A new model for cometary nuclei

A new model for the nucleus of comets is presented, hypothesizing formation at large heliocentric distances from many independent solid bodies. It is shown that such a configuration would collapse to a single assemblage if it is to survive into the inner solar system. Prior to collapse, the bodies would be subject to coating by interstellar gas and particles, which would form the material lost into the coma at subsequent inner solar system perihelia. Quantitative estimates place an upper limit to the body sizes of 2.3m and a lower limit of the number as 3 × 101° with sizes of a few tenths of a micron and numbers of about 10³³ most probable. The major structural and evolutionary features of such comet nuclei are consistent with the Whipple icy-conglomerate model.     

Do comets have satellites?

The recent evidence that many minor planets may have satellites, together with recently iscovered physical, chemical, and lightcurve similarities between minor planets and comets, lead naturally to the question, “Might comets have satellites also?” This paper explores several puzzling features of comets which do not fit easily into conventional cometary models, but which can be satisfactorily explained if it is assumed that comets have a full range of gravitationally bound masses, from dust size to the size of the nucleus, in orbit around the principal nucleus. This discussion also implies a higher probability of destruction of a spacecraft near a comet than is usually assumed.     

The non-existence of the Oort cometary shell

The procedures adopted as theory for a shell of comets are shown to be invalid. Any plot of numbers of Long-period comets against 1/a will automatically exhibit a peak at small values of this parameter, and cannot be inverted to demonstrate a high volume-density of aphelia in space. The positions of actual aphelion-points show no sign of any concentration at any range. Further, the aphelion-distance undergoes large almost random changes owing to planetary perturbations at each return, and present values can yield no indication of original positions. That a group of some forty, or even twenty, comets selected for other reasons would all be coming in for the first time has such evanescent probability as to be an entirely inadmissible assumption. Moreover, if comets were perturbed from almost circular motion in the supposed shell to the observed almost parabolic motions, the requisite changes of energy would in general displace the new aphelion-points far outside the shell, so that again the latest aphelion-distances could afford no indication of the position of the shell. To maintain the supply of observable long-period comets by means of stellar perturbations from the shell would require there to be hundreds of stars currently within distance of the order of a parsec from the Sun. The suggested origin for the comets of the shell as having being ‘born’ by unspecified process at planetary distances, from there to be ejected by planetary action to the domain of the shell, and for these orbits then to be rounded up by stellar action is contrary to every consideration of dynamical theory. A recent attempt to save the shell-theory by making use solely of comets with large perihelion-distance is shown to rest on exactly the same errors as have done all the earlier presentations. The plain conclusions emerge that the shell-theory is devoid of any support by facts, and that the alleged shell of comets is non-existent.     

A comparison of the continuum spectra of four comets

The continuum spectra of comets carry information concerning the physical and chemical properties of solid coma grains. Although it is not feasible to use the continuum spectra to uniquely characterize the solid grains, variations among the continua of different comets may reveal subtle differences in their respective grain populations. We have taken and reduced optical spectra of four comets in the wavelength range 3700–7300 Å using a single observing system and reduction procedure. The continua all appear reddened with respect to the solar spectrum. The amount of reddening is consistent with a prevalence of ～2-μm-sized grains in all four comets, if the refractive indices of the grains are approximately equal to those of terrestrial rocks. Significant color differences were measured among the comets. Different intrinsic grain properties are suggested since the scattering geometries were very similar. The amount of reddening does not appear to be correlated with the amount of dust in the coma.     

Composition of Comets: Observations and Models

We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N₂, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO₂, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.     

