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NASA NTRS2017

Large Impact Features on Icy Galilean Satellites

Impact crater morphology can be a very useful tool for probing planetary interiors, but nowhere in the solar system is a greater variety of crater morphologies observed (Fig. 1) than on the large icy Galilean satellites Ganymede and Callisto [e.g., 1- 3]. As on the rocky terrestrial planets, impact crater morphology becomes more complex with increasing size on these satellites. With increasing size, however, these same craters become less like their counterparts on the rocky planets. Several impact landforms and structures (multiring furrows, palimpsests, and central domes, for example), have no obvious analogs on any other planets. Further, several studies [e.g., 4-6] have drawn attention to impact landforms on Europa which are unusual, even by Galilean satellite standards. These radical differences in morphology suggest that impact into icy lithospheres that are mechanically distinct from silicate lithospheres may be responsible. As such, large impact structures may be important probes of the interiors of these bodies over time [e.g., 7]. The first goal of this work is to integrate and correlate the detailed morphologic and morphometric measurements and observations of craters on icy Galilean satellites [e.g., 4, 8-12] with new detailed mapping of these structures from Galileo high-resolution images. As a result, we put forward a revised crater taxonomy for Ganymede and Callisto in order to simplify the nonuniform impact crater nomenclature cluttering the literature. We develop and present an integrated model for the development of these unusual crater morphologies and their implications for the thermal evolution of these bodies.

crater morphologies↗

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 NTRS1993

Damage areas on selected LDEF aluminum surfaces

With the U.S. about to embark on a new space age, the effects of the space environment on a spacecraft during its mission lifetime become more relevant. Included among these potential effects are degradation and erosion due to micrometeoroid and debris impacts, atomic oxygen and ultraviolet light exposure as well as material alteration from thermal cycling, and electron and proton exposure. This paper focuses on the effects caused by micrometeoroid and debris impacts on several LDEF aluminum plates from four different bay locations: C-12, C-10, C-01, and E-09. Each plate was coated with either a white, black, or gray thermal paint. Since the plates were located at different orientations on the satellite, their responses to the hypervelocity impacts varied. Crater morphologies range from a series of craters, spall zones, domes, spaces, and rings to simple craters with little or no spall zones. In addition, each of these crater morphologies is associated with varying damage areas, which appear to be related to their respective bay locations and thus exposure angles. More than 5% of the exposed surface area examined was damaged by impact cratering and its coincident effects (i.e., spallation, delamination and blow-off). Thus, results from this analysis may be significant for mission and spacecraft planners and designers.

Coombs, Cassandra R.↗

NASA NTRS1989

Potential for observing and discriminating impact craters and comparable volcanic landforms on Magellan radar images

Observations of small terrestrial craters by Seasat synthetic aperture radar (SAR) at high resolution (approx. 25 m) and of comparatively large Venusian craters by Venera 15/16 images at low resolution (1000 to 2000 m) and shorter wavelength show similarities in the radar responses to crater morphology. At low incidence angles, the responses are dominated by large scale slope effects on the order of meters; consequently it is difficult to locate the precise position of crater rims on the images. Abrupt contrasts in radar response to changing slope (hence incidence angle) across a crater produce sharp tonal boundaries normal to the illumination. Crater morphology that is radially symmetrical appears on images to have bilateral symmetry parallel to the illumination vector. Craters are compressed in the distal sector and drawn out in the proximal sector. At higher incidence angles obtained with the viewing geometry of SIR-A, crater morphology appears less compressed on the images. At any radar incidence angle, the distortion of a crater outline is minimal across the medial sector, in a direction normal to the illumination. Radar bright halos surround some craters imaged by SIR-A and Venera 15 and 16. The brightness probably denotes the radar response to small scale surface roughness of the surrounding ejecta blankets. Similarities in the radar responses of small terrestrial impact craters and volcanic craters of comparable dimensions emphasize the difficulties in discriminating an impact origin from a volcanic origin in the images. Similar difficulties will probably apply in discriminating the origin of small Venusian craters, if they exist. Because of orbital considerations, the nominal incidence angel of Magellan radar at the center of the imaging swath will vary from about 45 deg at 10 deg N latitude to about 16 deg at the north pole and at 70 deg S latitude. Impact craters and comparable volcanic landforms will show bilateral symmetry parallel to the illumination vector and will appear increasingly compressed toward higher latitudes.

