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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 NTRS2005

The Explorer's Guide to Impact Craters

Impact cratering is a fundamental geologic process of our solar system. It competes with other processes, such as plate tectonics, volcanism, fluvial, glacial and eolian activity, in shaping the surfaces of planetary bodies. In some cases, like the Moon and Mercury, impact craters are the dominant landform. On other planetary bodies impact craters are being continuously erased by the action of other geological processes, like volcanism on Io, erosion and plate tectonics on the Earth, tectonic and volcanic resurfacing on Venus, or ancient erosion periods on Mars. The study of crater populations is one of the principal tools for understanding the geologic history of a planetary surface. Among the general public, impact cratering has drawn wide attention through its portrayal in several Hollywood movies. Questions that are raised after watching these movies include: How do scientists learn about impact cratering? , and What information do impact craters provide in understanding the evolution of a planetary surface? Fundamental approaches used by scientists to learn about impact cratering include field work at known terrestrial craters, remote sensing studies of craters on various solid surfaces of solar system bodies, and theoretical and laboratory studies using the known physics of impact cratering.

Chuang, F.↗

NASA NTRS2008

The Chesapeake Bay Impact Crater: An Educational Investigation for Students into the Planetary Impact Process and its Environmental Consequences

Planetary impact craters are a common surface feature of many planetary bodies, including the Earth, the Moon, Mars, Mercury, Venus, and Jupiter s moons, Ganymede and Callisto. The NASA Langley Research Center in Hampton, VA, is located about 5 km inside the outer rim of the Chesapeake Bay Impact Crater. The Chesapeake Bay Impact Crater, with a diameter of 85 km is the sixth largest impact crater on our planet. The U.S. Geological Survey (USGS), in collaboration with the NASA Langley Research Center, the Virginia Department of Environmental Quality (VDEQ), the Hampton Roads Planning District Commission (HRPDC), and the Department of Geology of the College of William and Mary (WM) drilled into and through the crater at the NASA Langley Research Center and obtained a continuous core to a depth of 2075.9 ft (632.73 meters) from the Chesapeake Bay Impact Crater. At the NASA Langley location, the granite basement depth was at 2046 ft (623.87 meters). This collaborative drilling activity provided a unique educational opportunity and ongoing educational partnership between USGS, NASA Langley and the other collaborators. NASA Langley has a decade-long, ongoing educational partnership with the Colonial Coast Council of the Girl Scouts. The core drilling and on site analysis and cataloguing of the core segments provided a unique opportunity for the Girl Scouts to learn how geologists work in the field, their tools for scientific investigation and evaluation, how they perform geological analyses of the cores in an on-site tent and learn about the formation of impact craters and the impact of impacting bodies on the sub-surface, the surface, the oceans and atmosphere of the target body. This was accomplished with a two-part activity. Girl Scout day camps and local Girl Scout troops were invited to Langley Research Center Conference Center, where more than 300 Girl Scouts, their leaders and adult personnel were given briefings by scientists and educators from the USGS, NASA, VDEQ, HRPDC and WM on the principles of geology, the formation of impact craters, the consequences of the impacting body on the atmosphere, ocean, surface and sub-surface, the geological, chemical and biological analyses of the core and the cataloguing and storage of the core segments, etc. After the briefings, the Girl Scouts visited the drilling site where they inspected the core drilling rig, examined the core samples and discussed the drilling procedures, cores and interpretation of the cores with scientists and educators from the organizations conducting the core drilling. Demonstrations at the drilling site included demonstrations of impacting objects hitting multi-colored layered mud targets at different angles of entry. The multi-colored layers of mud were instructive in mapping out the distribution of impact-ejected material around the impact crater. The presentation will include a series of photographs of the Girl Scout participating in activities at the Chesapeake Bay Impact Crater drill site, including retrieving cores from the drilling rig, inspecting the core samples and participating in the impact-crater formation demonstrations.

