The system of lunar craters,revised

A new catalogue of lunar crater diameters, depths and morphologies based on Orbiter photography is described. Preliminary shadow length reductions indicate that previous depths determined by ACIC and Baldwin are systematically about 500 m too shallow.Paper presented at the NATO Advanced Study Institute on Lunar Studies, Patras, Greece, September 1971.     

Crater frequencies on lava-covered areas related to the moon's thermal history

Making use of Orbiter and Apollo photographs, frequency counts of craters down to 2 km diam as indicators of the relative ages of lunar features, have been made on 264 areas, including 15 terrae, 27 recognized maria, 174 flat-floored craters and 48 lava-covered areas with indefinite boundaries designated as ‘marets’. Analysis of frequency counts on flat-floored craters on the basis of this data and re-assessment of former results, combined with the relatively restricted age range of lunar samples, make it unlikely that the present observations are able to reach back in time to impacts on an assumed primordial floating crust. The range of crater frequencies on the marets, together with their wide distribution over the lunar surface, suggest lava migrations to the surface within autonomous domains each with its own chronology, covering an extensive period of lunar history. The close association of marets with flat-floored craters provides a reasonable origin for the floor material of these latter objects. The lava migrations associated with the marets suggest that internal heating may be a more important factor in the origin of lunar surface features than had formerly been supposed. Kopal's views on the origin of the moon's multiple moments of intertia (1972) are considered to support the concept of autonomous domains. It is considered that the time sequence of separate lava flows represented by the marets may be a reflection of physical processes within the moon responsible for the successive lava flows associated with the larger maria.     

Ejecta size-velocity relation derived from the distribution of the secondary craters of kilometer-sized craters on Mars

The relation between the size and velocity of impact crater ejecta has been studied by both laboratory experiments and numerical modeling. An alternative method, used here, is to analyze the record of past impact events, such as the distribution of secondary craters on planetary surfaces, as described by Vickery (Icarus 67 (1986) 224; Geophys. Res. Lett. 14 (1987) 726). We first applied the method to lunar images taken by the CLEMENTINE mission, which revealed that the size-velocity relations of ejecta from craters 32 and 40 km in diameter were similar to those derived by Vickery for a crater 39 km in diameter. Next, we studied the distribution of small craters in the vicinity of kilometer-sized craters on three images from the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor (MGS). If these small craters are assumed to be secondaries ejected from the kilometer-sized crater in each image, the ejection velocities are of hundreds of meters per second. These data fill a gap between the previous results of Vickery and those of laboratory studies.     

Global survey of lunar regolith depths from LROC images

We performed the first global survey of lunar regolith depths using Lunar Reconnaissance Orbiter Camera (LROC) data and the crater morphology method for determining regolith depth. We find that on both the lunar farside and in the nearside, non-mare regions, the regolith depth is twice as deep as it is within the lunar maria. Our data compare favorably with previous studies where such data exist. We also find that regolith depth correlates well with density of large craters (>20 km diameter). This result is consistent with the gradual formation of regolith by rock fracture during impact events.     

Precision size-frequency distributions of craters for 12 selected areas of the lunar surface

Total crater populations in Lunar Orbiter photographs have been counted and measured for 12 selected areas of the lunar surface using precision techniques. Details of the counting procedure are described. Incremental and cumulative frequencies per km² (and their logarithms) are presented in graphical as well as tabular form for general use by other investigators. The data include 333,404 craters in areas totaling 10,833.3 km².     

On the depths and shapes of the freshest kilometer-scale simple craters on the lunar maria: A new crater shape model

Recent work on the shapes of small, simple impact craters on the Moon has shown that the parabolic ideal does not well represent the vast majority of these craters. They are hyperbolic in shape and usually resemble a cone more than a parabola. A parabolic shape also does not fit the most commonly held archetype for simple craters in general (Linné), which is also hyperbolic. In addition, Linné itself may not be the best model for fresh simple craters, in terms of cross-sectional shape, although shape data to compare it to have heretofore been lacking. Here, the “free shadowfront method” for determining the shapes of simple craters is used to measure 64 fresh simple craters on five lunar maria to test both assumptions. Laser altimetry cross sections, available for many of the craters measured herein, are used to complement and spot-check the shadow measurement results, and thereby demonstrate the efficacy of the free shadowfront method. A new shape model is established, and two craters that better fit this model than Linné are identified. These are located at 24.45° N/328.12° E and 31.35° N/296.46° E and have diameters of 1.40 and 2.73 km, respectively. An apparent dichotomy between fresh simple craters smaller than 2.5 km and those larger than this is observed. Flat floors are found to be ubiquitous among the larger craters, but rare and small in extent in smaller ones. A slide in one crater which appears to be an incipient flat floor suggests a major mode of formation for these flat floors.     

