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1.
A. V. Krivov L. L. Sokolov V. V. Dikarev 《Celestial Mechanics and Dynamical Astronomy》1995,63(3-4):313-339
This paper deals with dynamics of impact ejecta from Phobos and Deimos initially on near-circular equatorial orbits around Mars. For particles emitted in a wide size regime of 1 micron and greater, and taking into account the typical particle lifetimes to be less than 100 years, the motion is governed by two perturbing forces: solar radiation pressure and influence of Mars' oblateness. The equations of motion of particles in Lagrangian non-singular elements are deduced and solved, both analytically and numerically, for different-sized ejecta. We state that the coupled effect of both forces above is essential so that on no account can the oblateness of Mars are be neglected. The dynamics of grains prove to be quite different for the ejecta of Phobos and Deimos. For Deimos, the qualitative results are relatively simple and imply oscillations of eccentricity and long-term variations of orbital inclination, with amplitudes and periods both depending on grain size. For Phobos, the dynamics are shown to be much more complicated, and we discuss it in detail. We have found an intriguous peculiar behavior of debris near 300 µm in size. Another finding is that almost all the Phobos ejecta with radii less than 30 µm (against the values of 5 to 20 µm adopted earlier by many authors) should be rapidly lost by collisions with martian surface. The results of the paper may be the base for constructing an improved model of dust belts that presumably exist around Mars. 相似文献
2.
撞击坑是月表最典型的地质单元,其溅射物作为撞击坑的坑外组成部分可分布到距离坑中心10个直径距离之外的区域,因此撞击溅射物也是月球地质编图中最重要的表达要素之一。本文使用月球勘测轨道器(LRO)的激光高度计(LOLA)数据、广角相机(WAC)影像、窄角相机(NAC)影像以及Clementine的UVVIS多光谱数据,研究了哥白尼纪正面月海区直径31km的Kepler撞击坑和背面月陆区直径30km的Necho撞击坑。哥白尼纪撞击坑溅射沉积物可以分为三个相:连续溅射沉积相(CE)、不连续溅射沉积相(DE)和辐射纹(CR)。连续溅射沉积相分布在最大约2.6个半径范围之内,不连续溅射沉积相分布在最大近11个半径范围之内,辐射纹分布在最大近29个半径范围之内。本文强调了多源数据结合在识别撞击坑溅射沉积物中的作用,对Kepler坑和Necho坑溅射沉积物进行了填图,不对称分布的特征表明这两个坑可能形成于倾斜撞击。 相似文献
3.
Layers with high magnetic susceptibility contain heavy minerals that are the most likely to preserve signatures of impact in metamorphosed Precambrian rocks. The following impact indicators are resistant to alteration and metamorphism and are likely to be found in heavy mineral layers: impact diamond, SiC, shocked zircon, high-pressure TiO2 phases, reidite, and high-pressure chromite. Coesite and stishovite could also be present but are not likely to be preserved in rocks metamorphosed above the greenschist facies. Looking for layers with high magnetic susceptibility will also identify layers that might contain concentrations of zircon for dating. 相似文献
4.
Multidisciplinary research during the past 25 years has established that the Acraman impact structure in the 1.59 Ga Gawler Range Volcanics on the Gawler Craton, and an ejecta horizon found 240?–?540 km from Acraman in the ??580 Ma Bunyeroo Formation in the Adelaide Fold Belt and Dey Dey Mudstone in the Officer Basin, record a Late Neoproterozoic (Ediacaran) event of major environmental importance. Research since 1995 has verified Acraman as a complex impact structure that has undergone as much as 3?–?5 km of denudation and which originally had a transient cavity up to 40 km in diameter and a final structural rim possibly 85?–?90 km in diameter. The estimated impact energy of 5.2?×?106 Mt (TNT) for Acraman exceeds the threshold of 106 Mt nominally set for global catastrophe, and the impact probably caused a severe perturbation of the Ediacaran environment. The occurrence of the impact at a low palaeolatitude (12.5 +?7.1/???6.1°) may have magnified the environmental effects by perturbing the atmosphere in both hemispheres. These findings are consistent with independent data from the Ediacaran palynology of Australia and from isotope and biomarker chemostratigraphy that the Acraman impact induced major biotic change. Future research should seek geological, isotopic and biological imprints of the Acraman?–?Bunyeroo impact event across Australia and on other continents. 相似文献
5.
