Halley's Coma Research Articles

Fragmentation and Ion Desorption from Condensed Pyrimidine by Electron Impact: Implications for Cometary and Interstellar Heterocyclic Chemistry

The desorption process induced by photons, electrons, or ions onto icy surfaces is an important mechanism for leading neutral or ionized molecular species to the gas phase in interstellar and circumstellar environments. In this work, we report the results of high energy electron impact onto a condensed pyrimidine ice by means of electron-stimulated ion desorption (ESID). Desorbed cations from the icy surface were analyzed by time-of-flight mass spectrometry (TOF-MS). The most abundant desorbed ions are H+ and H2+, although several other fragments, such as C2H2+, HCN+, C3H3+, CHN2+, C2H2N2H+, and C4H4N2H+, have also been identified. The fragmentation pattern of pyrimidine was found to be characterized by six distinct regions on its desorption spectra, containing ionic fragments bearing one to six members of the original pyrimidine ring. Five-membered fragment ions were observed for the first time in electron impact experiments. Absolute values of desorption yield for several ions were determined. Desorbed ions observed in this experiment are in agreement with observations of the comet Halley coma composition and may act as markers of the presence of pyrimidine in the interstellar medium.

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.

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.

Small-Scale Dust Structures in Halley's Coma: Evidence from the Vega-2 Electric Field Records

Owing to simultaneous dust and plasma wave observations onboard the Vega mission to Comet Halley, previous studies have found that the two double probe antennas, short (of APV-N experiment) and long (APV-V), (i) responded to plasma clouds induced by impacts of relatively large particles, (ii) the target area was comparable to the whole spacecraft projection, and (iii) the mass thresholds depended on the ambient plasma conditions. Subsequently, the response mechanisms have been identified, and it was shown that if impacts became continuous, the sensitivity of the antennas to individual plasma clouds was reduced or even cancelled. In the present paper, about 30 short-time events of continuous impact (CIEs), recognized in the Vega-2 records from the two experiments mostly near the closest approach to (at ∼104 km from) the nucleus, are investigated. The high-resolution APV-N waveforms reveal that the respective dust formations were structured. A few types of structure, all belonging to one family, have been distinguished. The basic structure, as seen along the Vega-2 pass, is a sequence of particle clouds. CIEs have time scales shorter than or comparable to the time resolution of the dust experiments (spatial scale less than 200 km) and do not correlate with the SP-1 observations (m≤10−10 g) nor with the published SP-2 fluxes (m≤5.8×10−8 g). But, these dust data, combined with an integral criterion for continuous impact, provide a constraint which implies that the particles responsible were bigger than 10−9–10−8 g. The data from the DUCMA V-detector confirm positively this inference for about 1/3 (∼10) of CIEs and indicate that particles (much) bigger than 10−7 g were decisive in generating several other events. Using an argument from the dusty gas dynamics, it is shown that the small-scale dust structures were not jets but have originated from the disintegration of particle aggregates. An estimate of the total mass contained within a dust structure leads to values of 1–10 kg. Besides CIEs near closest approach, a pair of exceptionally prolonged events has been recorded by APV-V at relatively large distances (∼4×104 km). The dust data show that the mass distribution across the respective dust formations was highly variable.

