The seeding of life by comets.

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.     

Chemical interactions of solar wind with cometary nuclei and comae

Solar wind particles, especially H, C, N, O, S, and P-ions, may undergo specific chemical reactions with gaseous or solid matter of comets when in the energy region of a few 10 to some eV. Each component of the solar wind, even if not chemically reactive itself, creates a multiplicity of energetic secondary particles by knock-on processes with the cometary matter. These are responsible for the majority of the so called “hot” chemical processes. Endothermic reactions with high activation energy and atom molecule interactions are possible and may add to the classical exothermic ion-molecule or radical reactions. Other sources of hot atoms or ions in comets are: cosmic rays, acceleration or pick-up processes and turbulences in comae and gas or dust tails, and photon absorption induced dissociation. The products of hot chemical reactions, short period comets experience on their orbits, add to those formed in the individual component ice or dust grains by strong fluxes of energetic particles in times prior to the accretion to a comet.     

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.     

Crushing strength of porous ice-mineral bodies-relevance for comets

In laboratory investigations with fluffy, highly porous ice and ice-dust bodies a new mechanism could be identified which strengthens the porous bodies. The process takes place under isothermal conditions and leads to the formation of ice bridges between the ice (dust) particles. It is driven solely by the dependence of the partial pressure of water vapour on the curvature of the particles. This mechanism is generally called “sintering”. A theory for the crushing strength of a porous ice and ice-dust agglomerate is developed which describes the experimental results on isothermal changes is strength due to sintering quantitatively well. The relevance for the evolution of comets is discussed.     

Formation of amino acid precursors in cometary ice environments by cosmic radiation.

Cometary ices are believed to contain water, carbon monoxide, methane and ammonia, and are possible sites for the formation and preservation of organic compounds relating to the origin of life. Cosmic rays, together with ultraviolet light, are among the most effective energy sources for the formation of organic compounds in space. In order to study the possibility of the formation of amino acids in comets or their precursory bodies (interstellar dust grains), several types of ice mixtures made in a cryostat at 10 K ("simulated cometary ices") were irradiated with high energy protons. After irradiation, the volatile products were analyzed with a quadrupole mass spectrometer, while temperature of the cryostat was raised to room temperature. The non-volatile products remaining in the cryostat at room temperature were collected with water. They were acid-hydrolyzed, and analyzed by ion-exchange chromatography. When an ice mixture of carbon monoxide (or methane), ammonia and water was irradiated, some hydrocarbons were formed, and amino acids such as glycine and alanine were detected in the hydrolyzate. These results suggest the possible formation of "amino acid precursors" (compounds yielding amino acids after hydrolysis) in interstellar dust grains by cosmic radiation. We previously reported that amino acid precursors were formed when simulated primitive planetary atmospheres were irradiated with cosmic ray particles. It will be of great interest to compare the amount of bioorganic compounds that were formed in the primitive earth and that brought by comets to the earth.     

A laboratory study on the thermally induced transformation of hydrogen cyanide (HCN) to the cyanogen anion (CN) in Solar System analog ices

Mixtures of molecular nitrogen and methane have been identified in numerous outer Solar Systemices including the icy surfaces of Pluto and Triton. We have simulated the interaction of ionizing radiation in the Solar System by carrying out a radiolysis experiment on a methane – molecular nitrogen ice mixture with energetic electrons. We have identified the hydrogen cyanide molecule as the most prominent carbon–nitrogen-bearing reaction product formed. Upon warming the irradiated sample, we followed for the first time the kinetics and temporal evolution of the underlying acid–base chemistry which resulted in the formation of the cyanide ion from hydrogen cyanide. On the surfaces of Triton and Pluto and on comets in Oort’s cloud this sort of complex chemistry is likely to occur. In particular, hydrogen cyanide can be produced in low temperature environments (Oort cloud comets) and may be converted into cyanide ions once the comets reach the warmer regions of the Solar System.     

Thermomechanical stresses on comets

Thermal stresses due to temperature differences between the cometary surface and the core have been calculated for different models of cometary nuclei. It is shown, that for comets on P/Halley-type orbits thermomechanical stresses exceed the (cohesive) strength of water ice near to the cometary surface. Consequently, there should be cracks on cometary surfaces. The existence of line structures on the surface of P/Halley, which might be due to thermomechanical cracks, is demonstrated on the basis of VEGA-images from the surface of P/Halley.     

