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1.
Here we estimate the regions of stability, fragmentation, and destruction for cometary bodies versus rotational breakup in the radius-rotational period plane. By testing different plausible physical models of the cometary nucleus equation of state, we show that the plane is divided into 3 segments: the allowed, damaged, and forbidden regions. We then compare the location of well-observed comets with respect to the separation lines. The range of constituent material parameters from the literature for cometary nuclei are used to show that all the observed comets lie in the allowed region, except for Comet C/1995 O1 (Hale-Bopp), which resides in the damaged region (where the body is fractured and only held together gravitationally). We speculate that the extremely high activity demonstrated by Comet Hale-Bopp during the 1997 apparition may have been due to its highly fractured state. Comet Hyakutake, observed to emit fragments at perigee in 1996, may be near the boundary of the damaged region. Comet C/1999 S4 (LINEAR) was solidly in the rotationally allowed region, making its disintegration in July 2000 due to centrifugal forces unlikely. In contrast to the comets, the centaurs do not cluster in the allowed region, with the majority falling instead into the rotationally damaged and forbidden regions. The centaurs are only stable against breakup assuming much stronger solid water ice properties, strongly suggesting that on the whole, these bodies have different bulk physical properties than cometary nuclei. 相似文献
2.
JHK colors of 14 comets are correlated with cometary distance from the Sun. The correlation could be explained by (1) changes in coma particle size as comets approach the Sun, (2) decrease in the ice/dirt ratio in coma grains as comets approach the Sun, and/or (3) phase reddening. Short-term color changes in individual comets at fixed phase angles suggest that phase reddening does not explain all color changes. Short-term changes are consistent with jets injecting fresh (high ice/dirt) nuclear material into parts of the coma. All colorimetric data are consistent with pristine coma material being relatively low-albedo dirty ice grains colored by carbonaceous dirt like that in RD-type asteroids. Ice sublimation near the Sun may leave residual pure RD dirt grains, explaining the observed color changes. 相似文献
3.
The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright
comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion.
CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate
reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh.
Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus
and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other
phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma.
We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet
moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet
Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a
model with crystallization of amorphous ice close to the nuclear surface.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
5.
A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed. 相似文献
6.
M. E. Bailey 《Earth, Moon, and Planets》1996,72(1-3):57-68
The process of comet formation through the hierarchical aggregation of originally submicron-sized interstellar grains to form micron-sized particles and then larger bodies in the protoplanetary disc, culminating in the formation of planetesimals in the disc extending from Jupiter to beyond Neptune, is briefly reviewed. The ‘planetesimal’ theory for the origin of comets implies the existence of distinct cometary reservoirs, with implications for the immediate provenance of observed comets (both long-period and short-period) and their evolution as a result of planetary perturbations and physical decay, for example splitting and sublimation. The principal mode of cometary decay and collisional interaction with the terrestrial planets is through the formation and evolution of streams of cometary debris and hitherto undiscovered ‘families’ of cometary asteroids. Recent dynamical results, in particular the sungrazing and sun-colliding end-state for short-period comet and asteroid orbits, are briefly discussed. 相似文献
7.
Robert H. Brown Dante S. Lauretta Britney Schmidt John Moores 《Planetary and Space Science》2012,60(1):166-180
Recent ground-based and spacecraft observations of comets in the inner solar system reveal two distinct features that provide important insight into their origin and evolution. The first of these is the observation that the D/H ratio of water vapor in cometary comae is significantly higher than that in Vienna Standard Mean Ocean Water (VSMOW). The second observation is that cometary jets are bursty (i.e, roughly steady state emissions that are punctuated with short-lived outbursts of water vapor and other materials) (Hughes, 1990, Soderblom et al., 2002, Soderblom et al., 2004a, Soderblom et al., 2004b). We present an experimental and theoretical study of ice sublimation in a vacuum that reveals several heretofore unknown and fundamental characteristics about the kinetics and mechanisms of ice sublimation that may explain both of these observed phenomena. In particular, we observe quasi-periodic sublimation cascades on time scales of hours to days, the D/H ratio in the vapor issuing from the sample is in general different from that of the sample, and in many cases, quasi-periodic changes in the D/H ratio of the vapor accompany the sublimation cascades. Changes are also observed in the infrared spectrum of the sample before, during and after a cascade that are consistent with our hypothesis that vacuum sublimation of water ice is a diffusive process that works to leave behind the most strongly bound molecules. Finally, we speculate as to whether the effects observed in the lab can be extrapolated to cometary-nucleus-scale phenomena. 相似文献
8.
