Crater Density Research Articles

Constraints on the resurfacing history of Venus from the hypsometry and distribution of volcanism, tectonism, and impact craters

Improved measurements of the target elevations of 885 impact craters on Venus indicate that they are nearly random with respect to elevation. Although a slight deficit of craters at high elevations and an excess at low elevations is observed, the differences are marginally significant. Using a high-resolution digital map and database of all major volcanic, tectonic and impact features, we examine the distribution of impacts within volcanic and tectonic features, and the distribution of volcanism and tectonism with elevation. We show that the observed crater hypsometry results from resurfacing at higher elevations by volcanic and tectonic features superimposed on less active plains. The distribution of impacts in the map units has two distinct patterns: (1) the plains and shield fields (70%) have high crater densities and low proportions of tectonized or embayed craters; and (2) the remaining volcanic and tectonic features (30%) have low crater densities and high proportions of modified craters. The plains and shield fields appear to represent a much lower level of resurfacing activity. Simple area-balance calculations indicate that resurfacing at higher elevations by tectonic and volcanic features plausibly explains the observed crater hypsometry. However, the subtlety of the effects suggests that either (1) little resurfacing has occurred during the period of crater accumulation, or (2) resurfacing acts almost equally at all elevations. The apparent low activity of the plains and their abundance at lower elevations makes it unlikely that resurfacing is balanced with respect to elevation. It appears that the plains have been mostly quiescent since their emplacement, and that subsequent resurfacing occurs mostly in the highlands as a result of volcanism, corona formation, and rifting. We estimate that since the end of plains emplacement about 14% of Venus has been resurfaced by volcanism and about 6% by tectonic deformation.

Ejecta correlations with spatial crater density and Venus resurfacing history

We examine the alternative hypotheses that on Venus (i) the spatial randomness of the crater population reflects predominantly the spatially stochastic nature of impact crater formation or (ii) a significant amount of spatial variation in crater density is due to variations in the geological rate of crater removal. Three endmember models of resurfacing are formulated: Case I assumes that a single‐age production surface accounts for the spatial distribution of craters. Case II assumes that variations in spatial crater density reflect local production surface ages, while Case III assumes that individual craters are removed regionally, so that variations in spatial crater density reflect local retention ages. These endmember models are tested by examining correlations of crater properties with crater spatial density, ρ. We analyze the distribution of embayed and tectonized craters and the relative abundances of extended ejecta deposits (termed “halos”). The three endmembers provide specific predictions as to the occurrence of halos as a function of ρ. For ρ ≲ 1.5 craters per 106 km², Case III provides the best explanation for the deficiency of halos as well as the relative abundances of modified craters. For the interval 1.5 ≲ ρ ≲ 2.5, Case II is applicable but with only a modest spread in production ages. For densities ≳2.5, Case II is the only endmember model that matches the global observations, suggesting that regions with these high densities represent a series of distinct production surfaces with ages increasing from the planetary mean retention age.

Mean age of rifting and volcanism on Venus deduced from impact crater densities

UNLIKE the extensively cratered highlands of the Moon and Mars, the surface of Venus does not preserve a record of heavy bombard-ment from the early history of the Solar System1-3. Those craters that are found on Venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds, faults and lava flows1-3. Although the volcanic and tec-tonic history of Venus is still much debated2-5, there is mounting evidence for near-global resurfacing ~300-500 Myr ago1,2,6. Moreover, it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet, suggestive of significant activity after the resurfacing event7. It is not clear, however, whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet. To address this question, we have used the regional variations in crater density to date volcan-oes, rifts and coronae which, based on stratigraphic evidence, clearly post-date the main resurfacing event8-11. The calculated mean ages of 70-125 Myr exclude the possibility that the majority of these features represent the final stages of the global event.

Impact Crater Densities on Volcanoes and Coronae on Venus: Implications for Volcanic Resurfacing

The density of impact craters on large volcanoes on Venus is half the average crater density for the planet. The crater density on some classes of coronae is not significantly different from the global average density, but coronae with extensive associated volcanic deposits have lower crater densities. These results are inconsistent with both single-age and steady-state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on Venus over the last 500 million years.

