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At least 199 records · Page 11

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NASA NTRS1974

Morphology of lunar craters - A test of lunar erosional models

The present work is a test of published theoretical and experimental studies of crater erosion by micrometeorite bombardment which predict systematic variations in the morphology of lunar craters as a function of crater diameter and crater age. Numerical, ranking-type degradation classifications indicate that the craters on Mare Imbrium and Mare Tranquillitatis confirm these predictions by showing a systematic increase in degradation with decreasing diameter for craters smaller than a few kilometers in diameter but larger than the equilibrium diameter, and by showing fixed proportions of fresh, moderately degraded, and very degraded craters under equilibrium conditions. Furthermore, the relative ages of the two mare surfaces may be determined using a diameter/mean-degradation-number curve. These determinations of relative age and process of crater erosion are both essentially independent of the traditionally studied crater diameter/frequency relationships.

Mcgill, G. E.↗

NASA NTRS1974

Craters on Earth, Moon, and Mars - Multivariate classification and mode of origin

Testing extraterrestrial craters and candidate terrestrial analogs for morphologic similitude is treated as a problem in numerical taxonomy. According to a principal-components solution and a cluster analysis, 402 representative craters on the Earth, the Moon, and Mars divide into two major classes of contrasting shapes and modes of origin. Craters of net accumulation of material (cratered lunar domes, Martian calderas, and all terrestrial volcanoes except maars and tuff rings) group apart from craters of excavation (terrestrial meteorite impact and experimental explosion craters, typical Martian craters, and all other lunar craters). Maars and tuff rings belong to neither group but are transitional. The classification criteria are four independent attributes of topographic geometry derived from seven descriptive variables by the principal-components transformation. Morphometric differences between crater bowl and raised rim constitute the strongest of the four components.

Pike, R. J.↗

NASA NTRS1974

Smooth plains and continuous deposits of craters and basins

Apollo 16 photographic data are analyzed which suggest that smooth plains are related in origin to large primary cratering events, and that the plains material is mainly the ejecta of local and regional primary impact craters with only minor contributions from distant craters and basins. This material, then, has been emplaced in its present location by secondaries of distant craters and directly by primary craters near the site. Calculations indicate that the material excavated from large primary craters typically excavates much larger amounts of material from local terrain when it impacts in the secondary crater field. There is a correlation between the extent of development of smooth plains inside large high-land craters and the erosional state of the crater rims and walls. It is incorrect to consider the continuous deposits to be solely basin ejecta.

Oberbeck, V. R.↗

NASA NTRS1974

Problems in the interpretation of lunar mare stratigraphy and relative ages indicated by ejecta from small impact craters

The numbers of large ejecta blocks in excess of several meters in diameter ('blockiness') around the rims of small craters in southeastern Mare Serenitatis exceed those around similar craters in southern Mare Imbrium (and some other regions) at all but the final stages of crater degradation. Terrestrial explosion crater analogs, studies of impact processes, and a layered mare model suggest that the nature of the layering in the subsurface, including lavas, ejecta and buried regolith horizons, could account for the variable blockiness of crater ejecta and, possibly, for some variation in crater size-frequency distributions. Such effects would limit the reliability and utility of counting postmare craters for the purpose of estimating the relative ages of mare surfaces. Similarly, comparisons of the effects of progressive degradation on small impact craters to determine relative or absolute ages of individual craters may be limited by the influence of stratigraphy on ejecta fragment size distributions, which would in turn affect micrometeorite erosion rates and regolith production models.

Young, R. A.↗

NASA NTRS1976

Crater dimensions from Apollo data and supplemental sources

A catalog of crater dimensions that were compiled mostly from the Apollo-based Lunar Topographic Orthophotomaps is presented in its entirety. Values of crater diameter, depth, rim height, flank width, circularity, and floor diameter (where applicable) are tabulated for a sample of 484 craters on the moon and 22 craters on earth. Systematic techniques of mensuration are detailed. The lunar craters range in size from 400 m to 300 km across and include primary impact craters of the main sequence, secondary impact craters, craterlets atop domes and cones, and dark-halo craters. The terrestrial craters are between 10 m and 22.5 km in diameter and were formed by meteorite impact.

