中国自主木星冰卫星冰下液态海洋探测刍议

木星冰卫星(尤其是"木卫2")在整个太阳系中有非常独特的探测价值,也是当前国际深空探测最前沿、最受关注的领域之一。在对国外相关探测技术归纳总结的同时,结合木星冰卫星冰下海洋探测的若干科学问题,次表层模型、温度廓线及探测深度、电离层的影响等方面,提供了几种木星冰卫星冰下液态水海洋探测的新思路,并给出了一些初步的系统框架设计和次表层介电特性仿真分析等结果。通过对新方法、新模式的探索,有望对我国自主木星冰卫星的次表层液态海洋探测起到良好的借鉴作用。     

Thermo-mechanical design and testing of a microbalance for space applications

This work focuses on the thermo-mechanical design of the microbalance used for the VISTA (Volatile In Situ Thermogravimetry Analyzer) sensor. VISTA has been designed to operate in situ in different space environments (asteroids, Mars, icy satellites). In this paper we focus on its application on Mars, where the expected environmental conditions are the most challenging for the thermo-mechanical design.     

In situ analysis of Europa ices by short-range melting probes

A key aspect for understanding the astrobiological potential of planets and moons in the Solar system is the analysis of material embedded in or underneath icy layers on the surface. In particular in case of the icy crust of Jupiters moon Europa such investigation would be of greatest interest. For a Europa lander to be launched in the 2020–2030 timeframe, we propose to use a simplified instrumented melting probe which is able to access and sample depths of a few meters without the necessity of heavy and complicated drilling equipment.     

Mercury: can any ice exist at subpolar regions?

The heat transfer in a regolith subsurface layer of thickness 20 m has been theoretically simulated for the areas near Mercury's north pole aiming at the clarification of the possible existence of subsurface ice formations of different form. The paper considers different models of the icy regolith structure and composition: pure uniform amorphous ice; a porous dispersive system with ice-filled pores and voids; permafrost. For comparison the heat transfer in dry iceless regolith has been considered as well. It has been shown that the line of maximum distribution of subsurface icy formations depends on the icy regolith model, but for any one in the “hot” regions it does not go below 70°. For the “cool” regions this line has been shown to go from 5° to 10° southward than that for the “hot” ones. The possible thickness of icy regolith near the pole has been estimated for different models assuming an interior heat flow of 15 mW m⁻². It has been shown that the maximum thickness of this layer takes place at the pole and is equal to 10 km for any model.     

Sputtering processes: Erosion and chemical change

We review laboratory data and models on sputter-induced erosion and chemical alterations of ice films and apply the results to icy grains and satellites exposed to magnetospheric ion bombardment. We show that the source of the plasma in the inner magnetosphere of Saturn is likely to be the sputter erosion of the icy objects in this region and consider the sputter erosion and possible stabilization of the E-ring. Ion-induced polymerization is discussed as a source of the darkened rings of Uranus.     

From the interstellar clouds, through the inner to the outer solar system: a universally distributed complex organic chemistry. Preface.

High molecular weight organic compounds are involved in the chemistry and physics of many astrophysical and planetary objects. They are or should be present in interstellar dust, in comets and meteorites, in the Giant planets and Titan, in asteroids Triton and icy satellites. They represent a class of very complex organic material, part of which may have played a role in the origin of life on Earth. Thus they directly concern prebiotic chemistry and exobiology.     

Organic solids produced from simple C/H/O/N ices by charged particles: applications to the outer solar system.

