On the conditions for fluidisation in cometary mantles

In our current understanding, active cometary nuclei comprise a volatile-depleted outer crust covering a mixture of dust and ices. During each perihelion passage the thermal wave penetrates the crust and sublimates a portion of these ices, which then escape the nucleus, dragging with them dust particles that replenish the coma and dust tail. The flux of released gases is likely to vary as a complex function of solar distance, nucleus structure, spin rate, etc. It has been previously hypothesised that at some point a fluidised state could occur, in which the gas drag is approximately equal to the weight of overlying dust and ice grains. This state is well understood and used in industrial processes where extensive mixing of the gas and solid components is desired. The literature on fluidisation under reduced gravity and pressure conditions is here reviewed and published relations used to predict the conditions under which fluidisation could occur in the near-surface of a cometary nucleus.     

Models of P/Borrelly: Activity and dust mantle formation

Comet 19P/Borrelly was observed by Deep Space One spacecraft on September 22, 2001 (Soderblom et al., 2002).The DS1 images show a very dark and elongate nucleus with a complex topography; the IR spectra show a strong red-ward slope consistent with a very hot and dry surface (345K to 300K). During DS1 encounter the comet coma was dominated by a prominent jet but most of the comet was inactive, confirming the Earth-based observations that <10% of the surface is actively sublimating. We have developed a thermal evolution model of comet PBorrelly, using a numerical code that is able to solve the heat conduction and gas diffusion equations at the same time across an idealized spherical nucleus ( De Sanctis et al., 1999, 2000; Capria et al., 2000; Coradini et al., 1997a,b). The comet nucleus is composed by water, volatiles ices and dust in different proportions. The refractory component is made by grains that are embedded in the icy matrix. The code is able to account for the dust release, contributing to the dust flux, and the formation of dust mantles on the comet surface. The model was applied to a cometary nucleus with the estimated physical and dynamical characteristics of P/Borrelly in order to infer the status and activity level of a body on such an orbit during the DS1 observation. The comet gas flux, differentiation and thermal behavior were simulated and reproduced. The model results are in good agreement with the DS1 flyby results and the ground based observations, in terms of activity, dust coverage and temperatures of the surface.     

Interstellar hydrogen bonding

This paper reports the first extensive study of the existence and effects of interstellar hydrogen bonding. The reactions that occur on the surface of the interstellar dust grains are the dominant processes by which interstellar molecules are formed. Water molecules constitute about 70% of the interstellar ice. These water molecules serve as the platform for hydrogen bonding. High level quantum chemical simulations for the hydrogen bond interaction between 20 interstellar molecules (known and possible) and water are carried out using different ab-intio methods. It is evident that if the formation of these species is mainly governed by the ice phase reactions, there is a direct correlation between the binding energies of these complexes and the gas phase abundances of these interstellar molecules. Interstellar hydrogen bonding may cause lower gas abundance of the complex organic molecules (COMs) at the low temperature. From these results, ketenes whose less stable isomers that are more strongly bonded to the surface of the interstellar dust grains have been observed are proposed as suitable candidates for astronomical observations.     

On the possible detection of solid O2 and O3 interstellar grains with ISO

In various models of interstellar grain chemistry, solid O₂ is formed by accretion as well as by surface reactions on grains. In dense molecular cloud models, at a later stage of the evolution, the O₂ molecule may become a substantial grain mantle constituent. Since IR dipole vibrational transitions for the homonuclear diatomic molecule O₂ are forbidden, the abundance of this potentially important grain mantle component can not be determined. However, embedded in a dirty ice matrix, the fundamental vibration of O₂ at 1550 cm⁻¹ becomes observable at 10 K, due to interactions with surrounding molecules, which break the symmetry of molecular oxygen. This process might be applicable for the dust mantle environment of interstellar grains. We have studied the role of solid O₂ and O₃ in astrophysically relevant ice mixtures and discuss the possible detection of solid O₂ and its major photolysis product O₃ in interstellar grains, in dense molecular clouds. Both molecules represent a specific target to be observed by the ISO satellite in the near future.     

