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411.
A new technique is developed to compensate multiple-wavelength distortion in airborne antenna arrays. This approach exploits the phase information in microwave reflections from arbitrary terrain. To handle reflections incident over a broad angle, a range-Doppler preprocessor is used in each element channel to resolve wavefronts incident simultaneously from different directions. The phase information for each direction of arrival is compared between elements and processed by optimal estimators to determine the phase corrections needed to compensate the distortion. To develop the estimators, a statistical model of the complex baseband terrain reflections is developed. This is in turn used to generate conditional probability densities involving the range Doppler observations and the parameters to be estimated. These densities are subsequently used to develop minimum variance and maximum likelihood estimators. The new estimators use additional information that has not been exploited by previous techniques and therefore provide enhanced performance 相似文献
412.
B N Khare E L Bakes D Cruikshank C P McKay 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,27(2):299-307
Many bodies in the outer Solar System display the presence of low albedo materials. These materials, evident on the surface of asteroids, comets, Kuiper Belt objects and their intermediate evolutionary step, Centaurs, are related to macromolecular carbon bearing materials such as polycyclic aromatic hydrocarbons and organic materials such as methanol and related light hydrocarbons, embedded in a dark, refractory, photoprocessed matrix. Many planetary rings and satellites around the outer gaseous planets display such component materials. One example, Saturn's largest satellite, Titan, whose atmosphere is comprised of around 90% molecular nitrogen N2 and less than 10% methane CH4, displays this kind of low reflectivity material in its atmospheric haze. These materials were first recorded during the Voyager 1 and 2 flybys of Titan and showed up as an optically thick pinkish orange haze layer. These materials are broadly classified into a chemical group whose laboratory analogs are termed "tholins", after the Greek word for "muddy". Their analogs are produced in the laboratory via the irradiation of gas mixtures and ice mixtures by radiation simulating Solar ultraviolet (UV) photons or keV charged particles simulating particles trapped in Saturn's magnetosphere. Fair analogs of Titan tholin are produced by bombarding a 9:1 mixture of N2:CH4 with charged particles and its match to observations of both the spectrum and scattering properties of the Titan haze is very good over a wide range of wavelengths. In this paper, we describe the historical background of laboratory research on this kind of organic matter and how our laboratory investigations of Titan tholin compare. We comment on the probable existence of polycyclic aromatic hydrocarbons in the Titan Haze and how biological and nonbiological racemic amino acids produced from the acid hydrolysis of Titan tholins make these complex organic compounds prime candidates in the evolution of terrestrial life and extraterrestrial life in our own Solar System and beyond. Finally, we also compare the spectrum and scattering properties of our resulting tholin mixtures with those observed on Centaur 5145 Pholus and the dark hemisphere of Saturn's satellite Iapetus in order to demonstrate the widespread distribution of similar organics throughout the Solar System. 相似文献
413.
Effect of bacterial population density on germination wheat seeds and dynamics of simple artificial ecosystems. 总被引:1,自引:0,他引:1
L A Somova N S Pechurkin A B Sarangova T I Pisman 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,27(9):1611-1615
Effect of the size of rhizospheric bacterial populations on germination of seeds and development of simple terrestrial "wheat plants--rhizospheric microorganisms--artificial soil" and "wheat plants-artificial soil" systems has been studied. Experiments demonstrated that within specify ranges in the inoculate, the rhizospheric bacteria are capable of increasing the yield of germinated seeds and stimulate the growth of plantlets. Germination of seeds inoculated with bacteria was either stimulated, or inhibited or remained at control levels depending on the amount of bacteria. Plant biomass growth and total photoassimilation has been found to depend on the amount of bacteria on the plant roots: the higher the amount of bacteria on plant roots, the smaller is the biomass of plants but the total photoassimilation is, higher. Thus, depending on the amount of bacteria on the roots of plants the system either increases the biomass of plants or increases the total photoassimilation, i.e. "pumps" carbon through itself involving bacteria. Grant numbers: N99-04-96017, N15. 相似文献
414.
Water, vital for life, not only maintains the integrity of structural and metabolic biomolecules, it also transports them in solution or colloidal suspension. Any flow of water through a dormant or fossilized microbial community elutes molecules that are potentially recognizable as biomarkers. We hypothesize that the surface seepage channels emanating from crater walls and cliffs in Mars Orbiter Camera images results from fluvial erosion of the regolith as low-temperature hypersaline brines. We propose that, if such flows passed through extensive subsurface catchments containing buried and fossilized remains of microbial communities from the wet Hesperian period of early Mars (approximately 3.5 Ga ago), they would have eluted and concentrated relict biomolecules and delivered them to the surface. Life-supporting low-temperature hypersaline brines in Antarctic desert habitats provide a terrestrial analog for such a scenario. As in the Antarctic, salts would likely have accumulated in water-filled depressions on Mars by seasonal influx and evaporation. Liquid water in the Antarctic cold desert analogs occurs at -80 degrees C in the interstices of shallow hypersaline soils and at -50 degrees C in salt-saturated ponds. Similarly, hypersaline brines on Mars could have freezing points depressed below -50 degrees C. The presence of hypersaline brines on Mars would have extended the amount of time during which life might have evolved. Phototrophic communities are especially important for the search for life because the distinctive structures and longevity of their pigments make excellent biomarkers. The surface seepage channels are therefore not only of geomorphological significance, but also provide potential repositories for biomolecules that could be accessed by landers. 相似文献
415.
