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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A schematic model of crater modification by gravity

The morphology of craters found on planets and moons of the solar system is examined and a development model which can account for the observed crater characteristics is discussed. The prompt collapse of craters to form flat floors, terraced walls, and central peak structures is considered to be the result of an approximate Bingham plastic rheology of the material surrounding the crater. This rheology is induced dynamically by the strong incoherent acoustic 'noise' accompanying excavation of the crater. Central pits, peak rings, and other multiple symmetric-profile rings originate by oscillation of this fluid. Large craters with transient depths comparable to the lithosphere thickness are subject to collapse by fragmentation of the lithosphere as well as fluidization. The considered concepts are developed mathematically. A model emerges which appears capable of explaining most of the qualitative features of large impact structures.

Melosh, H. J.↗
NASA NTRS2016

Irregular Mare Patches (IMPs): 100 Ma or 3 Ga?

IMPs exhibit a perplexing combination of characteristics that are consistent with either an approximately 100 Ma or 3 Ga formation. Dozens of small-area IMPs have crisp morphologies and crater size-frequency distributions (SFDs) that denote relatively recent geologic activity (less than 100 Ma); however, the apparently well-developed regolith on portions of the IMPs are in conflict with such a young age [1]. To test possible formation hypotheses (e.g., [1-5]), which range from ancient volcanism to contemporary outgassing, we examined IMP morphology at the meter-scale with LROC NAC images and derived elevation models. We focused on the largest IMPs (Ina, Sosigenes, Cauchy, Maskelyne, and Nubium), where contacts between deposits are best developed. Most of our observations are consistent with multiple generations of inflation and breakouts (or squeeze-ups) of basaltic lavas that were affected by local slopes. Some of the extrusions coalesced into larger mounds or filled pre-existing craters. We did not observe evidence of large-scale void space (e.g., fissures, fractures, linear depressions, or pits) within or beneath the mounds or rougher deposits (e.g., [5]). But, small-scale voids may be signified by isolated pitted textures. We also did not detect evidence of the cooling fractures or lava plates expected in young lava flows and observed in lunar impact melt deposits. The smooth texture of the mounds is enigmatic. Block-less craters suggest at least 5 m of friable or poorly-cohesive material (such as regolith), yet mound margins exhibit slopes greater than 30 deg requiring significant material strength. Blocks are not common on the mounds, but are sometimes excavated by impacts (usually excavated from beneath the mounds). The uneven deposits are equally enigmatic and texturally varied (blocky, pitted, and crenulated). They are deficient in superposed craters compared to the mounds. If the mounds are indeed of similar age to the rougher units, then their different superposed crater morphologies and SFDs need to be explained by factors other than their ages. Any mounds originally composed of friable surface materials would evolve differently from more coherent deposits (e.g., [6-7]).

Stopar, Julie↗
NASA NTRS1993

Morphology of large impact craters and basins on Venus: Implications for ring formation

A nearly complete examination of the Magellan radar data for the Venusian surface reveals 72 unequivocal peak-ring craters and 4 larger structures that we interpret to be multiringed. This report updates our earlier studies and that of the Magellan team. The general morphology of peak-ring craters, decreasing ring diameter ratio trends with increasing crater diameter, and the general size-morphology progression from complex central-peak crater to peak-ring crater on Venus and the terrestrial planets suggest similar processes of peak-ring formation. Observations are consistent with a model of dynamic collapse, downward and outward, of an unstable central peak to form a ring. We interpret the four larger ringed structures (Klenova, Lise Meitner, Mead, and Isabella) to be morphologically similar to the Orientale Basin on the Moon, and thus, true multiringed basins.

