Halley's Nucleus Research Articles

Organics-glued dust aggregates in Halley's coma and some implications of the snow line inward motion for the formation of comets

In view of the solar nebula models, organics-glued dust aggregates (whose disintegration resulted in the two phenomena found in Halley's coma, the dust boundary and small-scale dust structures) originated due to coagulation of iceless dust particles somewhere within the snow line, and then were incorporated into Halley's nucleus as a consequence of the snow line inward motion. This implies that two types of comets exist: outer comets, formed entirely beyond the snow line, and inner comets, similar to Halley, which are bodies intermediate between outer comets and primitive asteroids. The presence of large iceless dust aggregates in nuclei of inner comets constrains the inward drift velocity of meter-sized dust bodies, which in turn implies that the radial transport of water in the solar nebula was predominantly outward. It is shown that in nuclei of inner comets: both the upper mass limit of iceless dust aggregates and the ice mantle thickness increase with decreasing formation heliocentric distance, while the cumulative mass distribution index decreases; the lower limit of the mass index is ∼0.8, and the upper limit of the ice mantle thickness is ∼10−3 cm (∼200 times the interstellar value); the lower limit of the latent heat of organics in organic mantles of submicron particles increases toward small heliocentric distances; the recondensation of organics combined with the growth of dust bodies leads to a fractionation of organics within iceless dust aggregates; last accreted sub-units of an aggregate are always glued by organics with the lowest value of the latent heat, which somewhat exceeds 60 kJ/mol. Based on in situ observations at Halley, the parameters characterizing iceless dust aggregates in that comet are calculated. Finally, feasible observational tests of the conclusions drawn are discussed.

Comet nuclei: Morphology and implied processes of surface modification

Surface morphology and related issues for nuclei of three comets: Halley, Borrelly and Wild 2, are considered in the paper. Joint consideration of publications and results of our analysis of the comets’ images led to conclusions, partly new, partly repeating conclusions published by other researchers. It was found that typical for all three nuclei is the presence of rather flat areas: floors of craters and other depressions, mesas and terraces. This implies that flattening surfaces or planation is a process typical for the comet nuclei. Planation seems to work through the sublimation-driven slope collapse and retreat. This requires effective sublimation so this process should work only when a comet is close to the Sun and if on the nucleus there are starting slopes, steep and high enough to support the “long-distance” avalanching of the collapsing material. If the surface had no starting slopes, then instead of planation, the formation of pitted-and-hilly surfaces should occur. An example of this could be the mottled terrain of the Borelly nucleus. Both ways of the sublimational evolution on the nucleus surface should lead to accumulation of cometary regolith. The thickness of the degassed regolith is not known, but it is obvious that in surface depressions, including the flat-floor ones, it should be larger compared with nondepression areas. This may have implications for the in situ study of comets by the Deep Impact and Rosetta missions. Our morphological analysis puts constraints on the applicability of the popular “rubble-pile comet nucleus” hypothesis (Weissman, 1986. Are cometery nuclei primordial rubble piles? Nature 320, 242–244.). We believe that the rubble pile hypothesis can be applicable to the blocky Halley nucleus. The Borelly and Wild 2 nuclei also could be rubble piles. But in these cases the “rubbles” have to be either smaller than 30–50 m (a requirement to keep lineament geometry close to ideal), or larger than 1–2 km (a requirement to form the rather extended smooth, flat surfaces of mesa tops and crater floors). Another option is that the Borelly and Wild 2 nuclei are not rubble piles. In relation to surface morphology we suggest that three end-member types of the comet nuclei may exist: (1) impact cratered “pristine” bodies, (2) non-cratered fragments of catastrophic disruption, and (3) highly Sun-ablated bodies. In this threefold classification, the Wild 2 nucleus is partially ablated primarily cratered body. Borrelly is significantly ablated and could be either primarily cratered or not-cratered fragment. Halley is certainly partially ablated but with the available images it is difficult to say if remnants of impact craters do exist on it. Recently published observations and early results of analysis of the Tempel 1 nucleus images taken by Deep Impact mission are in agreement with our conclusions on the processes responsible for the Halley, Borrelly and Wild 2 nuclei morphologies. In particular, we have now more grounds to suggest that decrease in crater numbers and increase of the role of smooth flat surfaces in the sequence Wild 2⩾Tempel 1⩾Borelli reflects a progress in the sublimational degradation of the nucleus surface during comet passages close to the Sun.