A model of the galactic tidal interaction with the oort comet cloud

We consider a model of the in situ Oort cloud which is isotropic with a random distrihution of perihelia directions and angular momenta. The energy distribution adopted has a continuous range of values appropriate for long-period (>200 yr) comets. Only the tidal torque of the Galaxy is included as a perturbation of comet orbits and it is approximated to be that due to a quasi-steady state distribution of matter with disk-like symmetry. The time evolution of all orbital elements can be analytically obtained for this case. In particular, the change in the perihelion distance per orbit and its dependence on other orbital elements is readily found. We further make the assumption that a comet whose perihelion distance was beyond 15 AU during its last passage through the Solar System would have orbit parameters that are essentially unchanged by planetary perturbations. Conversely, if the prior passage was inside 15 AU we assume that planetary perturbations would have removed the comet from the in situ energy distribution accessible by the galactic tide. Comets which had their perihelia changed from beyond 15 AU to within 5 AU in a single orbit are taken to be observable. We are able to track the evolution of 10⁶ comets as they are made observable by the galactic tidal touque. Detailed results are obtained for the predicted distribution of new (0 < 1/ < 10^–4 AU^–1) comets. Further, correlations between orbital elements can be studied. We present predictions of observed distributions and compare them with the random in situ results as well as with the actual observed distributions of class I comets. The predictions are in reasonable agreement with actual observations and, in many cases, are significantly different from random when perihelia directions are separated into galactic northern and southern hemispheres. However the well-known asymmetry in the north-south populations of perihelia remains to be explained. Such an asymmetry is consistent with the dominance of tidal torques today if a major stochastic event produced it in the past since tidal torques are unable to cause the migration of perihelia across the latitude barriers ±26°.6 in the disk model.     

Formaldehyde polymers in comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO) _n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material.     

Comet Splitting – Observations and Model Scenarios

Splitting events affect cometary nuclei to a different level of severity ranging from complete disruption of the nucleus (e.g., C/1999 S4 LINEAR) to separation of major fragments (e.g., 73P/Schwassmann-Wachmann 3) and spill-offs of smaller boulders (e.g., C/2001 A2 LINEAR).Fragmentation of comets produces secondary products over a wide range of sizes (from cometesimals to sub-micron dust). It is detectable through the presence of fragments (with own comae and tails) in the coma of the parent nucleus, through outbursts in its activity and through arc-lets (“coma wings”)associated with fragments. The secondaries have different life times and show different non-gravitational forces. Nucleus splitting is also considered to generate whole families of comets (Kreutz group) or — if gravitational bound — multiple nuclei (e.g., C/1995 O1 Hale-Bopp). It may explain the striae phenomena seen in dust tails of bright comets (C/1995 O1 Hale-Bopp) and the detection of chains of impact craters onother bodies in the solar system. As process of significant mass loss it is relevant for the scenario of nucleus extinction, at the same time it also plays a role for the number statistics of existing (observable) comets and for the size distribution of comet nuclei. Various model scenarios for nucleus splitting are proposed: tidal disruption, rotational splitting, break-up due to internal gas pressure, fragmentation due to collision with other bodies. Only in one case, Comet D/1993 F1Shoemaker-Levy 9, the physical process of fragmentation could be undoubtedly identified. In any case, comet splitting provides important insights inthe internal structure, surface layering and chemistry of comet nuclei.     

Long-term evolution of Oort Cloud comets: capture of comets

We test different possibilities for the origin of short-period comets captured from the Oort Cloud. We use an efficient Monte Carlo simulation method that takes into account non-gravitational forces, Galactic perturbations, observational selection effects, physical evolution and tidal splittings of comets. We confirm previous results and conclude that the Jupiter family comets cannot originate in the spherically distributed Oort Cloud, since there is no physically possible model of how these comets can be captured from the Oort Cloud flux and produce the observed inclination and Tisserand constant distributions. The extended model of the Oort Cloud predicted by the planetesimal theory consisting of a non-randomly distributed inner core and a classical Oort Cloud also cannot explain the observed distributions of Jupiter family comets. The number of comets captured from the outer region of the Solar system are too high compared with the observations if the inclination distribution of Jupiter family comets is matched with the observed distribution. It is very likely that the Halley-type comets are captured mainly from the classical Oort Cloud, since the distributions in inclination and Tisserand value can be fitted to the observed distributions with very high confidence. Also the expected number of comets is in agreement with the observations when physical evolution of the comets is included. However, the solution is not unique, and other more complicated models can also explain the observed properties of Halley-type comets. The existence of Jupiter family comets can be explained only if they are captured from the extended disc of comets with semimajor axes of the comets   a <5000 au  . The original flattened distribution of comets is conserved as the cometary orbits evolve from the outer Solar system era to the observed region.     