P. Gronkowski 《Astronomische Nachrichten》2004,325(4):343-352
The possibility of impacts and their results in relation to the cometary outbursts between comets and other small bodies in the solar system has been investigated. Taking into consideration certain physical features of cometary nuclei and impacting bodies, the probability of impacts of small bodies moving in the main asteroid belt with hypothetical comets which represent three types: Jupiter family comets, Halley family comets and long period comets has been computed. The probability of impacts between comets and meteoroids at large heliocentric distances has also been estimated. Potential consequences of these events in relation to outbursts of the cometary brightness have been discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
Predicted brightness temperatures for a variety of cometary nucleus models, consisting of homogeneous layers comprised of mixtures of water ice and refractory grains, are presented as functions of wavelength. These illustrative spectra are computed using simple radiative transfer techniques adapted from modeling of terrestrial ice and snow fields. The computed millimeter-wave spectra are sensitive to the values of physically significant nucleus parameters such as crust thickness, the subsurface temperature gradient, and the boundary temperature of the sublimating surface. It appears that millimeter-wave sensing from an interplanetary spacecraft is an effective means for distinguishing between alternate models of the nucleus and for inferring the rough physical state of substrata; modern theories on the nature of the nucleus indicate that sublimation from the substrata provides the gas phase cometary volatiles that are actually observed from ground-based and Earth-orbiting instruments. Antenna beam dilution is a major obstacle for ground-based molecular spectral line radio observations (e.g., water and ammonia) of comets but a modest millimeter-wave radiometer system in the near vicinity of the comet would not be subject to this problem. Such a system can make definitivebservations of several candidate parent molecules in the gas phase and should contribute to the understanding of the physics of the inner coma. 相似文献
10.
Diversity of the initial rocky planetary building materials at the edge of the solar system 
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下载免费PDF全文 Asteroids and comets are surviving members of the vast planetesimal population that was distributed across the early solar system. They appear to be a diverse set of bodies but we present evidence from comet samples that the body‐to‐body diversity of the initial rocky component mix in planetesimals may have declined with distance from the Sun. Laboratory measurements of the minor element Mn in olivine collected from Comet Wild 2 suggests that the micron‐sized rocky crystalline contents of this comet formed in numerous inner solar system environments. The results are consistent with a scenario where silicates such as olivine form at incandescent temperatures in multiple environments and then mix as they are transported to distant cold regions where silicates could accrete with ice and organics to form comets. Accreting far from silicate formation regions, many ice‐rich planetesimals are likely to have started with similar complex mixtures of diverse rocky components formed in various high‐temperature environments. This contrasts with asteroidal meteorite parent bodies whose silicates retain regional properties that give different chondrite classes their distinctive properties. 相似文献
11.
We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous-crystalline ice phase transition (T∼137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive. 相似文献
12.