Dust Spatial Distribution and Radial Profile in Halley's Inner Coma

Mosaic images of the brightness distribution of solar radiation scattered by dust in Halley's coma are constructed using the spectra obtained by the three-channel spectrometer TKS during the approach phase of the Vega 2 spacecraft. They cover a sector having a radius of 40,000 km centered at the nucleus with an angular extent of 50°. The dust scattered brightness is plotted as a function of the distance pbetween the nucleus and the line of sight. This distance pis also called impact parameter. The brightness varies as the inverse of pin the inner coma when pis less than 3000 km and larger than 7000 km. In the 3000–7000 km distance range, the brightness varies as p −1.52. At distances larger than 7000 km, two dust jets are clearly visible with a contrast comparable to the gaseous jets which appear in the OH, NH, CN, C 2, and C 3images (Clairemidi, J., G. Moreels, and V. A. Krasnopolsky, 1990, Astron. Astrophys.231, 235–240; 1990, Icarus86, 115–128). In the inner coma, the spatial distribution of dust seems to be more isotropic and less contrasted than the distribution of gaseous emissive species. A model is developed to calculate the scattered intensity integrated along a line of sight at a projected distance pfrom the nucleus. The model is based upon Mie theory and uses the data of the impact particle counter SP-2 on board the Vega spacecraft (Mazets, E. P., R. Z. Sagdeev, R. L. Aptekar, S. V. Golenetskii, Yu. A. Guryan, A. V. Dyachkov, V. N. Elyinskii, V. N. Panov, G. G. Petrov, A. V. Savvin, I. A. Sokolov, D. D. Frederiks, N. G. Khavenson, V. D. Shapiro, and V. I. Shevchenko, 1987, Astron. Astrophys.187, 699–706), extrapolated from 8030 to 440 km. The model takes into account the fountain effect due to the competition between solar gravitation and radiation pressure, the variation of phase function with scattering angle, and fragmentation processes. A simple method is used to simulate fragmentation: a particle in a mass decade class splits into particles of the lower mass decade, assuming that the mass conservation law is fulfilled. A single α( R) fragmentation coefficient is introduced, Rbeing the distance between the dust particle and the nucleus. A good agreement with the measured dust-scattered intensity radial profile can be reached if an efficient fragmentation process at R< 1000 km with α( R) = 0.87 is introduced, with the fragmentation coefficient decreasing to zero at R= 6000 km. Different values of the density ρ and complex index ( n− ik) of the grains are used. Two good fits with the model results are obtained for ρ = 2.2 − 1.4 a/( a+ 1) g cm −3, where ais expressed in micrometers (Lamy, P. L., E. Grün, and J. M. Perrin, 1987, Astron. Astrophys.187, 767–773) and both indices 1.387 − 0.031 i(Mukai, T., S. Mukai, and S. Kikuchi, 1987, Astron. Astrophys.187, 650–652) or 1.7 − 0.02 i(Khare, B. N., C. Sagan, E. T., Arakawa, F. Suits, T. A. Callcott, and M. W. Williams, 1984, Icarus60, 127–137). However, if a color index variation with pis considered, the agreement is much better when the complex index of tholin (1.7 − 0.02 i) is adopted.

Disintegration of Dust Aggregates as Origin of the Boundaries in Halley's Coma: Derivation of the Sublimation Parameters

This paper is focused on the pair of best pronounced boundaries (enhanced gradients of dust flux) which have been observed by the SP-1 and -2 detectors, one inbound and the other outbound, during both Vega encounters of Comet Halley. First, it is shown that the V-detector of the DUCMA instrument, also aboard the Vega spacecraft, served as an additional dust sensor, recording large particle impacts on the spacecraft and its$jytarget area (0.2–0.1 m2), as well as its two mass thresholds, VL ∼5 × 10−8g, and VH ∼10−6g, are derived. An extensive analysis of the boundaries based on the SP-1, SP-2, and V-detector data has led to the conclusion that the two dust boundaries: (1) are common for a very wide mass range, from ∼10−17g up to ∼10−7g; (2) are observed at nearly equal distances, inbound and outbound; (3) during the Vega-1 flyby were closer to the nucleus (∼60,000 km) than during the Vega-2 flyby (∼84,000 km). It is shown that none of these properties can be interpreted in terms of the fountain model. On the other hand, the first two can be easily explained as a consequence of disintegration of dust aggregates if only the aggregates were confined to a limited region around the nucleus. Assuming that the disintegration was due to sublimation of the volatile component gluing together dust particles, the condition for the aggregate confinement is formulated and a set of equations describing the formation and expansion of a dust boundary is derived. The boundary expansion speed drawn from the observations allows estimates for two sublimation parameters. In particular, the latent heat is determined to beL≈ 16 ± 1.5 kcal/mole. Using the derived parameters, some model calculations are made. They show that the dust boundary was first formed about 20 days after the comet perihelion at a distance of ∼45,000 km from the nucleus and expanded with a speed increasing from ∼35 m/sec to ∼140 m/sec during the next 20 days. The size of aggregates (at the time of emission from the nucleus) reaching the boundary was decreasing rapidly, by an order of magnitude in a 10-day interval, but the aggregate lifetime varied weakly. The expansion of the aggregate domain combined with the decrease of disintegration rate resulted in a rather fast decline of the boundary. The DID4 data from the Giotto encounter of Comet Halley give evidence in support of the model predictions.

