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1.
The comet 67P/Churyumov-Gerasimenko is the current target of the mission Rosetta, initially planned to investigate comet 46P/Wirtanen. These two comets have similar orbits, except the distance to the Sun at perihelion, but different orbital histories and different masses. Thus, structures of the nuclei can be significantly different. The evolution of comet Wirtanen was simulated by several authors, while comet Churyumov-Gerasimenko became an object of high interest only recently and is not well investigated. In the present work we simulate the evolution of the nucleus, down to tens of meters below the surface, using an extended version of the model previously applied for comet Wirtanen [Kossacki et al., 1999. Comet 46P/Wirtanen: evolution of the subsurface layer. Icarus 142, 202-218.]. The model includes strengthening of the nucleus due to sintering of the ice grains. Simulations are performed for different latitudes, accounting for the evolution of the orbit and for changes of the nucleus orientation, as well as diurnal and seasonal changes of insolation. The calculated loss of water vapor from the comet is integrated over the nucleus surface and is compared with the observational data. We have found, that the sublimation through the dust mantle can be large enough to reproduce the profile of the total water production as a function of time from perihelion. The required dependence of thickness of the dust layer on latitude qualitatively matches present distribution of the absorbed solar flux. The non-gravitational acceleration in the comet motion together with the simulated sublimation flux are used in order to estimate the mass and the bulk density of the nucleus.  相似文献   

2.
《Planetary and Space Science》1999,47(6-7):855-872
From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation.The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas.In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO2, CO- and of dust grains embedded in the ice matrix.By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed.Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.  相似文献   

3.
《Planetary and Space Science》1999,47(8-9):971-974
Astronomical observations of size and of related outgassing rates seem not to be compatible for the nucleus of comet 46 Pz.urule;Wirtanen, the target comet of the ROSETTA mission. This possible disagreement has caused speculations about peculiar properties of this comet nucleus. It is shown by model calculations which also takes into account vertical heat fluxes into the nucleus that there is a possibility to combine the results of astronomical observations within a model of a freely sublimating ice surface of this comet with an outgassing area of about half the dayside surface. The resulting half-size parameter (i.e. the radius of an equivalent sphere) can be shown to be of about R ≈ (725±230) m, and the nucleus is shown to have an active area of about half of the dayside surface, i.e. of about 25% of the total surface.  相似文献   

4.
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5.
Since many years cometary ions have been observed by the authors and their coworkers in order to study the comet-solar wind interaction. Comets with water production rates ranging from 1028(46P/Wirtanen) to 6 1030molecules s−1(C/1995 O1 Hale-Bopp) have been observed. In this paper we briefly introduce the physics of the comet-solar wind interaction. New observations of comet C/1996 Q1 (Tabur) are presented, where for the first time H2O+and CO+ions have been recorded exactly simultaneously with a two-channel system. They are compared with previous observations of comets C/1989 X1 (Austin), 46P (Wirtanen) and 109P (Swift-Tuttle). We use a new method of Wegmann et al. (1998), based on the MHD scaling law, to determine the water production of comet Tabur from its H2O+column density map and obtain a value of 3.3 1028water molecules s−1. Nonstationary phenomena like tail rays and so-called tail disconnections are very briefly reviewed. A movie of plasma envelopes observed in the light of OH+in comet 1995 O1 (Hale-Bopp) is presented on the attached CD-ROM. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
Akiva Bar-Nun  Diana Laufer 《Icarus》2003,161(1):157-163
In a unique machine, the first of its kind, large (200 cm2 × 10 cm) samples of gas-laden amorphous ice were prepared at 80 K and 10−5 Torr. The sample consisted of a fluffy agglomerate of 200-μm ice grains, similar to what is presumed to be the structure of comet nuclei. The sample was heated from above by IR radiation. The properties studied were gas content in the ice and its emanation from the ice upon warming and bearing on the gas/water vapor ratio observed in cometary comae vs this ratio in cometary nuclei and the effect of internal trapped gas on the thermal conductivity of the ice and the density and mechanical properties of pure ice vs gas-laden ice. These findings might have significance for the interpretation of comet observations, the forthcoming ESA’s Rosetta space mission to Comet 46P/Wirtanen in 2012, and to other comet missions.  相似文献   