Lunar craters evolution and meteoroidal flux in pre-mare and post-mare times

In order to study the geomorphic evolution and lifetimes of lunar craters, data were collected from (i) 32mare andterra provinces of the nearside of the Moon using the L.P.L. catalog; (ii) amare area in Sinus Medii, using direct observations of Lunar Orbiter photos, and (iii) aterra area on the farside using direct observations of Zond-8 photos. The theory presented in a previous publication is expanded and applied to the data.The following conclusions are obtained. (1) Steady-state conditions occur on the studiedmare surfaces for craters of diameter up to approximately 220 m, and on the studiedterra surfaces for craters of diameter up to at least 50 km. (2) The average lifetime of a crater, in addition of being a function of the meteoroidal flux, is a steep function of the diameter of the crater. (3) The correlation is good between a geomorphic classification of craters based on visual comparison with standard craters and a classification of craters based on their depth-diameter ratio, resulting in a coefficient of rank correlation of 0.64. (4) When craters are classified as young, mature, and old, the length of time spent as young is less than a few percent of the total lifetime of the crater; the time spent as mature is 10 to 30%; and as much as 80% is spent as an old crater. Within the error of the calculations, these values are independent of crater diameter and apply to both pre-mare and post-mare craters, indicating that they are also independent of the intensity of the meteoroidal flux. (5) The average lifetime of a 50 km crater in pre-mare times is estimated to be less than 0.3×10⁹ years. (6) The average lifetime of a 50 km crater in post-mare times is estimated to be between 3×10¹¹ and 10¹⁴ years. (7) The average meteoroidal flux in pre-mare times is estimated to be three to six orders of magnitude more intense than in post-mare times.     

Selenodetic control network

Identification details of the 66 first-order selenodetic control points - i.e., points consisting of craters having diameters larger than 8 km - are given. The distribution of all 211 reference points of the network on the Blagg-Müller maps is also presented. In the Appendix the corresponding Blagg-Müller and LPL Catalogue number for each crater as well as the frame numbers of Lunar Orbiter IV photographs in which the craters have been studied are included.On leave from the Astronomy Department, University of Athens, Greece.The Lunar Science Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration. This paper is Lunar Science Institute Contribution No. 237.     

Shallow and deep fresh impact craters in Hesperia Planum,Mars

The depths of 109 impact craters 2–16 km in diameter, located on the ridged plains materials of Hesperia Planum, Mars, have been measured from their shadow lengths using digital Viking Orbiter images (orbit numbers 417S–419S) and the PICS computer software. On the basis of their pristine morphology (very fresh lobate ejecta blankets, well preserved rim crests, and lack of superposed impact craters), 57 of these craters have been selected for detailed analysis of their spatial distribution and geometry. We find that south of 30°S, craters <6.0 km in diameter are markedly shallower than similar-sized craters equatorward of this latitude. No comparable relationship is observed for morphologically fresh craters >6.0 km diameter. We also find that two populations exist for older craters <6.0 km diameter. When craters that lack ejecta blankets are grouped on the basis of depth/diameter ratio, the deeper craters also typically lie equatorward of 30° S. We interpret the spatial variation in crater depth/diameter ratios as most likely due to a poleward increase in volatiles within the top 400 m of the surface at the times these craters were formed.     

Dust transport near electron beam impact and shadow boundaries

When the moon enters the plasma sheet of the earth, high energy electron fluxes are incident upon the lunar surface. Some regions are in the shadow of these fluxes due to topographic features. Large electric fields were found at similar shadow boundaries created by the electron beams incident upon an obstacle in the laboratory. Potentials on the beam-illuminated surface follow beam energies and were negative relative to potentials on the shadowed surface. Charged dust particles in the beam-illuminated region were observed to move into the shadow due to these electric fields. The oblique incidence of the electron fluxes upon craters can lead to a portion of the crater surface in the beam-illumination and another portion in the shadow. Dust particles on the slopes of the craters can thus experience large electric fields and transport downhill to fill the bottom of the craters. This mechanism may contribute to the formation of dust ponds observed by the NEAR-Shoemaker spacecraft at Eros, and might be at work on the lunar surface as well. In the laboratory, we used electron fluxes with energies up to 90 eV to bombard an insulating half-pipe. An angle of incidence was chosen so that the impact occurred on farside of the slope and left the bottom and the nearside slope in the shadow. Dust particles on the beam-illuminated slope moved down along the surface toward the bottom of the half-pipe and hopped to the bottom as well, while particles on the shadowed slope remained at rest.     