月尘静电迁移研究进展

月尘的静电悬浮和迁移是Apollo时期留下的最有争议的问题之一. 其既是研 究月球表面物质演化历史的重要线索, 也是探月工程必须考虑的重要因素. 月尘在月表环境下易因电子附着、光电效应、二次电子发射等过程带电, 并 在月球全球性静电场作用下发生迁移运动. 但目前对月尘静电迁移过程的认 识还不全面, 其主要原因在于对月尘静电特性的了解不够准确, 对静电迁移 过程的地面模拟不够充分以及对月球尘埃环境的探测较为缺乏. 未来需进一 步开展模拟月尘的研制, 月尘静电特性的分析, 静电迁移过程模拟以及尘埃 环境的探测等工作.      

Thermal model and thermo-mechanical stresses in cometary nuclei

Spherically symmetric radial temperature profiles of cometary nuclei have been determined numerically (and for simplified models analytically) in dependence on the orbital position of the periodic comet Halley. These temperature fields in the nucleus are connected with thermal stress fields which have been calculated with the assumption of elastic properties of cometary matter. The remarkable result is the possible existence of stresses, strong enough to cause internal cracking of the nucleus and break-ups of the cometary surface. This may be essential understanding normal cometary activity as well as outbursts and splits.     

The Comet Halley flyby I.R. sounder “I.K.S.”

An infrared sounder is being developed in France to observe in 1986 Comet Halley from the Soviet “VEGA” flyby probes. The instrument, called “I.K.S.”, has three measuring channels. Two of these channels will provide the spectrum of the comet emission in the spectral intervals 2.5–5.0 μ and 6–12 μ, at a constant resolution λ/Δλ = 50.The third channel analyzes the comet I.R. image at a spatial frequency of about 1 arc minute^?1; two I.R. colours are used in this channel: 7–10 μ and 10–14 μ. From the results expected, it is hoped that (1) most primary simple molecules emitted by the nucleus will be identified; (2) the chemical composition and perhaps crystalline structure of the dust grains and ices released by the comet will be derived; and (3) the diameter of the nucleus and its brightness temperatures will be measured.     

A comparison of the dust properties in recent periodic comets

Measurements of the thermal emission from the cometary dust coma can be used to derive the rate of dust production from the nucleus as well as the size distribution of absorbing grains. More than ten short-period comets have now been observed in the infrared over a wide range in heliocentric distance. Dust production rates are derived for these comets based on theoretical models of the thermal emission from small absorbing grains and calculations of dust grain velocities. The mean size and albedo of the dust grains is similar in these comets, with the exception of Comet Crommelin, which seems to have had larger, darker grains.     

Gamma astrometric measurement experiment (GAME) – Science case

We report here on the science case of a concept for a satellite orbiting at 1 AU from the Sun and using a baffled Fizeau interferometer to look as close as possible to its limb. This configuration, and the need for looking nearby the Sun, is required for the main scientific driver of the mission, namely the measure of the γ parameter of the Parameterized Post-Newtonian formulation to the 10⁻⁶–10⁻⁷ level at least. This would lead to an accurate test of the General Theory of Relativity against other alternative theories of gravity, and set stringent constraints on some of the most significant issues of Astrophysics like those involving exotic forms of dark matter and dark energy. Exploiting the possibilities offered by the observation strategy, it is also possible to target other interesting scientific goals. One is, again, in the realm of General Relativity and aims at measuring the light deflection nearby the Giant Planets to detect asymmetric effects induced by their quadrupoles, predicted by GR but never measured so far. Others can be found in the observation of selected extrasolar systems where, e.g., the astrometric and photometric capabilities of GAME will help to improve on the knowledge of the brown-dwarf regime and on the search for exo-planets with the transit method, respectively.     

Numerical simulations of the radiance from the comet 46P/Wirtanen in the various configurations of the measurements during “Rosetta” mission

The work we present deals with the spectrometric measurements of VIRTIS instrument of the Comet P/Wirtanen planned for the Rosetta mission. This spectrometer can monitor (VIRTIS M channel: 0.250μm – 0.980μm; Δκ=20cm⁻¹; 0.980 – 5.0 μm; Δκ = 5cm⁻¹; VIRTIS H channel: 2.0 μm – 5.0 μm; Δκ=2cm⁻¹) the nucleus and the coma in order to provide a general picture of coma's composition, the production of gas and dust, the relationship of coma production to surface composition and the structure and variation of mineralogy of the nucleus surface. During the mission the observation conditions of the spectroscopic investigation change due to different relative positions spacecraft/comet, and to the different illumination conditions of the surface at various distances of the comet to the Sun. The nucleus surface is continuously modified by the ice sublimation accompanied by gas and dust emission. Consequently the surface also its spectrophotometric properties changes and their monitoring can give a new insight. The important role of simulations is to predict the results of measurements in various experimental condition what, in the future, can help in interpretation of the measured data.