7.
We present a new method to study the long-term evolution of cometary nuclei in order to estimate their original size, and we consider the case of comets 46P/Wirtanen (hereafter 46P) and 67P/Churyumov–Gerasimenko (hereafter 67P). We calculate the past evolution of the orbital elements of both comets over 100 000 yr using a Bulirsch–Stoer integrator and over 450 000 yr using a Radau integrator, and we incorporate a realistic model of the erosion of their nucleus. Their long-term orbital evolution is prominently chaotic, resulting from several close encounters with planets, and this result is independent of the choice of the integrator and of the presence or not of non-gravitational forces. The dynamical lifetime of comet 46P is estimated at ∼133 000 yr and that of comet 67P at ∼105 000 yr. Our erosion model assumes a spherical nucleus composed of a macroscopic mixture of two thermally decoupled components, dust and pure water ice. Erosion strongly depends upon the active fraction and the density of the nucleus. It mainly takes place at heliocentric distances <4 au and lasts for only ∼7 per cent of the lifetime. Assuming a density of 300 kg m−3 and an average active fraction over time of 10 per cent, we find an initial radius of ∼1.3 km for 46P and ∼2.8 km for 67P. Upper limit are obtained assuming a density of 100 kg m−3 and an active fraction of 100 per cent, and amounts to 21 km for 46P and 25 km for 67P. Erosion acts as a rejuvenating process of the surface so that exposed materials on the surface may only contain very little quantities of primordial materials. However, materials located just under it (a few centimetres to metres) may still be much less evolved. We will apply this method to several other comets in the future.  相似文献   

8.
The micro-imaging dust analysis system (MIDAS) is an essential element among the scientific payload on the international Rosetta mission to comet 46P/Wirtanen. The MIDAS instrument based on an atomic force microscope (AFM) collects small particles drifting outwards from the nucleus surface. AFM is able to image small structures in 3D at nanometer-scale resolution. These images provide morphological and statistical information like grain size distribution on the dust population. In order to support the development of the flight hardware, optimisation of the control functions and consolidation of a proper scheme of data interpretation, laboratory studies with similar instruments were carried out. The obtained data demonstrate the capabilities of this technique. For the first time an instrument is able to observe the smallest (nm-sized) grains which are predicted by models and were to a certain extent deduced from previous measurements on the Giotto and Vega missions to comet 1P/Halley. On larger (μm-sized) particles the complex morphology will be visualised with high precision in 3D, and if present, within these aggregates crystalline materials with defined crystal faces can be identified.  相似文献   

9.
The Rosetta spacecraft (S/C), which is planned to meet comet 46P/Wirtanen in 2011, will carry a set of five wave and plasma instruments (i.e. the Rosetta Plasma Consortium). This is to measure the cometary plasma properties from the minimum value of activity of the comet to its maximum value at perihelion. The mutual impedance probe, MIP, is one of those (Trotignon et al., 1999) five. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
Biele  Jens 《Earth, Moon, and Planets》2002,90(1-4):445-458
As a part of ESA's cornerstone mission ``ROSETTA' to comet 46P/Wirtanen a 100 kgLander will bring a scientific payload of almost 27 kg to the surface of the nucleus.After a first scientific sequence it will operate for a considerable fraction of thecometary orbit around the sun (between 3 AU and 2 AU). Ten experiments with a number of sub-experiments are foreseen; this paper presents the current status of the Lander development and reviews the scientific capabilities of each of the experiments at a time when the Flight Model (FM) of the Lander is already delivered.  相似文献   

