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81.
The physics of scattering of electromagnetic waves by media in which the particles are in contact, such as planetary regoliths, has been thought to be relatively well understood when the particles are larger than the wavelength. However, this is not true when the particles are comparable with or smaller than the wavelength. We have measured the scattering parameters of planetary regolith analogs consisting of suites of well-sorted abrasives whose particles ranged from larger to smaller than the wavelength. We measured the variation of reflectance as the phase angle varied from 0.05° to 140°. The following parameters of the media were then deduced: the single scattering albedo, single scattering phase function, transport mean free path, and scattering, absorption, and extinction coefficients. A scattering model based on the equation of radiative transfer was empirically able to describe quantitatively the variation of intensity with angle for each sample. Thus, such models can be used to characterize scattering from regoliths even when the particles are smaller than the wavelength. The scattering parameters were remarkably insensitive to particle size. These results are contrary to theoretical predictions, but are consistent with earlier measurements of alumina abrasives that were restricted to small phase angles. They imply that a basic assumption made by virtually all regolith scattering models, that the regolith particles are the fundamental scattering units of the medium, is incorrect. Our understanding of scattering by regoliths appears to be incomplete, even when the particles are larger than the wavelength. 相似文献
82.
83.
Flavio Scappini Cesare Cecchi-Pestellini Harvey Smith William Klemperer Alexander Dalgarno 《Monthly notices of the Royal Astronomical Society》2003,341(2):657-661
We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H2 SO4 · n H2 O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects. 相似文献
84.
85.
Parasitic programs for the Search for Extraterrestrial Intelligence (SETI), carried out concurrently with conventional radio astronomical observing programs, can be an attractive and cost-effective means of exploring the large multidimensional search space intrinsic to this effort. We describe a microprocessor-based automated SETI acquisition system which searches for and records spectra of narrowband signals in the IF band of an observatory receiver. Data taken with this system over 35 days at the Hat Creek Radio Observatory at 1612 MHz are discussed. Out of approximately 105 spectra processed during this period, 4000 were identified by the system as containing narrowband signals and were recorded. Subsequent analysis indicates that over 3900 of these are due to local RF contamination. The remainder are undergoing further investigation. 相似文献
86.
The Solar–Stellar Irradiance Comparison Experiment {II (SOLSTICE {II), aboard the Solar Radiation and Climate Experiment (SORCE)
spacecraft, consists of a pair of identical scanning grating monochromators, which have the capability to observe both solar
spectral irradiance and stellar spectral irradiance using a single optical system. The SOLSTICE science objectives are to
measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy
of 5%, to determine its variability with a long-term relative accuracy of 0.5% per year during a 5-year nominal mission, and
to determine the ratio of solar irradiance to that of an ensemble of bright B and A stars to an accuracy of 2%. Those objectives
are met by calibrating instrument radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility
at the National Institute for Standards and Technology in Gaithersburg, Maryland. During orbital operations irradiance measurements
from an ensemble of bright, stable, main-sequence B and A stars are used to track instrument sensitivity. SORCE was launched
on 25 January 2003. After spacecraft and instrument check out, SOLSTICE {II first observed a series of three stars to establish
an on-orbit performance baseline. Since 6 March 2003, both instruments have been making daily measurements of both the Sun
and stars. This paper describes the pre-flight and in-flight calibration and characterization measurements that are required
to achieve the SOLSTICE science objectives and compares early SOLSTICE{II measurements of both solar and stellar irradiance
with those obtained by SOLSTICE {I on the Upper Atmosphere Research Satellite. 相似文献
87.
William P Dillon 《Marine Geology》1974,16(3):121-143
The structure of the continental shelf off southern Morocco was studied by means of 2,100 km of seismic reflection profiles, magnetic and bathymetric surveys, and dredge samples. The research area lies off four geologic divisions adjacent to the coast: the Atlas Mountains; the Souss Trough; the Anti-Atlas Mountains; and the Aaiun Basin. The continental shelf, along with the western Atlas Mountains, the western Souss Trough, and the entire Aaiun Basin, has subsided along a normal fault-flexure system. This system runs along the shore at the Anti-Atlas Mountains, and cuts off this cratonic block from the shelf subsidence. The shelf is narrow and characterized by out-building off the Anti-Atlas range, whereas it is broader and characterized by upbuilding to the north and south. Deposition was essentially continuous at least from Early Cretaceous through Eocene time. Published work suggests that the last cycle of sedimentation began during Permian rifting. After Eocene time, most sediments carried to the shelf must have bypassed it and gone to construct the slope and rise or to the deep sea. Tertiary orogenies caused extensive folding of Mesozoic and early Tertiary deposits off the Atlas Mountains. 相似文献
88.
Fourteen midwater trawl collections to depths of 450 m to 1,400 m were taken at eleven stations in the Bering Sea and adjoining regions of the northern North Pacific by the R/V Hakuho Maru during the summer of 1975. A total of 29 kinds of fishes were identified. Mesopelagic fishes of the families Myctophidae, Gonostomatidae and Bathylagidae predominated in the catches, contributing 14 species (94%) of the fishes caught.Seventeen species of fishes were caught in the Bering Sea, and all of these are known from nearby areas. The mesopelagic fish fauna of the Bering Sea is similar to that in adjoining regions of the northern North Pacific Ocean: endemic species are rare or absent.
Stenobrachius nannochir was usually the most common mesopelagic fish in our catches.Stenobrachius leucopsarus is a diel vertical migrant that is usually the dominant mesopelagic fish in modified Subarctic waters of the northeastern Pacific. The change in dominance fromS. nannochir in the western Bering Sea toS. leucopsarus in the eastern Bering Sea is related to differences in oceanographic conditions. 相似文献
89.
William H. Jefferys 《Celestial Mechanics and Dynamical Astronomy》2000,78(1-4):3-16
H. K. Eichhorn had a lively interest in statistics during his entire scientific career, and made a number of significant contributions to the statistical treatment of astrometric problems. In the past decade, a strong movement has taken place for the reintroduction of Bayesian methods of statistics into astronomy, driven by new understandings of the power of these methods as well as by the adoption of computationally-intensive simulation methods to the practical solution of Bayesian problems. In this paper I will discuss how Bayesian methods may be applied to the statistical discussion of astrometric data, with special reference to several problems that were of interest to Eichhorn. 相似文献
90.
The interferometer visibility of Jupiter, observed at a wavelength of 3.4 mm, is used to determine the global limb darkening of the planet's brightness. From a single-parameter fit to the visibility curve, we find an ammonia-to-molecular hydrogen mixing ratio of 6.4[+5.1, ?1.9] × 10?5, which corresponds to 35[+28, ?10]% of the solar nitrogen abundance if all of the nitrogen is in the form of ammonia. The fitting procedure uses a simple model atmosphere for the Jovian atmosphere which is based on other observations of the planet. The dependence of the result on the various model parameters is studied. 相似文献