Pike, R. J.↗

NASA NTRS1976

Lunar crater arcs

An analysis has been made of the tendency of large lunar craters to lie along circles. A catalog of the craters at least 50 km in diameter was prepared first, noting position, diameter, rim sharpness and completion, nature of underlying surface, and geological age. The subset of those craters 50-400 km in diameter was then used as input to computer programs which identified each 'family' of four or more craters of selected geological age lying on a circular arc. For comparison, families were also identified for randomized crater models in which the crater spatial density was matched to that on the moon, either overall or separately for mare and highland areas. The observed frequency of lunar arcuate families was statistically highly significantly greater than for the randomized models, for craters classified as either late-pre-Imbrian (Nectarian), middle pre-Imbrian, or early pre-Imbrian, as well as for a number of larger age-classes. The lunar families tend to center in specific areas of the moon; these lie in highlands rather than maria and are different for families of Nectarian craters than for pre-Nectarian. The origin of the arcuate crater groupings is not understood.

Jaffe, L. D.↗

NASA NTRS1976

Large scale cratering of the lunar highlands - Some Monte Carlo model considerations

In an attempt to understand the scale and intensity of the moon's early, large scale meteoritic bombardment, a Monte Carlo computer model simulated the effects of all lunar craters greater than 800 m in diameter, for example, the number of times and depths specific fractions of the entire lunar surface were cratered. The model used observed crater size frequencies and crater-geometries compatible with the suggestions of Pike (1974) and Dence (1973); it simulated bombardment histories up to a factor of 10 more intense than those reflected by the present-day crater number density of the lunar highlands. For the present-day cratering record the model yields the following: approximately 25% of the entire lunar surface has not been cratered deeper than 100 m; 50% may have been cratered to 2-3 km depth; less than 5% of the surface has been cratered deeper than about 15 km. A typical highland site has suffered 1-2 impacts. Corresponding values for more intense bombardment histories are also presented, though it must remain uncertain what the absolute intensity of the moon's early meteorite bombardment was.

Hoerz, F.↗

NASA NTRS1977

Crater evolutionary tracks

The technique proposed in the present paper for studying planetary surface processes is based on the measurement of crater rim height, h, depth, d, and diameter, D. The h/d and d/D ratios provide a quantitative description of crater morphology as well as a quantitative method for assessing the relative importance of competing crater modification processes (since h, d, and D change as a crater is degraded by surface processes, and h/d and d/D change with time). Different classes of processes produce distinctive evolutionary tracks on an h/d versus d/D diagram. Tracks for three general classes of crater modification (processes adding material to the crater; processes redistributing the material within the crater vicinity; and processes removing the material from the crater vicinity) are calculated, and h/d and d/D ratios for craters on the earth, moon, and Mars are compared.

Schubert, G.↗

NASA NTRS1977

Rayed craters on the moon and Mercury

Systems of bright rays are exhibited by many fresh craters on the moon and Mercury. Diameter/density distributions suggest that lunar-rayed craters represent the Class 1 craters, and that Mercurian rayed craters represent post-Caloris craters. Photogeological analyses of lunar imagery indicate that the ray systems are composed of finely divided material from the primary crater along with locally derived ejecta from secondary and tertiary craters. The primary ray material probably occurs in moderately thick (0.1-1 meter) deposits. The rate of darkening may depend more on the thickness of the ray material than on the rates of various darkening processes. Darkening rate may also be a function of crater size. It is observed that rays of craters more than 1 b.y. old remain bright, whereas those older than Class 1 generally fade to imperceptibility.

Allen, C. C.↗

NASA NTRS1978

Floor-fractured craters on the moon and Mars

Impact craters on the moon provided primary pathways for volcanic eruptions through shock-induced fractures that immediately or subsequently tap magma reservoirs. Evidence for this process includes unbreached lava-filled craters and fractured crater floors with associated eruptives. Over 200 craters with floor fractures have been identified on the moon. They commonly occur along the margins of irregular maria, thereby suggesting a link to this important phase of lunar volcanism. Topographic data reveal small elevation differences between the summit of the remnant central peaks and crater rim, and in many cases suggest that floor fracturing resulted from a tabular intrusion localized beneath the crater with uplift of the crater floor. A comparison of lunar and Martian crater photographs shows that some Martian craters also exhibit similar highly localized endogenic modifications.