11.
A simplified thermal model has been used to calculate thenon-gravitational forces acting on small irregular nuclei in the orbit ofComet 46P/Wirtanen. The torque of thenon-gravitational force has beencalculated and the Euler equations have been solved in order toinvestigate the rotational evolution of several irregular nuclei duringa single orbital step. Several initial spin axis orientations andactivity patterns on their surfaces have been considered. The nucleiconsidered have a mean radius of 1 km and their inertia moments havebeen calculated assuming a homogeneous bulk density of 500 kg/m3. Inall the simulations, the initial spin period is 6h and the nucleiinitially rotate around their shortest axis. Under these assumptions,significant changes in the angular momentum and in spin period have beenobtained in all the simulations, but the nucleus is found to practicallyremain in its spin state of lowest energy during the entire orbitalperiod.  相似文献   

12.
This work is dedicated to the application to 67P/Churyumov-Gerasimenko of a new quasi-3D approach for non-spherically shaped comet nuclei with the aim to interpret the current activity of the comet in terms of initial characteristics and to predict shape and internal stratification evolution of the nucleus. The model is applied to differently shaped nuclei taking into account the characteristics of Comet 67P/Churyumov-Gerasimenko deduced from observations. We focus our attention on the combined effects that shapes and obliquity have on the comet surface and sub-surface evolution. We discuss the results in terms of activity, local dust mantle formation and disruption, erosion of the surface and internal stratigraphy.The results show that differently shaped nuclei can have different internal structures leading to different activity patterns and behaviors. Our calculations have shown that local variations in the dust and gas fluxes can be induced by the nucleus shape. The distribution of “active” areas on Comet 67P/Churyumov-Gerasimenko is different because of different shapes, reflecting the illumination conditions on the surface. These shapes can influence the structure of the inner coma, but the coma far away from the nucleus is only marginally affected by the nucleus shape. However, different comet behaviors can arise from differently shaped comet nuclei, especially in terms of local activity, surface and sub-surface characteristics and properties. The water flux local distribution is the most influenced by the shape as it is directly linked to the illumination. Irregular shapes have large shadowing effects that can result in activity patterns on the comet surface.The effects of different pole directions are discussed to see the relations with the nucleus activity and internal structure. It is shown that the orientation of the rotation axis plays a strong role on the surface evolution of 67P/Churyumov-Gerasimenko, determining seasonal effects on the fluxes. The activity of the comet changes greatly with the nucleus obliquity leading to pre-post-perihelion differences in the activity and seasonal effects. The effects of the dust deposition and crust formation on the cometary activity have also been simulated and are discussed with respect to 67P/Churyumov-Gerasimenko observations. The dust mantling is also strongly obliquity dependent, with different surface distributions of the dust-covered regions according to the different comet pole orientations. Finally, we show that our model can reproduce the fluxes behavior near perihelion in terms of amplitude and asymmetry, and we estimate 20% of the illuminated surface to be active.  相似文献   

13.
Benkhoff  J.  Rauer  H. 《Earth, Moon, and Planets》1997,77(3):231-236
Sublimation of minor gases from ices inside of a porous comet nucleus strongly depends on the effective energy input. Our model meant to describe the gas flux inside and out of the porous nucleus has been used to study the influence of physical and structural parameters on the effective energy input. We solve the conservation equations for H2O and CO as the most abundant minor component of higher volatility under appropriate boundary conditions. From the calculations we obtain the gas flux from volatile, icy components inside the porous nucleus, temperature profiles, changes in relative chemical abundances, and the gas flux into the coma for each of the volatiles. We will show results from our calculations for a model comet in the orbit of Hale-Bopp (C/1995 O1). In this paper we focus on the energy balance at the surface. We will also relate measurements of molecule fluxes to available energies and try to provide hints about the evolutionary status of the comet. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

14.
《Planetary and Space Science》1999,47(6-7):827-837
A dust environment working group was encouraged by ESA to provide coma dust environment models useful to plan the ROSETTA operations around the nucleus of short period comet 46P/Wirtanen. Among the many parameters describing the dust released from the nucleus surface, special care was devoted to the dust size distribution. Its present uncertainty makes all environment models sensitive, mainly, to which actual size distribution is adopted. In fact, it must be stressed that no other cometary dust parameter can be derived, such as dust loss rate or dust to gas ratio, if the size distribution remains undetermined. This paper will focus, therefore, on the available information on cometary dust size distributions, starting from the in situ measurement cornerstone provided by the GIOTTO-DIDSY results. Available ground-based observations are then reviewed, in order to disentangle the real sensitivity of them to this quantity; the size distribution is always embedded together with other dust parameters, and its influence on the published results is often forgotten.  相似文献   

