Large Lunar Craters Research Articles

Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Materials Beneath Large Impact Craters on the Moon

AbstractIn this work, we examine variations in the mantle uplift associated with large lunar impact craters and basins between major terranes. This study is based on Bouguer gravity anomalies of 100–650‐km diameter impact craters using Gravity Recovery and Interior Laboratory (GRAIL) observations and the Lunar Orbiter Laser Altimeter (LOLA) crater database. The Bouguer gravity anomalies of 324 large impact craters analyzed herein are primarily controlled by the uplifted crust‐mantle (Moho) interface in the central region of these impact craters, while postimpact mare deposits that represent ∼25%–35% of the postimpact deposit column also contribute to the gravity anomalies of mare craters. The central uplift of the Moho interface is primarily controlled by impact energy and increases to ∼ 30 km for a 650‐km diameter crater. Analyses of nonmare craters in the Feldspathic Highlands Terrane (FHT) with varied crustal thickness (Tc) reveal that the onset crater diameter (Dmin) with an uplifted Moho interface is dependent on the local Tc: Dmin ∼ 150 + 1.3Tc (in a unit of km). This equation also provides a quantification of the depth‐dependent attenuation of impact‐induced structural uplift, using the Moho uplift as a proxy for the structural uplift. The Moho uplift of large craters in the South Pole‐Aitken (SPA) Terrane is not statistically different from the FHT craters, consistent with limited overall thermal difference between these two terranes and compensational thermal effects at crater collapse and viscoelastic relaxation stages.

The central pit and dome at Cerealia Facula bright deposit and floor deposits in Occator crater, Ceres: Morphology, comparisons and formation

The prominent bright deposit Cerealia Facula, Ceres, coincides with the central depression (or central pit) of the recently formed 92 km-wide complex crater Occator. The central pit is 9–10 km wide and up to 1 km deep and is partially filled with a 700 m-high 2 km-wide dome. The upper surface of the central dome is densely fractured but the flanks are not, indicating that uplift of the dome surface occurred after the bright deposit was emplaced, and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion. The pit is rimless except for two prominent massifs that flank it to the east and west. This pit-dome morphology bears a strong resemblance in morphology, topography, and dimensions to central pit craters observed on Ganymede and Callisto, but is essentially absent on the similar sized ice-rich moons of Uranus, Saturn, and Pluto. The lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of Ceres suggest that temperature is a significant controlling factor in central pit formation, beyond the canonical inverse-gravity scaling of complex crater transitions. While renewed modeling will benefit from these new constraints, the mapping of central pits on Ceres, including Occator, suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on Ceres (lack of similar high-resolution imaging on the Galilean icy satellites precludes definitive evaluation of models there). Large volumes of lobate floor-fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of Occator, and small outcrops of this unit are found perched high on closed topographic depressions on the Occator terrace zone and other higher areas of the crater floor. The swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as Tycho. These observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates, salts and/or carbonates, and large quantities of fragmented debris. Melt of ice phases within Ceres crust is also supported by independent numerical models of impact at Occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright Na-carbonates on the surface of the central pit at Occator, if the deposit formed within a million years or so of impact. The central dome finds analogs in salt domes on Earth and pingos (frost-mounds) on Earth and Mars, which form by water and solute migration and freeze expansion. Later freezing of a central reservoir of volatiles beneath the central pit at Occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome. Finally, no changes were detected in the Cerealia and Vinalia Facula bright deposits during the course of the prime Dawn mapping mission, placing additional constraints on their formation time and limits on ongoing activity.

Modeling the Historical Flux of Planetary Impactors

Abstract The impact cratering record of the Moon and the terrestrial planets provides important clues about the formation and evolution of the solar system. Especially intriguing is the epoch ≃3.8–3.9 Gyr ago (Ga), known as the Late Heavy Bombardment (LHB), when the youngest lunar basins such as Imbrium and Orientale formed. The LHB was suggested to originate from a slowly declining impactor flux or from a late dynamical instability. Here, we develop a model for the historical flux of large asteroid impacts and discuss how it depends on various parameters, including the time and nature of the planetary migration/instability. We find that the asteroid impact flux dropped by 1–2 orders of magnitude during the first 1 Gyr and remained relatively unchanged over the last 3 Gyr. The early impacts were produced by asteroids whose orbits became excited during the planetary migration/instability, and by those originating from the inner extension of the main belt. The profiles obtained for the early and late versions of the planetary instability initially differ, but end up being similar after ∼3 Ga. Thus, the time of the instability can only be determined by considering the cratering and other constraints during the first ≃1.5 Gyr of the solar system history. Our absolute calibration of the impact flux indicates that asteroids were probably not responsible for the LHB, independently of whether the instability happened early or late, because the calibrated flux is not large enough to explain Imbrium/Orientale and a significant proportion of large lunar craters.