Alexopoulos, Jim S.↗
NASA NTRS1985

The Nature of Martian Flow Ejecta Craters

Ejecta with flow features and discrete termini surround many fresh Martian craters. Several morphologies of the flow-like ejecta are observed; they are found globally and in nearly all terrains. It is suggested that the morphology of flow ejecta craters is related to the amount of subsurface volatiles and/or to atmospheric drag effects. It was attempted to constrain factors which could contribute to the ejecta morphology such as latitude, elevation, and terrain unit, however, they involved only one or two constrained variables or used global data sets based on Mariner 9 information. Viking-based data sets are becoming available and may provide a better base from which an understanding of the factors which governs ejecta morphology may be obtained. Block sizes of Martian flow ejecta may provide clues to the ejecta emplacement process.

Horner, V. M.↗
NASA NTRS1998

Gale Crater: An Amazonian Impact Crater Lake at the Plateau/Plain Boundary

Gale is a 140-km diameter impact crater located at the plateau/plain boundary in the Aeolis Northeast subquadrangle of Mars (5S/223W). The crater is bordered in the northward direction by the Elysium Basin, and in eastward direction by Hesperian channels and the Aeolis Mensae 2. The crater displays a rim with two distinct erosion stages: (a) though eroded, the south rim of Gale has an apparent crest line visible from the north to the southwest (b) the west and northwest rims are characterized by a strong erosion that, in some places, partially destroyed the rampart, leaving remnant pits embayed in smooth-like deposits. The same type of deposits is observed north, outside Gale, it also borders the Aeolis Mensae, covers the bottom of the plateau scarp, and the crater floor. The central part of Gale shows a 6400 km2 subround and asymmetrical deposit: (a) the south part is composed of smooth material, (b) the north part shows spectacular terraces, streamlines, and channels. The transition between the two parts of the deposit is characterized by a scarp ranging from 200 to 2000 in high. The highest point of the scarp is at the center of the crater, and probably corresponds to a central peak. Gale crater does not show a major channel directly inflowing. However, several large fluvi systems are bordering the crater, and could be at the origin of the flooding of the crater, or have contributed to. One fluvial system is entering the crater by the southwest rim but cannot be accounted alone for the volume of sediment deposited in the crater. This channel erodes the crater floor deposit, and ends in a irregular-shaped and dark albedo feature. Gale crater shows the morphology of a crater filled during sedimentation episodes, and then eroded Part of the lower sediment deposition contained in Gale might be ancient and not only aqueous in origin. According to the regional geologic history, the sedimentary deposit could be a mixture of aeolian and pyroclastic material, and aqueous sedimentary material that can originate both from drainage of the regional subsurface aquifer, and/or from surface flood. The central deposit shows three main levels: (a) the current crater floor (north of Gale), (b) an ancient level about 200 rn higher (south of Gale), and (c) the massive terraced deposits. A crater statistics on the 15,400 kM2 area of the crater floor and deposit [3,41 gave: 259+/-112.4 craters, most of them partly embayed in the sedimentary deposit, and all inferior to 5-km diameter. For superimposed crater population only, the result is 194+/-112. The deduced relative ages ranges from Early to Middle Amazonian. The population of craters are comparable for the three levels, implying that the last sedimentation/erosion episode on Gale was recent and affected the whole crater. The streamlined morphology of the border of the deposit, the layering, the channels, and the terraces are compatible with a significant fluvio-lacustrine history of the site. Multiple levels may suggest different episodes, but the common statistical age of the three levels shows that the last episode involved the whole crater. The origin of the lake water in Gale may have varied in time. Three major contributions have been proposed: (a) the drainage of the regional underground aquifer by Gale crater over an area of 110-km radius around the crater which would have provided approximately 1,600 cubic km of water, (b), surface drainage entering Gale by the south and north rims. In the south, a 250-km long system originates in the cratered uplands in a Noachian crater material plain (Nc), and crosses Hesperian and Amazonian crater material plains (AHc) northward [1]. Several fluvial systems originate in the Aeolis Mensae, east of Gale. They may had two functions in time: to recharge, the underground aquifer in the region of Gale, and to supply surface water in the crater by overspilling the northern rim, and (c) surface floods that originated from the rising of the water level in the Elysium Basin. According to the Amazonian age of Gale's floor, and the erosion direction in the crater, a flood from Elysium Basin is the most likely event to explain the material observed in Gale, and the formation of the last lake. This last flood may have been important enough to flood the central deposit up to about 1400 m above the crater floor, leaving two islands (non stream lined features) at the center of the deposit. Terrace spacing suggests a regular drop of the lake level in time. Fractures in terraces perpendicular to the shoreline can be interpreted either as: (a) the result of the drainage systems during the waning of the lake, or (b) traces of the pressure of an ice-covered sheet associated with subglacial drainage. The presence of a lake of such volume during the Amazonian period is one more evidence that water was still active on Mars relatively recently. Gale crater offers the rare opportunity to unveil a key-period of the martian history. The Amazonian might proved not as cold and dry as previously thought. The presence of large lakes and basins (Elysium Basin is large as the Mediterranean Sea), reinforces the model of an extensive water activity during the Amazonian that has still to be understood in the context of an assumed cooling and drying planet. The sediments and rocks that were left of this period in Gale keep the record of the climatic conditions of the Amazonian and the clues that are missing to understand the climatic evolution of Mars. In addition, Gale crater presents the advantage to be located at the plateau/plain boundary, which has never been studied and contains information about the two main martian geological units. As a conclusion, we propose a table that summarizes the worthiness of a mission in Gale crater, and the expected science return relative to the objectives to be met by the Surveyor Program. Additional information contained in the original.

