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1.
It is commonly believed that comets are made of primordial material. As a consequence, they can reveal more information about
the origin of our solar system. To interpret the coma composition measurements of comet Churyumov–Gerasimenko that will be
collected by the Rosetta mission, models of the coma chemistry have to be constructed. However, programming the chemistry
of a cometary coma is extremely complex due to the large number of species and reactions involved. Moreover, such a program
needs to be very flexible as one may want to extend, change, or update the set of species, reactions, and reaction rates.
Therefore, we developed software to manage a database of species and reactions and to generate code automatically to compute
source/loss balances. This database includes the data from the UMIST database and the ion–molecule reactions collected by
V.G. Anicich. To use all these databases together, a lot of practical problems need to be solved, but the result is an enormous
source of information about chemical reactions that can be used in chemical models, not only for comets but also for other
applications. 相似文献
2.
The chemical composition of comet nuclei derived from current data on interstellar dust ingredients and comet dust and coma
molecules are shown to be substantially consistent with each other in both refractory and volatile components. When limited
by relative cosmic abundances the water in comet nuclei is constrained to be close to 30% by mass and the refractory to volatile
ratio is close to 1:1. The morphological structure of comet nuclei, as deduced from comet dust infrared continuum and spectral
emission properties, is described by a fluffy (porous) aggregate of tenth micron silicate core-organic refractory mantle particle
on which outer mantles of predominantly H2O ices contain embedded carbonaceous and polycyclic aromatic hydrocarbon (PAH) type particles of size in the of 1 - 10nm range.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
3.
The main production process of species occurring in the coma of comets is the photodestruction of molecules initially present
in the nucleus ices and non-refractory grains or trapped inside the nucleus "material". Grains can also be a source of molecules
in the coma. Chemical reactions may occur between coma species. Consequently, although chances that an abundant coma species
has not been detected are now small, the coma composition is certainly quite different from that of the nucleus. Except for
the molecules released directly at the nucleus surface, all coma species are produced in an "extended region" or come from
"a distributed source". Since the recent literature is rich in reports on observations of molecules and species possibly not
initially present in the comet ices or not released at the nucleus, a general discussion of how coma species are stored, processed
or produced is presented, based mostly on observational results. What is at stake is the proper modeling of the coma structure,
hence an accurate derivation of the nucleus composition from coma observations.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
4.
Analysis of the polarization of light scattered by cometary particles reveals similarities amongst the phase curves, together
with some clear differences: i) comets with a strong silicate emission feature present a high maximum in polarization, ii)
the polarization is always slightly lower than the average in inner comae and stronger in jet-like structures. These results
are in excellent agreement with the Greenberg model of dust particles built up of fluffy aggregates of much smaller grains.
Also, they suggest the existence of different regions of formation, and of different stages of evolution for the scattering
particles inside a given cometary coma.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
5.
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
6.
The distribution of some molecules and radicals (H2CO, CO, HNC, CN,?…) in the atmosphere of several comets cannot be explained only by a direct sublimation from the nucleus, or by gas phase processes in the coma. Such molecules are in part the result of a distributed source in the coma, which could be the photo and thermal degradation of dust. We present a review of the degradation processes and discuss possible interpretations of the observations in which the degradation of solid complex organic material in dust particles seems to play a major role. The knowledge of such gas production mechanisms provides important clues on the chemical nature of the refractory organic material contained in comet nuclei. 相似文献
7.
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
8.
The initial structure of a comet nucleus is most probably a homogeneous, porous, fine-grained mixture of dust and ices, predominantly
water. The water ice is presumably amorphous and includes considerable fractions of occluded gases. This structure undergoes
significant changes during the early evolution of the nucleus at large heliocentric distances, due to internal radiogenic
heating. Structural changes occur mainly as a result of gas flow through the porous medium: the gas pressure that builds up
in the interior is capable of breaking the fragile structure and altering the pore sizes and porosity. These effects are modeled
and followed numerically, testing a large number of parameters.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
The Neutral Mass Spectrometer on the Giotto spacecraft established that H2O is the dominant species in Comet Halley's volatiles and determined the abundance of more than 10 parent species. The instrument
discovered strong extended H2CO and CO sources in the coma of Comet Halley. Polymerized H2CO associated with the cometary dust and evaporating slowly as the monomer is most likely the extended H2CO source. Photodissociation of the H2CO into CO fully accounts for the extended CO source.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
10.