Optical observations of comets and atmosphereless celestial bodies show that a change of sign of the linear polarization of scattered light from negative to positive at phase angles less than 20° is typical of the cometary coma, as well as of the regolith of Mercury, the Moon, planetary satellites, and asteroids. To explain a negative branch of polarization, this research suggests a unified approach to the treatment of cometary-dust particles and regolith grains as aggregate forms. A composite structure of aggregate particles resulting in the interaction of composing structural elements (monomers) in the light-scattering process is responsible for the negative polarization at small phase angles, if the monomer sizes are comparable to the wavelength. The characteristics of single scattering of light calculated for aggregates of this kind turned out to be close to the properties observed for cometary dust. Unlike the cometary coma, the regolith is an optically semi-infinite medium, where the interaction between particles is significant. To find the reflectance characteristics of regolith, the radiative-transfer equation should be solved for a regolith layer. In this case, the interaction between scatterers can be modeled to a certain extent by representing the regolith grains as aggregate structures consisting of several or many elements. Although real regolith grains are much larger than the particles considered here, laboratory measurements have shown that it is precisely the surface irregularities comparable to the wavelength that cause a negative branch of polarization. The main observed features of the phase and spectral dependence of the linear polarization of light scattered from comets and atmosphereless celestial bodies, which are due to the difference of the elementary scatterers in composition, size, and structure, can be successfully explained using the aggregate model of particles. 相似文献
13.
P. Gronkowski 《Astronomische Nachrichten》2009,330(8):784-790
One explanation of the sudden changes in the brightness of comets is proposed based on the author's earlier suggestions involving the fragmentation of cometary grains. Within the inner coma, a core‐mantle model of the structure of grains is assumed. The proposed mechanism is a combination of electrostatic stress and thermodynamical fragmentation of the cometary grains water‐ice mantle. It has been shown that the vapour pressure of volatile inclusions placed in the waterice mantle of grains can increase sufficiently to cause their fragmentation. It takes place before grains can completely sublime into the vacuum away. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. The proposed approach yields increases in cometary brightness consistent with observations of typical cometary outbursts. It is concluded that this approach can provide an explanation of the sudden change in activity of comets for a wide range of heliocentric distances (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
P. Nurmi M.J. Valtonen J.Q. Zheng 《Monthly notices of the Royal Astronomical Society》2001,327(4):1367-1376
Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater. 相似文献
15.
Andrew F. Cheng 《Icarus》2006,184(2):584-588
Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt. 相似文献
16.
A laboratory experiment has been made where a plasma stream collides with targets made of different materials of cosmic interest. The experiment can be viewed as a process simulation of the solar wind particle interaction with solid surfaces in space — e.g., cometary dust. Special interest is given to sputtering of OH and Na.It is also shown that the erosion of solid particles in interplanetary space at large heliocentric distances is most likely dominated by sputtering and by sublimation near the Sun. The heliocentric distance of the limit between the two regions is determined mainly by the material properties of the eroded surface, e.g., heat of sublimation and sputtering yield, a typical distance being 0.5 AU.It is concluded that the observations of Na in comets at large solar distances, in some cases also near the Sun, is most likely to be explained by solar wind sputtering. OH emission in space could be of importance also from dry, water-free, matter by means of molecule sputtering. The observed OH production rates in comets are however too large to be explained in this way and are certainly the results of sublimation and dissociation of H2O from an icy nucleus. 相似文献
17.
The nature of cometary volatile materials is subject to debate. Theoretical models of cometary nuclei and laboratory studies suggest that these objects could be made of amorphous water ice in addition to other volatile molecules and refractory grains. This water ice structure has the ability to encapsulate the gases of surrounding environment, reflecting the physical and chemical conditions during their deposition. Therefore, the knowledge of the chemical composition of volatile molecules trapped in amorphous water ice provides a tool for probing the formation environment of cometary ice grains. Experimental studies of gas trapping efficiency in amorphous water ice have been previously conducted mostly under kinetic conditions, where dynamic pumping and temperature gradients prevented rigorous calibrations. In this work, we investigated the trapping efficiencies of Ar, CO, CH4, Kr and N2 by depositing water vapor as ice in the presence of trace gases in a volume submerged in liquid nitrogen at 77 K. The gas trapping efficiencies were determined simply by monitoring the pressure difference of the trace gases before and after the deposition of a known amount of water molecules as amorphous ice.Our results show that the trapped gas to water molecule ratio in amorphous ice is controlled primarily by the partial pressure of the gas during water ice deposition, and is independent of the ice deposition rate as well as the gas to water ratio in the vapor phase. The trapping efficiencies of gases decrease in the order of Kr > CH4 > CO > Ar > N2 in accordance with previous studies. Assuming that the water ice structure of comets is at least partially amorphous water ice at the time of their formation, these results suggest that the total pressure and composition of the surrounding environment of amorphous ice formation are significant controlling factors of trace gas concentrations in cometary ice. This further indicates that the evolution of the solar nebula and timing of cometary ice condensation can also be important parameters in linking the volatile contents of comets and their formation process. 相似文献
18.
