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Temperature and material effects on high-velocity impact craters in phenolic-nylon and elastomeric ablative heat-shield materials
The graphs of diameter vs frequency plotted in the present study for primary impact craters produced in four time intervals exhibit an appreciable change in form with age. A deficiency of small craters relative to an extrapolation from the large-diameter parts of the curves is greatest and extends to the largest sizes in the oldest population (pre-Nectarian), whose curve approximates a log-normal form. Successively younger populations (Nectarian, Imbrian, Copernican plus Eratosthenian) have successively more small craters relative to larger ones and more nearly log-log distributions. Many of the craters thought to be small old primaries are here identified as secondary craters of basins, on the basis of previously developed criteria.
Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a database describing 842 craters on 89 percent of the planet's surface mapped through orbit 2578 (the craters range in diameter from 1.5 to 280 km). We have studied the distribution, size-frequency, morphology, and geology of these craters both in aggregate and, for some craters, in more detail. We have found the following: (1) the spatial distribution of craters is highly uniform; (2) the size-density distribution of craters with diameters greater than or equal to 35 km is consistent with a 'production' population having a surprisingly young age of about 0.5 Ga (based on the estimated population of Venus-crossing asteroids); (3) the spectrum of crater modification differs greatly from that on other planets--62 percent of all craters are pristine, only 4 percent volcanically embayed, and the remainder affected by tectonism, but none are severely and progressively depleted based on size-density distribution extrapolated from larger craters; (4) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (5) diffuse radar-bright or -dark features surround some craters, and about 370 similar diffuse 'splotches' with no central crater are observed whose size-density distribution is similar to that of small craters; and (6) other features unique to Venus include radar-bright or -dark parabolic arcs opening westward and extensive outflows originating in crater ejecta.
Lobate ejecta deposits surround many of the younger impact craters on Mars. Viking Orbiter images indicate the distal parts of the ejecta blankets of these lobate craters are characterized by ramparts. In the absence of detailed topographic data for characterizing the topology of these apparently fluidized ejecta deposits, physical models have relied upon their morphologic characteristics. The most widely accepted model for the formation of such rampart ejecta deposits on Mars invokes fluidization of the ejecta to produce one or more viscous flow lobes. The availability of high-precision topographic data from the Mars Orbiter Laser Altimeter [4,51 facilitates a more quantitative examination of the physical processes involved in the formation of rampart ejecta deposits on Mars. Here we investigate the emplacement constraints that can be developed from the dimensions, topography, and morphology of martian rampart craters. The primary assumptions we have adopted are: (1) the ejecta blanket is emplaced as a continuum flow over the martian surface, rather than an airfall deposit, and (2) that the observable dimensions of the deposits are indicative of flow dimensions during emplacement.
The lobate ejecta morphology surrounding most fresh Martian impact craters can be quantitatively analyzed to determine variations in ejecta sinuosity with diameter, latitude, longitude, and terrain. The results of such studies provide another clue to the question of how these morphologies formed: are they the results of vaporization of subsurface volatiles or caused by ejecta entrainment in atmospheric gases. Kargel provided a simple expression to determine the degree of non-circularity of an ejecta blanket. This measure of sinuosity, called 'lobateness', is given by the ratio of the ejecta perimeter to the perimeter of a circle with the same area as that of the ejecta. The Kargel study of 538 rampart craters in selected areas of Mars led to the suggestion that lobateness increased with increasing diameter, decreased at higher latitude, and showed no dependence on elevation or geologic unit. Major problems with the Kargel analysis are the limited size and distribution of the data set and the lack of discrimination among the different types of lobate ejecta morphologies. Bridges and Barlow undertook a new lobateness study of 1582 single lobe (SL) and 251 double lobe (DL) craters. The results are summarized. These results agree with the finding of Kargel that lobateness increases with increasing diameter, but found no indication of a latitude dependence for SL craters. The Bridges and Barlow study has now been extended to multiple lobe (ML) craters. Three hundred and eighty ML craters located across the entire Martian surface were studied. ML craters provide more complications to lobateness studies than do SL and DL craters - in particular, the ejecta lobes surrounding the crater are often incomplete. Since the lobateness formula compares the perimeter of the ejecta lobe to that of a circle, the analysis was restricted only to complete lobes. The lobes are defined sequentially starting with the outermost lobe and moving inward.