In this paper the first results of our simulation the radiance from the comet in the 0.25–5.0μm spectral range at two distances from the Sun (1AU and 3AU) are shown. The distance between the Rosetta orbiter and the nucleus surface as well as the sun zenith angles are taken into account according to the Rosetta mission phases. In fact the surface and coma properties vary along the comet orbit, and should be taken into account in our calculations. The optical parameters of the dust on the surface (e.g. reflectance) and in the coma (e.g. Q_ext) were calculated from optical constants of possible comet analogues. The thermodynamic parameters of the comet are taken from the models of comet evolution. Through this kind of modelling it is possible to identify the surface characteristics in spectra of the radiation from the surface of nucleus transmitted through the coma loaded with dust and gases.

Even if the “Rosetta mission” is postponed, with the consequence of a target change, we think that our idea and the method used for the simulations can be useful also for the new Rosetta target - the comet 67P/Churyumov Gerasimenko.     

Properties of the mesosphere and thermosphere and comparison with CIRA 72

Exospheric temperatures of several reference atmosphere are reviewed and a recommendation is made for the exospheric temperature of a proposed mean CIRA. One of the deficiencies of CIRA 72 and other present thermospheric models is the representation of density changes with geomagnetic activity. This deficiency is illustrated with samples of data. The data show the effects of geomagnetic activity, particle precipitation, a solar proton event, and gravity waves. An empirical model developed from the unique AFGL satellite density data bank using multiple linear regression is reviewed. The present model is for low to moderate solar flux and quiet geomagnetic conditions, but it is planned to extend the model to active conditions. Good progress has been made since CIRA 72 was specified in our knowledge and understanding of the properties of the lower thermosphere, although there are still some unresolved problems. The biggest progress has been made in the theory of tidal effects and of particulate energy deposition and of electrojet heating. On the other hand, it is still not possible to define adequately the systematic variations of the lower boundary conditions of thermospheric models. This is due to lack of knowledge of the systematic variations of the structure properties in the 100 to 120 km altitude region and inadequate information on the mesospheric turbulence profile and variations in the turbopause altitude.     

Planetary rings

The individual ring systems are described with dust/magnetosphere interactions high-lighted somewhat. Jupiter's main ring is tenuous and enveloped by the magnetosphere; it principally contains micron-sized silicate grains. A vertically-extended, radially-localized “halo” of submicron particles lies inward of the main ring while a newly-discovered very faint ring lies outside it. The classical Saturnian system is composed of water ice chunks with sizes principally between cm and meters. Satellite resonances determine some ring structure but most is not understood. The faint exterior rings (E, G, F and one just identified between the A and F rings) are intimately associated with magnetospheric particles and contain mainly small grains, which are also prominent in the “spokes” located in the dense, middle portion of the B ring. Most of the nine narrow Uranian rings are slightly inclined and eccentric, and presumably lie within the putative Uranian magnetosphere. Particles are likely carbonaceous; sizes are thought to be larger than microns.     

An attempt to determine the ideal psychological profiles for crews of long term space missions.

This paper reviews possible psychological criteria for selection at individual level (personality, psychological stability, competence, social skills) as well as at crew level (crew size, gender, compatability, group homeostasis). Once astronauts have been selected an important effort will have to be made pre-flight to prepare the crew to the autonomy necessary for a Mars trip. During the mission psychological support will be important, but probably limited by the mission constraints. At this stage, mission success will probably rely mainly on the capacity of the crew to prevent and manage crises internally. Post-flight psychological support is necessary to help astronauts to readapt to a normal way of life on Earth.     

Carbonaceous materials in cometary environments: the case of the 3-4 micrometers bands.