Amino acid survival in large cometary impacts

Abstract— A significant fraction of the Earth's prebiotic volatile inventory may have been delivered by asteroidal and cometary impacts during the period of heavy bombardment. The realization that comets are particularly rich in organic material seemed to strengthen this suggestion. Previous modeling studies, however, indicated that most organics would be entirely destroyed in large comet and asteroid impacts. The availability of new kinetic parameters for the thermal degradation of amino acids in the solid phase made it possible to readdress this question. We present the results of new high-resolution hydrocode simulations of asteroid and comet impact coupled with recent experimental data for amino acid pyrolysis in the solid phase. Differences due to impact velocity as well as projectile material have been investigated. Effects of angle of impacts were also addressed. The results suggest that some amino acids would survive the shock heating of large (kilometer-radius) cometary impacts. At the time of the origins of life on Earth, the steady-state oceanic concentration of certain amino acids (like aspartic and glutamic acid) delivered by comets could have equaled or substantially exceeded concentrations due to Miller-Urey synthesis in a CO₂-rich atmosphere. Furthermore, in the unlikely case of a grazing impact (impact angle ～5° from the horizontal), an amount of some amino acids comparable to that due to the background steady-state production or delivery would be delivered to the early Earth.     

Formaldehyde Polymers in Comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO)_n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Role of Giant Molecular Clouds in the Evolution of the Oort Comet Cloud

We estimated the gravitational influence of giant molecular clouds passing near the Solar system on the orbital evolution of Oort cloud comets. We performed a comparative analysis of the accuracies of the following two methods of allowance for the perturbations from giant molecular clouds: the impulse approximation and numerical integration. The impulse approximation yields fairly accurate estimates of the change in the energy of Oort cloud comets and the probability of their ejection under the influence of a molecular cloud if the path of the Solar system does not cross its boundary and if the molecular cloud may be treated as a point perturbing mass. The comet survival probability in the Oort cloud depends significantly on the internal structure of the perturbing molecular cloud and the impact parameter of the encounter. The most massive injection of comets into the planetary region and their ejection from the Oort cloud take place if the Solar system passes through a giant molecular cloud composed of several high-mass condensations. In this case, most of the comets injected into the planetary region were initially comets of the inner Oort cloud (a 10^–4 AU) with high orbital eccentricities.     

Origin of oort cloud comets in the interstellar space

Comets must form a major part of the interstellar medium. The solar system provides a flux of comets into the interstellar space and there is no reason to suspect that many other stars and their surrounding cometary systems would not make a similar contribution. Occasionally interstellar comets must pass through the inner solar system, but Whipple (1975) considers it unlikely that such a comet is among the known cases of apparently hyperbolic comets. Even so the upper limit for the density of unobserved interstellar comets is relatively high.In addition, we must consider the possibility that comets are a genuine component of interstellar medium, and that the Oort Cloud is merely a captured part of it (McCrea, 1975). Here we review various dynamical possibilities of two-way exchange of comet populations between the Solar System and the interstellar medium. We describe ways in which a traditional Oort Cloud (Oort, 1950) could be captured from the interstellar medium. However, we note that the so called Kuiper belt (Kuiper, 1951) of comets cannot arise through this process. Therefore we have to ask how necessary the concept of the yet unobserved Kuiper belt is for the theory of short period comets.There has been considerable debate about the question whether short period comets can be understood as a captured population of the Oort Cloud of comets or whether an additional source has to be postulated. The problem is made difficult by the long integration times of comet orbits through the age of the Solar System. It would be better to have an accurate treatment of comet-planet encounters in a statistical sense, in the form of cross sections, and to carry out Monte Carlo studies. Here we describe the plan of action and initial results of the work to derive cross sections by carrying out large numbers of comet — planet encounters and by deriving approximate analytic expressions for them. Initially comets follow parabolic orbits of arbitrary inclination and perihelion distance; cross sections are derived for obtaining orbits of given energy and inclination after the encounter. The results are used in subsequent work to make evolutionary models of the comet population.     

