Lunar Impact Craters Research Articles

Morphology of Lonar Crater, India: Comparisons and implications

Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30–35 m of rim rock uplift, 5–10 m of ejected debris); depth of 230–245 m (from rim crest to crater floor). The crater's circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest. In general, ‘fresh’ terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both ‘fresh’ terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift. A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the ‘fluidized craters’ of Mars.

The role of rim slumping in the modification of lunar impact craters

Wall failure has significantly altered the structure of virtually all large, fresh‐appearing lunar craters. Terrace blocks exposed upon a crater's interior walls are interpreted to be sections of the transient cavity rim that slumped into the cavity during the terminal stages of crater formation. Impact excavation cavities have been reconstructed by restoring the innermost terrace block exposed within a crater to its inferred original position at the cavity rim and accounting for the volume of material that slumped into the cavity. Critical model assumptions include (1) the radial variation of topography near the initial cavity rim crest, (2) the structure of the failure surface along which terrace blocks slumped into the cavity, and (3) the geometric shape of the initial cavity. This terrace restoration model has been applied to 12 fresh lunar craters with observed rim crest diameters D0 ranging from 19 to 137 km. Up to an initial cavity diameter Di of 30 km, reconstructed cavity depths are comparable to or greater than cavity depths extrapolated from nonterraced craters of less than 15 km in diameter. For a wide range of model parameters the reconstructed depths of impact cavities 15–30 km in diameter are significantly greater than depths predicted by small‐crater morphometry, implying that cavity depths inferred from depth/diameter ratios observed for small craters may substantially underestimate the depth of excavation of impact cratering events in this size class. Reconstructed cavity depths for Di > 70 km, however, are consistently less than cavity depths extrapolated from smaller craters. This indicates that the morphometric transition from small, relatively unmodified, bowl‐shaped craters (D0 < 15 km) to large, terraced, saucer‐shaped craters (D0 > 80–90 km) cannot be solely attributed to rim‐slumping modification. Ejecta fallback and basement rebound also play a role in modifying impact crater cavities; however, the manner in which the volume of fallback ejecta and basement rebound material varies with increasing crater size is unknown. The discrepancy between cavity depths extrapolated from small craters and those obtained from the terrace restoration model suggests that impact excavation cavities become relatively shallower at larger diameters. However, this cannot be conclusively demonstrated until the effects of rebound and ejecta fallback are quantitatively accounted for.

Concluding remarks

The idea of this meeting came to the organizers from the type of considerations set out by Dr Kelly in his opening remarks, namely that we see many apparent similarities between phenomena of large scale in geophysics and phenomena of small scale in metallurgy and we would like to understand their significance. Dr Kelly mentioned triple junctions between tectonic plates and between metal grains. Another example, which goes back many years, is that structures in mountains can be simulated by flow in materials such as plasticine, and Dr King has showed us how patterns of faulting look much the same over a wide range of scales. Yet another, by now well known, example is that lunar impact craters look almost indistinguishable over a range of sizes from a few micrometres up to hundreds of kilometres. Dr Kelly remarked to me this morning that the similarity between geophysics and metallurgy extends to terminology, for the geophysicist has his fault and the metallurgist his defect.

Lunar cratering

The form of the lunar impact crater size-frequency distribution is discussed. Latest results on the lunar cratering chronology in the first 1.5 Ga after its formation are reviewed. It is shown that most cratering arguments speak against an extraordinary high flux increase (‘cataclysm’) at ca . 4 Ga ago. From age determination by crater frequency measurements, it is concluded that the dominant process of the formation of light (Cayley) plains is not deposition of basin ejecta but an endogenic one.

A study of lunar impact crater size-distributions

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ⩽D ⩽ 20 km for regions with formation ages between ≈ 3 × 109 yr and ≳ 4 × 109 yr. A polynomial of 4th degree, valid in the size range 0.8 km ⩽D ⩽ 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ⩽D ⩽ ⩽ 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ∼D α(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.

Generation of magma at lunar impact crater sites

MANY lunar craters have lava flows associated with them. It is widely accepted that, in most cases, the craters are the result of meteoric impact and not volcanic activity but, if so, the occurrence of lavas needs explaining. It is commonly supposed that the lava flows are material that was melted during the impact process itself but this is not acceptable as a general explanation, for two reasons. First, in several cases the ages of the lavas have been found, by crater counting techniques, to be significantly younger than the craters themselves1 and so they cannot have been formed at the time of impact. Second, some of the lavas show morphology typical of terrestrial lava flows and it is possible to deduce from this that the lava came from sources similar to those of terrestrial volcanoes—that is, lava was supplied at a fairly steady rate over periods of several days and was not simply splash-out from a meteoric impact2. It therefore seems that these lava flows are a product of the presence of an impact crater and yet have occurred as normal volcanic effusions long after the formation of the craters.

Lunar and terrestrial impact crater spherules

Comparisons between the chemistry of impact debris, especially spherical droplets of impact-formed material, are made for lunar samples and for soil samples taken from terrestrial impact sites. Differences are assigned to differences in the chemistry of impacting bodies and in the surface rocks, and to the influence of the atmosphere in the terrestrial cases.

