![]() Top: A schematic drawing of the photosphere (shaded) and the surrounding atmosphere of a supernova. An example of the variety in spatial distribution found in the molecular emission from comets is shown in Fig. Since the intensity and distribution of a molecular species in a comet can be used to derive the physical conditions within the cometary nucleus and coma, studies of molecules in comets are a key element to an understanding of the physical structure of comets. It is theorized that parent species chemically react with other parent species and atoms in the coma gas to produce other molecules, called “daughter” species. Thus, measurements of parent molecular species in comets are a direct measure of the chemical properties of the material from which comets were made, which is thought to be the same material from which the solar system formed. These species are released into the coma of a comet when it receives enough heating from the sun to sublimate them into the gas phase, thus allowing them to be observable through their rotational transitions. Many of the molecules observed in comets are thought to be “parent” species, or species deposited onto icy grain mantles in the core during the formation of the comet. Molecular spectral line observations of approximately 28 molecular species have been made toward a number of comets, with the recent visitation by the extremely large comet Hale–Bopp accounting for over half of these. These measurements have shown that the dust particle sizes in these comets are greater than 1 mm and that the mass of dust contained in the radiating grains is 10 10 − 10 12 kg. ![]() For example, millimeter continuum observations of comets P/Halley and Hale–Bopp have been used to measure the dust particle properties of cometary nuclear matter.
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