Schultz, P. H.↗

NASA NTRS1981

Viscous relaxation of impact craters on icy planetary surfaces - Determination of viscosity variation with depth

Spacecraft images show that the icy Galilean satellites have surfaces with very low topographic relief. Impact craters on Ganymede and Callisto are anomalously shallow and are characterized by sharp well-defined rims and domed floors. These morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth. Under these conditions, large craters relax more rapidly than small craters, therefore explaining a possible underabundance of large craters. Viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters. The results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies.

Parmentier, E. M.↗

NASA NTRS1984

Martian Crater Size Distributions and Terrain Age

The crater size/frequency distributions of large ( 8 km) craters on the Moon and terrestrial planets display two very different curves representing two crater populations. The heavily cratered regions of the Moon, Mercury, and Mars show the same highly structured curve which cannot be represented by a single slope distribution function. In contrast, the lunar post mare crater population has a size/frequency distribution which differs significantly from that in the highlands over the same diameter range, and can be represented by a single-slope distribution function of -2.8 differential. On areas of martian lightly cratered northern plains, the crater population is essentially identical to that of the post mare population. This indicates that the same two families of impacting objects were responsible for the cratering records on both Moon and Mars. The thickness of mantling material varies among the various plains units, and can be calculated from the depth/diameter scaling relations for martian craters.

Nadine G Barlow↗

NASA NTRS1983

A proposed origin for palimpsests and anomalous pit craters on Ganymede and Callisto

The hypothesis that palimpsests and anomalous pit craters are essentially pristine crater forms derived from high-velocity impacts and/or impacts into an ice crust with preimpact temperatures near melting is explored. The observational data are briefly reviewed, and an impact model is proposed for the direct formation of a palimpsest from an impact when the modification flow which produces the final crater is dominated by 'wet' fluid flow, as opposed to the 'dry' granular flow which produces normal craters. Conditions of 'wet' modification occur when the volume of impact melt remaining in the transient crater attains a volume comparable to the transient crater. The normal crater-palimpsest transition is found to occur for sufficiently large impacts or sufficiently fast impactors. The range of crater diameters and morphological characteristics inferred from the impact model is consistent with the observed characteristics of palimpsests and anomalous pit craters.

Croft, S. K.↗

NASA NTRS1989

Radar scattering mechanisms within the meteor crater ejecta blanket: Geologic implications and relevance to Venus

Simple impact craters are known to occur on all of the terrestrial planets and the morphologic expression of their ejecta blankets is a reliable indicator of their relative ages on the Moon, Mars, Mercury, and most recently for Venus. It will be crucial for the interpretation of the geology of Venus to develop a reliable means of distinguishing smaller impact landforms from volcanic collapse and explosion craters, and further to use the observed SAR characteristics of crater ejecta blankets (CEB) as a means of relative age estimation. With these concepts in mind, a study was initiated of the quantitative SAR textural characteristics of the ejecta blanket preserved at Meteor Crater, Arizona, the well studied 1.2 km diameter simple crater that formed approx. 49,000 years ago from the impact of an octahedrite bolide. While Meteor Crater was formed as the result of an impact into wind and water lain sediments and has undergone recognizable water and wind related erosion, it nonetheless represents the only well studied simple impact crater on Earth with a reasonably preserved CEB. Whether the scattering behavior of the CEB can provide an independent perspective on its preservation state and style of erosion is explored. Finally, airborne laser altimeter profiles of the microtopography of the Meteor Crater CEB were used to further quantify the subradar pizel scale topographic slopes and RMS height variations for comparisons with the scattering mechanisms computed from SAR polarimetry. A preliminary assessment was summarized of the L-band radar scattering mechanisms within the Meteor Crater CEB as derived from a NASA/JPL DC-8 SAR Polarimetry dataset acquired in 1988, and the dominant scattering behavior was compared with microtopographic data (laser altimeter profiles and 1:10,000 scale topographic maps).