Cometopause revisited

The existence of the cometopause discovered by Vega-2 plasma observations in comet Halley's coma /1,2/ was recently questioned by Rème et al. /3/ based on their interpretation of Giotto data. This paper discusses the earlier presented Vega-2 observations with some new aspects supporting the existence of a boundary between the solar wind controlled cometosheath and the heavy cometary ion dominated plasma region.

Multiple Scattering of Light in a Spherical Cometary Atmosphere with an Axisymmetric Dust Jet: II. Image Simulation

A numerical solution for the multiple scattering of light in spherical axisymmetric geometry is used to simulate observations of a model comet atmosphere as it would be seen by a near-flying satellite or by general wide-angle imaging techniques that might resolve the nucleus. We consider a spherically symmetric dust distribution, an axisymmetric dust jet, and a hollow cone of dust about the subsolar point. We calculate the specific intensity of both single- and multiply scattered light. Nucleus visibility and overall coma appearance is examined for varying phase angle. The appearance of the hollow cone is found to be especially sensitive to viewing geometry; at a phase angle of 90° it bears a strong resemblance to the "northern area of activity" observed by Giotto at Comet Halley. We also calculate the azimuth-integrated intensity, used by Thomas and Keller (1990, Ann. Geophys. 8, 147-166), as a measure of particle fragmentation Halley's coma, and examine the extent to which line-of-sight attenuation depletes the azimuth-integrated intensity near the nucleus, and multiple scattering enhances it.

Simultaneous observations of quasistatic electric fields and large dust particles during the Vega-2 flyby of comet Halley

The quasistatic electric field records made in Halley's coma by the APV-V experiment represented a superposition of the overall negative variation with a number of positive spikes. A correlation has been found between the overall variation and the count rates of dust particles in the mass range 10 −11–10 −8 g. For interpretation purposes the impact-induced plasma cloud radius has been specifically defined. Owing to the asymmetry of the double-probe antenna with respect to the impact direction, the particles producing plasma clouds with radii greater than several metres induced impulsive negative potentials of different magnitudes on the two probes, thereby generating an apparent electric field. In the inner coma, the electron emission generated by impacts of cometary gas molecules maintained the sensitivity of the antenna to dust impacts, but implied that the lower mass limit of particles responsible for the apparent electric field varied proportionally with the secondary electron current. The linear dependence of the quasistatic signal on the particle flux breaks down for high fluxes ; hence, the positive spikes are interpreted as manifestations of smallscale, dense clouds of dust. The present study has shown that a long double-probe antenna can be used as a large dust particle detector, whose target area is comparable to the spacecraft projected area.

Impacts of dust particles m > 10 −9 g in Halley's coma as seen in the electric field waveforms of Vega 2

In this paper we draw some inferences on the response of the APV-N antenna of Vega 2 to plasma clouds induced by dust particles. We show that: (1) electric impulses were mostly generated by impacts of particles bigger than ∼10 −9 g on the whole spacecraft, (2) in the case of large particle impacts the impulse amplitude was weakly dependent on the particle mass, (3) the antenna response to the impact plasma depends on the ambient plasma parameters, and (4) the sensitivity to impacts decreases when the impact rate becomes quasicontinuous. The conclusions are based on an analysis of the wide impulses in the outbound pass of Vega 2 through the outer coma and on a comparison of the electric field spectral densities with the count rates of dust particles in various mass channels. The lower limit, down to which the amplitude vs mass dependence is weak, has been defined as the “least effective mass”; its value has been deduced to be ∼3 × 10 −9 g in the outer coma, and ∼10 −7 g in the inner coma, while near the point of closest approach to the comet it was determined to be more than ∼10 −6 g due to the continuous impact of small particles. The spectral density of impact-generated impulses should be roughly proportional to the integral flux of particles bigger than the least effective mass. Some new peculiarities of the large particle distribution in the coma are derived. The interpretation of the impulse generation involves the response of the antenna to a negative pulse of the spacecraft potential and to electric fields associated with the electron density gradients in the expanding plasma cloud. The correlation between the count rates of the light ( m < 10 −14 g) particles and of the impulses (found in a previous investigation) is reinterpreted in terms of the hypothesis that the light particles were originated by disintegration of the big ones.