CH4, CO, and CO2 are all potential one-carbon molecular repositories in primitive icy objects. These molecules are all found in the Comet Halley coma, and are probable but, (except for CH4 detected on Triton and Pluto) undetected subsurface constituents in icy outer solar system objects. We have investigated the effects of charged particle irradiation by cold plasma discharge upon surfaces of H2O:CH4 clathrate having a 200:1 ratio, as well as upon ices composed of H2O plus C2H6 or C2H2 (sometimes plus NH3) which are also plausible constituents. These materials color and darken noticeably after a dose 10(9) - 10(10) erg cm-2, which is deposited rapidly (< or = 10(4) yr.) in solar system environments. The chromophore is a yellowish to tan organic material (a tholin) which we have studied by UV-VIS reflection and transmission, and IR transmission spectroscopy. Its yield, -1 C keV-1, implies substantial production of organic solids by the action of cosmic rays and radionuclides in cometary crusts and interiors, as well as rapid production in satellite surfaces. This material shows alkane bands which Chyba and Sagan have shown to well match the Halley infrared emission spectrum near 3.4 microns, and also bands due to aldehyde, alcohol and perhaps alkene/aromatic functional groups. We compare the IR spectral properties of these tholins with the spectra of others produced by irradiation of gases and ices containing simple hydrocarbons.     

Analog environments for a Europa lander mission

This paper reviews the utility of analog environments in preparations for a Europa lander mission. Such analogs are useful in the demonstration and rehearsal of engineering functions such as sample acquisition from an icy surface, as well as in the exercise of the scientific protocols needed to identify organic, inorganic and possible biological impurities in ice. Particular attention is drawn to Antarctic and Arctic analog sites where progress in these latter areas has been significant in recent years.     

Miniaturized laser-induced plasma spectrometry for planetary in situ analysis – The case for Jupiter’s moon Europa

Jupiter’s icy moon Europa is one of most promising places in our Solar System where possible extraterrestrial life forms could exist either in the past or even presently. The Europa Lander mission, an exciting part of the international Europa Jupiter System Mission (EJSM/Laplace), considers in situ planetary exploration of the moon. The distance of Europa from the Earth and the Sun asks for autonomous analytical tools that maximize the scientific return at minimal resources, demanding new experimental concepts. We propose a novel instrument, based on the atomic spectroscopy of laser generated plasmas for the elemental analysis of Europa’s surface materials as far as it is in reach of the lander for example by a robotic arm or a mole, or just onboard the lander. The technique of laser-induced plasma spectrometry provides quantitative elemental analysis of all major and many trace elements. It is a fast technique, i.e. an analysis can be performed in a few seconds, which can be applied to many different types of material such as ice, dust or rocks and it does not require any sample preparation. The sensitivity is in the range of tens of ppm and high lateral resolution, down to 50 μm, is feasible. In addition, it provides the potential of depth profiling, up to 2 mm in rock material and up to a few cm in more transparent icy matrices. Key components of the instrument are presently developed in Germany for planetary in situ missions. This development program is accompanied by an in-depth methodical investigation of this technique under planetary environmental conditions.     

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。      

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.     

Return to Europa: Overview of the Jupiter Europa orbiter mission

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander.     

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.     

The formation of organic molecules in astronomical ices.

An absorption feature at 3.4 micrometers has been observed in various lines-of-sight through the diffuse interstellar medium. Its position and width lead to an identification with the C-H stretching mode of solid organic material. A possible mechanism for the production of organic solids in the interstellar medium is UV photoprocessing of icy mantles which accrete on dust grains in dense clouds. Furthermore, thermally induced reactions involving formaldehyde molecules in the mantles could be an important source of organics. Laboratory simulation of these processes shows that a large variety of oxygen- and nitrogen-rich species may be produced. It is shown that the occurrence of periodic transient heating events plays an important role in the production of organic material in the ice mantles. Finally, it is pointed out how future missions like the Infrared Space Observatory (ISO) as well as analysis of comet material by Rosetta may be able to clarify the nature and evolution of interstellar organics.     

Life in the solar system.

