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The physical characteristics of the fresh looking Aitken Crater and its surrounding environment are studied. Rim deposits and ejecta blanket indicate the crater has a probable impact origin or is formed during post-Aitken events. Data also indicate the crater's dark floor fill, its flow scarps, and smaller craters with domical structures are volcanic in origin. Landform varieties suggest episodic erruptions may also cause such craters.
Relative exposure ages based on agglutinate content are calculated for 26 Apollo 16 surface and core samples. These ages increase from the northern part of the traverse to the southern part and are in general agreement with cosmogenic gas ages and particle track ages. An apparent paradox exists in which presumed ray soil from South Ray Crater is much older than the age of South Ray Crater itself as determined by a variety of methods. The most likely explanation for the paradox is that the presumed South Ray Crater soil is not ejecta from South Ray Crater but is pre-existing regolith upon which blocks and fragments from South Ray Crater are scattered.
Approximately 10,000 microcraters were investigated using binocular microscope techniques on fifteen Apollo 16 rocks: crystalline rocks 60315, 60335, 61156, 62235, 62295, and 68415; breccias 60016, 61015, 61175, 66075, and 69935; and glass surfaces 60015, 60095, 60135, and 64455. Diameter measurements of the central glass-lined pits and surrounding spall zones were made. Ratios of spall to pit diameters may range from 3.0 to 4.5 for different rock surfaces. Crater size distributions obtained for production surfaces confirm and extend to larger crater sizes data published previously. The crater size distribution on lunar rocks in the pit diameter range, 10 to 1000 microns, is shown to depend on the average angle of impact which is a function of the exposure geometry. In contrast to results of earlier studies, a wide range of crater densities was observed on relatively heavily cratered surfaces.
Crater morphology and size play a major role in determining whether wind-blown streaks emanating from craters or dark splotches within craters will form. Both bright and dark streaks emanate almost exclusively from bowl-shaped craters. Dark splotches are found mainly in flat-floored craters, especially those that are deep and have high rim relief. Trends of dark splotches in the northern to southern midlatitudes closely follow those of bright streaks, suggesting both were formed by similar winds. In the high southern latitudes, on the other hand, dark splotch trends closely follow those of dark streaks. Qualitative models of streak and splotch formation are proposed on the basis of Mariner 9 data.
Between 1000 and 2000 infrared (eclipse) and radar anomalies have been mapped on the nearside hemisphere of the moon. A study of 52 of these anomalies indicates that most are related to impact craters and that the nature of the infrared and radar responses is compatible with a previously developed geologic model of crater aging processes. The youngest craters are pronounced thermal and radar anomalies; that is, they have enhanced eclipse temperatures and are strong radar scatterers. With increasing crater age, the associated thermal and radar responses become progressively less noticeable until they assume values for the average lunar surface. The last type of anomaly to disappear is radar enhancement at longer wavelengths. A few craters, however, have infrared and radar behaviors not predicted by the aging model. One previously unknown feature - a field strewn with centimeter-sized rock fragments - has been identified by this technique of comparing maps at the infrared, radar, and visual wavelengths.
It is shown that Mars experienced a brief episode of an increased obliteration rate contemporaneous with the formation of the cratered plains units. The obliteration rate during this episode was at least an order of magnitude greater than the preexisting rate. The obliteration rates prior to and after the event were identical within a factor of 3. Regional variations in the observed densities of degraded craters on the cratered terrain are shown to result from differences in the intensity of the obliteration rate during the event. The identification and characterization of the event are accomplished by using crater diameter-frequency distributions, not only for the total crater population but also for four morphological classes. Distributions are selected from 38 regions within the Martian equatorial latitudes.
In view of the discrepancies in published crater frequency data, effects modifying production size distributions of impact craters are discussed. The resulting criteria are applied to 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. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.
A review is presented of four hypotheses concerning the origin of lunar craters, taking into account the bubble hypothesis, the tide hypothesis, the volcanic hypothesis, and the impact hypothesis. A description is given of a series of experiments on impact craters and studies of a meteorite crater in Arizona are considered. It is concluded that the typical lunar craters can best be interpreted as impact craters.
Lavalike materials that were emplaced in a fluid state occur in and around lunar impact craters whose diameters range from 3 km to more than 200 km and whose ages span a time interval of at least 3.5 billion years. The distribution of the lavalike deposits conforms to asymmetries of other ejecta from the same craters, and the material is concentrated downrange to distances as great as a crater radius. The character and distribution of the lavalike materials support the idea that they formed by impact melting rather than by volcanism. Returned samples indicate that materials with appropriate physical characteristics are generated by partial melting of feldspathic rocks by impact. The geologic evidence at lunar craters suggests that there is more melt rock in and near the craters than is predicted by experiment and theory.
The depth/diameter ratio for 87 craters on Mars is determined using Mariner 9 ultraviolet-spectrometer (UVS) altimetry. The sample includes craters 12 to 100 km in diameter and 0.4 to 3.3 km in depth. The largest depth/diameter ratios on Mars are comparable to those of fresh craters on Mercury. However, more than half of the present sample consists of degraded craters whose depths are significantly shallower than those of fresh craters of similar diameter on Mercury, confirming the interpretations of earlier photoanalysts.