Oblique impact cratering experiments in brittle targets: Implications for elliptical craters on the Moon

Most impact craters observed on planetary bodies are the results of oblique impacts of meteoroids. To date, however, there have only been very few laboratory oblique impact experiments for analogue targets relevant to the surfaces of extraterrestrial bodies. In particular, there is a lack of laboratory oblique impact experiments into brittle targets with a material strength on the order of 1MPa, with the exception of ice. A strength on the order of 1MPa is considered to be the corresponding material strength for the formation of craters in the 100m size range on the Moon. Impact craters are elliptical if the meteoroid's trajectory is below a certain threshold angle of incidence, and it is known that the threshold angle depends largely on the material strength. Therefore, we examined the threshold angle required to produce elliptical craters in laboratory impact experiments into brittle targets. This work aims to constrain current interpretations of lunar elliptical craters and pit craters with sizes below a hundred meters. We produced mortar targets with compressive strength of 3.2MPa. A spherical nylon projectile (diameter 7.14mm) was shot into the target surface at a nominal velocity of 2.3km/s, with an impact angle of 5°‐90° from horizontal. The threshold angle of this experiment ranges from 15° to 20°. We confirmed that our experimental data agree with previous empirical equations in terms of the cratering efficiency and the threshold impact angle. In addition, in order to simulate the relatively large lunar pit craters related to underground cavities, we conducted a second series of experiments under similar impact conditions using targets with an underground rectangular cavity. Size and outline of craters that created a hole are similar to those of craters without a hole. Moreover, when observed from an oblique angle, a crater with a hole has a topography that resembles the lunar pit craters. The relation between the impact velocity of meteoroids on the Moon and the probability of elliptical crater formation was investigated based on our experimental results and an existing empirical equation. The results suggest a distinct possibility that most craters in the 100m size range on the Moon, given their elliptical shape, originated as secondary craters.

Quantifying the attenuation of structural uplift beneath large lunar craters

Terrestrial crater observations and laboratory experiments demonstrate that target material beneath complex impact craters is uplifted relative to its preimpact position. Current estimates suggest maximum uplift is one tenth of the final crater diameter for terrestrial complex craters and one tenth to one fifth for lunar central peak craters. These latter values are derived from an analytical model constrained by observations from small craters and may not be applicable to larger complex craters and basins. Here, using numerical modeling, we produce a set of relatively simple analytical equations that estimate the maximum amount of structural uplift and quantify the attenuation of uplift with depth beneath large lunar craters.

Go and catch a falling star

Patches of deposits containing unusual mafic minerals are observed in and around some large lunar impact craters. Numerical simulations suggest that in the slowest of these impacts, asteroidal material, alien to the Moon, could have survived.

Global detection of large lunar craters based on the CE-1 digital elevation model

Craters, one of the most significant features of the lunar surface, have been widely researched because they offer us the relative age of the surface unit as well as crucial geological information. Research on crater detection algorithms (CDAs) of the Moon and other planetary bodies has concentrated on detecting them from imagery data, but the computational cost of detecting large craters using images makes these CDAs impractical. This paper presents a new approach to crater detection that utilizes a digital elevation model instead of images; this enables fully automatic global detection of large craters. Craters were delineated by terrain attributes, and then thresholding maps of terrain attributes were used to transform topographic data into a binary image, finally craters were detected by using the Hough Transform from the binary image. By using the proposed algorithm, we produced a catalog of all craters ⩾10 km in diameter on the lunar surface and analyzed their distribution and population characteristics.

Ages of large lunar impact craters and implications for bombardment during the Moon’s middle age

Standard lunar chronologies, based on combining lunar sample radiometric ages with impact crater densities of inferred associated units, have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux. In particular, there has been increasing focus on the “middle age” of lunar bombardment, from the end of the Late Heavy Bombardment (∼3.8Ga) until comparatively recent times (∼1Ga). To gain a better understanding of impact flux in this time period, we determined and analyzed the cratering ages of selected terrains on the Moon. We required distinct terrains with random locations and areas large enough to achieve good statistics for the small, superposed crater size–frequency distributions to be compiled. Therefore, we selected 40 lunar craters with diameter ∼90km and determined the model ages of their floors by measuring the density of superposed craters using the Lunar Reconnaissance Orbiter Wide Angle Camera mosaic. Absolute model ages were computed using the Model Production Function of Marchi et al. (Marchi, S., Mottola, S., Cremonese, G., Massironi, M., Martellato, E. [2009]. Astron. J. 137, 4936–4948). We find that a majority (36 of 40) of our superposed crater size–frequency distributions are consistent with the Model Production Function. A histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from ∼3.8Ga until ∼3.0Ga, implying an extended tail to the Late Heavy Bombardment. For large craters, it also preliminarily suggests that between ∼3.0 and 1.0Ga bombardment may be characterized by long periods (>600Myr) of relatively few impacts (“lulls”) broken by a short duration (∼200Myr) of relatively more impacts (“spike”). While measuring superposed craters, we also noted if they were part of a cluster or chain (named “obvious secondary”), and analyzed these craters separately. Interestingly, we observe a wide variety of slopes to the differential size–frequency power-law, which demonstrates that there can be considerable variation in individual secondary crater field size–frequency distributions. Finally, four of the small, superposed crater size–frequency distributions are found to be inconsistent with the Model Production Function; possible reasons are: resurfacing has modified these distributions, unrecognized secondary craters, and/or the Model Production Function has incorrect inputs (such as the scaling law for the target terrain). The degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely.