Protosolar nebula and composition of comets

Comets seem to be composed of matter, which is supposed to have the same molecular composition as protosolar nebula. Although there are no unbiased evidence that cometary nuclei retain the molecular composition inherited from the protosolar cloud, the observed properties of comets indicate that there is at least a resemblance between cometary composition and the material properties of dense interstellar clouds. Therefore the origin of comets could be searched in the cold stages of the protosolar nebula and molecular abundances of grain mantles in this nebula may be similar to those in the cometary dust. It is suggested that comets may contain pristine, virtually unaltered protosolar material and their study might be very relevant way to more information about processes in early stages of the solar nebula. Our knowledge about composition of the cometary nucleus is still relatively scarce, but we can partly deduce it from data obtained either by ground-based spectroscopy or by in situ mass spectrometry from space experiments. Most important were the discovery of fluffy CHON particles composed partly or even completely from compounds containing light elements. No consensus concerning the presence of interstellar pristine matter in comet has been reached from various approaches to determine the relationship between comets and interstellar grains. Most of these studies are based on infrared spectroscopy. Another method is the comparison on the chemical models of the protosolar nebula with the volatile compounds of the cometary nuclei. Both gas-phase and grain-surface chemistry are considered and initial gas-phase atomic abundances are assumed to be protosolar. The cometary matter is certainly not identical with the typical material of dense interstellar cool dense clouds, but it is closer to it than any other type of matter in solar system so far accessible to us. The data from comets combined with models of chemical evolution of matter in environment similar as prevailed the early stage of presolar nebula may at least impose constrains on the condition for comet formation. Here presented study is a preliminary contribution to such studies.     

Periodicity in the crater formation rate and implications for astronomical modeling

Claim for periodicity in the crater formation rate is reinvestigated using a criterion proposed by Broadbent, and data sets of Rampino and Stothers and of Grieve are shown to satisfy the periodicity criterion (P 30 Myr).On the other hand, currently observed impactors are mainly asteroids, while long and short periodic comets whose fluxes may vary by external disturbances occupy only a small fraction. Using a Monte Carlo simulation, constraints are obtained for the dispersion Q(Myr) from an exact periodicity and for the periodic components (F _tp) in the signals for their periodicity to be detected. It is found that for = 5, 6 and 7 Myr, F _tp, would have to be 40% or greater, 60% or greater and 80% or greater, respectively. These constraints are used to discuss whether the giant molecular cloud perturbations can give rise to the periodicity in the impact events. The amplitude of the solar Z-motion need to be some 100pc for = 6 Myr, which requires the periodic component (SP and LP comets, if the former originate from the latter) to be 60%, while for = 7 Myr, the periodic component need to be 80%. The GMC perturbation model consistent with the periodicity appears to be the one where the amplitude is 100pc and the periodic component - 60% of the impactors. If SP comets mainly originate from a source such as the hypothetical Kuiper belt, the GMC perturbation would not be consistent with the periodicity.     

The composition of dust in Jupiter-family comets inferred from infrared spectroscopy

We review the composition of Jupiter-family comet (JFC) dust as inferred from infrared spectroscopy. We find that JFCs have silicate emission features with fluxes roughly 20-25% over the dust continuum (emission strength 1.20-1.25), similar to the weakest silicate features in Oort Cloud (OC) comets. We discuss the grain properties that alter the silicate emission feature (composition, size, and structure/shape), and emphasize that thermal emission from the comet nucleus can have significant influence on the derived silicate emission strength. Recent evidence suggests that grain porosity is the is different between JFCs and OC comets, but more observations and models of silicates in JFCs are needed to determine if a consistent set of grain parameters can explain their weak silicate emission features. Models of 8 m telescope and Spitzer Space Telescope observations have shown that JFCs have crystalline silicates with abundances similar to or less than those found in OC comets, although the crystalline silicate mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed with mid-infrared spectroscopy, and we review the evidence for heterogeneous dust properties in the nucleus of comet 9P/Tempel. Models of dust formation, mixing in the solar nebula, and comet formation must be able to explain the observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel, although more work is needed in order to understand to what extent do comets 9P/Tempel and Hale-Bopp represent comets as a whole.     