Cratering theory used to determine lunar surface layer thickness from lunar impact craters
The paper reports on a series of low-velocity impact experiments performed in ice and ice-saturated sand. It is found that crater diameters in ice-saturated sand were about 2 times larger than in the same energy and velocity range in competent blocks of granite, basalt and cement, while craters in ice were 3 times larger. It is shown that if this dependence of crater size on strength persists to large hypervelocity impact craters, then surface of geologic units composed of ice or ice-saturated soil would have greater crater count ages than rocky surfaces with identical influx histories. Among the conclusions are that Martian impact crater energy versus diameter scaling may also be a function of latitude.
The spatial density of impact craters larger than 100 m on the four great volcanic shields in the Tharsis region is analyzed, assuming that each observed crater population represents the total formed since the local surface was last renewed. Previous investigations of this type are reviewed, problems encountered in those studies are discussed, and an attempt is made to overcome these problems by applying explicit criteria for the separation of craters smaller than approximately 1 km in diameter according to their origin and by ignoring larger craters. Relative ages are determined by comparing crater populations at a common diameter, and such comparisons are carried out for six populations on Arsia Mons, one population on Pavonis Mons, several craters on Ascraeus Mons, and two populations on Olympus Mons. The results, presented in terms of 'crater ages', indicate that most dated surfaces on the great shields range in age from 0.5 to 1.2 billion years, that the eruption patterns of the volcanoes are similar to those of the Hawaiian basaltic shields, and that the most frequently renewed surfaces are on the middle-to-lower flanks as well as within the summit calderas. The youngest surfaces are estimated to date from the period between 0.5 billion years ago and the present; the oldest dated surfaces are shown to suggest that the volcanoes had attained essentially their present sizes over 1 billion years ago.
The structure in and around the Charlevoix and Deep Bay impact craters was analyzed by comparing SIR-B radar image lineament maps with maps derived from Landsat data and aerial photography. Relationships revealed include the discovery in the Deep Bay lineament distribution of a mode, of slightly different location and size to that determined by aerial photography, which correlates with the regional glacial trend. The Deep Bay regional glacial trend displays a much more prominent signature in the aerial photography than in the radar data, confirming the influence of radar look direction on the interpretation of lineament distribution and orientation. Effects from the impact event are noted in the radar images of both craters, with lineaments of one km or less appearing to be associated with the rim structure.
The most recent models concerning mechanical aspects of lunar regolith dynamics related to impact cratering use probabilistic approaches to account for the randomness of the meteorite environment in both space and time. Accordingly the absolute regolith thickness is strictly a function of total bombardment intensity and absolute regolith growth rate in nonlinear through geologic time. Regoliths of increasing median thickness will have larger and larger proportions of more and more deep seated materials. An especially active zone of reworking on the lunar surface of about 1 mm depth has been established. With increasing depth, the probability of excavation and regolith turnover decreases very rapidly. Thus small scale stratigraphy - observable in lunar core materials - is perfectly compatible with regolith gardening, though it is also demonstrated that any such stratigraphy does not necessarily present a complete record of the regolith's depositional history. At present, the lifetimes of exposed lunar rocks against comminution by impact processes can be modeled; it appears that catastrophic rupture dominates over single particle abrasion.
An expression is proposed for radial thickness variations in lunar impact crater ejecta. The expression was inferred from available cratering data and semi-empirical calculations for approximate modeling of variations in ejecta thickness with the increasing range of lunar craters. A table of predicted stratigraphic sections for the Apollo Landing sites is included. The predictions were made by using this expression. The results suggest that the pre-Nubium crustal material at upland Apollo sites lies at depths of above 280 up to 1940 m.
Models for the viscous relaxation of impact crater topography are used to constrain the crustal thickness (H) and the mean lithospheric thermal gradient beneath the craters on Venus. A general formulation for gravity-driven flow in a linearly viscous fluid has been obtained which incorporates the densities and temperature-dependent effective viscosities of distinct crust and mantle layers. An upper limit to the crustal volume of Venus of 10 to the 10th cu km is obtained which implies either that the average rate of crustal generation has been much smaller on Venus than on earth or that some form of crustal recycling has occurred on Venus.