Ford, J. P.↗

NASA NTRS1975

Size-morphology relations of lunar craters - Discussion

Morphologic as well as morphometric changes occur in the character of fresh lunar craters at a diameter range of about 10-20 km. The continuous size/morphology sequence advocated by Smith and Sanchez (1973) is sustained by their data only for craters between 10-20 km and about 100 km in diameter. The cumulative-morphology index, and the frequency-of-occurrence of wall terraces, central peaks, flat floors, and 'swirl texture' (Smith and Sanchez, 1973) support the existence of a morphologic transition at a crater diameter of 10-20 km, rather than a continuous gradation in shape within craters between 4 and 150 km in diameter.

Pike, R. J.↗

NASA NTRS2004

Lunar and Planetary Science XXXV: Impacts on Mars and Earth

The session "Impacts on Mars and Earth" included the following reports:A Timescale for Major Events in Early Mars Crustal Evolution; Impact Craters in Arabia Terra, Mars; Ancient Giant Basin/Aquifer System in the Arabia Region, Mars; Small Crater Morphology Within Gusev Crater and Isidis Planitia: Evidence for Widespread Secondaries on Mars; Martian Surface Properties: Inferences from Resolved Differences in Crater Geometries; Asymmetric Ramparts of Secondary Craters in Cerberus, Mars: Interpretation and Implications for Local Stratigraphy; Depth to Diameter Relationships of Craters in the High Latitudes (70 -80 ) of Mars: Implications for Geologic History of Those Areas; Impact Cratering on Mars: Search for Target Influence on Morphology; Starting Conditions for Hydrothermal Systems Underneath Martian Craters: Hydrocode Modeling; Impact-induced Hydrothermal System at the Sudbury Crater: Duration, Temperatures, Mechanics, and Biological Implications; Major and Trace Element Variations in Impact Crater Clay from Chicxulub, Lonar, and Mistastin, Implications for the Martian Soil; Stable Isotopes and Hydrothermal Fluid Source in the Yaxcopoil-1 Borehole, Chicxulub Impact Structure, Mexico; and Chicxulub Impact Melts: Geochemical Signatures of Target Lithology Mixing and Post-Impact Hydrothermal Fluid Processes.

Source record↗

NASA NTRS1987

Ice or liquid water in the Martian regolith. Morphologic indicators from rampart craters

The morphology of ejecta blankets around certain Martian craters carries implications for volatiles in either the Martian crust or the atmosphere or both. The evidence that rampart crater ejecta can be used to infer the physical condition of volatiles in target rocks at the time of impact is reviewed. It was concluded that ice, and not liquid water, was the main volatile state, although rare examples also suggest the presence of liquid water at the time the craters were formed.

Mouginismark, P. J.↗

NASA NTRS2003

Numerical Simulations of Silverpit Crater Collapse

The Silverpit crater is a recently discovered, 60-65 Myr old complex crater, which lies buried beneath the North Sea, about 150 km east of Britain. High-resolution images of Silverpit's subsurface structure, provided by three-dimensional seismic reflection data, reveal an inner-crater morphology similar to that expected for a 5-8 km diameter terrestrial crater. The crater walls show evidence of terrace-style slumping and there is a distinct central uplift, which may have produced a central peak in the pristine crater morphology. However, Silverpit is not a typical 5-km diameter terrestrial crater, because it exhibits multiple, concentric rings outside the main cavity. External concentric rings are normally associated with much larger impact structures, for example Chicxulub on Earth, or Orientale on the Moon. Furthermore, external rings associated with large impacts on the terrestrial planets and moons are widely-spaced, predominantly inwardly-facing, asymmetric scarps. However, the seismic data show that the external rings at Silverpit represent closely-spaced, concentric faultbound graben, with both inwardly and outwardly facing fault-scarps. This type of multi-ring structure directly analogous to the Valhalla-type multi-ring basins found on the icy satellites. Thus, the presence and style of the multiple rings at Silverpit is surprising given both the size of the crater and its planetary setting. A further curiosity of the Silverpit structure is that the external concentric rings appear to be extensional features on the West side of the crater and compressional features on the East side. The crater also lies in a local depression, thought to be created by postimpact movement of a salt layer buried beneath the crater.