Small-scale dust structures in Halley's coma: II. Disintegration of large dust bodies

Small-scale dust structures, SDSs, altogether ∼35 events with extent ∼30–220 km, have been recognized owing to electric field records, mostly near the closest approach of Vega-2 to Halley's nucleus. Several (8–9) morphological forms of SDS have been identified, and all they make one family. Among the family members, the key form (with respect to which, all other forms can be regarded as degenerate) is a sequence of 3–5 dust clouds. The morphological forms represent various Vega-2 passes through SDSs at different stages of development. SDSs observable as the key form consisted of several fairly regularly spaced dust subpopulations, whose plane of symmetry was parallel to the comet orbit plane. That regularity together with specific features of morphological forms strongly constrain disintegration scenarios and dynamics of fragments, and allow to draw a number of conclusions, the main of which are: SDS parent bodies were ice-free dust aggregates lifted from the nucleus near the comet perihelion, whose masses were in the range ∼0.1–1 of the biggest emitted mass (mass of a body accelerated to the escape velocity, i.e., ∼300–1500 kg); the disintegration scenario comprised a few steps, and the first-step disintegration consisted mainly in consecutive detachments of biggest first-step fragments (BF-SFs) from the parent body; a SDS observable as the key form included the dust minitail of parent body and a few BF-SF minitails, the former one being longer than the latter ones; SDS parent bodies had a fractal-like internal structure, and the BF-SF mass was a few percent of the parent body mass; the thermal conductivity of SDS parent body was less than ∼0.4 W m −1 K −1 or so, while the latent heat of gluing organics was roughly 80 kJ mol −1; the disintegration mechanism was a combination of sintering and sublimation of organics. The multistep disintegration of SDS parent bodies can be reconciled with the basically one-step disintegration of aggregates responsible for the dust boundary (Oberc, P., Icarus 1996, 124, 195–208). The fractal-like structure and the relation between BF-SF mass and parent body mass are in agreement with predictions from the Weidenschilling model of comet formation. Large ice-free dust bodies, in particular SDS parent bodies, can be identified with refractory boulders postulated by some comet nucleus models.

Size, physical properties, and orbital evolution of the comet Hale-Bopp (C/1995 O1) nucleus

AbstractThe equation of the energy balance for the comet Hale-Bopp nucleus is analyzed for the moment when the comet passed perihelion and the sublimation rate (for water) was 1031molecules s−1≈ 3 × 105kgs−1. The minimal value of the nucleus radiusRequ(for γ = 1.0, i.e. the total surface sublimation) equals 14.6 km; for γ = 0.2 (the comet Halley nucleus had approximately this γ value)Requ= 33 km; the thickness of the mineral crust equals ≈ 10−2m for the heat conductivity coefficient ƛ ≈ 0.2 Jm−1s−1K−1. The results of calculations of the orbital elements evolution of the comet Hale-Bopp during one rotation around the Sun show that the comet will not collide with Jupiter (both passages near Jupiter will take place at distances more than 4 AU).

Narrowband Photometry of Comet P/Halley: Variation with Heliocentric Distance, Season, and Solar Phase Angle

Narrowband filter photometry of Comet P/Halley from 109 nights in 1985, 1986, and 1987 is analyzed to discern the comet's variation in activity with changing heliocentric distance and season. The scattering phase function of dust in the coma is also investigated. Asymmetry about perihelion in the production of gas and dust is confirmed. However, we find the asymmetry to be significantly less for water than for the trace species and for dust grains. The relative abundances of CN, C2, and C3remain essentially constant with respect to each other throughout the apparition, but change markedly with respect to water. The behavior of NH also more closely tracks that of the carbon-based species than that of water. These phenomena are most readily understood in terms of seasonal effects on a compositionally variegated nucleus. However, this explanation may not work if Halley's nucleus is in a complex long-axis-mode rotational state as many believe to be the case. Observations of Halley's continuum reveal a substantial change near perihelion in the average radial distribution of dust in the inner coma, with the dust displaying a canonical 1/ρ fall-off after perihelion but a much steeper decrease with distance from the nucleus during 1985. The phase function of the dust shows pronounced curvature over the observed range of values (1.5° to 66°). The slope is 0.045 mag/degree near 0° phase angle but becomes nearly flat at angles greater than about 30°. The peak of the phase function at 0° is 2.9 times that at the minimum near 50°, compared with the 2× enhancement typically measured for the zodiacal light.

