Hot atoms in cosmic chemistry

High energy chemical reactions and atom molecule interactions might be important for cosmic chemistry with respect to the accelerated species in solar wind, cosmic rays, colliding gas and dust clouds and secondary knock-on particles in solids. “Hot” atoms with energies ranging from a few eV to some MeV can be generated via nuclear reactions and consequent recoil processes. The chemical fate of the radioactive atoms can be followed by radiochemical methods (radio GC or HPLC). Hot atom chemistry may serve for laboratory simulation of the reactions of energetic species with gaseous or solid interstellar matter. Due to the effective measurement of 10⁸–10¹⁰ atoms only it covers a low to medium dose regime and may add to the studies of ion implantation which due to the optical methods applied are necessarily in the high dose regime.

Experimental results are given for the systems: C/H₂O (gas), C/H₂O (solid, 77 K), N/CH₄ (solid, 77K) and C/NH₃ (solid, 77 K). Nuclear reactions used for the generation of 2 to 3 MeV atoms are: ¹⁴N(p, ) ¹¹C, ¹⁶O(p, pn) ¹¹C and ¹²C(d,n) ¹³N with 8 to 45 MeV protons or deuterons from a cyclotron. Typical reactions products are: CO, CO₂, CH₄, CH₂O, CH₃OH, HCOOH, NH₃, CH₃NH₂, cyanamide, formamidine, guanidine etc. Products of hot reactions in solids are more complex than in corresponding gaseous systems, which underlines the importance of solid state reactions for the build-up of precursors for biomolecules in space. As one of the major mechanisms for product formation, the simultaneous or fast consecutive reactions of a hot carbon with two target molecules (reaction complex) is discussed.     

On the evolution of dust in the solar vicinity.

The analysis of interplanetary dust shows that the majority of particles in out-of-ecliptic regions comes from comets and also that near solar dust, in the ecliptic regions, results most probably largely from comets. The intense radiation flux in the solar vicinity is expected to cause strong modifications in the material composition and surface structure of interplanetary dust particles and hence the analysis of near solar dust provides interesting insights into the evolution of meteoritic, especially cometary materials. Because of the lack of in-situ measurements our present knowledge concerning these processes derives from remote sensing, i.e. observations of the solar F-corona. In particular these are observations of albedo, polarization and colour temperature given in terms of average particle properties. For example the analysis of near infra-red F-corona data points to the existence of a strong component of irregularly structured silicate particles, most probably of cometary origin. The data may indicate a subsequent sublimation of different particles or different constituents of the particles. Here we compare particle properties derived from F-corona observations with model calculations of single particle properties and discuss perspectives of future analysis of cometary dust in the interplanetary cloud.     

Cometary constraints on the planet forming environment.

Molecular elemental and isotopic abundances of comets provide sensitive diagnostics for models of the primitive solar nebula. New measurements of the N2, NH and NH2 abundances in comets together with the in situ Giotto mass spectrometer and dust analyzer data provide new constraints for models of the comet forming environment in the solar nebula. An inventory of nitrogen-containing species in comet Halley indicates that NH3 and CN are the dominant N carriers observed in the coma gas. The elemental nitrogen abundance in the gas component of the coma is found to be depleted by a factor approximately 75 relative to the solar photosphere. Combined with the Giotto dust analyzer results for the coma dust component, we find for comet Halley Ngas + dust approximately 1/6 the solar value. The measurement of the CN carbon isotope ratio from the bulk coma gas and dust in comet Halley indicates a significantly lower value, 12C/13C = 65 +/- 9 than the solar system value of 89 +/- 2. Because the dominant CN carrier species in comets remains unidentified, it is not yet possible to attribute the low isotope ratio predominantly to the bulk gas or dust components. The large chemical and isotopic inhomogeneities discovered in the Halley dust particles on 1 mu scales are indicative of preserved circumstellar grains which survived processing in the interstellar clouds, and may be related to the presolar silicon carbide, diamond and graphite grains recently discovered in carbonaceous chondrites. Less than 0.1% of the bulk mass in the primitive meteorites studied consists of these cosmically important grains. A larger mass fraction (approximately 5%) of chemically heterogeneous organic grains is found in the nucleus of comet Halley. The isotopic anomalies discovered in the PUMA 1 Giotto data in comet Halley are probably also attributable to preserved circumstellar grains. Thus the extent of grain processing in the interstellar environment is much less than predicted by interstellar grain models, and a significant fraction of comet nuclei (approximately 5%) may be in the form of preserved circumstellar matter. Comet nuclei probably formed in much more benign environments than primitive meteorites.     