This paper presents a geometric model describing the geometry of the transient cavity of an impact crater and the subsequent collapse of its walls to form a crater filled by an interior breccia lens. The model is tested by comparing the volume of slump material calculated from known dimensional parameters with the volume of the breccia lens estimated on the basis of observational data. Results obtained from the model were found to be consistent with observational data, particularly in view of the highly sensitive nature of the model to input parameters.
Twenty-three samples covering a wide range of shock metamorphism from the Ries impact crater in southern Germany were analyzed for siderophile and volatile elements in order to determine the chemical nature of the impacting meteorite. Slight enrichments in the siderophile elements Ir and Os above the indigenous level of 0.015 ppb were observed in only eight samples, including a shocked, metal-bearing amphibolite and two weakly shocked biotite gneisses, whereas those samples expected to contain meteoritic material (heavily shocked glasses) do not exhibit a perceptible siderophile enrichment. The Ir, Os and Ni enrichments of the metal bearing amphibolite are found to be compatible with chondritic ratios, while in the other enriched samples, Ir, Ni and Se relationships suggest an achondritic meteoritic origin. An aubritic projectile is concluded to be most compatible with the data.
Several emplacement mechanisms were proposed for the unique flow features associated with 402 of the 912 impact craters found on Venus to date. Studies based on rheological models and laboratory results have suggested two separate depositional flow mechanisms: a turbulent emplacement and a lava like emplacement. Schultz proposed a progressive transition from the turbulent to the lava like mechanism during deceleration of some turbulent flows. He suggested that these composite flows were emplaced prior to the deposition of the normal ejecta. After an examination of the geomorphologic, stratigraphic, and topographic relations of the flows associated with the 402 craters, it is suggested that most of the outflows result from a two-stage emplacement process. In the first stage, a turbulent, proximal part is usually emplaced downrange of the impact site after the other ejecta materials. In the second stage, a distal part, composed of low-viscosity melt, slowly drains via dendritic channels from the proximal deposit and flows like lava. A few flows were identified that appear to have been emplaced prior to the ejecta; these flows likely result from a different mechanism.
Global geometric characteristics of topographically fresh impact craters have been assessed, for the first time, from gridded MOLA topography. Global trends of properties such as depth/diameter differ from previous estimates. Regional differences are observed.
El'gygytgyn crater, located within Eastern Siberia, is a Pliocene-aged (3.6 Ma), well-preserved impact crater with a rim diameter of roughly 23 km. The target rocks are a coherent assemblage of crystalline rocks ranging from andesite to basalt. At the time of impact the region was forested and the Arctic Ocean was nearly ice-free. A 15-km lake fills the center of the feature and water depths are approximately 175 m. Evidence of shock metamorphism, -- including coesite, fused mineral glasses, and planar deformation features in quartz -- has been reported. This feature is one of the youngest and best preserved complex craters on Earth. Because of its remote Arctic setting, however, El gygytgyn crater remains poorly investigated. The objectives of this three-year project are to establish and maintain a research partnership between scientists from Russia and the United States interested in the El gygytgyn crater. The principal institutions in the U.S. will be the Geophysical Institute, University of Alaska Fairbanks and the University of Massachusetts Amherst. The principal institution in Russia will be the North East Interdisciplinary Scientific Research Institute (NEISRI), which is the Far-East Branch of the Russian Academy of Science. Three science tasks are identified for the exchange program: (1) Evaluate impactite samples collected during previous field excursions for evidence of and level of shock deformation. (2) Build a high-resolution digital elevation model for the crater and its surroundings using interferometric synthetic aperture radar techniques on JERS-1, ERS-1, ERS-2, and/or RadarSat range-doppler data. (3) Gather all existing surface data available from Russian and U.S. institutions (DEM, remote sensing image data, field-based lithological and sample maps, and existing geophysical data) and assemble into a Geographic Information Systems database.
Collisions between asteroids and other forms of solar system debris are discussed, especially relatively high-energy, non-catastrophic collisions, the resulting craters, and the effects they should have on the surfaces and interiors of target bodies. Attention is given to the nature and formation of impact craters, as well as to shock waves and the energy (kinetic, internal) imparted through them, crater scaling, stress wave-surface interactions, impact melt, and the effects of non-escaping ejecta on the surface of the target body (rock, fine-grained regolith, porous media).