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.     

Gas permeability and flow characterization of simulated lunar regolith

Recent discoveries of water ice trapped within lunar topsoil (regolith) have placed a new emphasis on the recovery and utilization of water for future space exploration. Upon heating the lunar ice to sublimation, the resulting water vapor could theoretically transmit through the lunar regolith, to be captured on the surface. As the permeability of lunar regolith is essential to this process, this paper seeks to experimentally determine the permeability and flow characteristics of various gas species through simulated lunar regolith (SLR). Two different types of SLR were compacted and placed into the permeability setup to measure the flow-rate of transmitted gas through the sample. Darcy’s permeability constant was calculated for each sample and gas combination, and flow characteristics were determined from the results. The results show that Darcy’s permeability constant varies with SLR compaction density, and identified no major difference in permeable flow between the several tested gas species. Between the two tested SLR types, JSC-1A was shown to be more permeable than NU-LHT under similar conditions. In addition, a transition zone was identified in the flow when the gas pressure differential across the sample was less than ∼40 kPa.     

Searching for life in extreme environments relevant to Jovian’s Europa: Lessons from subglacial ice studies at Lake Vostok (East Antarctica)

The objective was to estimate the genuine microbial content of ice samples from refrozen water (accretion ice) from the subglacial Lake Vostok (Antarctica) buried beneath the 4-km thick East Antarctic ice sheet. The samples were extracted by heavy deep ice drilling from 3659 m below the surface. High pressure, a low carbon and chemical content, isolation, complete darkness and the probable excess of oxygen in water for millions of years characterize this extreme environment. A decontamination protocol was first applied to samples selected for the absence of cracks to remove the outer part contaminated by handling and drilling fluid. Preliminary indications showed the accretion ice samples to be almost gas free with a low impurity content. Flow cytometry showed the very low unevenly distributed biomass while repeated microscopic observations were unsuccessful.     

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.     

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.     

An overview of gas phenomena in comet Halley

Ground-based observations and in-situ measurements of the gas activities and coma structures of comet Halley have yielded many new insights to the expansion of atmospheric gas from the central nucleus. The first impression from these results is discussed in terms of present theoretical understanding of the different gas phenomena in the inner coma (r 10⁴ km). The need for developing a new generation of theoretical models capable of describing the observed highly anisotropic coma activities with spin-modulation in the outgassing rate as well as short-term outbursts is stressed.     

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.     

Cometary habitats for primitive life.

Comet Halley studies indicate most of the nucleus is covered by an insulating crust, presumed of pyrolysed organic material. The subcrust is warmed and percolated by gases within 2AU, so provides one habitat for primitive replicating organisms. Cracks and crevices within contaminated ice in the craters provides a habitat for photosynthesising organisms. Subsurface lakes on the Europa model, though insulated by some metres of ice, would require a trigger (perhaps meteorite impact and energy source (chemical or metabolic energy) to initiate and maintain a suitable-habitat on short period comets. Constraints on transfer between comets and other planetary bodies implies that radiation-resistant species with lengthy hibernation potential would be expected.     

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.     

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.     

The contemporary model of cometary nucleus and the prospects of its improvement by space research

No cometary nucleus has ever been observed directly. A model is deduced from ground-based and space data on cometary atmospheres. The main features of the chemical composition of cometary nuclei and the estimation of their sizes are described. The treatment of the process of vaporization of dusty ice shows, contrary to widespread opinion, that the islands on the non-volatile porous mantle are formed, not in perihelion but at large heliocentric distances and on the coldest parts of a nucleus. It is shown that the mantle does not disappear when the comet approaches the Sun, as it is often supposed, but is fluidized. The proposed model can give a number of properties of cometary nuclei but some of them can be established by direct space methods only. Such properties are the masses, the rotational velocities, the homogeneity of the dust-ice mixture, the internal structure, the power of the internal sources of energy.     

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.     

A quantitative model for comet nucleus topography

Topography of the surface of a comet nucleus is likely rough at all scales smaller than the mean effective radius. We present a flexible and easily scalable model for quantitative calculations simulating the effects of comet nucleus topography on gas release and dust mantle evolution. The topographic features we describe must be large enough (typically> 10 m) so that they will not erode in one orbit of the nucleus around the Sun. The maximum effective size of a hill is about 1/√2 times the effective radius of the nucleus. If it is larger, then an ellipsoidal shape of the nucleus is more appropriate. The procedure described here also permits for inhomogeneous composition of the topographic features, leading to locally different rates of gas production (e.g., jet-like features and filaments) or different thicknesses of the dust mantle. It also can give rise to different temperature patches, locally varying albedos and emissivities, and may explain the formation of permanent dust mantles.     

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.     

基于降阶模型的翼型结冰冰形预测方法

翼型结冰冰形的数值模拟预测通常比较复杂耗时,为了更加快速准确地预测冰形以减少计算资源消耗,建立了基于本征正交分解（POD）和Kriging模型的冰形快速预测方法。利用CFD数值模拟结果来构建样本空间,以飞行迎角为例详述了降阶模型的冰形预测的实现手段,并结合试验设计方法,完成了多参数的结冰冰形快速预测,同时研究了先进的Blind-Kriging模型的相关方法以及对于预测结果的改进。结果表明,降阶模型预测翼型结冰冰形与CFD数值模拟结果吻合较好,表明降阶模型可以快速、精确地应用于翼型结冰冰形预测。      

机翼防冰过程中冰脊问题的数值分析

对机翼防冰过程中冰脊的形成特点及冰脊对机翼气动特性的影响进行了计算分析.基于经典的Messinger结冰模型开发了多步结冰程序,对不同条件下过冷水滴的撞击结冰进行了热质耦合计算,计算结果和文献试验结果吻合较好,表明该热质耦合算法的正确性.在此基础上对不同环境温度、飞行速度和不同加热功率等条件下冰脊的生长特点和机翼的气动特性进行了计算分析.结果表明:在非霜冰条件下,冰脊主要在热防护极限外并紧挨着热防护极限的位置处形成和发展,而在霜冰条件下,冰脊主要在机翼下表面形成,但是在热防护区域内有显著的结冰出现,该条件下的结冰对机翼的气动特性具有较大的破坏性.     

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.     

基于润滑理论的二维积冰数值模拟

将基于润滑理论的水膜-冰层模型推广到在任意二维截面上建立的正交曲线坐标系,得到描述二维型线表面水膜流动和积冰的偏微分控制方程.采用一种隐式-显式有限差分格式建立控制方程组的代数形式,给出求解非稳态控制方程组的数值方法.为验证模型和数值解法的有效性,在典型的航空积冰和传输线积冰环境下对翼型和传输线表面的积冰算例进行数值模拟.将水膜-冰层模型的冰形计算结果与传统Messinger模型的模拟结果以及冰风洞试验结果进行对比,低温明冰条件下采用当前方法计算的翼型表面冰形接近于Messinger模型的模拟冰形曲线;相对高温条件下的计算结果比传统Messinger模型更为精确.低速积冰环境下Messinger模型难以模拟的传输线表面积冰同样可以采用水膜-冰层模型进行有效 预测.     

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.