First Images of Asteroid 243 Ida

The first images of the asteroid 243 Ida from Galileo show an irregular object measuring 56-kilometers by 24 kilometers by 21 kilometers. Its surface is rich in geologic features, including systems of grooves, blocks, chutes, albedo features, crater chains, and a full range of crater morphologies. The largest blocks may be distributed nonuniformly across the surface; lineaments and dark-floored craters also have preferential locations. Ida is interpreted to have a substantial regolith. The high crater density and size-frequency distribution (-3 differential power-law index) indicate a surface in equilibrium with saturated cratering. A minimum model crater age for Ida-and therefore for the Koronis family to which Ida belongs-is estimated at 1 billion years, older than expected.

Density and morphology of impact craters on Tessera Terrain, Venus

Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D<16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on‐tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.

An episodic hypothesis for Venusian tectonics

It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500 ± 200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near‐uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75–90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.

Keeping that youthful look

The record of impact cratering on Venus, recently revealed from high-resolution radar imaging by the Magellan spacecraft, has been interpreted as indicating that the planet underwent catastrophic global resurfacing about 500 million years ago. Although other interpretations of the crater characteristics have been suggested, the possibility of geologically rapid global resurfacing on the planet that most resembles our own in terms of mass, density, and bulk composition has generated widespread interest. The mechanisms that could cause such a catastrophe are discussed.

Geological correlations with the interior density structure of Venus

A model has been constructed by stipulating that Venusian long‐wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms. For convenience the latter are represented by a single Airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper. Mantle convection is represented at a single depth by mass perturbations that drive viscous flow. We invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near‐surface density anomalies. The mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings. The near‐surface density anomalies show no particular pattern. The inversion results have been compared with a global synthesis of Magellan data. Impact crater density does not correlate with the mantle convection pattern or the near‐surface density anomalies. Large volcanic structures are associated with upwellings. Planitiae are associated with downwellings. Coronae are anticorrelated with high‐amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion. Tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust. Tesserae show no definitive relationship to the present mantle convection pattern.

Martian parent craters for the SNC meteorites

The young ages (∼1.3 Ga) and the basaltic to ultramafic compositions of the shergottites, nakhlites, and chassignites meteorites severely restrict their potential source regions on Mars. We have used this age and compositional information, together with geologic data derived from Viking Orbiter images, to identify 25 candidate impact craters in the Tharsis region of Mars that could be the source crater for these meteorites. None of these craters are close to the size (∼100 km diameter) implied by the dynamical study of SNC ejection developed by Vickery and Melosh (1987). The craters in our study were selected because they are >10 km in diameter, have morphologies indicative of young craters, and satisfy both the petrologic criteria of the SNCs and the proposed 1.3 Ga crystallization ages. Of these 25 craters, only nine are found on geologic units believed to be young (crater density is less than 570 craters of greater than 1 km diameter per 106 km2). No crater exists to satisfy well the criteria of sampling both a 1.3 Ga surface (nakhlites and Chassigny) and a 180 Ma surface (shergottites) without at the same time imposing significant constraints on the chronology of Mars as inferred from the cumulative crater curves. The relatively young age (based on their inferred position in the stratigraphic column of Tharsis (Scott et al., 1981)) of the SNCs implies that volcanic activity on the plains of the Tharsis region extended well past 1.3 Ga.

Interpretation of the KRFM-infrared measurements of phobos

The multichannel radiometer KRFM on board the Phobos-2 spacecraft measured the thermal flux from Phobos' surface along two different ground tracks on March 25, 1989. The results for the first track are interpreted within a thermal model that takes into account the ellipsoidal shape and the surface roughness of Phobos. Information about the thermal properties and the grain size of the regolith is deduced by comparing measured with calculated thermal fluxes. It is found that a crater density of 0.9 and a heat conductivity 5 to 10 times higher than that of the Moon give the best agreement with the experimental data.