Charles Joseph Byrne 《Earth, Moon, and Planets》2007,101(3-4):153-188
The differences between the surface structure of the near side and the far side of the Moon have been topics of interest ever
since photographs of the far side have been available. One recurrent hypothesis is that a large impact on the near side has
deposited ejecta on the far side, resulting in thicker crust there. Specific proposals were made by P.H. Cadogan for the Gargantuan
Basin and by E.A. Whitaker for the Procellarum Basin. Despite considerable effort, no consensus has been reached on the existence
of these basins. The problem of searching for such a basin is one of finding its signature in a somewhat chaotic field of
basin and crater impacts. The search requires a model of the topographic shape of an impact basin and its ejecta field. Such
a model is described, based on elevation data of lunar basins collected by the Lidar instrument of the Clementine mission
and crustal thickness data derived from tracking Clementine and other spacecraft. The parameters of the model are scaled according
to the principles of dimensional analysis and isostatic compensation in the early Moon. The orbital dynamics of the ejecta
and the curvature of the Moon are also taken into account. Using such a scaled model, a search for the best fit for a large
basin led to identification of a basin whose cavity covers more than half the Moon, including the area of all of the impact
basins visible on the near side. The center of this basin is at 22 degrees east longitude and 8.5 degrees north latitude and
its average radius is approximately 3,160 km. It is a megabasin, a basin that contains other basins (the far side South Pole-Aitken
Basin also qualifies for that designation). It has been called the Near Side Megabasin. Much of the material ejected from
the basin escaped the Moon, but the remainder formed an ejecta blanket that covered all of the far side beyond the basin rim
to a depth of from 6 to 30 km. Isostatic compensation reduced the depth relative to the mean surface to a range of 1–5 km,
but the crustal thickness data reveals the full extent of the original ejecta. The elevation profile of the ejecta deposited
on the far side, together with modification for subsequent impacts by known basins (especially the far side South Pole-Aitken
Basin) matches the available topographic data to a high degree. The standard deviation of the residual elevations (after subtracting
the model from the measured elevations) is about one-half of the standard deviation of the measured elevations. A section
on implications discusses the relations of this giant basin to known variations in the composition, mineralogy, and elevations
of different lunar terranes. 相似文献
6.
We use a simple model of the formation, growth, coalescence and migration of veins of basaltic melt generated by partial melting in chondritic asteroids to deduce the sizes of, and pressures within, the fluid-filled dikes reaching the surfaces of such bodies. The gas contents ( 1000 ppm of mainly CO and N2) of the asteroids were high enough that bubbles of free gas trapped in the melt veins gave the basaltic melts significant buoyancy; expansion of these gases as a dike opened to the vacuum at the surface led to fragmentation of the melts into liquid droplets which were transported upwards by the accelerating gases to the surface. The sizes of these droplets and, hence, of the pyroclastic glass beads into which they cooled, are calculated to lie in the range 30 μm to 4 mm; this range is essentially independent of the size or gas content of the asteroid parent and only weakly dependent on the internal pressure of the erupting fluid. The fate of the pyroclasts, however, does depend on all of these factors. At very low internal pressures, significant separation of the gas and liquid in a rising dike may take place and not all of the liquid will be expelled from the dike when it opens to the surface. For relatively large ( 100 km radius) asteroids with relatively low ( 300 ppm) gas contents, the larger clasts are too heavy to be lifted from the level at which magma fragmentation takes place by the gas flow and so would also remain behind to form basaltic veins. The apparent absence of basaltic veins in meteorites then implies both that internal pressures in near-surface dikes were generally greater than 0.3 MPa and that low gas contents were not common. Finally, as long as pyroclasts are lofted from the magma fragmentation level, they will be accelerated to at least 90% of the final gas speed. If this speed exceeds the escape speed from the asteroid (as happens readily for high gas contents and small asteroids), the pyroclasts will be expelled into space and lost from the meteorite record. Otherwise (low gas contents or large asteroids), they will eventually fall back to be incorporated into the surface regolith, modifying the chemical and physical properties of meteorites subsequently derived from it. 相似文献
7.
