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751.
New spectrophotometry from 1.5 to 2.5 μm is reported for the Uranian satellites Titania, Oberon, and Umbriel. A spectrum of the rings of Uranus from 2.0 to 2.4 μm is also reported. No evidence is found for frost covering the surface of the ring material, consistent with the low albedo of the rings (PK = 0.03) previously reported by Nicholson and Jones (1980). The surfaces of the satellites are found to be covered by dirty water frost. Assuming albedos of the frost and gray components covering the Uranian satellites to be the same as the light and dark faces of Iapetus, radii are derived that are roughly twice those inferred from the assumption of a visual albedo of 0.5.  相似文献   
752.
753.
We present an analysis of the reported spectral features of NGC 4151 in X-rays. It is shown that the origin of X-rays from the source is inconsistent with a single production mechanism. We suggest a new two-component model in which soft X-rays arise from the black-body emission of a tiny hot nucleus withT2×107 K and the hard X-ray photons are generated in an extended region by inverse Compton scattering of electrons with the infrared photons.  相似文献   
754.
755.
A material balance is constructed for excess 210Pb (relative to 226Ra) as a test of the retentivity of Long Island Sound for a reactive heavy metal. Excess 210Pb is supplied to Long Island Sound chiefly by direct atmospheric deposition [1 ± 0.2(dis·min?1)cm?2·yr?1]. Rivers supply less than 20% of the atmospheric flux, and other inputs, from open ocean waters, 226Ra decay, groundwater seepage, and sewage discharge, appear to be negligible. The total input of excess 210Pb represents approximately the flux required to maintain the inventory of excess 210Pb measured in sediment cores from central Long Island Sound; that is, excess 210Pb is lost from Long Island Sound chiefly by radioactive decay. The retention of excess 210Pb within Long Island Sound is achieved in two steps: a rapid removal of soluble 210Pb onto suspended particles and the ongoing entrapment of particles in the basin by the residual bottom-water influx from the east.  相似文献   
756.
The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high-magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low-magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods. Using this criterion, an original low-magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high-magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high-magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty-seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes. Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0-03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite. Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate-reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric-phreatic origin of many ferroan calcite occurrences.  相似文献   
757.
Both spring-summer and fall-winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20-22 m using a radio-isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall-winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near-bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least-squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10?3 gm/cm/sec and 1.7 × 10?1 gm/cm/sec for ‘typical’ and extreme winter storm activity.  相似文献   
758.
759.
F.A. Gifford  R.P. Hosker  K.S. Rao 《Icarus》1978,36(1):133-146
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ~5° for light, and ~7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ~14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements.  相似文献   
760.
J. Veverka  K. Cook  J. Goguen 《Icarus》1978,33(3):466-482
A statistical study of all crater-related wind streaks visible on Mariner 9 A-camera frames between latitudes 0 and 30°N has been completed. Of the 2325 streaks identified 1914 (82%) are light tone streaks, 189 (8%) are dark tone, and the remaining 222 (10%) are of mixed tone. Nine parameters characterizing each streak and its associated crater were measured and intercorrelated. Because of the large number of light streaks in our sample fir findings for this type of streak are most significant statistically: light tone streaks occur preferentially in Pc terrain (heavily cratered plains); they are preferentially associated with fresh craters; the surface density of light streaks is not a strong function of elevation; a significant latitude effect does emerge—the density of light tone streaks reaches a maximum between 10 and 15°N, and drops off appreciably both toward the equator and toward higher latitudes; the mean angular width of light streaks is about 25°—long light streaks are significantly narrower than short ones; about 50% of streaks have streak length/crater diameter ratios of ?4; light streak directions conform closely to the wind regime expected at the season of global dust storms (southern summer). Generally speaking, the results for dark and mixed tone streaks in the northern equatorial zone are similar, with the following possible exceptions: dark streaks may show a slight preference to form at higher elecations; dark streaks may be slightly wider on average than light or mixed tone streaks; mixed tone streaks do not share the preference for sharp craters exhibited by light and dark streaks; in general, the directions of dark streaks do not conform to the general circulation pattern expected at the season of global dust storms as well as do those of the light streaks.  相似文献   
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