The dust tail of Comet 1P/Halley after its perihelion in 1986 and the rotation of the nucleus

The dust tail of Comet 1P/Halley in 1986 and the evolution of its streamers after perihelion are thoroughly investigated by means of direct modelling, determination of streamer densities, and synchrone and syndyne grids. For this purpose, 16 selected wide-field images of the comet in the red dust continuum, taken from 1986 February 22 to April 16, are used to cover the whole time interval of the observable dust tail evolution during its most active phase. In total, 10 significantly distinct streamers have been detected, which were emitted from 22 d before to 12 d after perihelion. The analysis of these streamers yields a synodic ‘basic period’ for the activity of the cometary nucleus of T0 = 2.10 ± 0.11 d. Moreover, ‘missing’ streamers indicate a complex interplay between the rotation of the nucleus and possibly its precession/nutation, and two or more distinct active regions on the nucleus. Periodicities of about 3.3 and 5.0 d or longer also seem to be present in the activity pattern. Thus the morphological analysis of the dust tail streamers independently confirms the complicated rotational pattern of Comet Halley's nucleus found from previous studies. The ‘decay’ of the dust tail and its streamers, i.e. the disappearance of the smallest particles from the tail, seems to be governed by a time-scale of ####t1/e ≈ 9 d. Typical ‘survival times’ of dust particles in the tail range from about 25 d for β = 1.0 (a ≈ 1 μm) to more than 65 d for β < 0.01 (a > 100 μm).

The COMET experiment

Abstract— The COMET program is a program for the collection of micron to submicron interplanetary dust particles in low Earth orbits. Since collection takes place as the Earth crosses a given meteor stream, the particles are mainly of cometary origin. The grain remnants, located at their impact positions on high purity metallic collectors, are analysed in the laboratory for chemical and isotopical identification. The COMET‐1 experiment took place in 1985 October, during encounter with the Draconid meteor stream, related to the Giacobini‐Zinner comet. The fluence of extraterrestrial grains that had impacted our detectors was ∼10x higher than the value of the mean meteroid fluence at ∼1AU, which suggests that most of the grains originated from the Giacobini‐Zinner comet. One of the most important results of their chemical analysis was that ∼90% of them are enriched in low Z elements (C and O have undoubedly been identified). They could contain a CHON phase similar to that observed in the close environment of Halley's nucleus. The first imagery of the grain remnants by field emission scanning electron microscopy suggests that they are very low density aggregates still present at the impacting positions which, in most cases, are very different from the impact craters observed for the same mean relative velocity for full grains of the same size. These results show that the COMET program has constituted an important step towards the analysis of cometary material and the understanding of the evolution of the early Solar System.

Temperature evolution of porous ice samples covered by a dust mantle

The existence of nonvolatile dust mantles covering the ices of cometary nuclei is suggested both from the behavior of many cometary lightcurves and from the direct observations of Comet Halley's nucleus by the Giotto and VEGA spacecraft in March 1986. In this paper we present a systematic study which sheds some light on the evolution of such dust-ice systems when they are irradiated by the sun. Hereby we assume that the dust mantle as well as the underlying ice are grainy, porous structures permeable to gases. It is found that both the surface temperature of the dust mantle and the temperature at the dust-ice interface are strong functions of the average pore radius of the materials and of the thermal conductivity of the mantle. For moderately insulating dust mantles (coal dust) with grain sizes in the micrometer range, high ice temperatures (up to the triple point of water ice) may be reached in response to irradiation with one solar constant. If the dust mantles are extremely bad heat conductors (assuming radiative heat transfer only) the ice temperature remains below 200 K in all cases considered. An increase of the dust mantle thickness may result either in a higher or in a lower ice temperature, depending upon the parameters chosen.