Possible lunar ring dikes

Terrestrial ring dike structures are features consisting of one or more series of concentric fracture systems along which the central block often subsided and up through which lavas intruded and extruded and other volcanic features formed. Before the lunar probe satellites, a search for lunar features that showed characteristics of terrestrial ring dikes was conducted using the LAC charts andKuiper Atlas photographs. More recently the search was extended on the nearside features and to the farside features using the Lunar Orbiter series of photographs resulting in a catalog of 559 nearside candidates and 82 farside. Features exhibiting one or more of the following four criteria were included as lunar analogs to terrestrial ring dikes: (1) inner ridge(s) approximately concentric with the crater wall, (2) inner rill(s) approximately concentric with the crater wall, (3) outer ridge(s) and/or rill(s) approximately concentric with the crater wall, and (4) interior and exterior slopes of the crater wall approximately equal (implying extrusion of lava along a ring fracture). Equal slopes are in contradistinction to a central source eruptive feature or an impact feature both of which usually produce craters with walls whose inner slopes are about twice as steep as their outer flanks, which characterize the vast majority of lunar craters. Features exhibiting each of the four criteria were found and some had combinations of two or more including rills merging into ridges, e.g., in Taruntius and Posidonius. Gambart is an example of equal inner and outer slopes, while Hesiodus A and Marth are two of the best examples of complete inner rings concentric with the outer rings. Ten percent of the candidates were probable impact craters but had subsequent volvanic activity of a ring dike nature. The initial search showed a distribution of the possible lunar ring dikes that was non-random and strongly associated with the margins of the maria, further implying that they are volcanic features. This relation was upheld when extended by the recent survey. The anticipated dearth of farside ring dikes was corroborated in our study and their distribution is restricted to those few mare-like areas on the farside, further supporting the volcanic nature of these features     

Increased iron abundances in slope avalanches of certain lunar craters

On the basis of the first images of certain areas of Lunar surface obtained by the Chang??e-2 spacecraft and materials of large-scale image acquisition from the LRO (Lunar Reconnaissance Orbiter) spacecraft, supplemented with remote spectral measurements performed from the Clementine spacecraft, the slope movements of material have been studied in lunar craters Daniell, Burg and Mauri A. It is established that despite a significantly different age of formation of these craters, the slope formations are of similar structure and differ by increased iron abundance in the soil surface layer. All objects are characterized with by increase in FeO abundance to 20 wt % at depths of several hundred meters from the surface. The material of the slope structures is distinguished by a low maturity rate. According to preliminary assessments using the optical maturity index and spectropolarimetric maturity index, the fresher slope formations can have an exposure age from several tens of years to several years.     

Prognosis of TiO2 abundance in lunar soil using a non-linear analysis of Clementine and LSCC data

We suggest a technique to determine the chemical and mineral composition of the lunar surface using artificial neural networks (ANNs). We demonstrate this powerful non-linear approach for prognosis of TiO₂ abundance using Clementine UV-VIS mosaics and Lunar Soil Characterization Consortium data. The ANN technique allows one to study correlations between spectral characteristics of lunar soils and composition parameters without any restrictions on the character of these correlations. The advantage of this method in comparison with the traditional linear regression method and the Lucey et al. approaches is shown. The results obtained could be useful for the strategy of analyzing lunar data that will be acquired in incoming lunar missions especially in case of the Chandrayaan-1 and Lunar Reconnaissance Orbiter missions.     

Identification,distribution and significance of lunar volcanic domes

Over 300 previously unrecognized volcanic domes were identified on Lunar Orbiter photographs using the following criteria: (1) the recognition of land forms on the Moon similar in morphology to terrestrial volcanic domes, (2) structural control, (3) geomorphic discordance, and (4) the recognition of land forms modified by dome-like swellings. Many terrestrial volcanic domes are similar in morphology to lunar domes. This analogy suggests that some lunar hills are in fact extrusive volcanic domes. Many of the domes identified in this paper seem to be related to basins and craters, and with the exception of local tectonic grid control few domes are related to any observable Moon-wide pattern. Domes are not uniquely found on maria. Dome formation probably spans a wide range of lunar time and activity in areas where domes are located may be continuing to the present as revealed by the close correlation of dome distribution with the distribution of lunar transient events. The overall morphology of a lunar dome is a poor indicator of the composition of the rock that forms the dome. Contribution No. 33 Planetary Geology Group, University of New Mexico.     