Levine, Arlene S.↗

NASA NTRS2012

Characterization of the Morphometry of Impact Craters Hosting Polar Deposits in Mercury's North Polar Region

Earth-based radar images of Mercury show radar-bright material inside impact craters near the planet s poles. A previous study indicated that the polar-deposit-hosting craters (PDCs) at Mercury s north pole are shallower than craters that lack such deposits. We use data acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on Mercury. We measured the depth and diameter of 537 craters located poleward of 45 N, evaluated the slopes of the northern and southern walls of 30 PDCs, and assessed the floor roughness of 94 craters, including nine PDCs. We find that the PDCs appear to have a fresher crater morphology than the non-PDCs and that the radar-bright material has no detectable influence on crater depths, wall slopes, or floor roughness. The statistical similarity of crater depth-diameter relations for the PDC and non-PDC populations places an upper limit on the thickness of the radar-bright material (< 170 m for a crater 11 km in diameter) that can be refined by future detailed analysis. Results of the current study are consistent with the view that the radar-bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters.

Talpe Matthieu↗

NASA NTRS2014

Gale Crater and Impact Processes - Curiosity's First 364 Sols on Mars

Impact processes at all scales have been involved in the formation and subsequent evolution of Gale crater. Small impact craters in the vicinity of the Curiosity MSL landing site and rover traverse during the 364 Sols after landing have been studied both from orbit and the surface. Evidence for the effect of impacts on basement outcrops may include loose blocks of sandstone and conglomerate, and disrupted (fractured) sedimentary layers, which are not obviously displaced by erosion. Impact ejecta blankets are likely to be present, but in the absence of distinct glass or impact melt phases are difficult to distinguish from sedimentary/volcaniclastic breccia and conglomerate deposits. The occurrence of individual blocks with diverse petrological characteristics, including igneous textures, have been identified across the surface of Bradbury Rise, and some of these blocks may represent distal ejecta from larger craters in the vicinity of Gale. Distal ejecta may also occur in the form of impact spherules identified in the sediments and drift material. Possible examples of impactites in the form of shatter cones, shocked rocks, and ropy textured fragments of materials that may have been molten have been observed, but cannot be uniquely confirmed. Modification by aeolian processes of craters smaller than 40 m in diameter observed in this study, are indicated by erosion of crater rims, and infill of craters with aeolian and airfall dust deposits. Estimates for resurfacing suggest that craters less than 15 m in diameter may represent steady state between production and destruction. The smallest candidate impact crater observed is ∼0.6 m in diameter. The observed crater record and other data are consistent with a resurfacing rate of the order of 10 mm/Myr; considerably greater than the rate from impact cratering alone, but remarkably lower than terrestrial erosion rates.

Curiosity↗

NASA NTRS1972

Simultaneous impact and lunar craters

The existence of large terrestrial impact crater doublets and crater doublets that have been inferred to be impact craters on Mars suggests that simultaneous impact of two or more bodies can occur at nearly the same point on planetary surfaces. An experimental study of simultaneous impact of two projectiles near one another shows that doublet craters with ridges perpendicular to the bilateral axis of symmetry result when separation between impact points relative to individual crater diameter is large. When separation is progressively less, elliptical craters with central ridges and peaks, and circular craters with deep round bottoms are produced. These craters are similar in structure to many of the large lunar craters. Results suggest that the simultaneous impact of meteoroids near one another may be an important mechanism for the production of central peaks in large lunar craters.

Oberbeck, V. R.↗

NASA NTRS1972

Ranger and other impact craters photographed by Apollo 16, part J

The Apollo 16 crew photographed an unusual variety of impact craters, including the two craters produced by the impacts of Ranger 7 and 9 spacecraft, small craters produced by boulders as they bounced downslope, craters with marked bilateral symmetry, and primary craters with a wide range of morphologies and sizes. Ranger impact craters and examples of other craters are briefly discussed. This photography has supplied important new data on craters in the lunar surface. Continued study of these craters and use of the metrical capabilities of the metric and panoramic camera photography will yield additional scientific results on lunar craters.

Moore, H. J.↗

NASA NTRS1973

Simultaneous impact and lunar craters.

The existence of large terrestrial impact crater doublets and Martian crater doublets that have been inferred to be impact craters demonstrates that simultaneous impact of two or more bodies occurs at nearly the same point on planetary surfaces. An experimental study of simultaneous impact of two projectiles near one another shows that doublet craters with ridges perpendicular to the bilateral axis of symmetry result when separation between impact points relative to individual crater diameter is large. When separation is progressively less, elliptical craters with central ridges and central peaks, circular craters with flat floors containing ridges and peaks, and circular craters with deep round bottoms are produced. These craters are similar in structure to many of the large lunar craters.