Triaxiality of Halley's Comet

There are many aspects of observational evidence that cometary nuclei have irregular or nonspherical shape. The triaxial figure of the Halley's Comet nucleus is a well known fact. Therefore, the nucleus shape plays a significant role in consideration of the formation and evolution of comets and several attempts have been made to explain their nonsphericity. These studies were mainly based on the random-walk schemes for the aggregation processes. Although some results indeed lead to irregularities and deviation from sphericity, the spherical or irregular shape seem to be prevailing results. On the other hand the triaxial figure can be formed by the tidal and rotational forces. Thus, the assumption that the shape of the cometary nucleus due to some of these effects is in principle acceptable. In here assumed scenario already evolved cometary nucleus is situated as a satellite in the gravitation field of a planetary-like body. Since the rigidity of the nucleus is low, it may be easily transferred in the state of a synchronous satellite and in its shape could be imprinted the dynamical effects from this epoch. Here presented results indicate, that such a possibility should be seriously considered. The theory of this process is applied to the nucleus of comet Halley. It is shown, that the nucleus might be synchronously orbiting around a planetary-like hypothetical body with a period of 0.7 days. The minimal bulk tensile strength of the cometary material of about 102 N m−2 is estimated.

Hydrogen cyanide polymers on comets

The original presence on cometary nuclei of frozen volatiles such as methane, ammonia and water makes them ideal sites for the formation and condensed-phase polymerization of hydrogen cyanide. We propose that the non-volatile black crust of comet Halley consists largely of such polymers. Dust emanating from Halley's nucleus, contributing to the coma and tail, would also arise partly from these solids. Indeed, secondary species such as CN have been widely detected, as well as HCN itself and particles consisting only of H, C and N. Our continuing investigations suggest that the yellow-orange-brown-black polymers are of two types: ladder structures with conjugated -CN- bonds, and polyamidines readily converted by water to polypeptides. These easily formed macromolecules could be major components of the dark matter observed on the giant planets Jupiter and Saturn, as well as on outer solar system bodies such as asteroids, moons and other comets. Implications for prebiotic chemistry are profound. Primitive Earth may have been covered by HCN polymers either through cometary bombardment or by terrestrial happenings of the kind that brought about the black crust of Halley. The resulting proteinaceous matrix could have promoted the molecular interactions leading to the emergence of life.

The spin state and homogeneity of comet Halley's nucleus

We determine a model for the spin state of Comet Halley's nucleus that simultaneously satisfies imaging data from the Vega and Giotto encounters, and a wide range of ground-based data including observations of CN-jets, CN-shells, C 2 production rates, and photometric variability. The model is that of an excited, axially symmetric, rotator whose shape is taken to be that of a prolate spheroid. The motion is assumed to be unaffected by jet-induced torques. The long-axis of the nucleus is inclined to the total angular momentum vector, M, by 66°.0, and rotates around M with a period, P φ , of 3.69 days. The component of spin around the long axis has a period, P Ψ , of 7.1 days which compounds with P φ to produce a total spin period, P T = 2.84 days. The total spin vector, S, is inclined to M by 21°.4 and freely precesses around the angular momentum vector with a period of 3.69 days. The model has a ratio of maximum to minimum moments of inertia of 2.28, which, when compared to the observed linear dimensions of the nucleus, implies an approximately constant density distribution throughout its interior. When compared to the sense of orbital motion, the spin is direct and M points toward right ascension, declination (1950) = (6°·2, -60°·7). The model is characterized by five, localized, areas on the nucleus which dominate the observed activity. One of these, located near the “waist” of the nucleus, appears to be solely responsible for the initiation of CN-shell boundaries seen propagating through the coma and may also be responsible for photometric activity seen at large (≥5 AU) heliocentric distances on approach to the sun. This behavior may indicate the presence of either a large-scale chemical inhomogeneity in the nucleus or, possibly, an extensive region on the surface with unusual physical structure.