Determination of Mars crater geometric data: Insights from high-resolution digital elevation models

We review the methods and data sets used to determine morphometric parameters related to the depth (e.g., rim height and cavity depth) and diameter of Martian craters over the past ~45 yr, and discuss the limitations of shadow length measurements, photoclinometry, Earth-based radar, and laser altimetry. We demonstrate that substantial errors are introduced into crater depth and diameter measurements that are inherent in the use of 128th-degree gridded Mars Orbiter Laser Altimeter (MOLA) topography. We also show that even the use of the raw MOLA Precision Engineering Data Record (PEDR) data can introduce errors in the measurement of craters a few kilometers in diameter. These errors are related to the longitudinal spacing of the MOLA profiles, the along-track spacing of the individual laser shots, and the MOLA spot size. Stereophotogrammetry provides an intrinsically more accurate method for measuring depth and diameter of craters on Mars when applied to high-resolution image pairs. Here, we use 20 stereo Context Camera (CTX) image pairs to create digital elevation models (DEMs) for 25 craters in the diameter range 1.5–25.6 km and cover the latitude range of 25^° S to 42^° N. These DEMs have a spatial scale of ~24 m per pixel. Six additional craters, 1.5–3.1 km in diameter, were studied using publically available DEMs produced from High-Resolution Imaging Science Experiment (HiRISE) image pairs. Depth/diameter and rim height were determined for each crater, as well as the azimuthal variation of crater rim height in 1-degree increments. These data indicate that morphologically fresh Martian craters at these diameters are significantly deeper for a given size than previously reported using Viking and MOLA data, most likely due to the improvement in spatial resolution provided by the CTX and HiRISE data.     

Structural disturbances of the lunar surface near the Lunokhod-1 spacecraft landing site

The American Lunar Reconnaissance Orbiter spacecraft acquired high-resolution images of the landing sites of the Apollo manned spaceships and the Luna automatic space probes. In the images taken with the LROC Narrow-Angle Camera, the traces of anthropogenic influence on the lunar surface are seen in these places. However, such traces are not always noticeable sufficiently well, since they are masked by inhomogeneities in the brightness of the examined surface region caused by its topographic features and albedo variations. To increase the potential of identifying the disturbances of the initial structure of the lunar surface, the data should be analyzed with so-called phase-ratio imaging. Its essence is that the ratio of two coinciding images of the same surface region obtained at different phase angles is calculated. This method was applied to the analysis of the landing site of the Soviet Luna-17 space probe that transported the Lunokhod-1 rover to the lunar surface. The structural disturbance caused by the impact of jet flows from the probe’s engines and the tracks of the Lunokhod-1 wheels, which are faintly discernible in the usual images, has been detected.     

Petrogenesis of lunar impact melt rock meteorite Oued Awlitis 001

Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth's atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced ¹⁴C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001.     

Probable swirls detected as photometric anomalies in Oceanus Procellarum

Images of the lunar nearside obtained by telescopes of Maidanak Observatory (Uzbekistan) and Simeiz Observatory (Crimea, Ukraine) equipped with Canon CMOS cameras and Sony CCD LineScan camera were used to study photometric properties of the lunar nearside in several spectral bands. A wide range of lunar phase angles was covered, and the method of phase ratios to assess the steepness of the phase function at different phase angles is applied. We found several areas with photometric anomalies in the south-west portion of the lunar disk that we refer to as Oceanus Procellarum anomalies. The areas being unique on the lunar nearside do not obey the inverse correlation between albedo and phase-curve slope, demonstrating high phase-curve slopes at intermediate albedo. Low-Sun images acquired with Lunar Orbiter IV and Apollo-16 cameras do not reveal anomalous topography of the regions, at least for scales larger than several tens of meters. The areas also do not have any thermal inertia, radar (70 and 3.8 cm), magnetic, or chemical/mineral peculiarities. On the other hand they exhibit a polarimetric signature that we interpret to be due to the presence of a porous regolith upper layer consisting of dust particles. The anomalies may be interpreted as regions of very fresh shallow regolith disturbances caused by impacts of meteoroid swarms consisting of rather small impactors. This origin is similar to one of the hypotheses for the origin of lunar swirls like the Reiner-γ formation. The photometric difference between the shallow and pervasive (Reiner-γ class) swirls is that the latter appear to have a significant amount of immature soils in the upper surface layers.