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At least 19 records
All sourcesNASA NTRSDOE OSTI
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 NTRS1985

Geomorphology and Geology of the Southwestern Margaritifer Sinus and Argyre Regions of Mars. Part 4: Flow Ejecta Crater Distribution

Flow ejecta craters - craters surrounded by lobate ejecta blankets - are found throughout the study area. The ratio of the crater's diameter to that of the flow ejecta in this region is approximately 40 to 45%. Flow ejecta craters are dominantly sharply defined craters, with slightly degraded craters being somewhat less common. This is probably indicative of the ejecta's relatively low resistence to weathering and susceptibility to burial. Flow ejecta craters here seem to occur within a narrow range of crater sizes - the smallest being about 4km in diameter and the largest being about 27km in diameter. Ejecta blankets of craters at 4km are easily seen and those of smaller craters are simply not seen even in images with better than average resolution for the region. This may be due to the depth of excavation of small impacting bodies being insufficient to reach volatile-rich material. Flow ejecta craters above 24km are rare, and those craters above 27km do not display flow ejecta blankets. This may be a result of an excavation depth so great that the volatile content of the ejecta is insufficient to form a fluid ejecta blanket. The geomorphic/geologic unit appears also to play an important role in the formation of flow ejecta craters. Given the typical size range for the occurrence of flow ejecta craters for most units, it can be seen that the percentage of flow ejecta craters to the total number of craters within this size range varies significantly from one unit to the next. The wide variance in flow ejecta crater density over this relatively small geographical area argues strongly for a lithologic control of their distribution.

Parker, T. J.↗
NASA NTRS2011

Small Rayed Crater Ejecta Retention Age Calculated from Current Crater Production Rates on Mars

Ejecta from impact craters, while extant, records erosive and depositional processes on their surfaces. Estimating ejecta retention age (Eret), the time span when ejecta remains recognizable around a crater, can be applied to estimate the timescale that surface processes operate on, thereby obtaining a history of geologic activity. However, the abundance of sub-kilometer diameter (D) craters identifiable in high resolution Mars imagery has led to questions of accuracy in absolute crater dating and hence ejecta retention ages (Eret). This research calculates the maximum Eret for small rayed impact craters (SRC) on Mars using estimates of the Martian impactor flux adjusted for meteorite ablation losses in the atmosphere. In addition, we utilize the diameter-distance relationship of secondary cratering to adjust crater counts in the vicinity of the large primary crater Zunil.

Rayed Craters↗
NASA NTRS1987

Effects of elevation and ridged plains thicknesses on Martian crater ejecta morphology

Relationships between the distribution of Martian crater ejecta morphologies and both the elevation and ridged plains thicknesses were investigated using individual Viking Orbiter images of 383 craters with uneroded ejecta termini located between +/- 25 deg latitude. Only a slight correlation was found between elevation and the distribution of ejecta types for the ridged plains. A stronger correlation was identified between the thicknesses of Lunae and Hesperia Plana and the complexity of ejecta morphologies. For a given crater size range, the complexity of the ejecta increases with decreasing thicknesses of the ridged plains. Possible explanations for the observed phenomena are discussed.

Horner, V. M.↗
NASA NTRS1987

Crater ejecta morphology and the presence of water on Mars

The possible effects of projectile, target, and environment on the cratering process is reviewed. It is suggested that contradictions in interpreting Martian crater ejecta morphologies reflect over simplifying the process as a singular consequence of buried water. It seem entirely possible that most ejecta facies could be produced without the presence of liquid water. However, the combination of extraordinary ejecta fluidity, absence of secondaries, and high ejection angles all would point to the combined effects of atmosphere and fluid rich substrates. Moreover, recent experiments revealing the broad scour zone associated with rapid vapor expansion may account for numerous craters in the circumpolar regions with subtle radial grooving extending 10 crater radii away with faint distal ramparts. Thus certain crater ejecta morphologies may yet provide fundamental clues for the presence of unbound water.

