Origin of organic matter in the protosolar nebula and in comets.

Comet organics are traced to their origin in interstellar space. Possible sources of comet organics from solar nebula chemistry are briefly discussed. The infrared spectra of interstellar dust are compared with spectra of solar (space) irradiated laboratory organic residues and with meteorites. The spectra compare very favorably. The atomic composition of first generation laboratory organic residues compares favorably with that of comet Halley organics if divided into appropriate "volatile" (less refractory) and "refractory" (more refractory) complex organics.     

A fine mist of very small comet dust particles

A comet nucleus considered as an aggregate of interstellar dust would produce a mist of very finely divided (radius ～ 0.01 μm) particles of carbon and metal oxides accompanying the larger dust grains. These small particles which are very abundant in the interstellar dust size spectrum would provide substantial physical effects because of their large surface area. They may show up strongly in particle detectors on the Halley probes. A strong basis for serious consideration of these particles comes from the other evidence that interstellar dust grains are the building blocks of comets; e.g. (1) the explanation of the “missing” carbon in comets; (2) The S₂ molecule detection which suggests that the comet solid ice materials have been previously subjected to ultraviolet radiation (as are interstellar grains) before aggregation into the comet; (3) the predicted dust to gas ratio.     

陨石、流星体与小行星及彗星的演化关系

小行星、彗星和流星体(meteoroid)都是绕太阳公转的小天体,它们只是在轨道特性和物理-化学性质方面有所不同,流星体泛指在行星际空间运行的、质量从10~(-16)克微流星体或微尘到10~8克的所有小天体,当它们闯入地球大气时与大气剧烈碰撞而产生发光的流星(meteor)现象,落到地面的流星体残余则称为陨石或陨星(meteorite)。     

Silicate core-organic refractory mantle particles as interstellar dust and as aggregated in comets and stellar disks.

The principal observational properties of silicate core-organic refractory mantle interstellar dust grains in the infrared at 3.4 microns and at 10 microns and 20 microns are discussed in terms of the cyclic evolution of particles forming in stellar atmospheres and undergoing subsequent accretion, photoprocessing and destruction (erosion). Laboratory plus space emulation of the photoprocessing of laboratory analog ices and refractories are discussed. The aggregated interstellar dust model of comets is summarized. The same properties required to explain the temperature and infrared properties of comet coma dust are shown to be needed to account for the infrared silicate and continuum emission of the beta Pictoris disk as produced by a cloud of comets orbiting the star.     

Cometary magnetospheres: a tutorial

The nucleus of an active comet, such as comet Halley near its perihelion, produces large quantities of gas and dust. The resulting cometary atmosphere, or coma, extends more than a million kilometers into space, where it interacts with the solar wind. An “induced” cometary magnetosphere is a consequence of this interaction. Cometary ion pick-up and mass loading of the solar wind starts to take place at very large cometocentric distances. Eventually this mass loading leads to the formation of a weak cometary bow shock. Even closer to the nucleus, collisional processes, such as ion-neutral chemistry, become important. Other features of the magnetosphere of an active comet include a magnetic barrier, a magnetotail, and a diamagnetic cavity near the nucleus. X-ray emission from comets is produced by the interaction of the solar wind with cometary neutrals and this topic is also discussed. A broad review of the cometary magnetosphere will be given in this paper.     

The Giotto mission to Halley''s comet

ESA's Giotto mission to Halley's comet is a fast flyby in March 1986, about four weeks after the comet's perihelion passage when it is most active. The scientific payload comprises 10 experiments with a total mass of about 60 kg: a camera for imaging the comet nucleus, three mass spectrometers for analysis of the elemental and isotopic composition of the cometary gas and dust environment, various dust impact detectors, a photopolarimeter for measurements of the coma brightness, and a set of plasma instruments for studies of the solar wind/comet interaction. In view of the high flyby velocity of 68 km/s the experiment active time is very short (only 4 hours) and all data are transmitted back to Earth in real time at a rate of 40 kbps. The Giotto spacecraft is spin-stabilised with a despun high gain parabolic dish antenna inclined at 44.3° to point at the Earth during the encounter while a specially designed dual-sheet bumper shield at the other end protects the spacecraft from being destroyed by hypervelocity dust impacts. The mission will probably end near the point of closest approach to the nucleus when the spacecraft attitude will be severely perturbed by impacting dust particles leading to a loss of the telecommunications link.     

In-situ photopolarimetric measurements of dust and gas in the coma of Halley''s comet

The Halley Optical Probe Experiment (HOPE) on board the Giotto spacecraft has provided the first in-situ measurements, both of the dust and of some gaseous species, from inside the coma of the comet. The instrument has already been described /1/, together with first results /2/. The purpose of this note is to show how optical measurements can lead to in-situ information, how those were obtained during the 13–14 march 1986 Halley fly-by, and what is the status of the data analysis.     