Isotopic analysis of cometary organic matter.

Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of 13C/12C values in those particles is quite uncertain. The D/H ratio in H2O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case.

Chemical evolution of primitive solar system bodies

In this paper we summarize some of the most salient observations made recently on the organic molecules and other compounds of the biogenic elements present in the interstellar medium and in the primitive bodies of the solar system. They include the discovery of the first phosphorus molecular species in dense interstellar clouds, the presence of complex organic ions in the dust and gas phase of Halley's coma, the finding of unusual, probably presolar, deuterium-hydrogen ratios in the amino acids of carbonaceous chondrites, and new developments on the chemical evolution of Titan, the primitive Earth, and early Mars. Some of the outstanding problems concerning the synthesis of organic molecules on different cosmic bodies are also discussed from an exobiological perspective.

Organic solids produced from simple C/H/O/N ices by charged particles: Applications to the outer solar system

CH 4, CO, and CO 2 are all potential one-carbon molecular repositories in primitive icy objects. These molecules are all found in the Comet Halley coma, and are probable but, (except for CH 4 detected on Triton and Pluto) undetected subsurface constituents in icy outer solar system objects. We have investigated the effects of charged particle irradiation by cold plasma discharge upon surfaces of H 2O:CH 4 clathrate having a 200:1 ratio, as well as upon ices composed of H 2O plus C 2H 6 or C 2H 2 (sometimes plus NH 3) which are also plausible constituents. These materials color and darken noticeably after a dose 10 9–10 10 erg cm −2, which is deposited rapidly (≤ 10 4 yr.) in solar system environments. The chromophore is a yellowish to tan organic material (a tholin) which we have studied by UV-VIS reflection and transmission, and IR transmission spectroscopy. Its yield, −1 C keV −1, implies substantial production of organic solids by the action of cosmic rays and radionuclides in cometary crusts and interiors, as well as rapid production in satellite surfaces. This material shows alkane bands which Chyba and Sagan have shown to well match the Halley infrared emission spectrum near 3.4 μm, and also bands due to aldehyde, alcohol and perhaps alkene/aromatic functional groups. We compare the IR spectral properties of these tholins with the spectra of others produced by irradiation of gases and ices containing simple hydrocarbons.

Infrared emission by organic grains in the coma of comet Halley

Spacecraft1 and ground-based2–5 observations of comet Halley in the near-infrared reveal a triple-peaked emission feature near 3.4 µm, characteristic of C–H stretching in hydrocarbons (–CH3 and –CH2– alkanes). Here we discuss transmission spectra of organic residues produced by laboratory irradiation of candidate cometary ices, which serve as the basis for a model of emitting cometary dust. The laboratory synthesis of solid organic residue from irradiated low-occupancy methane ice clathrate simulates the radiation processing experienced by comet Halley. The transmission spectrum of this residue, convolved with a simple two-component thermal emission model based on the spacecraft-determined dust distribution in the Halley coma, fits the 3.4 µm feature, provides optical depths in excellent agreement with those observationally determined, and accounts for the absence of features at longer wavelengths (despite their presence in transmission spectra of typical ice irradiation residues).

Magnetic field draping in the comet Halley coma: Comparison of Vega observations with computer simulations

During the VEGA‐1 encounter with comet Halley, the magnetometer observed draping and compression of the interplanetary magnetic field (IMF). These are reproduced well by a 3‐D MHD simulation of the cometary interaction. Rotations in the magnetic field similar to those at closest approach are also observed 2.75 hours earlier. We suggest that both rotations correpond to the same IMF interval and that the spacecraft had overtaken the plasma and encountered "older" magnetic field as it penetrated the coma. Analysis of the MHD model indicates that it should take ∼ 3 to 5 hours for a solar wind parcel to pass from the unperturbed solar wind to VEGA‐1 at closest approach. A simulated magnetic field profile composed of nested sections for different IMF orientations closely resembles the observations. This result supports the hypothesis of layered magnetic orientations in the coma.