Schultz, P. H.↗
NASA NTRS1991

Martian impact crater ejecta morphologies and their potential as indicators of subsurface volatile distribution

Many martian impact craters ejecta morphologies suggestive of fluidization during ejecta emplacement. Impact into subsurface volatile reserviors (i.e., water, ice, CO2, etc.) is the mechanism favored by many scientists, although acceptance of this mechanism is not unanimous. In recent years, a number of studies were undertaken to better understand possible relationships between ejecta morphology and latitude, longitude, crater diameter, and terrain. These results suggest that subsurface volatiles do influence the formation of specific ejecta morphologies and may provide clues to the vertical and horizontal distribution of volatiles in more localized regions of Mars. The location of these volatile reservoirs will be important to humans exploring and settling Mars in the future. Qualitative descriptions of ejecta morphology and quantitative analyses of ejecta sinuosity and ejecta lobe areal extent from the basis of the studies. Ejecta morphology studies indicate that morphology is correlated with crater diameter and latitude, and, using depth-diameter relationships, these correlations strongly suggest that changes in morphology are related to transition among subsurface layers with varying amounts of volatiles. Ejecta sinuosity studies reveal correlations between degree of sinuosity (lobateness) and crater morphology, diameter, latitude, and terrain. Lobateness, together with variations in areal extent of the lobate ejecta blanket with morphology and latitude, probably depends most directly on the ejecta emplacement process. The physical parameters measured here can be compared with those predicted by existing ejecta emplacement models. Some of these parameters are best reproduced by models requiring incorporation of volatiles within the ejecta. However, inconsistencies between other parameters and the models indicate that more detailed modeling is necessary before the location of volatile reservoirs can be confidently predicted based on ejecta morphology studies alone.

Barlow, Nadine G.↗
NASA NTRS1987

Crater ejecta morphology and the presence of water on Mars

The purpose of this contribution is to review the possible effects of projectile, target, and environment on the cratering process. The discussion presented suggests that contradictions in interpreting Martian crater ejecta morphologies reflect oversimplifying the process as a singular consequence of buried water. It seem entirely possible that most ejecta facies could be produced without the presence of liquid water. However, the combination of extraordinary ejecta fluidity, absence of secondaries, and high ejection angles all would point to the combined effects of atmosphere and fluid rich substrates. Moreover, recent experiments revealing the broad scour zone associated with rapid vapor expansion may account for numerous craters in the circum-polar regions with subtle radial grooving extending 10 crater radii away with faint distal ramparts. Thus certain crater ejecta morphologies may yet provide fundamental clues for the presence of unbound water.

Schultz, Peter H.↗
NASA NTRS1973

Radial thickness variation in impact crater ejecta - Implications for lunar basin deposits

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.

Mcgetchin, T. R.↗
NASA NTRS1976

Latitudinal variation of wind erosion of crater ejecta deposits on Mars

The characteristics of wind erosion as the dominant process involved in eroding crater ejecta deposits on Mars are studied. Present-day crater formation in mid to high latitudes involves impact into some thickness of aeolian debris, while impact in the equatorial zone is more likely to involve target materials consisting of coarse-grained aeolian lag deposits or even bedrock. Latitudinal variation dominates differences in ejecta emplacement mechanisms and probably differences in patterns of wind erosion of ejecta and surrounding intercrater materials. Escarpments develop as the deposits are eroded back toward crater rims. Erosion only takes places at escarpment edges where surface roughness may be low enough to allow particle entrainment. Preferential preservation of ejecta emplaced in thick debris may occur. An empirical model developed for wind erosion of ejecta deposits in nonmantled areas suggests that removal of ejecta materials on the average is exceedingly slow. Results suggest high differential aeolian erosion rates that are a function of both grain sizes and large-scale surface roughness.

Arvidson, R. E.↗
NASA NTRS1983

Crater ejecta scaling laws - Fundamental forms based on dimensional analysis

Self-consistent scaling laws are developed for meteoroid impact crater ejecta. Attention is given to the ejection velocity of material as a function of the impact point, the volume of ejecta with a threshold velocity, and the thickness of ejecta deposit in terms of the distance from the impact. Use is made of recently developed equations for energy and momentum coupling in cratering events. Consideration is given to scaling of laboratory trials up to real-world events and formulations are developed for calculating the ejection velocities and ejecta blanket profiles in the gravity and strength regimes of crater formation. It is concluded that, in the gravity regime, the thickness of an ejecta blanket is the same in all directions if the thickness and range are expressed in terms of the crater radius. In the strength regime, however, the ejecta velocities are independent of crater size, thereby allowing for asymmetric ejecta blankets. Controlled experiments are recommended for the gravity/strength transition.

Housen, K. R.↗
NASA NTRS1981

Ejecta emplacement and modes of formation of Martian fluidized ejecta craters

From an analysis of 1173 craters possessing single (type 1) and double (type 2) concentric ejecta deposits, type 2 craters are found to occur most frequently in areas that have also been described as possessing periglacial features. The frequency of occurrence of central peaks and wall failure (terraces plus scallops) within the craters indicates that, by analogy with previous analyses, type 1 craters form in more fragmental targets than type 2 craters. The maximum range of the outer ejecta deposits of type 2 craters, however, consistently extends about 0.8 crater radii further than ejecta deposits of type 1 craters, suggesting a greater degree of ejecta fluidization for the twin-lobed type 2 craters. Numerous characteristics of Ries Crater, West Germany, show similarities to craters on Mars, indicating that Martian fluidized ejecta craters may be closer analogs to this terrestrial crater than are lunar craters.