Ganymede crater dimensions – Implications for central peak and central pit formation and development

The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.

Global Distribution of Large Lunar Craters: Implications for Resurfacing and Impactor Populations

By using high-resolution altimetric measurements of the Moon, we produced a catalog of all impact craters ≥20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics. The most-densely cratered portion of the highlands reached a state of saturation equilibrium. Large impact events, such as Orientale Basin, locally modified the prebasin crater population to ~2 basin radii from the basin center. Basins such as Imbrium, Orientale, and Nectaris, which are important stratigraphic markers in lunar history, are temporally distinguishable on the basis of crater statistics. The characteristics of pre- and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the Orientale Basin event.

Highly Silicic Compositions on the Moon

Using data from the Diviner Lunar Radiometer Experiment, we show that four regions of the Moon previously described as "red spots" exhibit mid-infrared spectra best explained by quartz, silica-rich glass, or alkali feldspar. These lithologies are consistent with evolved rocks similar to lunar granites in the Apollo samples. The spectral character of these spots is distinct from surrounding mare and highlands material and from regions composed of pure plagioclase feldspar. The variety of landforms associated with the silicic spectral character suggests that both extrusive and intrusive silicic magmatism occurred on the Moon. Basaltic underplating is the preferred mechanism for silicic magma generation, leading to the formation of extrusive landforms. This mechanism or silicate liquid immiscibility could lead to the formation of intrusive bodies.

Global Silicate Mineralogy of the Moon from the Diviner Lunar Radiometer

We obtained direct global measurements of the lunar surface using multispectral thermal emission mapping with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Most lunar terrains have spectral signatures that are consistent with known lunar anorthosite and basalt compositions. However, the data have also revealed the presence of highly evolved, silica-rich lunar soils in kilometer-scale and larger exposures, expanded the compositional range of the anorthosites that dominate the lunar crust, and shown that pristine lunar mantle is not exposed at the lunar surface at the kilometer scale. Together, these observations provide compelling evidence that the Moon is a complex body that has experienced a diverse set of igneous processes.

Mantle deformation beneath the Chicxulub impact crater

The surface expression of impact craters is well-known from visual images of the Moon, Venus, and other planets and planetary bodies, but constraints on deep structure of these craters is largely limited to interpretations of gravity data. Although the gravity models are non-unique, they do suggest that large impact craters are associated with structure at the base of the crust. We use seismic data to map Moho (crust–mantle interface) topography beneath the Chicxulub crater, the youngest and best preserved of the three largest known terrestrial impact craters. The Moho is upwarped by ~ 1.5–2 km near the center of the Chicxulub crater, and depressed by ~ 0.5–1.0 km at a distance of ~ 30–55 km from the crater center. A comparison with numerical modeling results reveal that immediately following impact a transient crater reached a maximum depth of at least 30 km prior to collapse, and that subsequent collapse of the transient crater uplifted target material from deep below the crater floor. These results demonstrate that deformation from large terrestrial impacts can extend to the base of the continental crust. A similar Moho topography is also modeled for some large lunar and Martian craters, which suggests that mantle deformation may play a prominent role in large crater formation.

The Araguainha impact: a South American Permo–Triassic catastrophic event

The Araguainha meteorite impact was certainly one of the most catastrophic events in the history of the South American continent. The impact occurred around 250 Ma ago, when the region was covered by the estuarine waters of the Parana Basin in central parts of Brazil. The impacting body of approximately 2–3 km in diameter was sufficiently large to excavate a 2 km‐thick sedimentary sequence of the Parana Basin and to expose a 4 km‐wide core of basement crystalline rocks in the central part of the crater. The huge scar left by the meteorite collision is 40 km in diameter, the largest and best preserved impact crater on the continent. Combined field observations and remote sensing analysis demonstrates that the Araguainha impact structure preserves all morphological/structural features of large lunar craters, being thus an important analogue to study large extraterrestrial craters. The catastrophic energy released upon impact, close to 106 megatons of TNT, must have been disastrous for marine organisms living in the Parana Basin. Ongoing studies are currently evaluating the link between the Araguainha impact and the Permian–Triassic mass extinction, which is the greatest of the mass extinctions in Earth history.