Nighttime thermal maps of large, young lunar craters show an evolutionary sequence in the gross morphology of thermal patterns associated with the craters. The nature of the sequence is consistent with crater degradation models which predict a filling of large craters with material from the walls. Detailed comparison of a nighttime thermal map of Copernicus with a geologic map demonstrates that a smooth floor unit characterized as impact melt is more likely wall debris. Nighttime thermal maps will be particularly useful for quantifying crater modification which has occurred in the relatively quiescent post-basin period of lunar history.
The negative Bouguer anomalies (i.e., mass deficiencies) associated with four young lunar craters are analyzed. Model calculations based on generalizations made from studies of terrestrial impact structures suggest that the major contribution to the Bouguer anomaly for these lunar craters is due to a lens of brecciated material confined within the present crater rim crest and extending vertically to at least a depth of one-third the crater rim diameter. Calculations also reveal a systematic variation in the magnitude of the mass deficiencies with the cube of the crater diameter.
Geologic studies including mapping of the Kuiper quadrangle of Mercury suggest that secondary craters are much better preserved than those on the moon. Factors which may account for the apparent differences between lunar and Mercurian secondary crater morphology include: (1) the rapid isostatic adjustment of the parent crater, (2) different impact fluxes of the two planets, (3) the greater concentration of Mercurian secondaries around impact areas, and (4) differences in crater ejection velocities. It has been shown that the ejection velocities on Mercury are about 50% greater than those on the moon at equivalent ranges. This may account for morphologically enhanced secondary craters, and may explain their better preservation with time.
A preliminary investigation was conducted regarding the variations in the morphological characteristics of Mercurian craters that appear to be associated with a degradation of fresh craters. Craters are classified according to relative states of degradation in order to provide evidence on degradation styles and rates on Mercury, and to correlate degradation with major geologic events in the history of the planet. Processes and relative rates of degradation on Mercury and the moon are also compared. Degradation trends of crater morphology are found to be parallel on Mercury and the moon. The severe destruction of interior structures in lunar craters may have resulted from more widespread distribution of ejecta due to the moon's lower gravity. The possibility is considered that the smooth plains on the surface of Mercury were formed early in the history of that planet.
Some of the wide variations in the crater-size distributions in lunar photography and in the resulting statistics were interpreted as different degradation rates on different surfaces, different scaling laws in different targets, and a possible population of endogenic craters. These possibilities are reexamined for statistics of 26 different regions. In contrast to most other studies, crater diameters as small as 5 m were measured from enlarged Lunar Orbiter framelets. According to the results of the reported analysis, the different crater distribution types appear to be most consistent with the hypotheses of differential degradation and a superposed crater population. Differential degradation can account for the low level of equilibrium in incompetent materials such as ejecta deposits, mantle deposits, and deep regoliths where scaling law changes and catastrophic processes introduce contradictions with other observations.
The paper is a field report concerning a shallow core drilling program in the multicolored breccia deposits which constitute 90% of all the impact breccias beyond the outer rim of the Ries, a 26-km-diam impact crater. About 480 m of core was recovered from 11 locations with radial ranges between 16.5 and 35 km from the crater center. The cores consist of breccias, whose components are derived from the crater itself and the terrain outside the crater. The local components dominate the breccias at the larger ranges, and possibly constitute more than 90% of the breccia volume at the greatest distances investigated. The great depth of the Bunte Breccia (84 m at 27 km range), together with the preponderance of local components, necessitates an emplacement mechanism that ploughed up and mixed the crater surroundings to depths greater than 50 m.
We propose a model for the origin of impact-basin rings whereby the main topographic rim of a basin approximates the limit of excavation and inner rings approximate the rims of craters formed inside the transient crater by some perturbation in the cratering process. The cause of this complexity in transient cavities may be the presence of discontinuities in the target material. The second inward ring may have formed at the seismic discontinuity about 20 km deep in the lunar crust, and the third, innermost ring of a few large basins at the crust-mantle interface about 60 km deep. Slumping increased the original diameters of many rings and split some initially coherent rings into subsidiary or partial rings. Deformation outside the transient crater produced external arcs. This model differs from prevalent hypotheses of ring formation whereby an inner ring approximates the transient crater rim and major faulting of the flank produced the outer ring structures.
A brief review of Martian crater data is presented and a short summary is provided of Martian fresh crater morphology and morphometry based on pre-Viking information. The pre-Viking Mars missions are considered, taking into account the flyby of Mariner 4 in July 1965, the flyby of Mariner 6 in July 1969, the flyby of Mariner 7 in August 1969, and the insertion of Mariner 9 into a Martian orbit in November 1971. The 7329 photographs of the planet and its moons transmitted by Mariner 9 to earth provide a substantial basis for formulating a large body of data on crater morphometry, morphology, and location. In general, fresh Martian crater morphology follows a diameter-dependent progression. The simplest morphology is found in the smallest craters. The morphology remains essentially constant through diameters of approximately 10 km. Morphologies rapidly become more complex in the diameter range of 10-40 km.