P. Gronkowski 《Monthly notices of the Royal Astronomical Society》2007,379(3):1049-1052
The possibility of impacts between comets belonging to the Jupiter Family and other small bodies orbiting in the main asteroid belt, and the consequences in relation to cometary activity are discussed. The probability of such events and the jumps in cometary brightness caused by impacts are examined. The results are compared with the results of the Deep Impact mission to Comet 9P/Tempel 1. The main conclusion of this paper is in agreement with previous findings, namely that an impact mechanism cannot be the main cause of the outburst activity of comets. 相似文献
19.
The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii 1 km < R < 100 km was investigated. The radioisotopes considered were 26Al, 40K, 232Th, 235U, and 238U. Except for the 26Al abundance, which was varied, the other initial abundances were kept fixed, at values derived from those of chondritic meteorites and corresponding to a gas-to-dust ratio of 1. The initial models were homogeneous and isothermal (To = 10 K) amorphous ice spheres, in a circular orbit at 10(4) AU from the Sun. The main object of this study was to examine the conditions under which the transition temperature from amorphous into cubic ice (Ta = 137 K) would be reached. It was shown that the influence of the short-lived radionuclide 26Al dominates the effect of other radioactive species for bodies of radii up to approximately 50 km. Consequently, if we require comets to retain their ice in amorphous form, as suggested by observations, an upper limit of approximately 4 x 10(-9) is obtained for the initial 26Al abundance in comets, a factor of 100 lower than that of the inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of a coexistence of molten cometary cores and extended amorphous ice mantles is ruled out. Larger icy spheres (R > 100 km) reached Ta even in the absence of 26Al, due to the decay of the other radionuclides. As a result, a crystalline core formed whose relative size depended on the composition assumed. Thus the outermost icy satellites in the solar system, which might have been formed of ice in the amorphous state, have probably undergone crystallization and may have exhibited eruptive activity when the gas trapped in the amorphous ice was released (e.g., Miranda). 相似文献
20.
Ivano Bertini 《Planetary and Space Science》2011,59(5-6):365-377
Comets and asteroids have traditionally been considered two distinct separate populations of small bodies in the solar system, according to their different dynamical, observational, and compositional characteristics.The discovery of a new class of objects, the so-called, Main Belt Comets (MBCs), exhibiting a clear cometary activity but having at the same time orbits indistinguishable from the ones of asteroids in the Main Belt provided further evidence that asteroids and comets, rather than two distinct separate classes, represent the end-members of a continuum of small bodies, with compositions from the very rocky to the very icy.Their study is nowadays deepening our knowledge of the formation mechanisms of the solar system and of the distribution of volatile materials in the protoplanetary disk.In this paper the present knowledge of MBCs is reviewed in terms of physical properties derived from observations, dynamical studies about the origin and formation, thermal modeling of the nuclei, investigations about the activation mechanisms, and the eventual contribution to the presence of water on our planet. An overview of the large-scale surveys dedicated to their discovery and of the detection techniques used so far is also given. Moreover, open question and indications for future observations and modeling are outlined. 相似文献