Comet activity

The issue of amorphous ice is an important one for comet activity and evolution (Bar-Nun et al., 1987 Gronkowski, 2002). It is widely mentioned in the literature, but there is some debate on whether it exists in comets. Keller and Jorda (2002) outlines several arguments that it should not occur in comets. He uses the work of... 

The classic signature of erosion-corrosion is the formation of horseshoeshaped depressions, comet tads, grooves, or sand dunelike surface contours oriented along the direction of fluid flow (Figs. 11.1,11.2,11.3,11.5, and 11.8). Occasionally, erosion-corrosion will produce smooth, almost featureless, surface contours (Fig. 11.15), although even in this case oriented metal loss often exists around the perimeter of the affected region. If erosion-corrosion has been recently active, affected surfaces will be free of accumulated deposits and corrosion products. 

The comet s tail The tail only develops when the comet is inside the orbit of Mars and can reach a length of between 107 km and one AU. It is not always straight but is often curved. This happens when the comet is subject to strong solar winds, i.e., during periods of greater solar activity. Two types of tail can be distinguished ... 

Kurt Varmuza was bom in 1942 in Vienna, Austria. He studied chemistry at the Vienna University of Technology, Austria, where he wrote his doctoral thesis on mass spectrometry and his habilitation, which was devoted to the field of chemometrics. His research activities include applications of chemometric methods for spectra-structure relationships in mass spectrometry and infrared spectroscopy, for structure-property relationships, and in computer chemistry, archaeometry (especially with the Tyrolean Iceman), chemical engineering, botany, and cosmo chemistry (mission to a comet). Since 1992, he has been working as a professor at the Vienna University of Technology, currently at the Institute of Chemical Engineering. 

M. Venturi, R. J. Hambly, B. Glinghammar, J. J. Rafter and I. R. Rowland, Genotoxic activity in human faecal water and the role of bile acids a study using the alkaline comet assay. Carcinogenesis, 1997,18(12), 2353. 

There are reports indicating that arsenic compounds can damage cellular DNA, thereby acting as genotoxins, as assessed by the comet or single-cell gel assay. Some reports indicate that arsenite possesses this activity (5-7), whereas another indicates that although the methylated trivalent species are genotoxic, arsenite itself is not (8). 

DAB was genotoxic in the comet assay inducing DNA damage in the stomach, colon liver, bladder, lung, and bone marrow. It is also mutagenic to Salmonella in the Ames test. Because of its demonstrated carcinogenicity in animals, human exposure to DAB by any route should be avoided. In recent years, this compound has been used only in laboratories as a model of tumorigenic activity in animals. It is not produced commercially in the United States and is of little occupational health importance. 

Figure 19.5 Visualization of DNA damage induction in cultured human keratinocytes by photo-activated lomefloxacin using the comet assay. The presence of DNA breaks (induced either by ROS or by excision of DNA lesions) leads to fragmentation and electrophoretic migration to produce the comet tails, whereas bulky genomic DNA remains in the comets heads.

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. 

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. 

For all comets visited by spacecraft, the measured albedo is <5%, making comet nuclei among the darkest objects in the solar system. This coloration is due to black crusts that cover most of the nuclei. The materials now comprising the crusts were presumably spewed out of the interior at speeds below the escape velocity, so they can be considered lag deposits. The parts of surfaces covered by such deposits are inert, and active areas releasing gas and dust constitute only a small percentage of a comet s total surface area. 

Cometary activity occurring at great distance from the Sun (corresponding to temperatures <100 K) is probably controlled by ices more volatile than H20. For example, comet Hale-Bopp exhibited emission of highly volatile CO at great solar distances. Trapped CO was presumably released by crystallization of amorphous ice or sublimation of ice crystals at very low temperatures. 

At the time of encounter, Wild2 was at 1.86 AU, and thus was very active. Comets release thousands of tons of dust during cometary activity - the dust is there for the taking, the trick is to snatch it without destroying it or the spacecraft. Two technological achievements by scientists at the Jet Propulsion Laboratory made the collection of comet dust possible the clever design of a trajectory that allowed the spacecraft to encounter the comet coma at a relatively modest speed (6.1 kms ), and the development of a capture medium that slowed and trapped the particles without destroying them. 

What kinds of ices occur in comets, and how do they account for cometary activity ... 

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