Dust is an important constituent of cometary emission; its analysis is one of the major objectives of ESA’s Rosetta mission
to comet 67P/Churyumov-Gerasimenko (C–G). Several instruments aboard Rosetta are dedicated to studying various aspects of
dust in the cometary coma, all of which require a certain level of exposure to dust to achieve their goals. At the same time,
impacts of dust particles can constitute a hazard to the spacecraft. To conciliate the demands of dust collection instruments
and spacecraft safety, it is desirable to assess the dust environment in the coma even before the arrival of Rosetta. We describe
the present status of modelling the dust coma of 67P/C–G and predict the speed and flux of dust in the coma, the dust fluence
on a spacecraft along sample trajectories, and the radiation environment in the coma. The model will need to be refined when
more details of the coma are revealed by observations. An overview of astronomical observations of 67P/C–G is given, because
model parameters are derived from this data if possible. For quantities not yet measured for 67P/C–G, we use values obtained
for other comets, e.g. concerning the optical and compositional properties of the dust grains. One of the most important and
most controversial parameters is the dust mass distribution. We summarise the mass distribution functions derived from the
in-situ measurements at comet 1P/Halley in 1986. For 67P/C–G, constraining the mass distribution is currently only possible
by the analysis of astronomical images. We find that both the dust mass distribution and the time dependence of the dust production
rate of 67P/C–G are those of a fairly typical comet. 相似文献
11.
12.
C. M. Lisse M. F. A’Hearn T. L. Farnham O. Groussin K. J. Meech U. Fink D. G. Schleicher 《Space Science Reviews》2005,117(1-2):161-192
As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules
and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface
temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 102 km), it is in collisional equilibrium, at larger distances (≥104 km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply
dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics
of the coma, as well as with its variability with time, orbital position, and heliocentric distance.
The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive
measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several
gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986.
This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current
state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards
that the coma presents for the spacecraft.
An erratum to this article is available at . 相似文献
13.
During encounters with comet Halley, the experiment PICCA onboard GIOTTO measured the gas phase organic ion composition of the coma and the experiment PUMA onboard VEGA-1 measured the dust composition. Joining both results, we obtain a consistent picture of the parent organic matter from which dust and gas is produced. One recognizes a complex unsaturated polycondensate, which splits during coma-formation into the more refractory C=C,C-N-containing dust part, and the more volatile C=C,C-O-containing gas part. The responsible exothermal chemical reactions, which are triggered by the sunlight, may play a major role in the dynamics of coma formation.This paper is a shortened and upgraded version of Krueger, F.R., Korth, A., and Kissel, J.: 1989, in S. Chang (ed.) Proc. of the ROSETTA Conf., Milpitas CA, January 1989, submitted. 相似文献
14.
Isotopic ratios in comets provide keys for the understanding of the origin of cometary material, and the physical and chemical
conditions in the early Solar Nebula. We review here measurements acquired on the D/H, 12C/13C, 16O/18O, 14N/15N, 32S/34S ratios in dust and gases, and discuss their cosmogonic implications. The prospects for future measurements from cometary
space missions and remote sensing observations at millimeter and submillimeter wavelengths are presented.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
15.
L. Colangeli J. J. Lopez-Moreno P. Palumbo J. Rodriguez M. Cosi V. Della Corte F. Esposito M. Fulle M. Herranz J. M. Jeronimo A. Lopez-Jimenez E. Mazzotta Epifani R. Morales F. Moreno E. Palomba A. Rotundi 《Space Science Reviews》2007,128(1-4):803-821
The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted
to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the
dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and
time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation
or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The
coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma
evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure
mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10−10 ÷ 4.0 × 10−4 kg m s−1 for velocities up to about 300 m s−1. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10−10 g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties
in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts
demonstrate that GIADA is behaving as expected according to the design specifications.
The International GIADA Consortium (I, E, UK, F, D, USA). 相似文献
16.
R. Schulz 《Space Science Reviews》2008,138(1-4):225-235
This article presents some recent imaging and spectroscopic observations that led to results which are significant for understanding the properties of comet nuclei. The coma morphology and/or composition were investigated for 12 comets belonging to different dynamical classes. The data analysis showed that the coma morphology of three non-periodic comets is not consistent with the general assumption that dynamically new comets still have a relatively uniform nucleus surface and therefore do not exhibit gas and/or dust jets in their coma. The determination of carbon and nitrogen isotopic ratios revealed the same values for all comets investigated at various heliocentric distances. However, the relative abundance of the rare nitrogen isotope 15N is about twice as high as in the Earth’s atmosphere. Observations of comets at splitting events and during outbursts led to indications for differences between material from the nucleus surface and the interior. The monitoring of the induced outburst of 9P/Temple revealed that under non-steady state conditions the fast disintegration of species is detectable. 相似文献
17.