Cabrol, N. A.↗
NASA NTRS1986

An analysis of morphologic variation in simple lunar craters

Previous photogeologic classification of lunar craters have differentiated them into various morphologic types. In an attempt to identify the physical factors responsible for these morphologic variations, this study examined 44 fresh, simple lunar craters between 1.1 and 14 km in diameter on the maria. Photogeologic analysis was performed using Apollo stereo panoramic photography, while a quantitative analysis of cross-sectional shape was done using Fourier analysis. These analyses showed that the sample could not be split into two distinct types (flat- and round-floored) as had been done previously. The two crater types at best represent end-members of a distribution of morphologies. Some correlation was found between the degree of relative degradation and floor roundness, implying that flat floors are primary and round floors are degradational features. No difference was found between the average depth-to-diameter ratio of flat- and round-floored craters, indicating that the factor of two variation in depth-to-diameter ratio in this sample of fresh craters is a primary morphologic feature and that minor degradation of fresh craters has no significant effect on average crater depth.

Ravine, M. A.↗
NASA NTRS1982

Craters and basins on Ganymede and Callisto - Morphological indicators of crustal evolution

The morphologic characteristics of craters and palimpsests on Ganymede and Callisto are surveyed, and the crustal properties of these satellites and the evolution of the properties are studied. The morphology of bowl-shaped craters, smooth-floored craters, craters without central peaks, craters with central pits, chain craters on Callisto, the Gilgamesh and Western Equatorial Basins on Ganymede, crater palimpsests and penepalimpsests, multiring structures on Callisto, and the Galileo Regio rimmed furrow system on Ganymede are described individually. The crustal evolution is addressed by examining the development of the Galileo Regio system, the distribution of crater retention ages, the record of ray clusters, the thermal history of the lithosphere of Ganymede, and the origin of the central pits. It is suggested that as the lithosphere of each satellite cooled and thickened, crater retentivity spread as a wave from the polar regions and the antapex toward the apex; at any given location, progressively larger craters were retained with the passage of time.

Passey, Q. R.↗
NASA NTRS2002

New Evidence Supporting an Ice-covered Lake in Gusev Crater

The morphology and setting of a groove in Gusev strongly supports the hypothesis of a lateral channel generated by meltwater flowing against an ice covered lake margin. The size of the groove and the slope of the bed independent of the topography of the crater are consistent with modest discharges. Additional information is contained in the original extended abstract.