Expected characteristics of cometary meteorites

Abstract— Recent developments in our understanding of comets provide insights into the topic of cometary meteorites. These developments include the identification of comet-asteroid transition objects (such as 4015 Wilson-Harrington and 3200 Phaethon), information on the composition of cometary solids, and new ideas on the collisional history of Jupiter-family comets. In this work, we revisit this question, and we conclude that comets do indeed yield macroscopic meteorites, which either have not been found or have not been recognized. We also consider the expected characteristics of cometary meteorites, with an emphasis on those that may help identify and differentiate them from other types of meteorites. If cometary meteorites have preserved the main characteristics of cometary dust, the mineralogy would be dominated by highly unequilibrated anhydrous silicates, and the chemistry would be nearly chondritic but with a high abundance of C and N. On the other hand, if an unknown process produced extensive aqueous alteration in the material that formed cometary meteorites, they would resemble (or could even be) CI carbonaceous chondrites. We do not expect cometary meteorites to have chondrules. So far, no single meteorite looks unequivocally cometary. However, we have identified xenoliths in ordinary chondrite regolith breccias that meet most of our criteria for a cometary origin and deserve further study.     

Vaporization of comet nuclei: Light curves and life times

We have examined the effects of vaporization from the nucleus of a comet and show that a latitude dependence of vaporization can, in some cases, explain asymmetries in cometary light curves. We also find that a non-uniform distribution of solar radiation over a comet can considerably shorten the vaporization lifetime compared to the results normally obtained by assuming that the nuclear surface is isothermal.Independent of any latitude effects, comets with CO₂-dominated nuclei and with perihelion distances less than 0.5 AU have vaporization lifetimes less than or comparable to their dynamical ejection times. This may explain the observed deficit of comets with small perihelion distances. Similarly comets with CO₂-dominated nuclei and perihelia near Jupiter's orbit have vaporization lifetimes that are shorter than the time for capture into short-period orbits. We suggest, therefore, that at least some new comets are composed in large part of CO₂, while only H₂O-dominated comets, with lower vaporization rates, can survive to be captured into short-period orbits.     

彗星的CCD成像观测

近年来，对一系列彗星进行的宽带或带ＣＣＤ测光得到了许多有趣的结果，为深入地理解彗星的物理性质，结构，起源和演化等提供了丰富的信息。文中简要地介绍了利用ＣＣＤ成像观测在测量彗核的自转，大小，开头，质量和研究彗核的活动以及彗发的形成和演化等方面的进展。     

The Spatial Distribution of Gaseous Atomic Sodium in the Comae of Comets: Evidence for Direct Nucleus and Extended Plasma Sources

Sodium D-line emission (5890 and 5896 Å) has been observed in bright comets at small to moderate heliocentric distances for many years. We present here the first in depth study of a set of spatial profiles of the sodium D-line emission constructed from long-slit spectroscopic observations of Comets Bennett C/1969 Y1, Kohoutek C/1973 D1, and 1P/Halley. Preliminary analysis of these data lead to the suggestion by Combiet al.(1996,A Plasmagenic Source for Gaseous Sodium in Comets.Presented at Asteroids, Comets, Meteors) that a major fraction of the gaseous sodium was produced by an extended source in the tail and that the source was likely to be some charged species. Dissociative recombination of a molecular ion was suggested. The spatial profiles of sodium are not like typical neutral species. The inner region from the nucleus (<2 × 10⁴km) can be explained in terms of a model that accounts for collisional entrainment in the expanding coma and the heliocentric velocity dependent fluorescence rate and radiation pressure acceleration. This source comes either directly from the nucleus or has a very short-lived parent (?10³s). Away from the nucleus, down the tail and to the sides, the spatial profile slope flattens, indicating a second extended source. The striking similarity of the extended region of sodium spatial profiles with those of ions (H₂O⁺), both along and perpendicular to the tail, is highly suggestive that an ion source is responsible for the production of the extended component of gaseous sodium in the coma. The production rate of the highly variable extended source when present is four to five times that of the direct nucleus source. Observations (Schneideret al., 1991,Science253,1394–1397) and quantitative model analyses (Wilson and Schneider, 1994,Icarus111,31–34) have shown that a dissociative recombination of a sodium bearing molecular ion (NaX⁺) produces a peculiar component of the neutral sodium near Io. It displays a variable spatial morphology consistent with that of a molecular ion source “picked-up” in the plasma torus corotating with Jupiter's magnetic field. The rapid onset of the appearance of gaseous sodium in bright comets, its spatial distribution in the extended coma and near tail, and recent observations of sodium tails are all consistent with our original suggestion of this plasma source for sodium in comets.