Oberbeck, V. R.↗

NASA NTRS1977

Phobos - Surface density of impact craters

Revised crater counts for Phobos are presented which are based on uniform Mariner 9 imagery and Duxbury's (1974) map of the satellite. The contiguous portion of the satellite's surface on which all craters down to the limiting resolution of 0.2 to 0.3 km in diameter would be expected to be identified is delineated and found to contain 87 identifiable craters larger than 0.2 km in diameter. Analysis of the crater size distribution shows that the surface appears to be saturated for craters exceeding 1 km in diameter but the crater counts definitely fall below the saturation curve for smaller craters. Reasons for this fall-off are considered, and it is noted that too few craters are visible in Mariner 9 images of Deimos to permit meaningful crater counts on that satellite's surface. It is concluded that, contrary to a previous assertion, the surfaces of Phobos and Deimos are not known to be saturated with craters larger than 0.2 km in diameter.

Thomas, P.↗

NASA NTRS1978

A general cratering-history model and its implications for the lunar highlands

Through analysis of a large number of Monte Carlo and Markov Chain simulations, a model for determining crater accumulation and crater obliteration histories has been derived. The model generally applies to populations of large craters. It predicts that the following relationships hold for subequilibrium-density crater populations: (1) the more negative the production function's exponent, alpha (N near D super alpha) the lower the crater density at which the population size-frequency distribution will significantly depart from its production function; (2) the more negative the production function's exponent, the less obliteration a crater population will sustain after a set number of impacts. Application of the model to the lunar highlands implies (1) the production function for the large craters is highly structured, resembling the observed size-frequency distribution and not the function N near D to the -2; (2) even the densely cratered highlands have not attained crater saturation or equilibrium. Direct simulations of the highlands' crater population supports the model's implications.

Woronow, A.↗

NASA NTRS1975

Geomorphology of crater and basin deposits - Emplacement of the Fra Mauro formation

Characteristics of continuous deposits near lunar craters larger than about 1 km wide are considered, and it is concluded that (1) concentric dunes, radial ridges, and braided lineations result from deposition of the collision products of ejecta from adjacent pairs of similarly oriented secondary-crater chains and are, therefore, concentrations of secondary-crater ejecta; (2) intracrater ridges are produced within preexisting craters surrounding a fresh primary crater by ricocheting and focusing of secondary-crater ejecta from the preexisting craters' walls; and (3) secondary cratering has produced many of the structures of the continuous deposits of relatively small lunar craters and is the dominant process for emplacement of most of the radial facies of the continuous deposits of large lunar craters and basins. The percentages of Imbrium ejecta in deposits and the nature of Imbrium sculpturing are investigated.

Morrison, R. H.↗

NASA NTRS1980

Small impact craters in the lunar regolith - Their morphologies, relative ages, and rates of formation

The size frequency distributions, relative ages, and absolute age determination of lunar impact craters ranging in diameter from 1 to 100 m are discussed in relation to present-day and past impact rates. The size frequency distributions of craters produced by impacts on the lunar surface and of craters remaining after degradation with time and later impacts are examined, together with the steady state frequency curves for various crater morphologies. Relations between the relative ages of craters, their morphologies and their diameters are discussed, and the calibration of this kind of relationship with absolute crater ages by the location of individual craters with known absolute ages, diameters and frequency distribution statistics is considered. Estimates of present and past cratering rates based on crater size-frequency distributions are presented which suggest that the rate of impacting objects producing small lunar craters could have been constant over the last 100 million years.

Moore, H. J.↗

NASA NTRS1980

Formation of complex impact craters - Evidence from Mars and other planets

An analysis of the depth vs diameter data of Arthur (1980), is given along with geomorphic data for 73 Martian craters. The implications for the formation of complex impact craters on solid planets is discussed. The analysis integrates detailed morphological observations on planetary craters with geologic data from terrestrial meteorite and explosion craters. The simple to complex transition for impact craters on Mars appears at diameters in the range of 3 to 8 km. Five features appear sequentially with increasing crater size, flat floors, central peaks and shallower depths, scalloped rims, and terraced walls. This order suggests that a shallow depth of excavation and a rebound mechanism have produced the central peaks, not centripetal collapse and deep sliding. Simple craters are relatively uniform in shape from planet to planet, but complex craters vary considerably. Both the average onset diameter for complex impact craters on Mars and the average depth of complex craters vary inversely with gravitational acceleration on four planets.

