共查询到20条相似文献，搜索用时 31 毫秒
1.
An analysis of the spectra from the PUMA dust-impact mass spectrometers onboard the Vega-1 and Vega-2 spacecraft shows that a large number of the observed, unidentified small-amplitude peaks are produced by impacts of very-low-mass (from 10?17 to 10?20 g) particles. The mass flux of very fine particles accounts for a few percent of the total dust mass flux from comet Halley. The elemental composition of the finest cometary particles is identical to the composition of large particles (10?12–10?16 g), in agreement with present views about the nucleus of comet Halley as an aggregate of interstellar dust. 相似文献
2.
A numerical study of the contributions of dust source regions to the global dust budget 总被引：7，自引：1，他引：6
Contributions of the nine potential dust source regions (North and South Africa, the Arabian Peninsula, Central Asia, eastern and western China, North and South America, and Australia) to the global dust budget are investigated with a global dust transport model. A six-year simulation (1990 to 1995) indicates that the greatest contributor to the global dust budget is found to be North Africa (the Sahara Desert), which accounts for 58% of the total global dust emission and 62% of the total global dust load in the atmosphere. Australian dust dominates the southern hemisphere. The dust emission and atmospheric dust load originating from East Asia (eastern and western China) are estimated to be 214 Tg yr− 1 and 1.1 Tg, respectively, which are 11% and 6% of the total global dust emission and dust load. Dust from East Asia dominates the atmospheric load over China and Mongolia (about 70%), Korea (60%), Japan (50%), and the North Pacific Ocean (40%). The contribution of dust originating from regions other than East Asia to the dust load over these East Asian countries and the North Pacific Ocean cannot be ignored. The simulated total dust deposition flux on Greenland suggests a possible overestimation of the Saharan dust and an underestimation of the East Asian dust in the Arctic region, which may be a common problem with global dust transport models. Possible reasons for the underestimation of the East Asian dust are discussed. 相似文献
3.
The Ulysses spacecraft orbits the Sun on a highly inclined orbit, and the impact ionization dust detector on board continuously measures interstellar dust grains with masses up to , penetrating deep into the Solar System. The flow direction is close to the mean apex of the Sun's motion through the local interstellar cloud (LIC), and the grains act as tracers of the physical conditions in the LIC. Previous analysis gave a velocity dispersion of up to 40° for the interstellar grains. We partially re-analyzed the Ulysses interstellar dust data set, taking into account the detector's inner side walls. As the side walls have a sensitivity for dust impact detection almost identical to that of the instrument's target area, wall impactors must be taken into account for estimating the intrinsic velocity dispersion of the interstellar impactors and the interstellar dust flux value. Neglect of the sensor side walls overestimates the interstellar dust stream velocity dispersion by about 30% and the interstellar dust flux by about 20%. 相似文献
4.
We have studied to influence of grain porosity on the dust destruction mechanism in interstellar shocks. Our results show, that fluffy aggregates of dust grains can be broken up easily in shocks. 相似文献
5.
Martha S. Hanner 《Icarus》1980,43(3):373-380
The zodiacal light brightness and measured spatial density of the interplanetary dust lead to a mean geometric albedo of 0.24 for the dust particles near 1 AU; whereas the composition of collected micrometeroids suggests a geometric albedo ?0.1. The data do not support the very low albedo (?0.01) proposed by A. F. Cook [Icarus33 (1978), 349–360]. The evidence is against a change in the mean particle albedo between 0.1 and 2 AU. Beyond 2 AU the data are unclear and a change in albedo is not ruled out. 相似文献
6.
G.S. Golitsyn 《Icarus》1973,18(1):113-119
A short review is given of the observational data on the great Martian dust storms. It is noted that these storms are observed at the time of the great oppositions, when Mars is at its perihelion. It is then late spring-early summer in the Southern Hemisphere and the insolation is a maximum. Intense storms were observed during the 1892, 1924, 1941, 1956 and 1971 oppositions, but not in 1909 or 1939. These storms were of various duration and intensity which points out the importance of local time and space meteorological conditions at the moment of storm origin. Storm generation is caused by local conditions, but the fact that they frequently reach global scale implies that there are some feedback processes favoring the global spread of the dust. Possible such processes are described qualitatively together with the causes of storm declines. Those problems of Martian meteorology and micrometeorology are discussed which have to be studied for a better understanding of the generation, development and decay of a dust storm. 相似文献
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There are reasons to expect that Mars is surrounded by a region of dust, similar to rings, originating from the bombardment of Phobos and Deimos by meteroids. Using a simple radiative transfer model, we have investigated the angular distribution and the absolute values of the solar radiance scattered by such a dust region, to the purpose of assessing the possibilities and limitations of future photometric searches after the circummartian dust. Our model values of the number density of the dust grains in the space around Mars and of their size distribution have been derived from the results obtained by other authors. The single-scattering albedo of the dust grains has been deduced from the reflectance spectra of Phobos, taken by the spacecraft Phobos 2. Calculations, carried out for a few phenomenological phase functions, have shown that in the visible the radiance scattered by the rings is well within the detectability range of a modern sensible photometer, so that the prospectives for photometric search for the Martian dust rings are optimistic. Furthermore, our results confirm that the dust region could not be observed by the Viking cameras and this supports o our assumptions regarding the optical properties of the circummartian grains. 相似文献
10.