Collins, G. S.↗

NASA NTRS2003

Numerical Simulations of Silverpit Crater Collapse

The Silverpit crater is a recently discovered, 60-65 Myr old complex crater, which lies buried beneath the North Sea, about 150 km east of Britain. High-resolution images of Silverpit's subsurface structure, provided by three-dimensional seismic reflection data, reveal an inner-crater morphology similar to that expected for a 5-8 km diameter terrestrial crater. The crater walls show evidence of terracestyle slumping and there is a distinct central uplift, which may have produced a central peak in the pristine crater morphology. However, Silverpit is not a typical 5-km diameter terrestrial crater, because it exhibits multiple, concentric rings outside the main cavity. External concentric rings are normally associated with much larger impact structures, for example Chicxulub on Earth, or Orientale on the Moon. Furthermore, external rings associated with large impacts on the terrestrial planets and moons are widely-spaced, predominantly inwardly-facing, asymmetric scarps. However, the seismic data show that the external rings at Silverpit represent closely-spaced, concentric fault-bound graben, with both inwardly and outwardly facing faults-carps. This type of multi-ring structure is directly analogous to the Valhalla-type multi-ring basins found on the icy satellites. Thus, the presence and style of the multiple rings at Silverpit is surprising given both the size of the crater and its planetary setting.

Collins, G. S.↗

NASA NTRS1976

Comparison of the crater morphology-size relationship for Mars, moon, and Mercury

Morphology-size data are examined for Martian, Mercurian, and lunar craters. It is shown that both Martian and Mercurian craters exhibit central peaks at lower diameters than those on the moon and that gravity has affected the diameter at which central peaks form on Mars. A quantitative analysis of Martian crater statistics demonstrates that a complete range of crater shapes from small bowl craters to larger and more complex terraced craters occurs on Mars. The complexity of the shapes is found to increase as the sizes of craters increase. It is concluded that aeolian erosion on Mars does not completely mask differences in crater interior structure caused by differences in the gravitational forces on Mars and the moon.

Smith, E. I.↗

NASA NTRS1974

Cratering on Mars. I - Cratering and obliteration history. II Implications for future cratering studies from Mariner 4 reanalysis

It is pointed out that Mars is especially well adapted to statistical studies of crater morphologies for deciphering its geological history. A framework for understanding planetary geomorphological histories from the diameter-frequency relations of different morphological classes of craters described by Chapmam et al. (1970) is extended in order to understand Martian cratering, erosional, and depositional history. The cratering-obliteration history derived is compared with global interpretations considered by Hartman (1973) and Soderblom et al. (1974). An idealized dust-filling model is employed.

Chapman, C. R.↗

NASA NTRS1985

Clustered impacts - Experiments and implications

The characteristics of impact by clusters of projectiles are experimentally studied by launching grouped projectiles of aluminum shot, steel shot, iron filings, and sand. Cratering efficiency is considered as a function of a dimensionless parameter related to projectile size and impact velocity. The effects of different target and projectile densities on cratering efficiency are examined. Crater morphology is addressed by considering a typical example, reviewing the systematics between cluster dispersion and crater morphology for vertical impacts, and examining oblique angle impacts which have relevance for planetary secondary cratering processes. These results are compared with impacts by single bodies with different strengths. The evolution of the ejecta plume for clustered impacts is compared to that for single-body impacts for vertical and oblique impacts from 1.3 to 1.8 km/s. The experimental results are discussed in the context of planetary surface processes, emphasizing processes in an atmosphere-free environment and secondary impact cratering.