Velocity of finer fragments from impact

We describe the method and the result of a new experiment to obtain velocity distribution of fine ejecta fragments, from a few to a hundred microns in size, produced from basalt targets by impacts of nylon projectiles at a velocity of 3.7 km s −1. The size distribution of holes perforated by the ejecta fragments on thin films and foils placed around the targets was investigated, and the size-velocity relation was determined with the aid of an empirical formula for threshold penetration (McDonnell and Sullivan, Hypervelocity Impacts in Space, Unit for Space Sciences, University of Kent, 1992). The velocity of the fastest fragments, at a given size, is from the extrapolation of the size-velocity relation for 1–100 mm fragments (Nakamura and Fujiwara, Icarus 92, 132–146, 1991; Nakamura et al, Icarus 100, 127–135, 1992). The laboratory results are also compared with those obtained from the study of secondary craters around large lunar craters (Vickery, Icarus 67, 224–236, 1986, Geophys. Res. Lett. 14, 726–729, 1987). All these data provide a smooth size-velocity relationship in the normalized fragment size range of four orders of magnitude.

Impacts and tectonism in Earth and Moon history of the past 3800 million years

The Moon's surface, unlike the Earth's, displays a comparatively clear record of its past bombardment history for the last 3800 Myr, the time since active lunar tectonism under the massive pre-mare bombardment ended. From Baldwin's tabulation of estimated ages for a representative sample of large lunar craters younger than 3800 Ma, six major cratering episodes can be discerned. These six bombardment episodes, which must have affected the Earth too, appear to match in time the six major episodes of orogenic tectonism on Earth, despite typical resolution errors of ±100 Myr and the great uncertainties of the two chronologies. Since more highly resolved events during the Cenozoic and Mesozoic Eras suggest the same correlation, it is possible that large impacts have influenced plate tectonics and other aspects of geologic history, perhaps by triggering flood basalt eruptions.

Near-infrared spectroscopy of probable impact melt from three large lunar highland craters

The composition of small areas within three large lunar highland craters are compared using near-infrared, Earth-based telescopic data. Spectra from many areas in the walls and central peaks of the craters indicate the presence of crystalline components which include olivine, two types of pyroxene, and sometimes Fe-bearing feldspar. Spectra from other regions suspected of being impact melt deposits, located on the floors and walls of the three craters, have remarkably similar, yet anomalous features. These features are interpreted to indicate the presence of pyroxene, Fe-bearing glass, and Fe-bearing feldspar. Pyroxene and feldspar are believed to occur primarily in the form of lithic clasts and rapidly recrystallized impact melt. The Fe-bearing glass is interpreted as impact melt glass.

Clustered impacts: Experiments and implications

Impact by clusters of projectiles rather than a single projectile can result from several processes: atmospheric breakup, tidal breakup, and ejecta from a large primary impact. Experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities (15 m/s to 6 km/s). At very low impact velocities (15–200 m/s), clustered impacts were produced by launching a grouped projectiles of aluminum shot, steel shot, iron filings, and sand. At moderate to high velocities (0.8–6 km/s), pyrex spheres were shattered above the target during passage through aluminum foil or paper, thereby forming a well‐defined cluster of fragments of overall radius rc. The ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion. Sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength. The experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body. “Tight” clusters (rc/rs < 3) displace a factor of 2 less than a single impactor of the same mass, “open” clusters (rc/rs ∼ 9) displace a factor of 5 less mass, and “dispersed” clusters (rc/rs > 20) displace a factor of 10 less mass. This reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio (diameter/depth) as large as 30 for open clusters. The size and velocity of the cluster (as well as the density and strength contrast between the impactor and target) can dramatically affect crater morphology. Open clusters impacting compacted pumice produce a flat, hummocky floor with an incipient multiring pattern, whereas a tight cluster impacting the same target produces a central floor mound. Oblique impacts by symmetrical clusters form a characteristic array of V‐shaped ridges pointing uprange. The apex angle of the ridges depends on the cluster dispersion and impact angle. Inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction. The amount of projectile material retained on the surfaces increases with decreasing impact angle (from the horizontal) and with increasing strength of the target relative to the projectile. Clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features. On this basis and on the basis of reasonable models of ejecta curtain structure, we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters. When the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain, then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90% primary material, Such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single, noninteracting impact events. Beyond the continuous ejecta facies, impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays.

Large lunar secondary craters: Size‐range relationships

The relationship between primary crater diameter and mean maximum secondary crater diameter for lunar craters is essentially linear for primary diameters from 0.5 to 260 km. The mean range from the primary to the largest secondary craters is also nearly linearly related to primary diameter. Extrapolation of these results to lunar basin scale supports the contention that basin secondaries of large diameter are to be found across much of the Moon.

Lunar crater arcs

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