W. M. Napier 《Monthly notices of the Royal Astronomical Society》2001,321(3):463-470
A Markov chain model is constructed to investigate fluctuations in the mass of the zodiacal cloud. The cloud is specified by a three-dimensional grid, each element of which contains the numbers of dust particles as a function of semimajor axis, eccentricity and mass. The evolutionary pathways of dust particles owing to radiation pressure are described by fixed transition probabilities connecting the grid elements. Other elements are absorbing states representing infall to the Sun or ejection to infinity: particles entering these states are removed from the system. Particles are injected through the breakup of comets entering short-period, high-eccentricity orbits at random times, and are subject to the PoyntingRobertson effect and removal through collisional disintegration and radiation pressure. The main conclusions are that the cometary component of the zodiacal cloud is highly variable, and that in the wake of giant comet entry into a short-period, near-Earth orbit, the dust influx to the Earth's atmosphere may acquire a climatically significant optical depth. 相似文献
11.
V.H. Tyne A. Evans B. Smalley T.R. Geballe S.P.S. Eyres 《Astrophysics and Space Science》2002,279(1-2):139-147
We follow the evolution of the infrared excess and the dustshell of Sakurai's Object by modelling the dust emission. The optical depth,dust temperature, and shell thickness parameters from the models are presented.Fits of DUSTY models to infrared spectra between 1997 May and 1999September illustrate the development of the dust from small grains to a rangeof grain sizes with an almost constant 60% graphite – 40% amorphous carbonmixture. The size of the infrared emitting region continues to growand may now be big enough to resolve. This may help to answerquestions such as the morphology of the dust shell, and whether the dustshell is `blobby' or smooth. 相似文献
12.
W. J. Baggaley 《Earth, Moon, and Planets》2004,95(1-4):197-209
Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary
formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about
the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic
exploration. 相似文献
13.
We have considered the electrodynamic effects on small Al2O3 spherules dumped into the Earth's magnetosphere in large quantities during solid rocket propellant burns. The charges acquired by these grains in all regions of the terrestrial environment (plasmasphere, magnetosphere, and solar wind) are modest. Consequently electrodynamic effects are significant only at the lower end of the dust size spectrum (R
g0.1 m). In that case, the electrodynamic forces conspire with solar radiation pressure to eliminate the grains from the magnetosphere in a comparatively short time. Although not studied here in detail, we anticipate a similar fate for fine micrometeoroids entering the Earth's magnetosphere, with the electro-dynamic effects playing an even more important role.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
14.
The variation of the dust temperature with heliocentric distance for a comet is calculated using the optical constants of an astronomically important silicate. 相似文献
15.