IRAS observations of cometary dust

The large beam size of the Infrared Astronomy Satellite (IRAS) focal plane detector array is well suited to measuring the low level thermal emission from cometary dust. Eight comets discovered in 1983 and nine previously known periodic comets were observed by IRAS during its ten month active lifetime. Dust production rates are derived for a wide range of heliocentric distances. Grain properties are inferred from application of simple models to the long wavelength spectral energy distribution.     

论彗星的起源

彗星富含挥发物,这表明它们形成并长期保存于太阳系外部低温区。本文分析了星云盘外缘区的结构,论证了那里不可能存在彗星形成带。作者认为,彗星是木星到海王星这个区域中的残存星子演变而成的。      

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)。     

Cometary ephemerides for spacecraft flyby missions

For spacecraft without on-board navigation capability, their ability to fly close to target comets is limited primarily by the comet's ephemeris uncertainty. Factors contributing to cometary ephemeris uncertainties include measurement errors, star catalog errors, and offsets between the comet's center of mass and its observed center of light. The situation is further complicated by nongravitational forces acting upon a comet's nucleus and the paucity of observers currently making astrometric observations of comets. For comet Halley, the nongravitational forces affecting this comet's motion are consistent with the rocket effect of an outgassing water ice nucleus; the nucleus is apparently rotating in a direct sense about a stable spin axis. Accurate comet Halley ephemerides for close spacecraft flybys will require continued efforts to refine the existing nongravitational force model. In addition, the various flyby missions to comet Halley will require a well organized network of astrometric observers. These observers must rapidly reduce their observations in early 1986, thus allowing continuous updates to the comet's ephemeris just prior to the spacecraft flybys in March 1986.     

The flux of Earth-crossing and moon-cratering interplanetary bodies

Methods of determining the present flux and total number of kilometer-sized Earth-crossing objects are discussed, including (1) probability considerations based on the frequency of chance rediscoveries of the lost objects, (2) evaluation of large-scale photographic surveys for the detection of fast moving objects, and (3) evaluation of close encounters of interplanetary bodies with the Earth. The results are interfaced with the lunar and terrestrial cratering history. It is shown that the discrepancies between these two independent lines of evidence are still within the margin of uncertainty set by observational biases, cratering efficiencies, and surface reflectivities of the objects, in particular as regards extinct cometary nuclei. Impacts of active comets can only be held responsible for a very small fraction of the craters, and impacts of high-albedo Apollo asteroids are consistent with a steady state. There is no definite enhancement of the present-day flux as compared with the average level of cratering during the last 3 Gyr which would require significant variations in the stellar environment of the solar system, affecting the rate of delivery of new comets from the Oort cloud. There is also no evidence of a recent major collisional event in the asteroid belt. Deviations from an equilibrium between source and sink only become effective in the size range of meteor particles, where no long-term cratering record is available. They are apparently due to a very limited number of parent objects, and appear on a time scale which is very short compared with the age of the solar system.     

Ultraviolet spectroscopy of cometary comae: An update

Since its launch in 1978 the $\text{International Ultraviolet Explorer}$ ($\text{IUE}$) satellite observatory has been used to record ultraviolet spectra of nearly two dozen comets. These observations have been applied principally to studies of the composition, chemistry and evolution of the gaseous coma and more recently, with the substantially increased data base, to comparative analyses. The observations of Comets Bowell (1982 I) and Cernis (1983?) at a heliocentric distance of ≈ 3.4 AU show these two comets to be virtually identical and pose problems for water ice vaporization models. The most significant recent result from $\text{IUE}$ was the discovery of S₂ in the Earth-approaching comet IRAS-Araki-Alcock (1983d) and the use of the S₂ emission as a monitor of short-term variations in cometary activity. In early 1984, periodic comet Encke was observed for the second time by $\text{IUE}$, this time post-perihelion.     

Organics produced by ion irradiation of ices: some recent results.

Some results, recently obtained from laboratory experiments of ion irradiation of ice mixtures containing H, C, N, and O, are here summarized. They are relevant to the formation and evolution of complex organics on interstellar dust, comets and other small bodies in the external Solar System. In particular the formation of CN-bearing species is discussed. Interstellar dust incorporated into primitive Solar System bodies and subsequently delivered to the early Earth, may have contributed to the origin of life. The delivery of CN-bearing species seems to have been necessary because molecules containing the cyanogen bond are difficult to be produced in an environment that is not strongly reducing as that of the early Earth probably was. Moreover we report on an ongoing research program concerning the interaction between refractory materials produced by ion irradiation of simple ices and biological materials (amino acids, proteins, cells).     