Mouginis-Mark, P.↗
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 NTRS2014

Evidence for Crater Ejecta on Venus Tessera Terrain from Earth-Based Radar Images

We combine Earth-based radar maps of Venus from the 1988 and 2012 inferior conjunctions, which had similar viewing geometries. Processing of both datasets with better image focusing and co-registration techniques, and summing over multiple looks, yields maps with 1-2 km spatial resolution and improved signal to noise ratio, especially in the weaker same-sense circular (SC) polarization. The SC maps are unique to Earth-based observations, and offer a different view of surface properties from orbital mapping using same-sense linear (HH or VV) polarization. Highland or tessera terrains on Venus, which may retain a record of crustal differentiation and processes occurring prior to the loss of water, are of great interest for future spacecraft landings. The Earth-based radar images reveal multiple examples of tessera mantling by impact ''parabolas'' or ''haloes'', and can extend mapping of locally thick material from Magellan data by revealing thinner deposits over much larger areas. Of particular interest is an ejecta deposit from Stuart crater that we infer to mantle much of eastern Alpha Regio. Some radar-dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains. We suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites.

surface↗
NASA NTRS1998

Inferences on the Emplacement Dynamics of Martian Impact Crater Ejecta: Constraints from Mola Topography

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.

Garvin, J. B.↗
NASA NTRS1986

Phobos, Deimos, and the moon - Size and distribution of crater ejecta blocks

Ejecta block characteristics observed on Phobos, Deimos, and the moon are examined. The analyzed source craters on Deimos are 0.8-2.3 km in diameter, those on Phobos are 1.5-10 km, and the lunar craters are between 0.2-3.5 km in diameter. The size and radial distribution of the ejecta blocks for the three bodies are compared. It is observed that the size distribution of the ejecta blocks surrounding craters on the three objects are basically similar, and the radial distribution of the blocks for Phobos and the moon are the same (within 2 radii of the crater center); however, the ejecta on Deimos are more dispersed (greater than or equal to 2 radii from the crater center).

Lee, S. W.↗
NASA NTRS1983

Impact and explosion crater ejecta, fragment size, and velocity

A model was developed for the mass distribution of fragments that are ejected at a given velocity for impact and explosion craters. The model is semi-empirical in nature and is derived from (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter and an assumption on the functional form for the distribution of fragements ejected at a given velocity. This model implies that for planetary impacts into competent rock, the distribution of fragments ejected at a given velocity are nearly monodisperse, e.g., 20% of the mass of the ejecta at a given velocity contain fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. Using this model, the largest fragment that can be ejected from asteroids, the moon, Mars, and Earth is calculated as a function of crater diameter. In addition, the internal energy of ejecta versus ejecta velocity is found. The internal energy of fragments having velocities exceeding the escape velocity of the moon will exceed the energy required for incipient melting for solid silicates and thus, constrains the maximum ejected solid fragment size.

Okeefe, J. D.↗
NASA NTRS2015

Lunar Crater Ejecta: Physical Properties Revealed by Radar and Thermal Infrared Observations

We investigate the physical properties, and changes through time, of lunar impact ejecta using radar and thermal infrared data. We use data from two instruments on the Lunar Reconnaissance Orbiter (LRO) - the Diviner thermal radiometer and the Miniature Radio Frequency (Mini-RF) radar instrument - together with Earth-based radar observations. We use this multiwavelength intercomparison to constrain block sizes and to distinguish surface from buried rocks in proximal ejecta deposits. We find that radar-detectable rocks buried within the upper meter of regolith can remain undisturbed by surface processes such as micrometeorite bombardment for greater than 3 Gyr. We also investigate the thermophysical properties of radar-dark haloes, comprised of fine-grained, rock-poor ejecta distal to the blocky proximal ejecta. Using Diviner data, we confirm that the halo material is depleted in surface rocks, but show that it is otherwise thermophysically indistinct from background regolith. We also find that radar-dark haloes, like the blocky ejecta, remain visible in radar observations for craters with ages greater than 3 Ga, indicating that regolith overturn processes cannot replenish their block populations on that timescale.

Ghent, R. R.↗