The sodium emissions have been observed in several new and long-period comets, but only for comet Mrkos 1957d (Nguyen-Huu-Doan,
1960) was a sodium tail detected on a Schmidt plate obtained with a objective prism. Comet Hale-Bopp 1995 O1 offered the first
great opportunity to get an image of a long sodium tail. It was more than 3 × 107 km long, defined as a third type of tail, as it was composed only of neutral atoms (Cremonese, 1997a). After the discovery
of the sodium tail another team announced it had observed it (Wilson et al., 1998), but it was soon realized they had seen a different sodium tail. The image of Wilson et al. (1998) showed a very diffuse sodium tail superimposed on the dust tail, most likely due to the release of sodium atoms from
dust particles. It was different from the narrow tail found in the image obtained by the European Hale-Bopp Team and its position
angle was 15-20 degrees lower. Spectroscopic observations have been performed on the dust tail, at different beta values,
and along the narrow sodium tail showing that the sodium emissions had very different line profiles. The analysis of these
profiles will yield important insights into the sources in the inner coma and in the dust tail. This work will report on preliminary
analysis of both sodium tails and emphasize the high-resolution spectroscopy performed on the dust tail.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
18.
Following on from IRAS, ISO has provided a huge advancement in our knowledge of the phenomenology of the infrared (IR) emission
of normal galaxies and the underlying physical processes. Highlights include the discovery of an extended cold dust emission
component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation
of the spectral energy distribution in the IR, revealing the channels through which stars power the IR light; the derivation
of realistic geometries for stars and dust from ISO imaging; the discovery of cold dust associated with H I extending beyond the optical body of galaxies; the remarkable similarity of the near-IR (NIR)/mid-IR (MIR) SEDs for spiral
galaxies, revealing the importance of the photo-dissociation regions in the energy budget for that wavelength range; the importance
of the emission from the central regions in shaping up the intensity and the colour of the global MIR luminosity; the discovery
of the “hot” NIR continuum emission component of interstellar dust; the predominance of the diffuse cold neutral medium as
the origin for the main interstellar cooling line, [C II] 158 μm, in normal galaxies.
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries:
France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA. 相似文献
19.
Fred Goesmann Helmut Rosenbauer Reinhard Roll Cyril Szopa Francois Raulin Robert Sternberg Guy Israel Uwe Meierhenrich Wolfram Thiemann Guillermo Munoz-Caro 《Space Science Reviews》2007,128(1-4):257-280
Comets are thought to preserve the most pristine material currently present in the solar system, as they are formed by agglomeration
of dust particles in the solar nebula, far from the Sun, and their interiors have remained cold. By approaching the Sun, volatile
components and dust particles are released forming the cometary coma. During the phase of Heavy Bombardment, 3.8--4 billion
years ago, cometary matter was delivered to the Early Earth. Precise knowledge on the physico-chemical composition of comets
is crucial to understand the formation of the Solar System, the evolution of Earth and particularly the starting conditions
for the origin of life on Earth. Here, we report on the COSAC instrument, part of the ESA cometary mission Rosetta, which
is designed to characterize, identify, and quantify volatile cometary compounds, including larger organic molecules, by in
situ measurements of surface and subsurface cometary samples. The technical concept of a multi-column enantio-selective gas
chromatograph (GC) coupled to a linear reflectron time-of-flight mass-spectrometer instrument is presented together with its
realisation under the scientific guidance of the Max-Planck-Institute for Solar System Research in Katlenburg-Lindau, Germany.
The instrument's technical data are given; first measurements making use of standard samples are presented. The cometary science
community is looking forward to receive fascinating data from COSAC cometary in situ measurements in 2014. 相似文献
20.
CO observations have shown that many lenticular and elliptical galaxies contain significant amounts of cool dense gas. This review summarizes the observational results related to the neutral gas phase and presents a systematic comparison with other interstellar and stellar data. The discovery of very dense molecular gas in the nuclear regions of early-type galaxies, the possible existence of a dust component neither seen optically nor in CO, internal inconsistencies of cooling flow scenarios, the origin of the cool gas, the presence of massive stars, aspects of galaxy evolution, and possibilities for future research are discussed in the light of the new data. 相似文献