Structure of Sierra Madera, Texas, as a Guide to Central Peaks of Lunar Craters

The central peaks of Copernicus or a similar lunar crater have been considered as a target for manned exploration, partly on the supposition that the peaks expose rock uplifted from beneath the crater floor. This supposition is based on an analogy with central uplifts of terrestrial cryptoexplosion structures. Sierra Madera in west Texas is one of these structures. The structure of its central uplift is described as a proposed analog of the central peaks of large lunar impact craters. Setting and stratigraphy of the Sierra Madera are discussed together with structural framework, the central uplift, fold patterns, and fault patterns. It is concluded that the central uplifts of Sierra Madera and similar cryptoexplosion structures appear to be analogous to central peaks of large lunar impact craters.

Geology: Craters

We investigate the spatial distributions of boulders ejected from 18 lunar impact craters that are hundreds of meters in diameter. To accomplish this goal, we measured the diameters of 13,955 ejected boulders and the distance of each boulder from the crater center. Using the boulder distances, we calculated ejection velocities for the boulders. We compare these data with previously published data on larger craters and use this information to determine how boulder ejection velocity scales with crater diameter. We also measured regolith depths in the areas surrounding many of the craters, for comparison with the boulder distributions. These results contribute to understanding boulder ejection velocities, to determining whether there is a relationship between the quantity of ejected boulders and lunar regolith depths, and to understanding the distributions of secondary craters in the Solar System. Understanding distributions of blocky ejecta is an important consideration for landing site selection on both the Moon and Mars.

Thickness determinations of the lunar surface layer from lunar impact craters

Small, fresh lunar craters with normal, central-mound, flat-bottomed, and concentric geometry are widespread on maria surfaces. The same types of craters have been produced in the laboratory by impacting projectiles against targets consisting of loose, granular, noncohesive materials overlying cohesive substrates. The mechanics of formation of each laboratory crater type is described, and evidence is offered that the corresponding types of lunar craters are of impact origin. Extensive studies of the effects of lunar impact variables on the conditions of formation of these crater types show that a previously described statistical method can be used to determine the thickness of the lunar surfaces layer within narrow limits. Two independent methods for determining the layer thickness at specific points are presented. Thickness estimates of the Surveyor 1 site obtained previously from study of medium-resolution Orbiter 1 photographs are re-evaluated by using subsequently obtained high-resolution photographs, and thickness determinations of two additional areas are presented. The different areas examined have different surface layer thickness. The fragments of the surface are certainly partly of impact origin, but volcanic contributions may also be present. The maria substrates are probably composed of volcanic flow rocks with interbeds of fragmental material.

Pseudo‐volcanism and lunar impact craters

One of the ‘freshest’ heavily rayed craters on the Moon is the large nearly circular Copernicus. Copernicus is generally considered to be of impact origin [Shoemaker, 1962; Schmidt et al., 1967], and many of the small craters associated with it, including some of those arranged in chains, are ascribed to impacts of secondary fragments. Other authorities, however, cite many features of volcanic origin, including some within Copernicus itself [Green, 1967]. Volcanic features are even suggested by those generally advocating the predominance of impact mechanisms [Schmidt et al., 1967].

Lunar Landscape Luminescence.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lunar Landscape Luminescence. Cohen, Alvin J. Abstract Let us consider the possibility that the moon has never been differenti~ted. With this premise, it is likely that the lunar surface was originally composed largely of magnesium silicates. Magnesium silicate meteorites namely, the enstatite achondrites may well approximate the lunar surface and, in fact, may be of lunar origin. Many of these achondrites have undergone shock-brecciation similar to rocks in such craters in Quebec as Lac Couture, Clearwater Lakes, Manicouagan and New Quebec, and could well have been thrown off the moon during production of similar craters. The enstatite achondrites exhibit both red and blue luminescence (Derham, Geake, and Walker, Nature 203, 134, 1964). The red luminescence is similar to that observed near Kepler after a solar flare (Kopal nud Rackham, Nature 201, 239, 1964) and could be due to excitation of either lunar matter or stony meteoritic debris from an asteroid or nucleus of a comet. The red luminescence is probably due to the substitution of Mn II for Mg II in the symmetrical Mg octahedral site in the orthoenstatite structure. It is not improbable that the blue luminescence is related to the substitution of Mn II for Mg II in the distorted Mg octahedral position in the orthoenstatite structure. It has recently been observed that the ratio of blue/red luminescence increases in disordered enstatite achondrites compared to enstatite achondrites that have not become disordered (Reid, Bunch, Cohen, and Pollack, Nature 204, 1292, 1964). Disorder is interpreted here as resulting from a hypervelocity shock wave produced by impact collision which distorts in some way the environment of the symmetrical Mn II 50 that the red luminescing environment is either destroyed or converted to a blue type. It is suggested that magnesium silicates on the moon may exhibit both red and blue luminescence as do enstatite achondrites and that the ratio of blue to red luminescence as detected from probes may possibly be used as a guide for mapping the amount of shocked matter around lunar impact craters. The amounts of Fe II probably present in darker lunar areas would prevent the luminescence in these regions. Detection of large areas of luminescence of the types described in the lighter regions would preclude a granite moon, but not lay to final rest the old hypothesis of the separation of the moon from the earth by tidal attraction. A nongranite moon would make the earth the only possible source of tektite parent material. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109577 Bibcode: 1965AJ.....70R.135C full text sources ADS |