Europa, tidally heated oceans, and habitable zones around giant planets.

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth.     

Remote infrared observations of parent volatiles in comets: A window on the early solar system

Organic volatiles and water in Oort Cloud comets were investigated at infrared wavelengths. The detected species include H₂O, CO, CH₃OH, CH₄, C₂H₂, C₂H₆, OCS, HCN, NH₃, and H₂CO. Several daughter fragments (CN, OH, NH₂, etc.) are also measured, and OH prompt emission provides a proxy for water. Long-slit spectra are taken at high spectral dispersion and high spatial resolution, eliminating several sources of systematic error. The resulting parent volatile production rates are highly robust, permitting a sensitive search for compositional diversity among comets. Here, seven OC comets are compared. Six (including Halley) exhibit similar compositions (excepting CO and CH₄). Their low formation temperatures (30 K) suggest this group probably formed beyond 30 AU from the young sun. However, C/1999 S4 is severely depleted in hypervolatiles and also in methanol, and it likely formed near 5–10 AU. C/2001 A2 is discussed briefly to illustrate future prospects.     

Hydrogen coma of comet P/Halley

The main molecular processes to produce the hydrogen comae of comets are now well known: Water, the main constituent of cometary atmospheres, is photodissociated by the solar ultraviolet radiation to form the high (20 km s⁻¹) and low (8 km s⁻¹) velocity components of the atomic hydrogen. The hydrogen clouds of various fresh comets have been observed in 1216Å by a number of spacecrafts. Ultraviolet observations of short period comets are, however, rather rare. Consequently Comet P/Halley in this apparition is a good object to obtain new physics of the hydrogen coma. Strong breathing of the hydrogen coma of this comet found by “Suisei” provides just such an example. The rotational period of Comet Halley's nucleus, its activity in the form of outbursts alone, and the position of jet sources etc. are determined from the breathing phenomena. Atomic hydrogen from organic compounds with a velocity of 11 km s⁻¹ play an important role in that analysis. The time variations of the water production rate of Comet Halley during this apparition observed by various spacecrafts appear to be in agreement with each other and are about 1.5–2 times larger than the standard model. The difficulty of the calibration problem was emphasized.     

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.     

Space observations of comets during solar flares: A possible explanation for comet brightness outbursts

Problems connected with mechanisms for comet brightness outbursts as well as for gamma-ray bursts remain open. Meantime, calculations show that irradiation of a certain class of comet nuclei, having high specific electric resistance, by intense fluxes of energetic protons and positively charged ions with kinetic energies more than 1 MeV/nucleon, ejected from the Sun during strong solar flares, can produce a macroscopic high-voltage electric double layer with positive charge in the subsurface zone of the nucleus, during irradiation times of the order of 10–100 h at heliocentric distances around 1–10 AU. The maximum electric energy accumulated in such layer will be restricted by the electric discharge potential of the layer material. For comet nuclei with typical radii of the order of 1–10 km the accumulated energy of such natural electric capacitor is comparable to the energy of large comet outbursts that are estimated on the basis of ground based optical observations. The impulse gamma and X-ray radiation together with optical burst from the comet nucleus during solar flares, anticipated due to high-voltage electric discharge, may serve as an indicator of realization of the processes above considered. Multi-wavelength observations of comets and pseudo-asteroids of cometary origin, having brightness correlation with solar activity, using ground based optical telescopes as well as space gamma and X-ray observatories, during strong solar flares, are very interesting for the physics of comets as well as for high energy astrophysics.     

Modifications of Martian ice-saturated regolith due to meteoroid impact

The possibility of water ice in the Martian subsurface regolith continues to present an intriguing enigma. We submit that any exo-martian source of energy, i.e. meteorite, asteroid or cometary impact, must disturb the equilibrium within the regolith surrounding an impact site. We farther propose impactors with the size range of 100 m and larger with velocities of 10 km s⁻¹ are effective in causing significant temperature increases in the regolith and subsequent ice melting. We present our studies of the crater formation within Martian surface layers that presumably are a solid mixture of regolith and water ice. Mass ratio of rocks to ice, as well as the thermal gradient in the crust, are the parameters. We have completed numerical simulations of a cratering event by means of two-dimensional, axialsymmetric hydrocodes involving a free particles' method in order to provide a simulation of an impact cratering. A fraction of subsurface ice melts and the crater partially forms from mud-like material. Comparison of a calculated crater and observed Martian crater is presented. The simplified analytical estimates concerning melting of ground ice in the regolith surrounding a Martian impact crater are presented.