Life, defined as a chemical system capable of transferring its molecular information via self-replication and also capable of evolving, must develop within a liquid to take advantage of the diffusion of complex molecules. On Earth, life probably originated from the evolution of reduced organic molecules in liquid water. Organic matter might have been formed in the primitive Earth's atmosphere or near hydrothermal vents. A large fraction of prebiotic organic molecules might have been brought by extraterrestrial-meteoritic and cometary dust grains decelerated by the atmosphere. Any celestial body harboring permanent liquid water may therefore accumulate the ingredients that generated life on the primitive Earth. The possibility that life might have evolved on early Mars when water existed on the surface marks it as a prime candidate in a search for bacterial life beyond the Earth. Europa has an icy carapace. However, cryovolcanic flows at the surface point to a possible water subsurface region which might harbor a basic life form. The atmosphere and surface components of Titan are also of interest to exobiology for insight into a hydrocarbon-rich chemically evolving world. One-handed complex molecules and preferential isotopic fractionation of carbon, common to all terrestrial life forms, can be used as basic indicators when searching for life beyond the Earth.     

The sticking probability of a hydrogen atom on icy mantle.

In the present work, we investigate the sticking process of a hydrogen atom on the surface of dust grains. As a realistic model for the icy mantle of dust grains, we produced slab-shaped amorphous water ice with infinite area by classical molecular dynamics (MD) computational simulation using two-dimensional periodic boundary condition. The resulting amorphous water ice slabs at 10 K and 70 K were found to be in good agreement with the experimental high-density and low-density amorphous water ice, respectively. Then, we investigated the dynamical behaviors of an impinging H atom on the surface of it by MD simulations. The sticking probabilities of incident H atoms with several initial temperatures on 10 K and 70 K ice were obtained. It was found that most of H atoms colliding with the 10 K ice stuck on the surface of it. After having stuck, the impinging H atoms diffused on the surface of ice and became trapped in one of potential wells on the surface. The mobility of a H atom on the surface of the amorphous water ice was found to depend only upon the temperature of ice.     

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.     

Dust emission phenomena of cometary analogues

Experiments with cometary analogues are reported, which were performed at DLR - Köln in a small vacuum chamber and in the Space Simulator. The results apply to the emission processes of ice-dust grains from the surface of mineral-ice mixtures during insolation. The emission phenomena were observed with CCD-video-cameras. The camera used at the small chamber was equipped with a magnifying (10x) macro-lens. The magnified pictures show that the ice-dust agglomerates carry tiny ice particles on their surfaces. These may play the role of interlinking bonds between one grain and its neighbors. In the enlarged view it can be seen that the ice-dust grains do not erupt with high velocity out of the surface. The upstreaming gas jet originating from the sublimation of the volatile components first erodes the interlinking bonds between the particles. When the drag force exerted on the particles exceeds the force of gravity, the particles lift off. In most of the observed cases this initial lift off velocity is very small e.g. a few cm/s.

During a comet-simulation experiment in the Space-Simulator in November 1989 the trajectories of emitted dust particles were recorded for the first time with a high speed shutter video system 60 cm from the model comet. The video tapes were evaluated with a computerized image processing system. Unique insights in the gas-dust-interaction immediately at and above the insolated icy body were obtained. The trajectories differ markedly in all cases from the the ballistic parabolas calculated from the measured starting conditions: elevation angle and speed. The trajectory is mainly dominated by the momentum transfer from the gas jet to the dust particles. Many of the particles have millimeter sizes. The observed velocities at the end of the visible trajectory are in the order of a few meters per second.     

Nusselt number for convection driven by tidal and radiogenic heating

The ‘traditional’ formulas giving the Nusselt number Nu as a function of Rayleigh number Ra cannot be used for low and moderate values of Ra. Moreover, the recent progress in 3D numerical modeling makes possible to determine the Nusselt number Nu as a function of Rayleigh number Ra for convection driven by radiogenic and tidal heating. We found that for low and moderate Ra: Nu(Ra) = ε(Ra+ξ)^λ where λ depends on rheology and boundary conditions, ε depends only on the mode of heating, and ξ = ε^−1/λ · Nu(Ra) makes possible to develop a parameterized theory of convection in medium size icy satellites. We also indicate some differences between convection driven by tidal and radiogenic heating and convection driven exclusively by radiogenic heating.