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Mark S. Bentley Norbert I. KömleGünter Kargl Erika KaufmannErika Hütter 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
In our current understanding, active cometary nuclei comprise a volatile-depleted outer crust covering a mixture of dust and ices. During each perihelion passage the thermal wave penetrates the crust and sublimates a portion of these ices, which then escape the nucleus, dragging with them dust particles that replenish the coma and dust tail. The flux of released gases is likely to vary as a complex function of solar distance, nucleus structure, spin rate, etc. It has been previously hypothesised that at some point a fluidised state could occur, in which the gas drag is approximately equal to the weight of overlying dust and ice grains. This state is well understood and used in industrial processes where extensive mixing of the gas and solid components is desired. The literature on fluidisation under reduced gravity and pressure conditions is here reviewed and published relations used to predict the conditions under which fluidisation could occur in the near-surface of a cometary nucleus. 相似文献
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A major goal of comet research is to determine conditions in the outer solar nebula based on the chemical composition and
structure of comet nuclei. The old view was to use coma abundances directly for the chemical composition of the nucleus. However,
since the composition of the coma changes with heliocentric distance, r, the new view is that the nucleus composition msut be determined from analysis of coma mixing ratios as a function of r. Taking advantage of new observing technology and the early detection of the very active Comet Hale-Bopp (C/1995 O1) allows
us to determine the coma mixing ratios over a large range of heliocentric distances.
In our analysis we assume three sources for the coma gas: (1) the surface of the nucleus (releasing water vapor), (2) the
interior of the porous nucleus (releasing many species more volatile than water), and (3) the distributed source (releasing
gases from ices and hydrocarbon polycondensates trapped and contained in coma dust). Molecules diffusing inside the nucleus
are sublimated by heat transported into the interior. The mixing ratios in the coma are modeled assuming various chemical
compositions and structural parameters of the spinning nucleus as it moves in its orbit from large heliocentric distance through
perihelion.
We have combined several sets of observational data of Comet Hale-Bopp for H2O (from OH) and CO, covering the spectrum range from radio to UV. Many inconsistencies in the data were uncovered and reported
to the observers for a reanalysis. Since post-perihelion data are still sparse, we have combined pre- and post-perihelion
data. The resulting mixing ratio of CO relative to H2O as a function of r is presented with a preliminary analysis that still needs to be expanded further. Our fit to the data indicates that the
total CO release rate (from the nucleus and distributed sources) relative to that of H2O is 30% near perihelion.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
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