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921.
We analyzed tree rings in wood samples collected from some of the few surviving trees found close to the epicenter (within 4-5 km) of the Tunguska event that occurred on the last day of June 1908. Tree-ring growth shows a depression starting in the year after the event and continuing during a 4-5-year period. The most remarkable traces of the event were found in the rings' anatomical structure: (1) formation of "light" rings and a reduction of maximum density in 1908; (2) non-thickened tracheids (the cells that make up most of the wood volume) in the transition and latewood zones (the middle and last-formed parts of the ring, respectively); and (3) deformed tracheids, which are located on the 1908 annual ring outer boundary. In the majority of samples, normal earlywood and latewood tracheids were formed in all annual rings after 1908. The observed anomalies in wood anatomy suggest two main impacts of the Tunguska event on surviving trees--(1) defoliation and (2) direct mechanical stress on active xylem tissue. The mechanical stress needed to fell trees is less than the stress needed to cause the deformation of differentiating tracheids observed in trees close to the epicenter. In order to resolve this apparent contradiction, work is suggested on possible topographic modification of the overpressure experienced by these trees, as is an experimental test of the effects of such stresses on precisely analogous growing trees. 相似文献
922.
Trainer MG Pavlov AA Curtis DB McKay CP Worsnop DR Delia AE Toohey DW Toon OB Tolbert MA 《Astrobiology》2004,4(4):409-419
An organic haze layer in the upper atmosphere of Titan plays a crucial role in the atmospheric composition and climate of that moon. Such a haze layer may also have existed on the early Earth, providing an ultraviolet shield for greenhouse gases needed to warm the planet enough for life to arise and evolve. Despite the implications of such a haze layer, little is known about the organic material produced under early Earth conditions when both CO(2) and CH(4) may have been abundant in the atmosphere. For the first time, we experimentally demonstrate that organic haze can be generated in different CH(4)/CO(2) ratios. Here, we show that haze aerosols are able to form at CH(4) mixing ratios of 1,000 ppmv, a level likely to be present on early Earth. In addition, we find that organic hazes will form at C/O ratios as low as 0.6, which is lower than the predicted value of unity. We also show that as the C/O ratio decreases, the organic particles produced are more oxidized and contain biologically labile compounds. After life arose, the haze may thus have provided food for biota. 相似文献
923.
L Narici V Bidoli M Casolino M P De Pascale G Furano A Morselli P Picozza E Reali R Sparvoli S Licoccia P Romagnoli E Traversa W G Sannita A Loizzo A Galper A Khodarovich M G Korotkov A Popov N Vavilov S Avdeev V P Salnitskii O I Shevchenko V P Petrov K A Trukhanov M Boezio W Bonvicini A Vacchi N Zampa R Battiston G Mazzenga M Ricci P Spillantini G Castellini P Carlson C Fuglesang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(1):141-146
The ALTEA project participates to the quest for increasing the safety of manned space flights. It addresses the problems related to possible functional damage to neural cells and circuits due to particle radiation in space environment. Specifically it aims at studying the functionality of the astronauts' Central Nervous Systems (CNS) during long space flights and relating it to the peculiar environments in space, with a particular focus on the particle flux impinging in the head. The project is a large international and multidisciplinary collaboration. Competences in particle physics, neurophysiology, psychophysiology, electronics, space environment, data analyses will work together to construct the fully integrated vision electrophysiology and particle analyser system which is the core device of the project: an helmet-shaped multi-sensor device that will measure concurrently the dynamics of the functional status of the visual system and passage of each particle through the brain within a pre-determined energy window. ALTEA is scheduled to fly in the International Space Station in late 2002. One part of the multi-sensor device, one of the advanced silicon telescopes, will be launched in the ISS in early 2002 and serve as test for the final device and as discriminating dosimeter for the particle fluences within the ISS. 相似文献
924.
Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous species. Some of these compounds are amphiphilic, having polar and nonpolar groups on the same molecule. Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. The goal of future investigations will be to fabricate artificial cells as models of the origin of life. 相似文献
925.
A. P. Tyutnev A. N. Doronin V. S. Saenko D. N. Sadovnichii E. D. Pozhidaev 《Cosmic Research》2002,40(2):129-132
A simplified theory of the electrification of polymer films, which permits estimation of the internal electric fields of films irradiated by monoenergetic electrons simulating real streams of plasma electrons during magnetic substorms at a geosynchronous orbit, is suggested. To determine the depth behavior of the forward current and the dosage rate in the irradiated polymer, numerical calculations are made by the Monte Carlo technique. The appearance of electrostatic discharges in thin (5 m) mylar films is shown to be very probable in the orbit's shadow sector under magnetic substorm conditions. 相似文献
926.