Grin, E. A.↗
NASA NTRSDate not supplied

THE INFLUENCE OF A GRANULAR TARGET'S PACKING DENSITY ON CRATER MORPHOMETRY AND EJECTA KINEMATICS: EXPLORATORY EXPERIMENTATION

With current orbital cameras providing images of planetary surfaces at ever increasing fidelity, it is now possible to resolve meter-scale craters (e.g. [1]). As the morphology and morphometry of small craters have been demonstrated to be sensitive to the physical properties of the impactor and target in both laboratory-based [2,3] and modeling studies [4,5],these newly resolvable features offer an opportunity to probe the near-surface properties of impacted planetary surfaces. In order to robustly infer near-surface structure from craters formed on complex surfaces with poorly known physical properties, it is imperative to understand how small changes in target properties are manifested in the final morphometry of an impact crater. Here, we present early experimental results on the effects of changes in bulk density of granular tar-gets(inclusive of porosity) on crater morphology and the cratering process.

C J Cline↗
NASA NTRS1977

Radial variation of lunar crater rim topography

The paper presents results of determinations of variations of rim topography as a function of range from the crater rim for a group of morphologically fresh craters. It is found that the rate at which exterior crater topography converges with the surrounding surface is highly variable among different radial directions at individual craters as well as between different craters, and that at several craters, oblique impact appears to have contributed to azimuthal elevation/range variations.

Settle, M.↗
NASA NTRS1991

Cratering of the Uranian satellites

Available crater counts and their interpretations are reviewed, with emphasis on essential scaling considerations and comparisons with hypotheses developed for interpreting the cratering records on other planets and satellites. New approaches are employed to scaling based on new measurements of crater depths and morphology, which show craters in ice to be unexpectedly different from those in rock. It is found that the published crater counts on the Uranian satellites, despite mutual inconsistencies, can be interpreted as compatible with cratering by the heliocentric population of cometary bodies that was responsible for much of the cratering of the satellites of Jupiter and Saturn. Scaling arguments are applied to the catastrophic breakup of icy satellites and ring particles. The importance of large-scale collisions in disrupting the inner Uranian satellites is found to depend on the shape of the size distribution of cometary bodies at large sizes.

Mckinnon, William B.↗
NASA NTRS2011

The Impact History of Vesta: New Views from the Dawn Mission

The Dawn mission has completed its Survey and High-Altitude Mapping Orbit (HAMO) phases at Vesta, resulting in 60-70 meter per pixel imaging, high-resolution image-derived topography, and visual and infrared spectral data covering up to approx.50 degrees north latitude (the north pole was in shadow during these mission phases). These data have provided unprecedented views of the south polar impact structure first detected in HST imaging [1], now named Rheasilvia, and in addition hint at the existence of a population of ancient basins. Smaller craters are seen at all stages from fresh to highly-eroded, with some exposing atypically bright or dark material. The morphology of some craters has been strongly influenced by regional slope. Detailed studies of crater morphology are underway. We have begun making crater counts to constrain the relative ages of different regions of the surface, and are working towards developing an absolute cratering chronology for Vesta's surface.

OBrien, D. P.↗
NASA NTRS2025

Large Impact Features on Ganymede and Callisto as Revealed By Geological Mapping and Morphometry

