PHYSICAL NATURE OF
COMETS Nucleus and Coma Almost the entire mass of a comet is concentrated in its nucleus. The diameter
of the
nucleus is on the order of a few kilometers. The density, between 0.1 and 1 g/cm7 (6 and 60 lb/ft7), indicates that the nucleus is tenuous. According to Fred L. Whipple's "dirty snowball" modelÑconfirmed by recent observationsÑthe nucleus consists of a conglomerate of such compounds as water, carbon monoxide (CO), methanol (see methyl alcohol), ammonia, and methane, all frozen and mixed with dust. When the comet approaches the Sun, the frozen matter sublimes and forms a cloud of gas and gritÑcalled the comaÑaround the nucleus. Closer to the Sun, the production of gases increases. The particles are repelled from the nucleus by the solar radiation pressure and the solar wind (a stream of charged particles), forming the tail. The average diameter of the coma is about 100,000 km (62,000 mi), but its mass is small. Some
molecules are decomposed and ionized (see ion and ionization) by ultraviolet light on their way from the nucleus to the tail. The chemical composition of the nucleus can be discovered indirectly by spectral analysis of this released gas. The main products observed are hydrogen and oxygen atoms, water, and hydroxyl (OH) radicals (see hydroxyl group). Molecules of carbon and carbon compounds (CN, CO+, and two- and three-carbon molecules) are present in concentrations about 100 times lower, and the amounts of NH, NHH, CH, and charged nitrogen molecules are 1,000 times lower. Carbon monosulfide (CS) and atomic and molecular sulfur have also been detected, and ethane was observed in comet Hyakutake. The coma of a comet generally becomes smaller as it nears the Sun, and its molecules, decomposed more rapidly by the solar wind, are pulled into the tail. The apparent brightness of a comet depends on its distance from the Sun and from the Earth: the brightness depends on about the fourth power of the distance from the Sun, which indicates that a comet not only reflects light, but also absorbs and then itself emits light. Consequently the brightness may increase quite rapidly upon approach to the Sun. Some comets show abrupt, striking increases in brightness in the vicinity of the Sun, the result of temporary increases in solar activity. Other comets fade away upon approaching the Sun, probably because the nucleus disintegrates. In some short-period comets the brightness decreases slightly with each revolution, probably owing to loss of matter. Tail When a bright comet becomes visible, the most noticeable feature is the tail. At the appearance of Halley's comet in 1910, the tail stretched for more than 90¡ over the celestial sphere. During Halley's most recent appearance in 1985Ð86, however, this elongation took place while the comet was on the far side of the Sun, so that the show made in Earth's night sky was undramatic. The length of the tail ranges from 1 million to 100 million km (0.62 million to 62.14 million mi). Usually it first appears at a distance of about 1.5 AU from the Sun. Despite its size, it usually contains less material in 1 cu km than in 1 cu mm of ordinary air. The tail is formed of gas from the coma and always points away from the Sun. It was initially thought that the solar radiation pressure alone was responsible for driving the tail away; it is now clear that the solar wind has a far greater effect. The solar wind consists of charged particles ejected from the Sun. The force exerted by these particles on the gas molecules of the coma is about 100 times stronger than the gravitational force of the Sun, so that the molecules are pulled along by the wind. The solar wind is not constant, and its variations are responsible for the threadlike structure of the tail. Solar flares or other perturbations on the Sun can sometimes be seen to make the tail turbulent or to bend it. A comet may have one of two types of tail, and many comets have both typesÑa double tail. One is eated and almost straight, has a fibrous structure, and consists of ionized gases. This is called a Type I tail; comet Hale-Bopp's was notably blue. Type II tails, or dust tails, are more strongly curved and hazier; they consist of dust repelled by sunlight. A comet may possess several dust tails in addition to a gas tail, and Hale-Bopp had a sodium tail as well (observable only by very large telescopes). Some comets have an anomalous tail, or antitail, which points toward the Sun (for example, the Arend-Roland comet, 1957 III). The antitail is a very thin layer of dust expelled at an earlier stage and lying along the orbit of the comet. The gases that predominate in the tail are CO§, charged nitrogen molecules, CH§, carbon dioxide, and OH§. These ions, like those in the coma, are formed when larger molecules are split by the solar wind.