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231.
Wolfgang Schulte Stefan Hofer Peter Hofmann Hans Thiele Ralf von Heise-Rotenburg Jan Toporski Petra Rettberg 《Acta Astronautica》2007,60(12):966-973
For more than a decade Kayser-Threde, a medium-sized enterprise of the German space industry, has been involved in astrobiology research in partnership with a variety of scientific institutes from all over Europe. Previous projects include exobiology research platforms in low Earth orbit on retrievable carriers and onboard the Space Station. More recently, exobiology payloads for in situ experimentation on Mars have been studied by Kayser-Threde under ESA contracts, specifically the ExoMars Pasteur Payload. These studies included work on a sample preparation and distribution systems for Martian rock/regolith samples, instrument concepts such as Raman spectroscopy and a Life Marker Chip, advanced microscope systems as well as robotic tools for astrobiology missions. The status of the funded technical studies and major results are presented. The reported industrial work was funded by ESA and the German Aerospace Center (DLR). 相似文献
232.
233.
Rosemary Killen Gabrielle Cremonese Helmut Lammer Stefano Orsini Andrew E. Potter Ann L. Sprague Peter Wurz Maxim L. Khodachenko Herbert I. M. Lichtenegger Anna Milillo Alessandro Mura 《Space Science Reviews》2007,132(2-4):433-509
It has been speculated that the composition of the exosphere is related to the composition of Mercury’s crustal materials.
If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric
study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been
believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485–5499, 1981), Na and K come from volatilized materials partitioned between Mercury’s crust and meteoritic impactors (Hunten, D.M., et
al. in Mercury, pp. 562–612, 1988; Morgan, T.H., et al. in Icarus 74:156–170, 1988; Killen, R.M., et al. in Icarus 171:1–19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization,
photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal
and spatial dependence. The exosphere is strongly influenced by Mercury’s highly elliptical orbit and rapid orbital speed.
As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude.
Because there is no inclination of the orbital axis, there are regions at extreme northern and southern latitudes that are
never exposed to direct sunlight. These cold regions may serve as traps for exospheric constituents or for material that is
brought in by exogenic sources such as comets, interplanetary dust, or solar wind, etc. The source rates are dependent not
only on temperature and composition of the surface, but also on such factors as porosity, mineralogy, and space weathering.
They are not independent of each other. For instance, ion impact may create crystal defects which enhance diffusion of atoms
through the grain, and in turn enhance the efficiency of PSD. The impact flux and the size distribution of impactors affects
regolith turnover rates (gardening) and the depth dependence of vaporization rates. Gardening serves both as a sink for material
and as a source for fresh material. This is extremely important in bounding the rates of the other processes. Space weathering
effects, such as the creation of needle-like structures in the regolith, will limit the ejection of atoms by such processes
as PSD and ion-sputtering. Therefore, the use of laboratory rates in estimates of exospheric source rates can be helpful but
also are often inaccurate if not modified appropriately. Porosity effects may reduce yields by a factor of three (Cassidy,
T.A., and Johnson, R.E. in Icarus 176:499–507, 2005). The loss of all atomic species from Mercury’s exosphere other than H and He must be by non-thermal escape. The relative
rates of photo-ionization, loss of photo-ions to the solar wind, entrainment of ions in the magnetosphere and direct impact
of photo-ions to the surface are an area of active research. These source and loss processes will be discussed in this chapter. 相似文献
234.
The Neutral Mass Spectrometer on the Giotto spacecraft established that H2O is the dominant species in Comet Halley's volatiles and determined the abundance of more than 10 parent species. The instrument
discovered strong extended H2CO and CO sources in the coma of Comet Halley. Polymerized H2CO associated with the cometary dust and evaporating slowly as the monomer is most likely the extended H2CO source. Photodissociation of the H2CO into CO fully accounts for the extended CO source.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
235.
Guanghua Zheng 《Russian Aeronautics (Iz VUZ)》2010,53(4):475-478
A convective-film system of high pressure-differential turbine blade cooling is presented. The results of calculating the
thermal-hydraulic blade state using the KW3D software are given. 相似文献
236.
Recent experimental studies with laser supported absorption waves, arising from intense laser radiation incident upon various surface materials, are discussed. The propagation characteristics of both subsonic (laser supported combustion), intensity about 105 W/cm2, and supersonic (laser supported detonation), intensity greater than 106 W/cm2, waves are described, including the dependence of wave speed on laser intensity, beam diameter, gas density and laser wave length. Measurements of the plasma properties in such waves are described, and the analysis of these data allows conclusions identifying the dominant transport mechanism in such waves. It is also concluded that in the LSC (subsonic) case, radial flow ahead of and within the wave are dominant features of the wave structure. The equipment and laser facilities used in the experiments are also discussed. 相似文献
237.