Schultz, P. H.↗

NASA NTRS1990

Incidence angle and resolution - Potential effects on interpreting Venusian impact craters in Magellan radar images

The effects of incidence angle and spatial resolution are analyzed in order to evaluate the types of information that Magellan imaging radar may provide about impact craters on Venus. Seasat synthetic-aperture radar images of small terrestrial craters at high resolution (about 25 m) and Soviet Venera images of comparatively large Venusian craters at low resolution (1000 to 2000 m) and shorter wavelength show comparable radar responses to crater morphology. At low incidence angles, it is difficult to locate the precise position of crater rims on images, while at higher incidence angles, images show less distortion of crater morphology. Magellan will obtain images through a range of incidence angles varying from about 45 deg at low latitudes to 16 deg at polar latitudes. Radially symmetrical landforms such as impact craters are expected to show outlines that vary with the incidence angle and to appear bilaterally symmetrical and increasingly compressed at higher latitude.

Ford, J. P.↗

NASA NTRS1980

Lunar and Planetary Science Conference, 11th, Houston, TX, March 17-21, 1980, Proceedings. Volume 3 - Physical processes

Geophysical investigations are discussed, taking into account laboratory measurements, planetary measurements, and structural implications and models. Impact processes are also examined. Experimental studies are considered along with aspects of crater morphology and frequency, and models theory. Volcanic-tectonic processes are investigated and topics related to the study of planetary atmospheres are examined. Attention is given to shallow moonquakes, the focal mechanism of deep moonquakes, lunar polar wandering, the search for an intrinsic magnetic field of Venus, the early global melting of the terrestrial planets, the first few hundred years of evolution of a moon of fission origin, the control of crater morphology by gravity and target type, crater peaks in Mercurian craters, lunar cold traps and their influence on argon-40, and solar wind sputtering effects in the atmospheres of Mars and Venus.

Merrill, R. B.↗

NASA NTRS1993

Collisional and dynamical history of Gaspra

Interpretation of the impact record on Gaspra requires understanding of the effects of collisions on a target body of Gaspra's size and shape, recognition of impact features that may have different morphologies from craters on larger planets, and models of the geological processes that erase and modify impact features. Crater counts on the 140 sq km of Gaspra imaged at highest resolution by the Galileo spacecraft show a steep size-frequency distribution (cumulative power-law index near -3.5) from the smallest resolvable size (150 m diameter) up through the large feature (1.5 km diameter crater) of familiar crater-like morphology. In addition, there appear to be as many as eight roughly circular concavities with diameters greater than 3 km visible on the asteroid. If we restrict our crater counts to features with traditionally recognized crater morphologies, these concavities would not be included. However, if we define craters to include any concave structures that may represent local or regional damage at an impact size, then the larger features on Gaspra are candidates for consideration. Acceptance of the multi-km features as craters has been cautious for several reasons. First, scaling laws (the physically plausible algorithms for extrapolating from experimental data) indicate that Gaspra could not have sustained such large-crater-forming impacts without being disrupted; second, aside from concavity, the larger structures have no other features (e.g. rims) that can be identified with known impact craters; and third, extrapolation of the power-law size distribution for smaller craters predicts no craters larger than 3 km over the entire surface. On the other hand, recent hydrocode modeling of impacts shows that for given impact (albeit into a sphere), the crater size is much larger than given by scaling laws. Gaspra-size bodies can sustain formation of up to 8-km craters without disruption. Besides allowing larger impact craters, this result doubles the lifetime since the last catastrophic fragmentation event up to one billion years. Events that create multi-km craters also globally damage the material structure, such that regolith is produced, whether or not Gaspra 'initially' had a regolith, contrary to other models in which initial regolith is required in order to allow current regolith. Because the globally destructive shock wave precedes basin formation, crater size is closer to the large size extrapolated from gravity-scaling rather than the strength-scaling that had earlier been assumed for such small bodies. This mechanism may also help explain the existence of Stickney on Phobos. Moreover, rejection of the large concavities as craters based on unfamiliar morphology would be premature, because (aside from Stickney) we have no other data on such large impact structures on such a small, irregular body. The eight candidate concavities cover an area greater than that counted for smaller craters, because they are most apparent where small craters cannot be seen: on low resolution images and at the limb on high resolution images. We estimate that there are at least two with diameter greater than 4 km per 140 sq km, which would have to be accounted for in any model that claims these are impact craters.