The derivation of Halley parameters from observations

A set of nominal model parameters for P/Halley is derived from its light curve and spectra. In those cases where Halley observations are not sufficient, the average value derived from a large set of other comets has been used, or data from comet Bennett, Halley's best analogue has been taken. The derived parameters include nucleus mass, size, density, albedo, rotation period, axial inclination, and surface temperature, the composition of the parent molecules, the total gas and dust production rates, distributions for the dust size and bulk density as well as various other parameters.     

Cometary atmosphere (gas and dust)

There is important progress now in the identifications and measurements of primary (parent) molecules in the inner coma of Comet Halley. H₂O, CO₂ and CO are definitely in the list, CH and some complicate organic molecules are suspected. Gas production rate for water vapor is Q_H2O 10³⁰ s⁻¹. The bulk of data doesn't contradict to the Whipple model of nucleus (with clathrate modification). Pronounced spatial structure of gaseous flow in the coma was observed, but in general measured properties of neutral gas in the coma of Comet Halley are not very different from predicted. Situation for dust is different. In situ dust measurements show that size spectrum and optical properties of particles in coma are substantively declining from predicted on the base of groundbased photometry. However there are discrepancies between Vega and Giotto dust counter data. Dust in the inner coma didn't prevent the succesful imaging of nucleus by TV on Vega 1 and 2.     

Solar wind interaction with comet Halley

In March 6 and 9, 1986 the spacecrafts ‘Vega-1’ and ‘Vega-2’ have flown through the coma of comet Halley and have carried measurements of plasma, energetic particles, magnetic field and plasma waves along its trajectory. A short review of these measurements and its comparison with theoretical models of solar wind interaction with comets are given.

The spacecrafts ‘Vega-1’ and ‘Vega-2’ have studied the solar wind loading by cometary ions, the structure of cometary bow shock and the processes in the inner coma of comet Halley. Exactly in this sequence we discuss the results of measurements and compare them with the theory.     

Comparative study of the dust emission of 19P/Borrelly (Deep space 1) and 1P/Halley

Images obtained by the Miniature Integrated Camera and Imaging Spectrometer (MICAS) experiment onboard the Deep Space 1 spacecraft which encountered comet 19P/Borrelly on September 22nd 2001 show a dust coma dominated by jets. In particular a major collimated dust jet on the sunward side of the nucleus was observed. Our approach to analyse these features is to integrate the observed intensity in concentric envelopes around the nucleus. The same procedures has been used on the Halley Multicolour Camera images of comet 1P/Halley acquired on March 14th 1986. We are able to show that at Borrelly the dust brightness dependence as a function of radial distance is different to that of Halley. At large distances both comets show constant values as the size of the concentric envelopes increases (as one would expect for force free radial outflow). For Halley the integral decreases as one gets closer to the nucleus. Borrelly shows opposite behaviour. The main cause for Halley's intensity distribution is either high optical thickness or particle fragmentation. For Borrelly, we have constructed a simple model of the brightness distribution near the nucleus. This indicates that the influence of deviations from point source geometry is insufficient to explain the observed steepening of the intensity profile close to the nucleus. Dust acceleration or fragmentation into submicron particles appear to be required. We also estimate the dust production rate of Borrelly with respect to Halley and compare their dust to gas ratios.     

Dirty ice grains in comets

Based on the computed equilibrium temperature of evaporating dirty water-ice grains, dirty water-ice halo is examined, taking into account of a size dependence of terminal velocity of dust at P/Halley. It is found that due to an enhanced grain's temperature caused by dirtiness, icy halo cannot extend over 100 km from the nucleus when comet approaches inside a solar distance r of 1 AU. Therefore, it is unlikely that the ice bands in the near infrared wavelengths could be detected in the cometary coma at r<1 AU.     

Comets and life.

Some of the chemical species which have been detected in comets include H2O, HCN, CH3CN, CO, CO2, NH3, CS, C2 and C3. All of these have also been detected in the interstellar medium, indicating a probable relationship between interstellar dust and gas clouds and comets. Laboratory experiments carried out with different mixtures of these molecules give rise to the formation of the biochemical compounds which are necessary for life, such as amino acids, purines, pyrimidines, monosaccharides, etc. However, in spite of suggestions to the contrary, the presence of life in comets is unlikely. On the other hand, the capture of cometary matter by the primitive Earth is considered essential for the development of life on this planet. The amount of cometary carbon-containing matter captured by the Earth, as calculated by different authors, is several times larger than the total amount of organic matter present in the biosphere (10(18)g). The major classes of reactions which were probably involved in the formation of key biochemical compounds are discussed. Our tentative conclusions are that: 1) comets played a predominant role in the emergence of life on our planet, and 2) they are the cosmic connection with extraterrestrial life.     

