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Surface temperature and the available effective energy strongly influence the mass flux of H2O and minor volatiles from the nucleus. We perform computer simulations to model the gas flux from volatile, icy components
in porous ice-dust surfaces, in order to better understand results from observations of comets. Our model assumes a porous
body containing dust, one major ice component (H2O) and up to eight minor components of higher volatility (e.g. CO, CH4, CH3OH, HCN, C2H2, H2S), The body's porous structure is modeled as a bundle of tubes with a given tortuosity and an initially constant pore diameter.
Heat is conducted by the matrix and carried by the vapors. The model includes radially inward and outward flowing vapor within
the body, escape of outward flowing gas from the body, complete depletion of less volatile ices in outer layers, and recondensation
of vapor in deeper, cooler layers. From the calculations we obtain temperature profiles and changes in relative chemical abundances,
porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for
each of the volatiles at various positions of the body in its orbit.
In this paper we relate the observed relative molecular abundances in the coma of Comet C/1995 O1 (Hale-Bopp) and of Comet
46P/Wirtanen to molecular fluxes at the surface calculated from our model.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
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
3.
Tilman Spohn Karsten Seiferlin Axel Hagermann Jörg Knollenberg Andrew J. Ball Marek Banaszkiewicz Johannes Benkhoff Stanislaw Gadomski Wojciech Gregorczyk Jerzy Grygorczuk Marek Hlond Günter Kargl Ekkehard Kührt Norbert Kömle Jacek Krasowski Wojciech Marczewski John C. Zarnecki 《Space Science Reviews》2007,128(1-4):339-362
MUPUS, the multi purpose sensor package onboard the Rosetta lander Philae, will measure the energy balance and the physical parameters in the near-surface layers – up to about 30 cm depth- of the
nucleus of Rosetta’s target comet Churyumov-Gerasimenko. Moreover it will monitor changes in these parameters over time as
the comet approaches the sun. Among the parameters studied are the density, the porosity, cohesion, the thermal diffusivity
and conductivity, and temperature. The data should increase our knowledge of how comets work, and how the coma gases form.
The data may also be used to constrain the microstructure of the nucleus material. Changes with time of physical properties
will reveal timescales and possibly the nature of processes that modify the material close to the surface. Thereby, the data
will indicate how pristine cometary matter sampled and analysed by other experiments on Philae really is. 相似文献
4.
Mitrofanov I. G. Kozyrev A. S. Lisov D. I. Litvak M. L. Malakhov A. A. Mokrousov M. I. Benkhoff J. Owens A. Schulz R. Quarati F. 《Space Science Reviews》2021,217(5):1-47
Space Science Reviews - Volatiles and refractories represent the two end-members in the volatility range of species in any surface-bounded exosphere. Volatiles include elements that do not interact... 相似文献
5.
A Coradini F. Capaccioni P. Drossart G. Arnold E. Ammannito F. Angrilli A. Barucci G. Bellucci J. Benkhoff G. Bianchini J. P. Bibring M. Blecka D. Bockelee-Morvan M. T. Capria R. Carlson U. Carsenty P. Cerroni L. Colangeli M. Combes M. Combi J. Crovisier M. C. Desanctis E. T. Encrenaz S. Erard C. Federico G. Filacchione U. Fink S. Fonti V. Formisano W. H. Ip R. Jaumann E. Kuehrt Y. Langevin G. Magni T. Mccord V. Mennella S. Mottola G. Neukum P. Palumbo G. Piccioni H. Rauer B. Saggin B. Schmitt D. Tiphene G. Tozzi 《Space Science Reviews》2007,128(1-4):529-559
The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe
for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as
a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus
Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully
observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars
in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison
– using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative
of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship
with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed
to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the
comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will
be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload
of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral
bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification
of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent
of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied. 相似文献
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