Layne CS Lange GW Pruett CJ McDonald PV Merkle LA Mulavara AP Smith SL Kozlovskaya IB Bloomberg JJ 《Acta Astronautica》1998,43(3-6):107-119
The precise neuromuscular control needed for optimal locomotion, particularly around heel strike and toe off, is known to he compromised after short duration (8- to 15-day) space flight. We hypothesized here that longer exposure to weightlessness would result in maladaptive neuromuscular activation during postflight treadmill walking. We also hypothesized that space flight would affect the ability of the sensory-motor control system to generate adaptive neuromuscular activation patterns in response to changes in visual target distance during postflight treadmill walking. Seven crewmembers, who completed 3- to 6-month missions, walked on a motorized treadmill while visually fixating on a target placed 30 cm (NEAR) or 2 m (FAR) from the subject's eyes. Electronic foot switch data and surface electromyography were collected from selected muscles of the right lower limb. Results indicate that the phasic features of neuromuscular activation were moderately affected and the relative amplitude of activity in the tibialis anterior and rectus femoris around toe off changed after space flight. Changes also were evident after space flight in how these muscles adapted to the shift in visual target distance. 相似文献
927.
Bird M.K. Dutta-Roy R. Heyl M. Allison M. Asmar S.W. Folkner W.M. Preston R.A. Atkinson D.H. Edenhofer P. Plettemeier D. Wohlmuth R. Iess L. Tyler G.L. 《Space Science Reviews》2002,104(1-4):613-640
A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction
and strength of Titan's zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probe's wind-induced horizontal motion will
be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit
and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas,
thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information
on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after
Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted
signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout,
as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests),
are described. Ongoing efforts to measure and model Titan's winds using various Earth-based techniques are briefly reviewed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
928.
Geometric accuracy in airborne SAR images 总被引:1,自引:0,他引:1
Blacknell D. Freeman A. Quegan S. Ward I.A. Finley I.P. Oliver C.J. White R.G. Wood J.W. 《IEEE transactions on aerospace and electronic systems》1989,25(2):241-258
Uncorrected across-track motions of a synthetic aperture radar (SAR) platform can cause both a severe loss of azimuthal positioning accuracy in, and defocusing of, the resultant SAR image. It is shown how the results of an autofocus procedure can be incorporated in the azimuth processing to produce a fully focused image that is geometrically accurate in azimuth. Range positioning accuracy is also discussed, leading to a comprehensive treatment of all aspects of geometric accuracy. The system considered is an X -band SAR 相似文献
929.
Modeling and Design of Switching Regulators 总被引:1,自引:0,他引:1
Various building blocks of a switching regulator are described in detail and mathematical models are developed for all building blocks in terms of transfer functions, which enable one to design a switching regulator for stability, desirable bandwidth, line rejection, and transient response. A step-by-step procedure to design compensation is illustrated using two examples. Various networks for compensation and their transfer functions are presented which the author hopes will be very handy to use and will become the reference source. 相似文献
930.
A. Sprague J. Warell G. Cremonese Y. Langevin J. Helbert P. Wurz I. Veselovsky S. Orsini A. Milillo 《Space Science Reviews》2007,132(2-4):399-431
Mercury’s surface is thought to be covered with highly space-weathered silicate material. The regolith is composed of material
accumulated during the time of planetary formation, and subsequently from comets, meteorites, and the Sun. Ground-based observations
indicate a heterogeneous surface composition with SiO2 content ranging from 39 to 57 wt%. Visible and near-infrared spectra, multi-spectral imaging, and modeling indicate expanses
of feldspathic, well-comminuted surface with some smooth regions that are likely to be magmatic in origin with many widely
distributed crystalline impact ejecta rays and blocky deposits. Pyroxene spectral signatures have been recorded at four locations.
Although highly space weathered, there is little evidence for the conversion of FeO to nanophase metallic iron particles (npFe0), or “iron blebs,” as at the Moon. Near- and mid-infrared spectroscopy indicate clino- and ortho-pyroxene are present at
different locations. There is some evidence for no- or low-iron alkali basalts and feldspathoids. All evidence, including
microwave studies, point to a low iron and low titanium surface. There may be a link between the surface and the exosphere
that may be diagnostic of the true crustal composition of Mercury. A structural global dichotomy exists with a huge basin
on the side not imaged by Mariner 10. This paper briefly describes the implications for this dichotomy on the magnetic field
and the 3 : 2 spin : orbit coupling. All other points made above are detailed here with an account of the observations, the
analysis of the observations, and theoretical modeling, where appropriate, that supports the stated conclusions. 相似文献