15.
A model of cometary activity is developed which integrates the feedback processes involving heat, gas, and dust transport, and dust mantle development. The model includes the effects of latitude, rotation, and spin axis orientation. Results are obtained for various grain size distributions, dust-to-ice ratios, and spin axis orientations. Attention is focused on the development, change of structure and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. In this model the dust mantle controls the mechanism of gas transport not onlu by its effect on the temperature but, more importantly, by its own dynamic stability. Results suggest that an initially homogeneous short-period comet with a “cosmic” dust-to-water ice ratio, typical orbit, rotation rate, and grain size distribution would develop at most only a thin (<1 mm) cyclic mantle at all points on the nucleus. Such a fully developed temporary mantle would exist throughout the diurnal cycle only beyond ～4AU. Thus, cyclic behavior would be expected for such an idealized comet, at least for most of its lifetime. Long-term irreversible mantle development on comets with typical rotation rates was not found except regionally on Encke and also on objects with perihelia ?1.5 AU. Even in these cases, free silicate exists, after a few cycles, only as relatively rare large grains and agglomerates with radii ～1 cm scattered over exposed ice. Full mantle development would require hundreds to thousands of cycles. In the case of an initially homogeneous comet Encke, this slow incipient mantle development is shown to be the direct result of its peculiar axial orientation. High obliquity appears required for long-term mantle development for typical rotation rates and perihelia ?1.5 AU. Heat conduction into the nucleus for an incompletely mantled or bald comet has been found to be very important in maintaining relatively higher ice surface temperatures, and hence fluxes, during those portions of the diurnal and orbital cycles which would otherwise be cooler. It is also shown to be at least one cause of post perihelion brightness asymmetries, especially in lower obliquity comets. Maximum heliocentric distances at which 1-μm dust, sand, pebbles, cobbles, and boulders can be permanently ejected from the subsolar point by H2O (CO2) are (in AU): 6.9 (16.8), 5.2 (11.5), 1.8 (3.0), 0.21 (0.34) and 0.07 (0.11), respectively. A detailed anatomy of temperature, gas and dust fluxes vs latitude and longitude for a homogeneous rotating comet with fixed axis is given for comparison with future observations. Most H2O flux histories deduced from brightness data are found to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. A clear exception is Encke. It is shown that the large discrepancy between Encke's observed and model predicted fluxes, based on radar cross section, can be used to evaluate the extent of exposed ice (<10%). The model is then used to place an active area so as to explain a reported sharp drop in flux on approach to the Sun at 0.78 AU. An active area or areas, <10% of the comet's surface, centered near 65°N latitude appears indicated. Although cyclic mantles are generally indicated for the set of parameters we used, our results show that a global mantle only 1 to 3 cm thick (depending on the orbit) consisting of a full range of grain sizes can cause irresversible evolution to a noncometary body. We investigated the long-term evolution of such a postulated initially thinly mantled cometary object. It was found that after the first few passes and until the end of its dynamic lifetime the object averaged <3 × 10?12 g cm?1 sec?1 H2O flux. Therefore, if cometary objects evolve into Apollo asteroids, ice should always be accessible within 10 m of the surface despite numerous close perihelion passages. The possible impact of factors not included in the model, such as initial inhomogeneities, coma scattering of radiation, and global redistribution of ejected silicate around the nucleus, are discussed.  相似文献   

16.
《Planetary and Space Science》1999,47(6-7):725-733
The SWAN instrument on board SOHO is a Lyman-α photometer able to map the sky intensity with a resolution of 1°, primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background. In addition SWAN was extensively used to map the Lyman α emission of several comets since launch in December 1995. Here we report observations of Comet 46 P/Wirtanen near perihelion. From the recorded Lyman α intensity the H2O production rate was derived for 45 observations from 21 December 1996–17 May 1997, with a peak of 1.6±0.4×1028 mol/s just before perihelion. This should help to constrain the physical models of 46 P/Wirtanen for Rosetta mission planning purposes.  相似文献   