The composition and plasma signature of a large dust impact on the Giotto spacecraft

At about 14,800 km from the comet Halley nucleus, on the inbound leg, at least six of the sensors onboard the Giotto spacecraft observed an unusual, brief (∼30 to 500 ms) event: The ion mass spectrometer data show a brief flow of energetic (up to several hundred electron volts) plasma consisting of protons, water group, and heavier ions. The Johnstone plasma analyzer data show a short burst of plasma, while the dust impact detector system data show an impact event in four of its detectors. The magnetometer signature of the event shows two brief dips in the field. The sudden change in the spacecraft attitude and spin rate observed by the camera at that same time has been interpreted as the result of a large (≥5 mg) dust particle impact on the front bumper shield of the spacecraft. In addition, at about the same time the spacecraft star‐tracker suffered damage. Here we give the first report combining direct measurements of the composition and dynamics of a dust impact plasma cloud, the dust particle mass and the location of the impact on the spacecraft. Analysis of the data indicate that the impacting particle was water or ice‐bearing, possibly loosely compacted, and was composed of one or more of: carbon, nitrogen, and silicon.

Nitrogen abundance in Comet Halley

Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion.

The Forest and the Trees

AbstractSome 24 points are made concerning the author’s understanding of the cometary nucleus as developed in this colloquium. Comparisons with an earlier report are noted, and most of the earlier advances in concept are substantiated. The mean density of the Halley nucleus may have been underestimated. The dust/gas ratio apparently needs to be increased by as much as two times, perhaps to unity or higher. The nature of the rotation remains puzzling. CHON grains now appear to be more important sources of gas than thought earlier. A very low temperature at, and subsequent to, the formation of the nucleus continues to receive support, as does a solar-system composition. The 12C/i:3C ratio may, however, be lower than expected.

The comet halley nucleus: random jets

The Comet Halley nucleus is modeled as a homogeneous, triaxial ellipsoid precessing under the torque of nine randomly placed cometographically fixed jets. We employ the ansatz of random jets to elicit the dominant periods in the solar-heating-induced emission spectrum and to test the permanence of the spin state against disruption by the torque from the jets. The precessing nucleus is followed numerically using nonsingular, matrix differential equations. Eight models in the literature are evaluated in relation to the constraints imposed by: (i) the observed long-axis directions at the spacecraft encounters, (ii) the observed ground-based emission periods harmonically related to ≈7.4 days ( M. J. S. Belton 1990, Icarus 86, 30–51), (iii) the need for a 2-day total spin period in models of the jet morphology, (iv) the limitation on the net rotation (roll) around the long axis between the Vega 2 and Giotto spacecraft encounters ( Smith et al. 1987 , Nature (London) 326, 573–574), (v) the resistance to spin-state change from the torque of the jets, and (vi) the return of the nucleus to the same spatial orientation roughly every 7.4 days. All eight models were constructed to satisfy spacecraft-encounter long-axis directions. The ground-based emission periods disfavor the five models (numbered 1, 2, 6, 7, and 8 below) that were derived from a 2.2-day long-axis precession. The same models fail to repeat their orientation every 7.4 days. The three remaining models (3, 4, and 5 below) proposed by Belton (1990) are based on a 3.7-day long-axis precession and his Assumption SV1G2 that the long axis is reversed at Vega 1 and reflected in the plane of the sky as seen from Giotto. Models 1, 3, and 8 fail the test of a permanent spin state. The extremely inhomogeneous LAM Model 4 satisfies the six constraints. But, the emission spectrum of Model 4 is weighted toward the fifth harmonic of 7.4 days while that of Model 5 is weighted toward the third. Models 1, 2, 5, and 7 require reexamination of the Smith et al (1987 , Nature 326, 573–574) constraint. Models 3 and 5 require reanalysis of the jet morphology with a 3-day spin period.

Periodic variations in the activity of Comet P/Halley during the 1985/1986 apparition

A search for periodic variation in the production of gas and dust by Comet Halley has been performed using narrowband photometric measurements from four sites - Lowell Observatory, Mauna Kea Observatory, Perth Observatory, and Cerro Tololo Inter-American Observatory. The method of phase dispersion minimization was applied to observations made during 164 observing nights between September 1985 and June 1986. A clear-cut variation, with a period near 7.4 days, was present throughout the postperihelion window. Less conclusive evidence of a similar period has been found in the pre-perihelion data. No indication of a shorter period or of strong sporadic activity exists in the data. The observations require that Halley's nucleus returns to essentially the same orientation with respect to the sun approximately every 7.4 days except for longer-timescale seasonal evolution. This fact precludes certain proposed models of nuclear motion.