Bruce W. Fouke Aubrey L. Zerkle Walter Alvarez† Kevin O. Pope‡ Adriana C. OcampO§ Richard J. Wachtman Jose Manuel Grajales Nishimura¶ Phillipe Claeys & Alfred G. Fischer†† 《Sedimentology》2002,49(1):117-138
The depositional and diagenetic history of Cretaceous–Tertiary (KT) impact ejecta deposited 360 km from the Chicxulub crater, at Albion Island, Belize, has been investigated using integrated cathodoluminescence and isotopic analyses. A quarry exposes 26 m of Upper Cretaceous Barton Creek Formation dolomitized marine limestone overlain by 16 m of dolomitized Albion Formation impact ejecta. The Albion Formation consists of a lower fine‐grained ≈1‐m‐thick spheroid bed and an upper 15‐m‐thick coarse conglomeratic diamictite bed. A 14‐event paragenetic sequence has been documented and used as a temporal framework to interpret chemostratigraphic trends in bulk rock δ18O, δ13C and 87Sr/86Sr. The uppermost surface of the Barton Creek Formation was subaerially exposed before the KT impact, as indicated by a brecciated palaeosol that caps upsection decreases in δ13C and δ18O. Small 1‐cm‐diameter spheroids in the spheroid bed exhibit vermicular crystalline textures but lack the concentric zonations common to accretionary lapilli. These spheroids are hypothesized originally to have been impact glass or reactive Ca and Mg oxide dusts that adhered to water vapour particles condensing from the cooling impact vapour cloud. The spheroids were dolomitized soon after deposition. The earliest dolomitization in the matrix sediments of the Albion Formation was also post‐depositional, replacing clays formed by devitrification of impact glass. Dolomite and clay 87Sr/86Sr exhibit a distinct symmetrical distribution in the spheroid bed ranging from 0·707745 to 0·707872. Although unproven, this may represent primary changes in the chemical composition of the impact glass. The limestone clasts in the diamictite bed were dolomitized before the KT impact and exhibit upsection decreases in bulk rock 87Sr/86Sr. This suggests that the clasts were excavated from strata equivalent in age or older than the Barton Creek Formation at locations closer to, or in, the Chicxulub crater. 相似文献
8.
Sixteen elements (Ca, K, Mg, Na, Al, Fe, Mn, P, Co, Cu, Li, Ni, Rb, Sr, Ti, Zn, determined by atomic absorption) were identified in 453 pumice fragments recovered from Holocene strandplains in southeast Queensland and New South Wales. Eight pumice groups and 13 subgroups are recognised by numerical analysis. Some pumices record known eruptions. Others come from known centres in Tonga and Vanuatu. Several pumice eruptions have occurred from some centres, but there are instances of single episodes. The numerical analyses, combined with carbon dating and soil identification, identify marker horizons in the development of the strandplains. These horizons provide a time‐scale for soil development. Pumice that occurs in middens has an archaeological value. Coke was found with recent pumice. It conveniently identifies the modern industrial age. 相似文献
9.
The Wittenoom Formation (Hamersley Group, Western Australia) is a well-preserved Neoarchaean unit deposited in a deep shelf to upper slope setting. The upper part contains several laterally persistent marker beds, one of which is rich in well-preserved spherules of former silicate melt showing a diverse suite of internal textures. We quantified the relative abundances of these textures by point counting in spherule-rich samples selected from seven sites and found them to be surprisingly uniform for a lateral distance of?>?350 km. They also appear to be uniform at one site where the layer is thicker and contains multiple zones rich in spherules. Given this homogeneity and by comparison to experimentally produced textures and K/T impact spherules, we infer that: (i) the homogeneously diverse nature of the ejecta is most consistent with an impact origin; (ii) the spherules were partially crystallised at the time they were deposited and therefore classifiable as microkrystites; (iii) the original impact melt was roughly basaltic in composition; (iv) the spherules were generated by a single impact then deposited in multiple pulses; (v) the K/T impact model is not directly applicable to the Wittenoom spherule layer; and (vi) the Wittenoom spherule layer was not formed by the same impact as the Carawine layer. 相似文献
10.
《Chemie der Erde / Geochemistry》2019,79(1):1-61
The Earth’s impact record is known to be rather limited in both time and space. There are ca. 190 impact structures currently known on Earth, representing a minor fraction of all the impact events that contributed to the initial formation of our protoplanet, and then to formation and modification of the surface of the planet. Moreover, the distribution of impact structures on Earth is manifestly uneven. One continent that stands out for its relatively small number of confirmed impact structures and impact ejecta occurrences is South America. The limited impact record for this large continent makes a robust case that there is a significant potential for further discoveries. Significant information on the impact record of South America is dispersed in different types of publications (journal articles, books, conferences abstracts, etc.), and in several languages, making it difficult to access and disseminate it among the geoscientific community. We aim to present a summary of the current knowledge of the impact record of this continent, encompassing the existing literature on the subject. It is published in two parts, with the first one covering an up-to-date introduction to impact cratering processes and to the criteria to identify/confirm an impact structure and related deposits. This is followed by a comprehensive analysis of the Brazilian impact structures. The Brazilian impact record accounts for the totality of the large structures of this kind currently confirmed in South America. The second part will examine the impact record of other countries in South America, provide information about a number of proposed impact structures, and review those that already have been discarded as not being formed by impact. 相似文献