Garvin, J. B.↗

NASA NTRS1990

Martian impact craters: Continuing analysis of lobate ejecta sinuosity

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.

Barlow, Nadine G.↗

NASA NTRS1991

Degradation studies of Martian impact craters

The amount of obliteration suffered by Martian impact craters is quantified by comparing measurable attributes of the current crater shape to those values expected for a fresh crater of identical size. Crater diameters are measured from profiles obtained using photoclinometry across the structure. The relationship between the diameter of a fresh crater and a crater depth, floor width, rim height, central peak height, etc. was determined by empirical studies performed on fresh Martian impact craters. We utilized the changes in crater depth and rim height to judge the degree of obliteration suffered by Martian impact craters.

Barlow, N. G.↗

NASA NTRS1992

Melt production in large-scale impact events: Implications and observations at terrestrial craters

The volume of impact melt relative to the volume of the transient cavity increases with the size of the impact event. Here, we use the impact of chondrite into granite at 15, 25, and 50 km s(sup -1) to model impact-melt volumes at terrestrial craters in crystalline targets and explore the implications for terrestrial craters. Figures are presented that illustrate the relationships between melt volume and final crater diameter D(sub R) for observed terrestrial craters in crystalline targets; also included are model curves for the three different impact velocities. One implication of the increase in melt volumes with increasing crater size is that the depth of melting will also increase. This requires that shock effects occurring at the base of the cavity in simple craters and in the uplifted peaks of central structures at complex craters record progressively higher pressures with increasing crater size, up to a maximum of partial melting (approx. 45 GPa). Higher pressures cannot be recorded in the parautochthonous rocks of the cavity floor as they will be represented by impact melt, which will not remain in place. We have estimated maximum recorded pressures from a review of the literature, using such observations as planar features in quartz and feldspar, diaplectic glasses of feldspar and quartz, and partial fusion and vesiculation, as calibrated with estimates of the pressures required for their formation. Erosion complicates the picture by removing the surficial (most highly shocked) rocks in uplifted structures, thereby reducing the maximum shock pressures observed. In addition, the range of pressures that can be recorded is limited. Nevertheless, the data define a trend to higher recorded pressures with crater diameter, which is consistent with the implications of the model. A second implication is that, as the limit of melting intersects the base of the cavity, central topographic peaks will be modified in appearance and ultimately will not occur. That is, the peak will first develop a central depression, due to the flow of low-strength melted materials, when the melt volume begins to intersect the transient-cavity base.

Grieve, Richard A. F.↗

NASA NTRS1992

Effect of impact angle on central-peak/peak-ring formation and crater collapse on Venus

Although asymmetry in ejecta patterns and craters shape-in-plan are commonly cited as diagnostic features of impact angle, the early-time transfer of energy from impactor to target also creates distinctive asymmetries in crater profile with the greatest depth uprange. In order to simulate gravity-controlled crater-growth, laboratory experiments use loose particulate targets as analogs for low-strength material properties following passage of the shock. As a result, impact crater diameter D in laboratory experiments generally is many times greater than the impactor diameter 2r (factor of 40), and early-time asymmetries in energy transfer from oblique impacts are consumed by subsequent symmetrical crater growth, except at the lowest angles (less than 25 deg). Such asymmetry is evident for oblique (less than 60 deg from horizontal) impacts into aluminum where D/2r is only 2 to 4. Because cratering efficiency decreases with increasing crater size and decreasing impact angle, large scale planetary craters (4080 km) should have transient excavation diameters only 6-10 times larger than the impactor. At basin scales, D/2r is predicted to be only 3-5, i.e., approaching values for impacts into aluminum in laboratory experiments. As a result, evidence for early-time asymmetry in impactor energy transfer should become evident on planetary surfaces, yet craters generally retain a circular outline for all but the lowest impact angles.

Schultz, Peter H.↗