We present in this paper a simulation of cometary spectra between 3 and 4 micrometers performed by using the optical properties of various carbon-based materials measured at different temperatures in the range 300-520 K. In our computations we have used new laboratory data obtained for hydrogenated amorphous carbon (HAC) grains and three kinds of polycyclic aromatic hydrocarbons (PAHs). All these materials show significant features in the near IR region; however, only the synthetic spectra obtained from HAC grains show a satisfactory agreement with the profile of the cometary bands.     

增益对星载微波辐射计系统线性度的影响分析

针对星载微波辐射计中检波器的工作特性进行分析. 观测固定目标时, 通过调节AGC (Automatic Gain Control)的大小, 定量分析系统伏安特性曲线, 对星载微波辐射计系统线性度进行定性分析. 结果表明, 通过调节AGC大小, 系统增益发生变化, 导致检波器工作区间发生微弱变化, 通过两点定标不能完全消除由于增益引起的非线性误差. 对一定亮温范围的观测目标进行分析, 结果证明, AGC变化时反算亮温与实际目标亮温差值随观测亮温的变化而成非线性变化. 从而证明, 系统增益变化导致的亮温变化通过两点线性定标并不能完全消除. 在系统正常工作或实时定标过程中, 要尽量避免调节AGC值, 保证微波辐射计系统工作在线性区间.      

Polarization imaging of dust cloud particles: Improvements and applications of the PROGRA2 instrument

The imaging system of the -PROGRA2 instrument allows to obtain maps of polarization and brightness of levitating dust clouds with a theoretical resolution of 10 μm per pixel. The measurements are conducted in microgravity during parabolic flights and on the ground by air-draught. It is then possible to measure the contribution of individual particles (grains, aggregates and agglomerates.) The size distribution can be retrieved, as well as the variation of polarization for a given phase angle with size for particles larger than 10 microns. Two different kinds of particles are considered: compact grains and (aggregates and agglomerates of) fluffy particles. Opposite results are obtained for these two kinds of particles, concerning the dependence of polarization with size and color in the visible domain for gray materials. These results, coupled with such remote sensing observations in the solar system, can then help to better understand the physical properties of solid particles and their variation in cometary comae, as well as in the Earth's atmosphere.     

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.     

Collection of cometary dust

Rendezvous Missions to Comets lead to low velocities at the nucleus of the comet. The resulting impact velocity of the cometary dust on a target will range between 10 and 400 m/s. The dust particle which impacts on a target can be collected for a subsequent in-situ analysis.

The collection efficiency of a target depends in addition to obvious geometrical conditions upon the surface of the target. The surface characteristics can be divided into two groups:

• “dirty” surfaces, covered with silicate or hydrocarbon compounds (for example vacuum grease),

• “clean” surfaces, like gold (with additional sputtering).

This paper deals with the experimental and theoretical investigation of the collection efficiency of “clean” targets. Laboratory experiments are described which were conducted at the Technische Universität München, Lehrstuhl für Raumfahrttechnik, and the Max-Planck-Institut für Kernphysik, Heidelberg. In both experiments an electromagnetic accelerator is used to accelerate different types of dust in vacuum to velocities between 10 and 400 m/s.

The target is then examined under the microscope and a secondary ion mass spectrometer (which is a model of the laboratory carried on board of the spacecraft for “in situ” analysis). The adhesion of the dust grains at the target is evaluated experimentally in an ultracentrifuge.     

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.     

Production and evolution of carbonaceous material by ion irradiation in space.

We review recent experimental studies concerning the evolution, driven by ion irradiation, of carbonaceous material from frozen gas to a refractory molecular solid. Under further irradiation the latter changes to a polymer-like material and ultimately to amorphous carbon. Most of the results have been obtained by "in situ" and remote IR and Raman spectroscopy. The results have been applied to demonstrate that molecular solids may be easily formed by irradiation of frozen mantles in dense interstellar clouds. Polymer-like material and amorphous carbons may result by further irradiation of organic mantles on grains in the diffuse interstellar medium. Those grains, during the aggregation to form extended bodies like comets (T-Tau phase of the Sun), are further modified. These latter are also irradiated, after the comet formation, during their long stay in the Oort cloud. In particular it has been suggested that comet may develop an ion-produced cometary organic crust that laboratory evidences show to be stable against temperature increases experienced during passages near the Sun. The comparison between the Raman spectra of some IDP (Interplanetary Dust Particles) and the Raman spectra of some ion-produced amorphous carbons, is also discussed.