IUE observations of comet Halley during the Vega and Giotto encounters

The observations of comet Halley from the International Ultraviolet Explorer (IUE) satellite from 9 to 16 March 1986 allow a comprehensive study of gas and dust production by the comet in the week during which the fleet of six spacecraft from four space agencies encountered the comet. The gaseous output of the nucleus was found to vary over timescales of the order of one or two hours. The Vega 2 probe crossed Halley's coma at the time of, or shortly after, the onset of a strong outburst. The Giotto spacecraft performed its measurements while the cometary nucleus was near a minimum of activity. The ultraviolet spectrum of comet Halley differs from that of other comets observed by the IUE satellite in that the continuum emission is very strong relative to the gaseous emissions, and the CO2+ violet bands, if present, are weak. The rate of water production was estimated to be 5.6×1029 and 5.2×1029 molecules per second at the time of the Vega 2 and Giotto encounters, respectively, while atomic carbon was a few per cent and CO ∼10–20% of these values.

Dust counter and mass analyser (DUCMA) measurements of comet Halley's coma from Vega spacecraft

The Vega encounters with comet Halley allowed the first direct measurements of the spatial and temporal distributions of the masses and fluxes of dust particles originating from the comet nucleus. These data are fundamental for establishing the physical processes of dust emission from the nucleus, their propagation to form a coma, and the behaviour of dust jets. The measurements reported here were made with instruments employing a new principle of dust detection1,2, which have a high time-resolution (∼4µs) over a large range of dust fluxes and masses. The dust coma, whether quiescent (as seen by Vega 2) or containing a major jet structure (as seen by Vega 1), displays large, short-term variations throughout, which are at times quasi-periodic. The integral mass spectra increase in intensity to the lowest masses measured (contrary to some theoretical models), and the flux levels lie approximately in the ranges estimated previously from ground-based observations6. The coma is highly dynamical on all spatial and temporal scales, suggesting a complex structure of localized regions of dust emission from the nucleus.

Magnetic field observations in comet Halley's coma

During the encounter with comet Halley, the magnetometer (MISCHA) aboard the Vega 1 spacecraft observed an increased level of magnetic field turbulence, resulting from an upstream bow wave. Both Vega spacecraft measured a peak field strength of 70–80 nT and observed draping of magnetic field lines around the cometary obstacle. An unexpected rotation of the magnetic field vector was observed, which may reflect either penetration of magnetic field lines into a diffuse layer related to the contact surface separating the solar-wind and cometary plasma, or the persistence of pre-existing interplanetary field structures.

An improved thermal model for cometary nuclei

We have modified our earlier thermal model of cometary nuclei by incorporating a more accurate calculation of the opacity of the cometary dust coma. The new model uses a modified Sekanina‐Miller dust particle size distribution and a Delsemme‐Miller gas outflow dynamics model. The improved model has been applied to the case of the 1986 apparition of Halley's Comet, also incorporating a revised radius and albedo for the Halley nucleus. The estimated opacity of the Halley coma is reduced from that of the earlier model. The revised opacity is 0.12 at 1.0 AU and 0.43 at perihelion, 0.587 AU. This results in a decrease in the energy supplied to the nucleus from the coma by multiply scattered radiation and thermal emission, and brings the model into better agreement with the results of others. The coma feedback mechanism enhances nucleus gas production by a factor of 1.4 at 1.0 AU over that for a bare ice nucleus with no dust coma, and by a factor of 2.3 at perihelion. Predicted gas production at 0.9 AU postperihelion, the expected heliocentric encounter distance for the Giotto mission, is 4.4×1029 molecules per second. This agrees well with a recent estimate of 3.9×1029 molecules s−1 for the Halley gas production rate at 0.9 AU during the 1910 apparition.

A spectral photopolarimeter for Giotto: Halley optical probe experiment

The Halley Optical Probe Experiment (HOPE) is designed to provide in situ photopolarimetric data on both the dust cloud and the gaseous atmosphere in Halley's coma. The optical probe concept is presented here, together with a description of the instrumentation and with the possibilities for cross-checks between HOPE results and those of other space and ground-based experiments.