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.     

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.     

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.     

Microgravity particle research on the space station: the Gas-Grain Simulation Facility.

In the gravitational field on Earth, the large settling rate of micron-sized particles and the effects of gravity-induced convection prohibit many interesting studies of phenomena such as coagulation, collisions, and mutual interactions of droplets, dust grains and other particles. Examples of exobiology experiments involving these phenomena are the simulation of organic aerosol formation in Titan's atmosphere, studies of the role of comets in prebiotic chemical evolution, and simulations of carbon grain interactions in various astrophysical environments. The Gas-Grain Simulation Facility (GGSF) is a proposed Earth-orbital laboratory that will allow present ground-based experimental programs which study processes involving small particles and weak interactions to be extended to a new domain. Physics issues that scientists wishing to propose GGSF experiments must consider are reviewed in this paper. Specifically, coagulation, motion in gases and vacua, and wall deposition of particles in a microgravity environment are discussed.     

Comet-solar wind interactions: A dusty point of view

Anticipating the new results from the space missions to Comet Halley and Comet Giacobini-Zinner, we make a brief review of recent theoretical and observational studies of dust-plasma environment. In order to relate different disciplines in cometary research in the context of comet-solar wind interaction, two separate issues: (a) surface processes and (b) plasma processes are considered to indicate how various kinds of observations of cometary dust comas and tails may be used to infer the conditions of solar wind - comet interaction and the corresponding plasma processes in the cometary ionospheres and ion tails (and vice-versa). In particular, it is suggested that the narrow sunward-pointing dust streamers emitted from the cometary nuclei could be related to the electrostatic transport of sub-micron dust over the nuclear surfaces at large heliocentric distances; and the striae sometimes observed in cometary dust tails at smaller heliocentric distances could be the consequence of electrostatic fragmentation of fluffy dust particles in the ion tails.     

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.     

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.     

月球表面太空风化作用及其效应

太空风化作用普遍发生在月球、小行星等一些无大气层的天体表面, 主要包括太阳风离子辐射和微陨石撞击等作用. 通过对真实月球样品太空风化作用的研究成果以及低能离子和激光照射等地面模拟实验结果的系统综述和分析, 重点分析了月壤颗粒中非晶质层和纳米铁的成因, 指出太空风化研究中存在的太阳风离子辐射和微陨石撞击贡献难以区分及模拟实验模拟效果尚不充分的问题.进而提出月球太空风化研究不能简单套用其他天体的模型, 并建议未来开展更多月球和陨石样品的精细化学分析.      

Interplanetary medium – A dusty plasma

The average mass of dust per volume in space equals that of the solar wind so that the interplanetary medium should provide an obvious region to study dust plasma interactions. While dust collective behavior is typically not observed in the interplanetary medium, the dust component rather consists of isolated grains screened by and interacting with the plasma. Space measurements have revealed several phenomena possibly resulting from dust plasma interactions, but most of the dust plasma interactions are at present not quantified. Examples are the production of neutrals and pick-up ions from the dust, dust impact generated field variations at spacecraft and magnetic field variations possibly caused by solar wind interacting with dust trails. Since dust particles carry a surface charge, they are exposed to the Lorentz force in the interplanetary magnetic field and for grains of sub-micrometer sizes acceleration can be substantial.     

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.     

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.     

Plasma flow variations and energetic protons upstream of the earth’s bow shock: A statistical study

Foreshock is a special region located upstream of the Earth’s bow shock characterized by the presence of various plasma waves and fluctuations caused by the interaction of the solar wind plasma with particles reflected from the bow shock or escaping from the magnetosphere. On the other hand, foreshock fluctuations may modify the bow shock structure and, being carried through the magnetosheath, influence the magnetopause. During the years 1995–2000, the INTERBALL-1 satellite made over 10,000 hours of plasma and energetic particles measurements in the solar wind upstream of the Earth’s bow shock. We have sorted intervals according to the level of solar wind ion flux fluctuations and/or according to the flux of back-streaming energetic protons. An analysis of connection between a level of ion flux fluctuations and fluxes of high-energy protons and their relation to the IMF orientation is presented.     