Greenberg, R.↗

DOE OSTI2020

The role of ambient gas confinement, plasma chemistry, and focusing conditions on emission features of femtosecond laser-produced plasmas

Ultrafast laser ablation coupled with optical emission spectroscopy is currently under development for standoff detection of elements and their isotopes for a wide range of application areas. In this work, we explore the role of ambient air pressure during the interaction of an ultrafast laser beam with two metallic targets (brass and uranium) with significantly different oxygen gas-phase reactivity. Plasma plumes were generated by focusing ~800 nm, ~35 fs pulses from an ultrafast laser system using a long focal length lens with f = 1 m. Two-dimensional spectral imaging was performed over the pressure range of 30 mTorr-700 Torr air to evaluate emission dynamics, plasma chemistry, signal-to-background ratio, and characteristic parameters (i.e. excitation temperature, electron density). An increase in ambient air pressure during ultrafast laser ablation leads to plume confinement and subsequent changes in the emission dynamics due to plasma chemistry as well as changes in plasma generation conditions (i.e. focused fs laser ablation versus filament ablation). Ablation crater morphologies were also investigated via scanning electron microscopy. Results indicate atomic emission intensity and signal-to-background ratios peak at moderate pressure levels (~50-100 Torr air) for both targets studied, although plasma chemistry influences uranium emission signatures. The emission features of the uranium plasma at pressures = 10 Torr showed the presence of atoms and oxide molecules which are co-located within the plume. We also find filament ablation leads to wider, more shallow craters compared to focused laser ablation. Our study provides unique insight into the interplay between plume dynamics, confinement, and oxygen chemistry of fs laser-produced plasmas and how these phenomena evolve with changing ambient air pressure.

Laser-produced plasma↗

NASA NTRS1981

Planetary geological studies

A global data base was assembled for the study of Mars crater ejecta morphology. The craters were classified as to morhology using individual photographic prints of Viking orbiter frames. Positional and scale information were derived by fitting digitized mosaic coordinates to lattitude-longitude coordinates of surface features from the Mars geodetic control net and feature coordinates from the U.S.G.S. series of 1:5,00,000 scale shaded relief maps. Crater morphology characteristics recorded are of two classes - attributes of each ejecta deposit and other crater charactersitics. Preliminary efforts to check the data base with findings of other workers are described.

Blasius, K. R.↗

NASA NTRSDate not supplied

Impacts into a Strength‐Layered Target: The Time Evolution of the Excavation‐Stage Flow

Impact cratering has dominated the evolution and modification of planetary surfaces throughout the history of the solar system. Impact craters can serve as probes to understanding the details of a planetary subsurface; for example, Oberbeck and Quaide suggested that crater morphology can be used to estimate the thickness of a regolith layer on top of a more competent unit. Lunar craters show a morphological progression from a simple bowl shape to flat‐floored and concentric craters as diameter increases for a given regolith thickness. The final shape of the impact crater is a result of the subsurface flow‐field initiated as the projectile transfers its energy and momentum to the target surface at the moment of impact. Therefore, when a regolith layer is present over a stronger substrate, such as is the case on the lunar mare, the substrate modifies the flow‐field resulting in the distinctive final crater morphology. Here we report on a series of experimental impacts into targets composed of a thin (2 cm) layer of loose sand on top of a stronger substrate. We use the Ejection‐Velocity Measurement System (EVMS, Fig.1) developed by [3] to examine the ejecta kinematics at specific times during the formation of these craters in strength‐layered targets.

J. L. B. Anderson↗