Pike, R. J.↗

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 NTRS1985

A Parmetric Study of Crater-saturation Simulations

Computer models of cratered surfaces often use inputs of uncertain nature and importance. This work evaluates the sensitivity of the resulting crater-saturation estimates to the input parameters, principally applicable to the study of craters upward from 8km diameter. In order of decreasing importance, crater saturation simulations are found to be sensitive to: (1) the dynamic range of crater diameters used; (2) the effectiveness of ejecta-blanket obliteration assumed; and (3) the number of points taken to describe the crater rim. The size of the largest crater in proportion to the size of the simulated surface has no effect on the results when the edges of the simulated surface are correctly treated and craters are counted by the fraction of the crater within the simulated area. A similar procedure is recommended when gathering crater size-density data from images.

Woronow, A.↗

NASA NTRS1985

A Monte Carlo study of parameters affecting computer simulations of crater saturation density

Computer models of cratered surfaces often use inputs of uncertain nature and importance. This work evaluates the sensitivity of the resulting crater-saturation estimates to the input parameters, principally applicable to the study of craters upward from 8 km diameter. In order of decreasing importance, crater saturation simulations are found to be sensitive to: (1) the dynamic range of crater diameters used, (2) the effectiveness of ejecta-blanket obliteration assumed, and (3) the number of points taken to describe the crater rim. The size of the largest crater in proportion to the size of the simulated surface has no effect on the results when the edges of the simulated surface are correctly treated and craters are not counted simply by integers. Craters should be counted by their fractions lying within the simulated area. A similar procedure is recommended when gathering crater size-density data from images.

Woronow, A.↗

NASA NTRS1992

Geology and distribution of impact craters on Venus - What are they telling us?

Data on the distribution of impact craters on Venus are analyzed in relation to the geologic history of Venus and the effects of high ambient pressures and temperatures on the cratering process itself. The cratering record of Venus is discussed in terms of size, number, spatial distribution, and state of preservation of craters. The morphology of Venusian craters is examined as a function of the diameter, and features unique to these craters are discussed. Attention is also given to several craters that illustrate important features of Venusian crater; surface 'splotches' that are closely related in origin to impact craters; and implications of the distribution of severely modified craters for Venus' recent geologic history.

Schaber, G. G.↗

NASA NTRS1992

What can we learn about impact mechanics from large craters on Venus?

More than 50 unequivocal peak-ring craters and multiringed impact basins have been identified on Venus from Earth-based Arecibo, Venera 15/16, and Magellan radar images. These ringed craters are relatively pristine, and so serve as an important new dataset that will further understanding of the structural and rheological properties of the venusian surface and of impact mechanics in general. They are also the most direct analogues for craters formed on the Earth in Phanerozoic time. Finite-element simulations of basin collapse and ring formation were undertaken in collaboration with V. J. Hillgren (University of Arizona). These calculations used an axisymmetric version of the viscoelastic finite element code TECTON, modeled structures on the scale of Klenova or Meitner, and demonstrated two major points. First, viscous flow and ring formation are possible on the timescale of crater collapse for the sizes of multiringed basins seen on Venus and heat flows appropriate to the plant. Second, an elastic lithosphere overlying a Newtonian viscous asthenosphere results mainly in uplift beneath the crater. Inward asthenospheric flow mainly occurs at deeper levels. Lithospheric response is dominantly vertical and flexural. Tensional stress maxima occur and ring formation by normal faulting is predicted in some cases, but these predicted rings occur too far out to explain observed ring spacings on Venus (or on the Moon). Overall, these estimates and models suggest that multiringed basin formation is indeed possible at the scales observed on Venus. Furthermore, due to the strong inverse dependence of solid-state viscosity on stress, the absence of Cordilleran-style ring faulting in craters smaller than Meitner or Klenova makes sense. The apparent increase in viscosity of shock-fluidized rock with crater diameter, greater interior temperatures accessed by larger, deeper craters, and decreased non-Newtonian viscosity associated with larger craters may conspire to make the transition with diameter from peak-ring crater to Orientale-type multiringed basin rather abrupt.

Mckinnon, William B.↗