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1.
Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth. 相似文献
2.
Jonathan F. Stebbins Emily V. Dubinsky Koji Kanehashi Kimberly E. Kelsey 《Geochimica et cosmochimica acta》2008,72(3):910-925
Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl2Si2O8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al2O3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well. 相似文献
3.
In an earlier paper, values of exospheric density were obtained from the orbit of Echo 2 for the years 1964–1965. The results indicated a semi-annual variation in density by a factor of between 2 and 3, considerably larger than predicted by existing atmospheric models.
These studies have now been extended to the beginning of 1967, using both Echo 2 and Calsphere 1, to show how the density is responding to increasing solar activity. Variations in density during 1964 have been analysed in more detail. The long-term variation associated with the solar cycle and the short-term variations associated with magnetic and solar disturbances agree with the variations expected on the basis of current models. The semi-annual variation is persisting to higher levels of solar activity, and although its amplitude is diminishing the factor of variation was still 1.6 in 1966. 相似文献
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Comparison is made between the run of number density of meteoroids from penetration detectors aboard Helios A (masses below 10?8 g) and Pioneer 10 (masses near and above 3 × 10?9 g), the source function of the zodiacal light deduced from photometric observations aboard Helios A and Pioneer 10, counts versus brightness of objects passing by Pioneer 10 from the Sisyphus experiment and the distribution of meteoroids deduced from radar and optical meteors at the Earth. The Sisyphus experiment on Pioneer 10 observed reflecting glints on meteoroids rather than the meteoroids themselves and the counting statistics refer not to the effective radii of the meteoroids but to the effective radii of curvature of the reflecting glints on the meteoroids. The penetration detectors appear to find some increase in number density toward the Sun and a flat distribution outward to 5.2 AU. The overall behavior of the zodiacal light is that the relative distribution over direction is unchanged while the source scattering function diminishes as the inverse 1.4 power of distance from the Sun. The fit to the brightness of the zodiacal light obtained from these statistics can be combined with the mass distribution results from the optical meteors to deduce a mean geometric albedo of meteoroids of 0.006 at 1 AU from the Sun. Combination of the space distribution from radar meteors with the scattering source function of the zodiacal light yields geometric albedos for meteoroids running from 0.07 at 0.1 AU, from the Sun through 0.006 at 1 AU down to about 0.0001 at 3.3 AU which may run flat thence outward. This result is imposed by the indicated modest increase in density of meteoroids very near the Sun, a minimum between the Sun and the Earth near 0.4 AU and rising density outward to somewhere beyond 3.3 AU which is very different from the inverse 1.4 power of the distance shown for scatterers (product of number density and albedo) by the zodiacal light. A check on the distribution at very large sizes is possible if a search is made for fireballs in Jupiter's atmosphere by the Mariner Jupiter Saturn 1977 television cameras during the two encounters with Jupiter in 1979. An easy detection of such activity would put the maximum in the meteoroid distribution out near Jupiter and lend further confirmation to the indicated drop in albedo. 相似文献
9.
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
10.
Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay 总被引:2,自引:0,他引:2
Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the
development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic
perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic
and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates
were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative
substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds
of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution.
On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production
(or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO3 m−2d−1. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210
to −1.900 g CaCO3 m−2 night−1. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day
on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but
only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution
in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration.
Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study
indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr−1 and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida
Bay. 相似文献