17.
《Planetary and Space Science》1999,47(6-7):765-772
Imaging of comet 46P/Wirtanen was performed in the standard VRI filters on 10 and 11 November 1996 with the 1.0 m telescope of the South African Astronomical Observatory (SAAO). After proper processing and calibration, the images have been used to derive information about the isophote distribution, magnitude and colour indices. We observed a coma extending to at most ∼5×103 km. We derive absolute magnitudes of the coma for the different filters and the colours V−R = 0.18±0.17 and R−I = 0.39±0.16. The Afρ value (related to the dust production rate) for the R filter is 10.5±0.6 cm. The radial profile (azimuthally averaged) of the surface brightness in the R filter has a slope of −0.9, which is consistent with a steady-state dust production. The results are compared with other observations in different periods.  相似文献   

18.
The aim of this modelling work is to assess shape changes of cometary nuclei caused by sublimation of ices. The simplest possible model is assumed with the nucleus being initially spherical and its thermal conductivity being neglected. We have calculated the time-dependent sublimation flux versus cometographic latitude. If the rotation axis of the comet is inclined to the orbital plane, then sublimation leads to non-symmetrical changes of the nucleus shape. Calculations were performed for the nuclei of comets Hale–Bopp and Wirtanen.  相似文献   

19.
Helium and neon distributions are reported for a variety of Stardust comet 81P/Wild 2 samples, including particle tracks and terminal particles, cell surface and subsurface slices from the comet coma and interstellar particle collection trays, and numerous small aerogel blocks extracted from comet cells C2044 and C2086. Discussions and conclusions in several abstracts published during the course of the investigation are included, along with the relevant data. Measured isotope ratios span a broad range, implying a similar range for noble gas carriers in the Wild 2 coma. The meteoritic phase Q‐20Ne/22Ne ratio was observed in several samples. Some of these, and others, exhibit 21Ne excesses too large for attribution to spallation by galactic cosmic ray irradiation, suggesting exposure to a solar proton flux greatly enhanced above current levels in an early near‐Sun environment. Still others display evidence for a solar wind component, particularly one C2086 block with large abundances of isotopically solar‐like helium and neon. Eighty‐nine small aerogel samples were cut from depths up to several millimeters below the cell C2044 surface and several millimeters away from the axis of major track T41. A fraction of these yielded measurable and variable helium and neon abundances and isotope ratios, although none contained visible tracks or carrier particle fragments and their locations were beyond estimated penetration ranges for small particles or ions incident on the cell surface, or for lateral ejecta from T41. Finding plausible emplacement mechanisms and sources for these gases is a significant challenge raised by this study.  相似文献   

20.
We suggest that the regions of smooth terrain which were observed on Comet 9P/Tempel 1 by the Deep Impact spacecraft were formed by blowing ice grains in an outburst of gas from the comet interior. When gas is released from 10 to 20 m deep layers which were heated to 135 K, it is released quiescently onto the surface by individual conduits. If large amounts of gas are released, the drainage system cannot release them fast enough and wider interconnected channels are formed, leading to sudden outburst of gas. Instability triggering a sudden shift of flow is well known in subglacial drainage of water. The ballistic trajectory of the ice particles reach a distance of 3 km in the atmosphereless comet, whose gravity is 0.034 cm s−1, if ejected at an angle of 45° at a speed of 95 cm s−1. This speed is close to the speeds measured in laboratory experiments: 167, 140×sini and 167 cm s−1, for particles of 0.3, 1000 and 14-650 μm, respectively. Blowing of ice grains can overcome the 1650 m long horizontal section of smooth terrain i1 (Fig. 1), whereas simple flow of material downhill would stop close to the foot of the hill. The ice particles at the end of their trajectory have a horizontal velocity component and this low velocity ballistic sedimentation would lead to formation of lineaments on the smooth terrain, like in solid-particulate volcanic eruptions.  相似文献   

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