The icy Galilean satellites are host to a broad range of impact feature morphologies. Hypotheses seeking to explain the diversity of these impact features consider the effects of impact melt, the physical state of the subsurface at the time of impact, and the impactor characteristics. As part of a larger effort to assess the role of these factors in the formation and evolution of these impact features, we have performed topographic and geological mapping of 19 large impact features on Ganymede and Callisto. These are divided into two main morphological groups: craters (subdivided into pit, dome, and anomalous dome craters), and penepalimpsests/palimpsests. The transitions from pit, dome, to anomalous dome craters appear to be size-dependent up to diameters of ∼170 km. The morphologies of pit and dome craters appear to be independent of their age or geologic context. The impacts that formed them only affected a cold, rigid ice layer, with the development of pits and raised annuli on their floors possibly stemming from the evolution of a pocket of impact melt. The subdued rims and floors of anomalous dome craters indicate the increasing effect of a weak, warm ice layer on impact feature morphology with increasing size, but their prominent annuli and pits indicate that mobilization of impact melt is also a factor. The very low topographic relief of older penepalimpsests and palimpsests indicates that their impacts penetrated the ice shell to mobilize very large volumes of pre-existing liquid from a subsurface layer, with little contribution to the final feature morphology from impact melt. Penepalimpsests are distinguished from palimpsests by the higher frequency of concentric ridges within their interiors, indicating a generally more robust state of the subsurface that could better support the rotation and uplift of solid material during impact, even if a crater-like depression could not be supported. A few impact features seem to be transitional between anomalous dome craters and penepalimpsests, and the overlap of anomalous dome craters, penepalimpsests, and palimpsests in terms of diameter as well as age indicates that impactor size and subsurface properties over time are major factors in determining which of these morphologies emerges.

Oliver L White↗
NASA NTRS1993

Morphologic and morphometric studies of impact craters in the northern plains of Mars

Fresh impact craters in the northern plains of Mars display a variety of morphologic and morphometric properties. Ejecta morphologies range from radial to fluidized, interior features include central peaks and central pits, fluidized morphologies display a range of sinuosities, and depth-diameter ratios are being measured to determine regional variations. Studies of the martian northern plains over the past five years have concentrated in three areas: (1) determining correlations of ejecta morphologies with crater diameter, latitude, and underlying terrain; (2) determining variations in fluidized ejecta blanket sinuosity across the planet; and (3) measurement of depth-diameter ratios and determination of regional variations in this ratio.

Barlow, N. G.↗
NASA NTRS1976

Hollow lunar spherules and microcratering

Craters were generated by micrometer-size, thin-walled hollow glass spheres impacting glass and stainless steel targets to determine morphological features distinguishing craters formed by hollow spheres from those formed by solid projectiles. The craters formed in both targets differed from those formed by solid spherical projectiles in that the central pit is an annular rather than a cup-shaped depression. The existence of an annular crater morphology on a surface exposed to cosmic dust would indicate the presence of thin-walled hollow spherules. Penetrations of thin-walled hollow glass beads by high velocity, solid, micrometer-size spheres are characterized by inward and outward flowing lips showing asymmetries dependent on the angle of impact.

Vedder, J. F.↗
NASA NTRS1993

Fractal dimensions of rampart impact craters on Mars

Ejecta blanket morphologies of Martian rampart craters may yield important clues to the atmospheric densities during impact, and the nature of target materials (e.g., hard rock, fine-grained sediments, presence of volatiles). In general, the morphologies of such craters suggest emplacement by a fluidized, ground hugging flow instead of ballistic emplacement by dry ejecta. We have quantitatively characterized the shape of the margins of the ejecta blankets of 15 rampart craters using fractal geometry. Our preliminary results suggest that the craters are fractals and are self-similar over scales of approximately 0.1 km to 30 km. Fractal dimensions (a measure of the extent to which a line fills a plane) range from 1.06 to 1.31. No correlations of fractal dimension with target type, elevation, or crater size were observed, though the data base is small. The range in fractal dimension and lack of correlation may be due to a complex interplay of target properties (grain size, volatile content), atmospheric pressure, and crater size. The mere fact that the ejecta margins are fractals, however, indicates that viscosity and yield strength of the ejecta were at least as low as those of basalts, because silicic lava flows are not generally fractals.

Ching, Delwyn↗
NASA NTRS1974

Morphologic classification of Martian craters and some implications

The establishment of the absolute age range for the unmodified crater class is considered in an attempt to estimate the change of crater degradation rates as a function of time. It is assumed that both the Moon and Mars have had similar time rate of changes in impacting rates. This means that both were subjected to intense bombardment in early history. The rate rapidly declined to low rates around 3.5 b.y. ago.

Arvidson, R. E.↗