Dust optical properties: A comparison between cometary and interplanetary grains

A better understanding of cometary dust optical properties has been derived from extensive observations of comet Halley, complemented by other cometary observations at large phase angles and/or in the infrared. Also, further analysis of IRAS observations and improvements in inversion techniques for zodiacal light have led to some progress in our knowledge of interplanetary dust.

Synthetic curves for phase angle dependence of intensity and polarization are presented, together with typical albedo values. The results obtained for interplanetary dust are quite reminiscent of those found for comets. However, the heterogeneity of the interplanetary dust cloud is demonstrated by the radial dependence of its local polarization and albedo; these parameters are also found to vary with inclination of the dust grains' orbits with respect to the ecliptic. Such results suggest drastic alterations with temperature in the texture of cometary dust, and would favor an important asteroidal component in the zodiacal cloud.     

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.     

Solar cosmic rays in the near Earth space and the atmosphere

Solar energetic particles (SEPs) constitute a distinct population of energetic charged particles, which can be often observed in the near Earth space. SEP penetration into the Earth’s magnetosphere is a complicated process depending on particle magnetic rigidity and geomagnetic field structure. Particles in the several MeV energy range can only access to periphery of the magnetosphere and the polar cap regions, while the GeV particles can arrive at equatorial latitudes. Solar protons with energies higher than 100 MeV may be observed in the atmosphere above ∼30 km, and those with energies more than 1 GeV may be recorded even at the sea level. There are some observational evidences of SEP influence on atmospheric processes. Intruding into the atmosphere, SEPs affect middle atmosphere odd-nitrogen and ozone chemistry. Since spatial and temporal variations of SEP fluxes in the near Earth space are controlled by solar activity, SEPs may present an important link between solar activity and climate. The paper outlines dynamics of SEP fluxes in the near Earth space during the last decades. This can be useful for tracing relationship between SEPs and atmospheric processes.     

Probing heliospheric diffusion coefficients with solar energetic particles

The dynamics of solar particle events provide a direct link to the understanding of properties of wave–particle interactions, and to the nature of the solar wind fluctuations. Depending on their energy, the often simultaneously observed electrons, protons and ions interact with different wavenumber ranges of the fluctuations, and are sensitive to various aspects of the dynamical nature of the solar wind turbulence. In general, the evolution of particle events is also sensitive to the spatial variation of the transport parameters between the Sun and a few AU. Together with in situ plasma and magnetic field observations this information can be used to extrapolate the properties of transport parameters into the more distant Heliosphere. Recent developments in the theory of parallel transport of energetic particles, and examples for the modelling of solar particle events and the derivation of transport parameters are considered. A dynamical quasi-linear theory is presented which gives special emphasis to the geometry and dynamic nature of the fluctuations, and which is able to provide particle mean free paths solely from observed plasma parameters, in good agreement with those derived by the modelling. Possibilities to apply the above results to the study of other energetic particle processes in the Heliosphere are discussed.     

Towards multi-dimensional space weather modeling for energetic oxygen ions in the earth''s inner magnetosphere: Equilibrium configuration

We report the first 3+1 dimensional model development for energetic atomic oxygen ions in the Earth's radiation belts. Energetic Oxygen ions cans be supplied to the Earth's Inner magnetosphere from the sun (as a component of solar wind and solar energetic particles), from anomalous cosmic rays, and from acceleration processes acting on ionospheric atomic oxygen ions. We have built a multi-dimensional oxygen ion model in the following free parameters: geomagnetic L-shell, the magnetic moment, the second adiabatic invariant, and the discrete charge state number. Quiet time, steady state oxygen ion distributions have been obtained numerically from an assumed outer radiation zone boundary condition at L=7, average values of the radial diffusion coefficients, and standard values for the exospheric neutral densities due to the MSIS-86 upper atmosphere and exosphere neutral thermal particle density model. Average distributions of free electrons in the plasmasphere were also assumed with a mean plasmapause location just beyond L=4. We included the six lowest ionic charge states of atomic oxygen (¹⁶O) based on an existing charge exchange cross section compilation by Spjeldvik and Fritz (1978). Computed oxygen ion distributions include the resulting equilibrium structure of energy oxygen ions between 10 KeV and 100 MeV.