C S Cockell 《Acta Astronautica》2001,49(12):693-706
The Martian polar ice caps are regions of substantial scientific interest, being the most dynamic regions of Mars. They are volatile sinks and thus closely linked to Martian climatic conditions. Because of their scale and the precedent set by the past history of polar exploration on Earth, it is likely that an age of polar exploration will emerge on the surface of Mars after the establishment of a capable support structure at lower latitudes. Expeditions might be launched either from a lower latitude base camp or from a human-tended polar base. Based on previously presented expeditionary routes to the Martian poles, in this paper a "spiral in-spiral out" unsupported transpolar assault on the Martian north geographical pole is used as a Reference expedition to propose new types of equipment for the human polar exploration of Mars. Martian polar "ball" tents and "hover" modifications to the Nansen sledge for sledging on CO2-containing water ice substrates under low atmospheric pressures are suggested as elements for the success of these endeavours.Other challenges faced by these expeditions are quantitatively and qualitatively addressed. 相似文献
416.
Following an enthusiastic start in 1985, ESA's life support technology development programme was re-assessed in the mid- to late-1990s to reflect the strong reduction in European manned space ambitions which occurred at that time. Further development was essentially restricted to activities that could constitute ISS upgrades or enhancements, or support ISS utilisation/operations, together with a single, limited, activity (MELISSA) aimed at bioregenerative life support, in the continuing hope that there might be "life after Station". The paper describes the current status of these activities and summarises the main priorities for future development that were identified at the April 1999 Workshop on Advanced Life Support. 相似文献
417.
The Martian surface is exposed to both UVC radiation (<280 nm) and higher doses of UVB (280-315 nm) compared to the surface of the Earth. Terrestrial organisms have not evolved to cope with such high levels of UVC and UVB and thus any attempts to introduce organisms to Mars, particularly in closed-loop life support systems that use ambient sunlight, must address this problem. Here we examine the UV radiation environment of Mars with respect to biological systems. Action spectra and UV surface fluxes are used to estimate the UV stress that both DNA and chloroplasts would experience. From this vantage point it is possible to consider appropriate measures to address the problem of the Martian UV environment for future long term human exploration and settlement strategies. Some prospects for improving the UV tolerance of organisms are also discussed. Existing artificial ecosystems such as Biosphere 2 can provide some insights into design strategies pertinent to high UV environments. Some prospects for improving the UV tolerance of organisms are also discussed. The data also have implications for the establishment of closed-loop ecosystems using natural sunlight on the lunar surface and elsewhere in the Solar System. 相似文献
418.
F Raulin P Coll N Smith Y Benilan P Bruston M C Gazeau 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(4):453-460
Titan, the largest satellite of Saturn, with a dense atmosphere very rich in organics, and many couplings in the various parts of its "geofluid", is a reference for studying prebiotic chemistry on a planetary scale. New data have been obtained from experiments simulating this organic chemistry (gas and aerosol phases), within the right ranges of temperature and a careful avoiding of any chemical contamination. They show a very good agreement with the observational data, demonstrating for the first time the formation of all the organic species already detected in Titan atmosphere including, at last, C4N2, together with many other species not yet detected in Titan. This strongly suggests the presence of more complex organics in Titan's atmosphere and surface, including high molecular weight polyynes and cyanopolyynes. The NASA-ESA Cassini-Huygens mission has been successfully launched in October 1997. The Cassini spacecraft will reach the Saturn system in 2004 and become an orbiter around Saturn, while the Huygens probe will penetrate into Titan's atmosphere. In situ measurements, in particular from Huygens GC-MS and ACP instruments, will provide a detailed analysis of the organics present in the air, aerosols, and surface. This very ambitious mission should yield much information of crucial importance for our knowledge of the complexity of Titan's chemistry, and, more generally for the field of exobiology. 相似文献
419.
Exobiology is not only the study of the origin, distribution and evolution of life in the universe, but also of structures
— including at the molecular level, and processes — including organic chemical transformations — related to life. In that
respect, with its dense nitrogen atmosphere, which includes a noticeable fraction of methane, and the many organic compounds
which are present in the gas and aerosols phases, Titan appears to be a planetary object of prime interest for exobiology
in the Solar system, allowing the study of chemical organic evolution in a planetary environment over a long time scale. We
describe here some aspects of this extraterrestrial organic chemistry which involves many physical and chemical couplings
in the different parts of what can be called ‘Titan's geofluid’ (gas phase, aerosol phases and surface solid and maybe liquid
phases). The three complementary approaches which can be followed to study such chemistry of exobiological interest are considered.
Those are experimental simulations in the laboratory, chemical and photochemical modeling and of course observation, using
both remote sensing and in situ measurements, which is an essential approach. The Cassini-Huygens mission, that offers a unique
opportunity to study in detail the many aspects of Titan's organic chemistry, is discussed and the many expected exobiological
returns from the different instruments of the Cassini orbiter and the Huygens probe are considered.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
420.
C. Done 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):211-219
The X-ray properties of Active Galactic Nuclei (AGN) support recent theories which unify the Seyfert 1 nuclei with the Seyfert 2's and radio-quiet quasars. In these objects the underlying spectrum is strongly distorted by the effects of reflection from the accretion flow and by absorption in partially ionised material. These obscure any intrinsic changes in the spectrum, making it difficult (though not impossible) to constrain the nature of the emission process. Conversely, there is no evidence for either of these spectral distortions in the radio-loud AGN, supporting the hypothesis that the X-rays are dominated by beamed emission from the relativistic jet. 相似文献