Rotation states of the nucleus of Comet Halley compatible with spacecraft images

The spacecraft images of Comet Halley's nucleus have been analyzed to characterize the positions of the nucleus during the Vega 1, Vega 2, and Giotto flybys. Then the mean instantaneous rotations axes compatible with each pair of positions were calculated. The comparison with ground-based estimates of the angular momentum or the instantaneous rotation axis of the nucleus shows that the instantaneous rotation axis during the space-craft encounters was moving. Thus it demonstrates that the rotation state of the nucleus cannot be simple. However, the quick prograde rotation of 54 hr always appears as the dominant motion. The 7.4-day period could be explained by oscillations around the long axis and a nodding motion, with small amplitudes.

Indication for precession of comet Halley's nucleus from dust tail analysis

The distinct structures ta comet Halley's dust tail around the beginning of March 1986 are analyzed by means of a computer simulation based on nucleus data obtained by the Giotto mission. It is shown that the assumption of a considerable free precession is required to understand the ground based dust tail observations from that time supposing a rotational period of some 50 hr. But a precession-free rotation with a period of about 7 days does not contradict an analysis of the dust tail structure. In both cases, an asymmetric distribution of the relevant emission sources is required.

Rotation of Halley's comet

The two distinct periodicities of 2.2 days and 7.4 days inferred from the coma structure and lightcurves of Halley's comet together with the irregular shape of its nucleus suggest that the nucleus is not in a state of principal axis rotation. We model this situation using numerical simulations supported by analytic calculations. It is easy to numerically generate lightcurves from modulated jets of material which exhibit both periodicities if we choose initial conditions for a representative nucleus such that the shorter period is the rotation period and the longer period is that of precession of the spin vector in the body frame of reference. The dominant spectral power in such curves most often corresponds to the 7.4 day period unless the amplitude of precession is very small. Although curves similar to those observed over short time intervals are obtained for precession about either of the axes with extreme moments of inertia, the observed marked seasonal changes in the lightcurve are only produced for precession about the axis of maximum moment of inertia. The generation and stability of a wobble-type rotation state is investigated by numerically solving Euler's equations for the reaction torque from a jet of ejected material. A nonprincipal axis rotation is easy to excite from an initial rotation about the axis of maximum moment of inertia (minimum energy) only if the nucleus is nearly axisymmetric. Except for almost perfect axial symmetry, excitation of a precession about the axis of minimum moment of inertia is always frustrated by an initial precession about the axis of maximum moment of inertia. In the more probable case of significant triaxiality, even a precession of the spin about the axis of maximum moment of inertia is difficult to excite from an initial minimum energy state, and it can change only slowly from the jet-induced torques. However, this slow change could suffice to generate a substantial amplitude wobble from the cumulative effects of many apparitions. The damping time for the wobble is O(10 6 t 10 8) years, which rules out a primordial origin of a wobble but allows a gradual excitation. Significant changes in the spin angular momentum can occur in a single apparition, but the stability of the amplitude of precession about the axis of maximum moment of inertia means that a rotation state which is relatively stable over many apparitions is not unreasonable. The improbability of exciting a spin precession about the axis of minimum moment of inertia, the relative instability of this state to the jet-induced torques, and the smaller probability of observing significant seasonal changes in the lightcurve in this state all favor the model in which Halley's nucleus precesses about the axis of maximum moment of inertia.

In situ exploration of the dusty coma of comet P/Halley at Giotto's encounter: Flux rates and time profiles from 10 −19kg To 10 −5kg

Flux rates and time profiles from the Giotto DIDSY and PIA experiments are presented in the light of more refined data analysis and increased data coverage. The methods of data analysis for both experiments are explained, together with the relevant behaviour of their in-flight performance. A mass distribution from 237 000km to 11 500km from comet Halley's nucleus is given over a range of masses from ∼ 10 −19kg up to 10 −5kg.