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81.
Fred L. Whipple 《Earth, Moon, and Planets》1996,72(1-3):69-78
This study is based primarily on the calculations of comet orbits over ~ 106 years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated “ablation AGES”. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater “AGES” tend to be less active and to show less dust in their spectra than comets of lesser “AGES”. 相似文献
82.
A.G.G.M. Tielens L.B.F.M. Waters F.J. Molster K. Justtanont 《Astrophysics and Space Science》1997,255(1-2):415-426
This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal
the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known
10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source
to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are
dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting
grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed
to amorphous silicate grains.
These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence
of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation
sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that
the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates
in these outflows, resulting in amorphous grains with little characterizing spectral detail.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
83.
R. Wieler Th. Graf P. Signer S. Vogt G. F. Herzog C. Tuniz D. Fink L. K. Fifield J. Klein R. Middleton A. J. T. Jull P. Pellas J. Masarik G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ~20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ~65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. 相似文献
84.
Andrew J. L. Harris Anna L. Butterworth Richard W. Carlton Ian Downey Peter Miller Pedro Navarro David A. Rothery 《Bulletin of Volcanology》1997,59(1):49-64
Satellite data offer a means of supplementing ground-based monitoring during volcanic eruptions, especially at times or locations
where ground-based monitoring is difficult. Being directly and freely available several times a day, data from the advanced
very high resolution radiometer (AVHRR) offers great potential for near real-time monitoring of all volcanoes across large
(3000×3000 km) areas. Herein we describe techniques to detect and locate activity; estimate lava area, thermal flux, effusion
rates and cumulative volume; and distinguish types of activity. Application is demonstrated using data for active lavas at
Krafla, Etna, Fogo, Cerro Negro and Erebus; a pyroclastic flow at Lascar; and open vent systems at Etna and Stromboli. Automated
near real-time analysis of AVHRR data could be achieved at existing, or cheap to install, receiving stations, offering a supplement
to conventional monitoring methods.
Received: 21 January 1997 / Accepted: 3 April 1997 相似文献
85.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
86.
Results of three years of directional wave measurement at the Eastern Mediterranean coast of Haifa, Israel are presented. The wave-height and energy-flux distributions reveal a moderately high-energy coast with a bimodal annual cycle.The rate of wave-induced longshore sediment transport is estimated from the directional energy flux distributions. It describes an annual cycle with a maximum northward transport of 75 ± 14 × 103 m3/month in midwinter and a southward transport of 26 ± 5 × 103 m3/month in summer. The net annual transport is northward and computed at 110 ± 100 × 103 m3/yr.We show that a wave-induced transport is sufficient in explaining the apparent transport of sediments in the Nile Littoral Cell, from the Nile Delta source to the Haifa Bay sink. 相似文献
87.
G.H.L. Read 《Estuarine, Coastal and Shelf Science》1985,21(3):313-324
A dry (1979–1980) and a wet (1980–1981) season had a marked effect on the freshwater inflow into the Keiskamma estuary. Under low inflow conditions, which results in elevated salinities in the upper reaches, an upstream migration of adult Macrobrachium petersi (Hilgendorf) to freshwater takes place. During periods of increased river inflow adult M. petersi move downstream to the more saline reaches of the estuary. These two migratory responses have been interpreted as (a) a breeding migration under high inflow conditions which ensures that larvae are in close proximity to salinities that favour growth and development, and (b) an adult upstream migration back to freshwater to escape elevated estuarine salinities as a result of the low freshwater inflow. 相似文献
88.
89.
L. Trafton 《Icarus》1985,63(3):374-405
We report the results of monitoring Saturn's H2 quadrupole and CH4 band absorptions outside of the equatorial zone over one-half of Saturn's year. This interval covers most of the perihelion half of Saturn's elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH4 absorption accompanied by weakly opposite changes in the H2 absorption. Around the 1980 equinox, the H2 and CH4 absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturn's troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH4 is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH4 bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturn's south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH4/H2 mixing ratio of (4.2 ± 0.4) × 10?3, a haze particle albedo of , and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 Å NH3 band made annually since the 1980 equinox. A 20 ± 4% increase in the NH3 absorption at south temperate latitudes has occurred since 1973–1976 and the NH3 absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH3 crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturn's atmosphere. An apparently anomalous ortho-para H2 ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant. 相似文献
90.
Sunetra Giridhar N. Kameswara Rao David L. Lambert 《Journal of Astrophysics and Astronomy》1996,17(1-2):23-40
Two high resolution spectra of the hot RCrB star DY Cen in the red region are compared. The photospheric absorption lines
show a radial velocity variation of 12 kms-1 between 1989 July and 1992 May. Emission components to some CII lines present
in 1989 are almost entirely absent in 1992. Nebular forbidden lines of [OI], [NII] and [SII] appear unchanged from 1989 to
1992 相似文献