstellar medium. IL Isotope effects in the formation of linear and cyclic C3H and C3D radicals in interstellar environments. Astrophys. J. 1999, 510, 784—788. Kaiser, R.L Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. The formation of HCS and HCSH molecules and their role in the collision of comet Shoemaker-Levy 9 with Jupiter. Science 1998, 279, 1181-1184. Kaiser R.L Stranges D. Lee Y.T. Suits A.G. Neutral-neutral reactions in the interstellar medium. 1. Formation of carbon hydride radicals via reaction of carbon atoms with unsaturated hydrocarbons. Astrophys. J. 1997, 477, 982-989. 

The most visually distinctive part of a comet is its long tail, which always extends from the nucleus and coma in a direction away from the Sun. The tail consists of two parts, one that extends outward in a straight line away from the nucleus, and one that has a curved "ballistic" shape. The first of these two tails, the ion or gas tail, is formed when solar radiation ionizes neutral atoms and molecules in the coma and nucleus. Ions found in cometary tails include CO+, CH+, OH+, CN+, C02+, and N2+. These ions are then pushed away from the nucleus and coma by the solar wind, which consists of charged particles emitted from the Sun. The ion tail is visible partly because... 

Photodissociation is far more likely to occur than photoionization, however, because the energy needed to bring about dissociation is much less than that required for photoionization. In fact, once a water molecule escapes from the comet nucleus into the coma, the average time in which it is likely to he broken apart by solar radiation is about 25 seconds. By contrast, the average amount of time during which a water molecule is ionized by solar radiation is likely to be a few hours. Such reactions are more likely to occur when water molecules have trailed off into the comet s tail at distances of about 1 AU. Consequently, the coma of a comet can he expected to consist largely of neutral remnants of the photolysis of water molecules, such as H and OH, while ionized species, such as H+ and free electrons, are more likely to be found in the comet s tail. 

OH radical and neutral H2O (recombination of the H20 ion with electrons leads essentially to dissociation to OH + H). In solid or gaseous form water has been found in a variety of astrophysical sites besides the ISM planets, satellites, comets, circumstellar disks, other galaxies and in our Sun and on our Moon. It also forms a matrix for trapping gases, as clathrates in which guest molecules are trapped within polyhedral water cages the most prominent example is that of methane hydrates which occur on ocean floors and in permafrost. 

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