Investigation of dynamics of discrete framed structures by a numerical wave-based method and an analytical homogenization approach

In this article, the analytical homogenization method of periodic discrete media (HPDM) and the numerical condensed wave finite element method (CWFEM) are employed to study the lon-gitudinal and transverse vibrations of framed structures. The valid frequency range of the HPDM is re-evaluated using the wave propagation feature identified by the CWFEM. The relative error of the wavenumber by the HPDM compared to that by the CWFEM is illustrated in functions of fre-quency and scale ratio. A parametric study on the thickness of the structure is carried out where the dispersion relation and the relative error are given for three different thicknesses. The dynamics of a finite structure such as natural frequency and forced response are also investigated using the HPDM and the CWFEM.     

Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites

Of the terrestrial planets, Earth and Mercury have self-sustained fields while Mars and Venus do not. Magnetic field data recorded at Ganymede have been interpreted as evidence of a self-generated magnetic field. The other icy Galilean satellites have magnetic fields induced in their subsurface oceans while Io and the Saturnian satellite Titan apparently are lacking magnetic fields of internal origin altogether. Parts of the lunar crust are remanently magnetized as are parts of the crust of Mars. While it is widely accepted that the magnetization of the Martian crust has been caused by an early magnetic field, for the Moon alternative explanations link the magnetization to plasma generated by large impacts. The necessary conditions for a dynamo in the terrestrial planets and satellites are the existence of an iron-rich core that is undergoing intense fluid motion. It is widely accepted that the fluid motion is caused by convection driven either by thermal buoyancy or by chemical buoyancy or by both. The chemical buoyancy is released upon the growth of an inner core. The latter requires a light alloying element in the core that is enriched in the outer core as the solid inner core grows. In most models, the light alloying element is assumed to be sulfur, but other elements such as, e.g., oxygen, silicon, and hydrogen are possible. The existence of cores in the terrestrial planets is either proven beyond reasonable doubt (Earth, Mars, and Mercury) or the case for a core is compelling as for Venus and the Moon. The Galilean satellites Io and Ganymede are likely to have cores judging from Galileo radio tracking data of the gravity fields of these satellites. The case is less clear cut for Europa. Callisto is widely taken as undifferentiated or only partially differentiated, thereby lacking an iron-rich core. Whether or not Titan has a core is not known at the present time. The terrestrial planets that do have magnetic fields either have a well-established inner core with known radius and density such as Earth or are widely agreed to have an inner core such as Mercury. The absence of an inner core in Venus, Mars, and the Moon (terrestrial bodies that lack fields) is not as well established although considered likely. The composition of the Martian core may be close to the Fe–FeS eutectic which would prevent an inner core to grow as long as the core has not cooled to temperatures around 1500 Kelvin. Venus may be on the verge of growing an inner core in which case a chemical dynamo may begin to operate in the geologically near future. The remanent magnetization of the Martian and the lunar crust is evidence for a dynamo in Mars’ and possibly the Moon’s early evolution and suggests that powerful thermally driven dynamos are possible. Both the thermally and the chemically driven dynamo require that the core is cooled at a sufficient rate by the mantle. For the thermally driven dynamo, the heat flow from the core into the mantle must by larger than the heat conducted along the core adiabat to allow a convecting core. This threshold is a few mW?m^?2 for small planets such as Mercury, Ganymede, and the Moon but can be as large as a few tens mW?m^?2 for Earth and Venus. The buoyancy for both dynamos must be sufficiently strong to overcome Ohmic dissipation. On Earth, plate tectonics and mantle convection cool the core efficiently. Stagnant lid convection on Mars and Venus are less efficient to cool the core but it is possible and has been suggested that Mars had plate tectonics in its early evolution and that Venus has experienced episodic resurfacing and mantle turnover. Both may have had profound implications for the evolution of the cores of these planets. It is even possible that inner cores started to grow in Mars and Venus but that the growth was frustrated as the mantles heated following the cessation of plate tectonics and resurfacing. The generation of Ganymede’s magnetic field is widely debated. Models range from magneto-hydrodynamic convection in which case the field will not be self-sustained to chemical and thermally-driven dynamos. The wide range of possible compositions for Ganymede’s core allows models with a completely liquid near eutectic Fe–FeS composition as well as models with Fe inner cores or cores in with iron snowfall.     

Physics of Solar Prominences: I—Spectral Diagnostics and Non-LTE Modelling

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (i.e. when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.     

The definition of life: a brief history of an elusive scientific endeavor

In spite of the spectacular developments in our understanding of the molecular basis that underlies biological phenomena, we still lack a generally agreed-upon definition of life, but this is not for want of trying. Life is an empirical concept; and, as suggested by the many unsuccessful efforts to define it, this task is likely to remain, at best, a work in progress. Although phenomenological characterizations of life are feasible, a precise definition of life remains an elusive intellectual endeavor. This is not surprising: as Nietszche once wrote, there are concepts that can be defined, whereas others only have a history. The purpose of this essay is to discuss some of the manifold (and often unsatisfactory) definitions of life that have been attempted from different intellectual and scientific perspectives and reflect, at least in part, the key role that historical frameworks play. Although some efforts have been more fruitful, the lack of an all-embracing, generally agreed-upon definition of life sometimes gives the impression that what is meant by life's origin is defined in somewhat imprecise terms and that several entirely different questions are often confused. The many attempts made to reduce the nature of living systems to a single living compound imply that life can be so well defined that the exact point at which it started can be established with the sudden appearance of the first replicating molecule. On the other hand, if the emergence of life is seen as the stepwise (but not necessarily slow) evolutionary transition between the non-living and the living, then it may be meaningless to draw a strict line between them.     

Space industry: the scene in 1992

Space Policy is here reprinting the Executive Summary of the 1993 edition of Euroconsult's World Space Industry Survey, Ten Year Outlook, which summarizes world-wide trends in space activities and covers prospects for industrial and commercial development, for the different areas of applications and for launch systems. World-wide, the space industry is expected to stand still for the next few years, with growth in applications slowed and dependent on new markets. Copies of the 410-page report can be obtained from Euroconsult, 71 boulevard Richard Lenoir, 75011 Paris, France.     

Future for CIS space industry

The following is the executive summary of Volume 1 of Euroconsult's four volume study, Space Industries and Markets in Russia and Other Countries of the Former Soviet Union. The material reproduced covers space policy and industry in CIS countries, providing a transversal view of CIS space activities and organizations. Generic problems across all fields of applications are analysed; the final focus is on trade with foreign countries. The other volumes cover prospects for satellite communications in the CIS to 2000; prospects for Earth observation satellite systems in Russia and Ukraine to 2000; and prospects for space transportation systems in the CIS to 2000. Information on the whole — some 650 pages — may be obtained from Pauline Byrne at Euroconsult, 71 Boulevard Richard Lenoir, 75011 Paris, France.     

Water, Life, and Planetary Geodynamical Evolution

In our search for life on other planets over the past decades, we have come to understand that the solid terrestrial planets provide much more than merely a substrate on which life may develop. Large-scale exchange of heat and volatile species between planetary interiors and hydrospheres/atmospheres, as well as the presence of a magnetic field, are important factors contributing to the habitability of a planet. This chapter reviews these processes, their mutual interactions, and the role life plays in regulating or modulating them.