Interpretation of the measurements of secondary electron currents induced by impacts during the flyby of comet Halley

The Giotto, Vega-1 and Vega-2 spacecraft flew through the environment of comet Halley at a relatively close range with velocities of the order of 70–80 km/s. The fore sections of their surface were bombarded by neutral molecules and dust grains which caused the emission of secondary electrons and sputtered ions. This paper makes use of the secondary electron current measurements performed on Vega-1 to infer some characteristic features of the cometary atmosphere. The total gas production rate is estimated to be of the order of 10³⁰ molecules/s and is found to vary with time; the presence of a major jet is also detected at closest approach.     

The Halley dust model

The properties of dust ejecta from Comet Halley are studied on the basis of (a) evidence from the comet's past apparitions and (b) analogy with recent, physically similar comets. Specifically discussed are the light curve and spectrum, discrete phenomena in the head, the physical properties of the nucleus (size, albedo, rotation, surface temperature, and morphology), and an interaction between the nucleus and dust atmosphere. Also reviewed are constraints on the size and mass distributions of dust particles, information on submicron-size and submillimeter-size grains from the comet's dust tail and antitail, and the apparent existence of more than one particle type. Similarities between the jet patterns of Halley and the parent comet of the Perseid meteor stream are depicted, and effects of the surface heterogeneity (discrete active regions) on the dust flow are assessed. Current dust models for Halley are summarized and the existence of short-term variations in the dust content in the comet's atmosphere is suggested.     

Giotto extended mission

The navigation of the ESA spacecraft Giotto to its encounter with comet P/Halley on 14 March 1986 required just 10% of the fuel available. Although the spacecraft was damaged by dust impacts during its close flyby at the nucleus of P/Halley it was retargeted to return close to Earth to maintain the option to extend the mission to encounter another comet, P/Grigg-Skjellerup on 10 July 1992.

On 2 April 1986 the spacecraft was put into hibernation configuration and had been orbiting the Sun in the ecliptic with an orbital period of 10 months. On 19 February 1990 it was reactivated, spacecraft subsystems and the payload checked out to determine its health status.

On 2 July 1990 Giotto performed succesfully the first-ever Earth gravity assist manoeuvre of a spacecraft approaching the Earth from deep space and was retargeted for comet P/Grigg--Skjellerup. It was concluded that the spacecraft is ready to provide valuable data during a potential encounter with a second comet.     

Thermal modeling of Halley's comet

The comet thermal model of Weissman and Kieffer is used to calculate gas production rates and other parameters for the 1986 perihelion passage of Halley's Comet. Gas production estimates are very close to revised pre-perihelion estimates by Newburn based on 1910 observations of Halley; the increase in observed gas production post-perihelion may be explained by a variety of factors. The energy contribution from multiply scattered sunlight and thermal emission by coma dust increases the total energy reaching the Halley nucleus at perihelion by a factor of 2.4. The high obliquity of the Halley nucleus found by Sekanina and Larson may help to explain the asymmetry in Halley's gas production rates around perihelion.     

Physical properties of dust grains deduced by optical probing techniques

In-situ space observations of dust in the solar system are seldom possible. On the opposite, remote observations of solar light scattered by dust are relatively easy to perform from Earth- or satellite-based observatories; the evolution of the polarization of light scattered by dust particles as a function of the phase angle may provide information on the physical properties of these particles. Unfortunately, since remote observations are integrated along the line-of-sight of the observer, they can hardly be used to determine local physical properties. We have precisely developed Optical Probe techniques to forge the link between the numerous remote observations and the unique in-situ measurements. A short review of the remote observations of light scattered by cometary dust is first presented. Then, the Optical Probe concept is analyzed. Finally, the OPE instrument, which had been designed to optically probe the inner coma of comet Halley is described; its limitations and its achievements during Halley and Grigg-Skjellerup encounters are discussed.     

Planet-A mission to Halley

Looking at the chance of the next apparition of the Halley comet in 1986, ISAS decided to send a first Japasanese interplanetary spacecraft for the study of cometary hydrogen coma and solar wind. The Planet-A spacecraft which carries VUV imaging camera and solar wind plasma analyser will be launched in August 1985 and flyby the Halley comet in early March 1986 with the distance of several million kilometers from the comet nucleus. This mission is not only self-consistent but collaborative with other space mission as well as earth-bound observations. In the present paper, the Planet-A mission to Halley is described with brief explanation of the spacecraft.     

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.