Quantification of spatial structure of human proximal tibial bone biopsies using 3D measures of complexity 总被引:2,自引:0,他引:2
Saparin PI Thomsen JS Prohaska S Zaikin A Kurths J Hege HC Gowin W 《Acta Astronautica》2005,56(9-12):820-830
Changes in trabecular bone composition during development of osteoporosis are used as a model for bone loss in microgravity conditions during a space flight. Symbolic dynamics and measures of complexity are proposed and applied to assess quantitatively the structural composition of bone tissue from 3D data sets of human tibia bone biopsies acquired by a micro-CT scanner. In order to justify the newly proposed approach, the measures of complexity of the bone architecture were compared with the results of traditional 2D bone histomorphometry. The proposed technique is able to quantify the structural loss of the bone tissue and may help to diagnose and to monitor changes in bone structure of patients on Earth as well as of the space-flying personnel. 相似文献
238.
Roland Diehl 《Space Science Reviews》2001,99(1-4):197-208
Gamma-ray lines arise from radioactivities produced in nucleosynthesis sites, and from deexcitation of nuclei which have been activated through energetic particle collisions. Since the bulk of nucleosynthesis activity relates to activities inside massive stars, both these processes are related to the likely sources of cosmic rays: Supernova remnants show radioactivity afterglows at time scales which bracket their likely phases of relevance as CR acceleration sites; 26Al radioactivity may trace regions of intense wind interactions from groups of massive stars, and also encode information about the possible injection of matter into CR acceleration environments through interstellar dust grains. The status of -ray line measurements after the Compton Observatory mission is presented, with models and interpretations of current results, and the prospects of upcoming measurements. 相似文献
239.
240.
Peter Mazur Elso S. Barghoorn Harlyn O. Halvorson Thomas H. Jukes Isaac R. Kaplan Lynn Margulis 《Space Science Reviews》1978,22(1):3-34
A central purpose of Viking was to search for evidence that life exists on Mars or may have existed in the past. The missions carried three biology experiments the prime purpose of which was to seek for existing microbial life. In addition the results of a number of the other experiments have biological implications: (1) The elemental analyses of the atmosphere and the regolith showed or implied that the elements generally considered essential to terrestrial biology are present. (2) But unexpectedly, no organic compounds were detected in Martian samples by an instrument that easily detected organic materials in the most barren of terrestrial soils. (3) Liquid water is believed to be an absolute requisite for life. Viking obtained direct evidence for the presence of water vapor and water ice, and it obtained strong inferential evidence for the existence of large amounts of subsurface permafrost now and in the Martain past. However it obtained no evidence for the current existence of liquid water possessing the high chemical potential required for at least terrestrial life, a result that is consistent with the known pressure-temperature relations on the planet's surface. On the other hand, the mission did obtain strong indications from both atmospheric analyses and orbital photographs that large quantities of liquid water flowed episodically on the Martian surface 0.5 to 2.5 G years ago.The three biology experiments produced clear evidence of chemical reactivity in soil samples, but it is becoming increasingly clear that the chemical reactions were nonbiological in origin. The unexpected release of oxygen by soil moistened with water vapor in the Gas Exchange experiment together with the negative findings of the organic analysis experiment lead to the conclusion that the surface contains powerful oxidants. This conclusion is consistent with models of the atmosphere. The oxidants appear also to have been responsible for the decarboxylation of the organic nutrients that were introduced in the Label Release experiment. The major results of the GEX and LR experiments have been simulated at least qualitatively on Earth. The third, Pyrolytic Release, experiment obtained evidence for organic synthesis by soil samples. Although the mechanism of the synthesis is obscure, the thermal stability of the reaction makes a biological explanation most unlikely. Furthermore, the response of soil samples in all three experiments to the addition of water is not consistent with a biological interpretation.The conditions now known to exist at and below the Martian surface are such that no known terrestrial organism could grow and function. Although the evidence does not absolutely rule out the existence of favourable oases, it renders their existence extremely unlikely. The limiting conditions for the functioning of terrestrial organisms are not the limits for conceivable life elsewhere, and accordingly one cannot exclude the possibility that indigenous life forms may currently exist somewhere on Mars or may have existed sometime in the past. Nevertheless, the available information about the present Martian environment puts severe constraints and presents formidable challenges to any putative Martian organisms. The Martian environment in the past, on the other hand, appears to have been considerably less hostile biologically, and it might possibly have permitted the origin and transient establishment of a biota. 相似文献