Infrared photometry has been used to determine the physical characteristics of cometary solids. Observations were made of the reflected and thermal parts of the spectra of seven comets. Two of these comets, Bowell and West, were nonperiodic; the other five, Chernyhk, Encke, Kearns-Kwee, Stephan-Oterma, and Tuttle, were periodic. Observations in the 3 μm region of the spectrum of Comet Bowell provide the first direct evidence for the presence of H₂O ice in a comet. This detection represents one of the strongest possible confirmations of Whipple's (1950) icy conglomerate model of cometary nuclei. The observations of the periodic comets have yielded the following picture of the dust in this type of objects: grains with a size distribution ranging from about 0.3 μm to 10 μm, and peaking around a few microns. These grains were made up of at least two components, a silicate material and an absorbing material. These characteristics are remarkably similar to those of the dust in nonperiodic comets. This indicates that the type of dust a comet ejects does not change with age, and supports the absence of large scale differentiation in cometary nuclei. Comet West is the first case of a splitting comet in which the fragments were observed to have differences in their dusty component. These observations suggest that the nucleus of this comet did not have an "onion skin" or layered structure but rather had pockets containing dust grains with different size distributions. Based on the results presented, the relation between cometary and interstellar dust, and the origin of comets are discussed.

This study addressed the problem of water vapor production and distribution in comet P/Halley, based upon interpretation of observational data obtained during the recent 1985-86 apparition. The data was acquired using the Lunar and Planetary Laboratory charge-coupled device (CCD) on the 154-cm Catalina telescope of the University of Arizona Observatories. Our data acquisition system was employed in two modes. The long-slit (∼200") spectroscopy mode covered the wavelength range 5200-10400 Å at a spectral resolution ∼14 Å. The narrow band filter imaging mode allowed two-dimensional mapping of selected cometary emission features, as well as the continuum, with a field of view of roughly 10 arc-min. Both neutral and ionized (H₂O⁺) water species were studied, with emphasis on the ion distribution. This involved comparing long-slit spatial profiles obtained ∼UT 1986 March 05.5, as well as cuts across filter images (∼ March 06.5) centered on the H₂O⁺ 0,8,0-band emission, with the Vega-1 spacecraft in situ ion density measurements (∼ March 06.3). Our March 05 spectroscopic data revealed a central dip, of order 30% relative to the profile peak, in H₂O⁺ column density in the inner coma (inside ∼ 2 x 10⁴ km from the nucleus), which filled in farther tailward. Similarly the BD - 3 plasma detector aboard Vega-1 measured a decrease in local ion density, of roughly 60% at the closest approach distance (∼ 9000 km sunward of the nucleus), relative to the inbound maximum density at R ≃ 12000 km from the nucleus. These results suggest a bimodal flow of ions out of the coma and/or an extended region over which the H₂0 molecules were being ionized. Our imaging data showed that, while the falloff in ion density was relatively rapid sunward of the nucleus, it was much more gradual in the anti-solar direction. This is due to the solar wind sweeping ions from the head of the comet into the plasma tail, whose width was of order 10⁵ km in the inner coma, diverging slowly and breaking up into a ray pattern farther tailward. The distribution of neutral water was mapped out using the [O I] λ6300 emission as diagnostic probe. In contrast to the ions, the H₂0 molecules were mainly confined to the inner few x 10⁴ km of the coma, and exhibited a much more symmetrical distribution. Integration of the [O I] slit profiles, assuming azimuthal symmetry, allowed calculation of the H₂0 production rate, which ranged from ∼ 10²⁸ molecules s⁻¹, when Halley was at a distance r≳ 2 AU from the sun, to a value of ∼ 1.5 x 10³⁰ molecules s⁻¹ for 1986 March 05 (r ≃ 0.78 AU). Using the latter production rate, and assuming a 100/1 production ratio of H₂0/ H₂O⁺, a spatially-averaged, tailward flow speed of ions out of the inner coma, < v⁺ > ≃ 16 km s⁻¹, was derived by integrating our March 05 H₂O⁺ profile, for which the slit was oriented across the coma, just tailward of the nucleus.