《Planetary and Space Science》2006,54(9-10):1024-1032
The Enceladus flybys of the Cassini spacecraft are changing our understanding of the origin and sustainment of Saturn's E ring. Surprisingly, beyond the widely accepted dust production caused by micrometeoroid impacts onto the atmosphereless satellites (the impactor-ejecta process), geophysical activities have been detected at the south pole of Enceladus, providing an additional, efficient dust source. The dust detector data obtained during the flyby E11 are used to identify the amount of dust produced in the impactor-ejecta process and to improve related modeling [Spahn, F., Schmidt, J., Albers, N., Hörning, M., Makuch, M., Seiß, M., Kempf, S., Srama, R., Dikarev, V.V., Helfert, S., Moragas-Klostermeyer, G., Krivov, A.V., Sremčević, M., Tuzzolino, A., Economou, T., Grün, E., 2006. Cassini dust measurements at Enceladus: implications for Saturn's E ring. Science, in press]. With this, we estimate the impact-generated dust contributions of the other E ring satellites and find significant differences in the dust ejection efficiency by two projectile families—the E ring particles (ERPs) and the interplanetary dust particles (IDPs). Together with the Enceladus south-pole source, the ERP impacts play a crucial role in the inner region, whereas the IDP impacts dominate the particle production in the outer E ring, possibly accounting for its large radial extent. Our results can be verified in future Cassini flybys of the E ring satellites. In this way poorly known parameters of the dust particle production in hypervelocity impacts can be constrained by comparison of the data and theory. 相似文献
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Richard W. Zurek 《Icarus》1981,45(1):202-215
A δ-Eddington radiative transfer algorithm is used to compute the thermal tidal heating of a dusty Martian atmosphere for a given set of dust optical depth, effective single scattering albedo, and phase function asymmetry parameter. The resulting thermal tidal forcing is used in a classical atmospheric tidal model to compute the amplitudes of the surface pressure oscillations at the Viking Lander 1 site for the two 1977 Martian great dust storms. Parametric studies show that the dust opacities and optical parameters derived from the Viking Lander imaging data are roughly representative of the global dust haze needed to reproduce the tidal surface pressure amplitudes also observed at Lander 1, except that the model-inferred asymmetry parameter is smaller during the onset of a great storm. The observed preferential enhancement during dust-storm onset of the semidiurnal tide at Viking Lander 1 relative to its diurnal counterpart is shown to be due primarily to the elevation of the tidal heating source in a very dusty atmosphere, although resonant enhancement of the main semidiurnal tidal mode makes an important secondary contribution. 相似文献
18.
T.J. Stubbs D.A. Glenar W.M. Farrell R.R. Vondrak M.R. Collier J.S. Halekas G.T. Delory 《Planetary and Space Science》2011,59(13):1659-1664
Radio occultation measurements from the Soviet Luna 19 mission suggest that electron concentrations above the sunlit lunar surface can be significantly higher than that expected from either the photo-ionization of exospheric neutrals or any other well-known process. These measurements were used to infer the electron column concentrations above the lunar limb as a function of tangent height, which surprisingly indicated peak concentrations of ∼103 cm−3 at ∼5 km altitude. It has been speculated that electrically charged exospheric dust could contribute to such electron populations. This possibility is examined here using the exospheric dust abundances inferred from Apollo 15 coronal photographs to estimate the concentration of electrons produced by photo- and secondary emission from dust. These estimates far exceed the electron concentrations predicted by any other suggested mechanism, and are within a factor of ≈20 of those inferred from the Luna 19 measurements. It is possible that this discrepancy is due to an under-estimate in dust grain capacitances and/or the presence of much higher exospheric dust abundances during the Luna 19 measurements. These results suggest that electrons emitted from exospheric dust could be responsible for the Luna 19 measurements, and that this process could dominate the formation and evolution of the lunar ionosphere. 相似文献
19.
Yang Liu Darren W. Schnare Benjamin C. Eimer Lawrence A. Taylor 《Planetary and Space Science》2008,56(11):1517-1523
In order to study the toxicity of lunar dust, the respirable size fraction of lunar soil needed to be separated with an apparatus that possesses the following capabilities: no use of liquid; fully recoverable sample; and use of only small sample quantities (<1 gm). We report the design of a simple apparatus that meets these requirements and implements an inertial-impaction mechanism established in aerosol science. Lunar soil was agitated at a frequency of 100 Hz using a vibration table with a containment chamber under a constant air flow. The air flow carried the lofted lunar dust particles past four impactors (four T-junction connectors), upon which a fraction of large particles were captured during the impaction. The fine particles in the air flow were then collected by an end-of-the line membrane filter. Detailed examination of particles on the filter showed that the majority (∼80-90 wt%) are <3 μm (geometric diameter), suggesting a high level of effectiveness for the apparatus. 相似文献
20.
ISOPHOT has been used to perform high-resolution 60-μm scans of Vega, and these have been compared with those from γDra, to obtain a Gaussian width of 22±2 arcsec. The dust disc around Vega has been mapped, resolving it at 60 and 90 μm with ISOPHOT. At 90 μm a Gaussian width of 36±3 arsec has been derived. In addition, multi-filter photometry is presented, at 25, 60, 80, 100, 120, 150, 170 and 200 μm. The data are fitted by a modified blackbody with a temperature of 73 K [ Q (λ)∝1/λ1.1 ]. The dust disc has a luminosity L IR / L * ∼3×10−5 . Using a distance of 7.8 pc, 22 arcsec corresponds to a distance of 86 au, and 36 arcsec to a distance of 140 au. 相似文献