Ancient glaciation on Mars

A large number of anomalous landforms on Mars can be attributed to glaciation, including the action of ice and meltwater. Glacial landscapes are concentrated south of lat -33° and in the Northern Plains suggesting vast Austral and Boreal ice sheets. Crater densities on the glaciated terrains indicate that the final glacial epoch occurred late in Martian history. Thus, Mars may have had a relatively warm, moist climate and dense atmosphere much later than previously believed.

Pulsed laser treatment of plasma-sprayed thermal barrier coatings: effect of pulse duration and energy input

Pulsed laser treatments of plasma-sprayed thermal barrier coatings can provide good corrosion resistance of protected components without impairing thermal fatigue resistance of the ceramic layers. Laser treatments are performed over a wide range of pulse durations and energy inputs, and their effects on microstructure, crystalline grain size and chemical composition of the remelted thin upper layer are investigated. Particular attention is given to macro and microcracking originating on the surface, gas bubble motion inside the melted layer and consequent surface crater formation. Density, shape, dimension and distribution of craters in the laser-irradiated zone are correlated with pulse duration and energy input of the laser beam. A numerical simulation of temperature distributions and heat phenomena originating in the ceramic coating during laser irradiation is presented, in order to explain the influence of laser characteristics on the quality of the coating surface.

Geology and tectonics of Beta Regio, Guinevere Planitia, Sedna Planitia, and Western Eistla Regio, Venus: Results from Arecibo image data

New radar images (resolution 1.5–2.0 km) obtained from the Arecibo Observatory are used to assess the geology of a portion of the equatorial region of Venus (1‡ S to 45‡ N and from 270‡ eastward to 30‡). Nine geologic units are mapped on the basis of their radar characteristics and their distribution and correspondences with topography are examined. Plains are the most abundant unit, make up 80%; of the area imaged, and are divided into bright, dark, and mottled. Mottled plains contain abundant lava flows and domes suggesting that volcanism forming plains is a significant process in the equatorial region of Venus. Tesserae are found primarily on Beta Regio and its eastern flank and are interpreted to be locally stratigraphically older units, predating episodes of faulting and plains formation. Isolated regions of tesserae concentrated to the north of Western Eistla Regio are interpreted to predate the formation of plains in this area. The volcanoes Sif Mons, Gula Mons, Sappho, Theia Mons, and Rhea Mons, are found exclusively in highland regions and their deposits are interpreted as contributing only a small percentage to the overall volume of the regional topography. The northern 15‡ of the image data overlap with Venera 15/16 images making it possible to examine the characteristics of geologic units mapped under various illumination directions and incidence angles. Surface panoramas and geochemical data obtained from Venera landers provide ground truth for map units, evidence that plains are made up of basaltic lava flows, and that linear deformation zones contain abundant blocks and cobbles. On the basis of spatial and temporal relationships between geologic units, the highlands of Beta Regio and Western Eistla Regio are interpreted to have formed in association with areas of mantle upwelling which uplift plains, cause rifting, and in the case of Beta Regio, disrupt a large region of tessera. Zones of linear belt deformation in Beta Regio and Western Eistla Regio are interpreted to be extensional and indicate that at least limited extension has occurred in both regions. The images reveal for the first time that southern Devana Chasma is a region of overlapping rift valleys separated by a distance of 600 km. Linear deformation zones in Guinevere Planitia, separating Beta Regio and Eistla Regio, converge at a region of ovoids forming a discontinuous zone of disruption and completes an equatorial encompassing network of highlands and tectonic features. The similarity between ovoids and coronae suggests a mechanism of formation associated with hotspots or mantle plumes. Analysis of the distribution and density of impact craters suggests a surface age for this part of the planet similar to or slightly less than that determined for the northern high latitudes from Venera 15/16 data (0.3 to 1.5 by) and comparable to that calculated for the southern hemisphere.

Evolution of lunar topography by impact processes

Using a simple model of lunar topographic evolution resulting from the formation of impact craters, we try to constrain the total number of impactors striking the Moon after the solidification of the lunar crust. The numerical results indicate that the evolution of the Moon's hypsometric curve is strongly influenced by the total number of craters and is less affected by their size distribution. It is suggested that the total number of craters necessary to explain the standard deviation of the present hypsometric curve is roughly equivalent to the number density of craters on the oldest lunar crust, corresponding to about 10−4 lunar mass of impactors. This may be the total mass of impactors striking after solidification of the lunar crust, more specifically, after about 4.4 to 4.5 Ga ago when the viscous degradation of craters became ineffective.

