Other
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........41a&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF MICHIGAN, 1993.Source: Dissertation Abstracts International, Volume: 54-11, Section: B, page:
Other
Scientific paper
A theoretical comet nucleus model has been developed to simulate the efflux from a cometary nucleus. The model employs 3 volatiles; CO, CO_2, and H_2O, and a siliceous dust component. Separation into mantle layers is assumed, then the heat flow equation and interface heat balance expressions are solved for temperatures while expressions for the growth and shrinkage of the mantles are integrated in time until a steady state is reached. New steady states at other positions in space, making use of the old steady state as an initial condition, are calculated along an orbit trajectory. The result constitutes a hysterisis curve of the output along the orbit. The model allows for variations in the equation of state of ices; the Clausius/Clapeyron equation as well as other equations based on the latest thermodynamic data; the Brown/Ziegler (1989) relationship for CO_2 and CO at low pressure and the Lowe (1977) expression for water vapor at low pressure. It allows for variations in the physics of the outflow, from a mechanism of sublimation directly to space, consideration of the Knudsen regime, to the Dusty-Gas Dynamic model (Cunningham and Williams, (1980), Mason and Malinauskas, (1983)). Variations in density, porosity, friability, tortuosity, ice phase, pore radius size, and orbital parameters are explored. Results are compared with measured cometary light curves from Halley's comet, comet Tempel2, and comet Bradfield (Feldman, et al., (1987), Roettger, et al., (1990)). Temperature variations over each orbit, the hysterisis curves of the growth and shrinkage of each mantle, the gas production rates for each species, as well as the overall gas flux rate are shown. None of the variations in parameter space so far considered produce a result that compares well with the comet data. All models yield a total gas production between 1027 and 10 28 sec^{ -1} (low). This suggests that secondary mechanisms peripheral to this modeling effort, such as the opening and closing of cracks, a conductivity expression which depends upon porosity as well as temperature, etc., may play a more important role in the cometary efflux process than previously thought.
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