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area, Venus: Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 ‡ S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/106 km2, it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).

Microstructures of TiN films grown by various physical vapour deposition techniques

A study has been made of TiN coatings deposited on steel substrates by five commercially available physical vapour deposition (PVD) methods; low voltage electron beam evaporation, triode high voltage electron beam evaporation, random-arc evaporation, steered-arc evaporation and magnetron sputtering. The microstructure and substrate-film interfacial microchemistry of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDX), cross-sectional transmission electron microscopy (XTEM) and scanning transmission electron microscopy (STEM) combined with EDX analyses of XTEM samples. The XRD analyses showed that all the films were in a state of compressive stress with interplanar distances as much as 1.7% higher than reference bulk values. SEM examination revealed only minor variations in surface roughness among the samples except for the arc-evaporated films which contained large droplets and craters resulting from the detachment of droplets. The number density and average sizes of droplets and craters were lower in the steered-arc sample than in the random-arc sample. XTEM analyses showed that all the films had columnar structures with clearly defined substrate-film interfacial layers. The films appeared dense except for the magnetron-sputtered sample which exhibited intercolumnar porosity. STEM-EDX analyses showed large variations in the microchemistry of the substrate-film interfacial regions which consisted, depending on the sample, of renucleated near-surface substrate grains, intentionally (or, in at least one case, unintentionally) introduced foreign material or gas-bubble-like inclusions. However, the microchemistry of these interfacial regions was, in most cases, understandable on the basis of the substrate pretreatment and/or choice of film growth parameters.

Venus Southern Hemisphere: Geologic Character and Age of Terrains in the Themis-Alpha-Lada Region

Arecibo high-resolution radar images of the southern hemisphere of Venus extending to 78 degrees S show that the surface of the Themis-Alpha-Lada region is characterized by linear deformation zones with volcanoes and corona-like features and by regional volcanic deposits (primarily plains, small shields, and large edifices). Large-scale areal deformation is limited to the tessera of Alpha Regio. Lada Terra, in the southern high latitudes, contains several large coronae, in contrast to Ishtar Terra in the northern high latitudes. The density of craters of possible impact origin is somewhat lower than that observed in the Venera 15 and 16 coverage; these data extend to 43 percent of the areas of the surface of Venus with ages of less than about 1 billion years.

Theoretical interpretation of infrared measurements at Deimos in the framework of crater radiation

Ground-based infrared photometric data of Deimos at 4.8, 10, and 20 μm are not consistent with the predictions by the standard thermal model (STM). Contrary to the STM which is a smooth surface model with an artificial beaming factor, this paper attempts to interpret the observations in the framework of a cratered surface model. Heat conduction and the ellipsoidal shape of Deimos are taken into account. The best fit to the observed fluxes is achieved at a relative crater density of 0.9 for nearly hemispherical craters (millimeter to centimeter scale) and a lunar-like heat conduction coefficient of the regolith. The resulting flux ratios observed flux/calculated flux are 0.86, 1.16, and 1.05 at 4.8, 10, and 20 μm, respectively.

The Impact Cratering Record on Triton

Impact craters on Triton are scarce owing to the relatively recent resurfacing by icy melts. The most heavily cratered surface has a crater density about the same as the lunar maria. The transition diameter from simple to complex craters occurs at a diameter of about 11 kilometers, and the depth-diameter relationship is similar to that of other icy satellites when gravity is taken into account. The crater size-frequency distribution has a differential -3 slope (cumulative -2 slope) and is the same as that for the fresh crater population on Miranda. The most heavily cratered region is on the leading hemisphere in Triton's orbit. Triton may have a leading-trailing asymmetry in its crater population. Based primarily on the similarity of size distributions on Triton and Miranda and the relatively young surface on Triton, the source of Triton's craters is probably comets. The very peculiar size distribution of sharp craters on the "cantaloupe" terrain and other evidence suggests they are volcanic explosion craters.