Query: craters moon

title: Analyzing the ages of south polar craters on the Moon: Implications for the sources and evolution of surface water ice.
abstract: Surface water ice in the permanently shadowed polar regions of the Moon has a patchy surficial distribution and is not found within all available cold trapping areas. To date it is not well understood when the ice was delivered, which has important implications for the surficial characteristics of the ice as well as for possible delivery mechanisms. Here we present absolute model ages for 20 south polar craters that host surface water ice, providing maximum estimates of the ages of surface ice contained within these craters. We quantify the amount of available cold trapping surface area that is occupied by water ice in order to examine the relationship between the patchiness of ice within each crater and the age of each host crater. The majority of surface ice is contained in old craters ~3.1 Gyr, where the majority of cold trapping area on the pole exists. The ice is these ancient craters is very patchy in surficial distribution, occupying Siegler et al. (2016) If true polar wander did occur on the Moon, then the ages of ice bearing craters presented here set an upper limit for the age of post true polar wander hydrogen emplacement of 4.1 0.1 Gyr.

title: Thick ice deposits in shallow simple craters on the Moon and Mercury
abstract: Permanently shadowed regions near the poles of Mercury and the Moon may cold trap water ice for geologic time periods. In past studies, thick ice deposits have been detected on Mercury, but not on the Moon, despite their similar thermal environments. Here we report evidence for thick ice deposits inside permanently shadowed simple craters on both Mercury and the Moon. We measure the depth/diameter ratio of approximately 2,000 simple craters near the north pole of Mercury using Mercury Laser Altimeter data. We find that these craters become distinctly shallower at higher latitudes, where ice is known to have accumulated on their floors. This shallowing corresponds to a maximum infill of around 50 m, consistent with previous estimates. A parallel investigation of approximately 12,000 lunar craters using Lunar Reconnaissance Orbiter data reveals a similar morphological trend near the south pole of the Moon, which we conclude is also due to the presence of thick ice deposits. We find that previously detected surface ice deposits in the south polar region of the Moon are spatially correlated with shallow craters, indicating that the surface ice may be exhumed or linked to the subsurface via diffusion. The family of lunar craters that we identify are promising targets for future missions, and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on Mercury and the Moon.Ice deposits up to around 50 m thick infill some craters near the Moon's south pole and Mercury's north pole, as inferred from the poleward shallowing of simple craters.

title: Morphometric studies of the Copernicus and Tycho secondary craters on the moon: Dependence of crater degradation rate on crater size
abstract: This paper considers depth/diameter ratios and values of maximum inner slopes of two groups of relatively small secondary impact craters on the Moon associated with the 800 Ma old Copernicus crater and 100 Ma old Tycho crater. Secondary craters have the same formation age as their primary craters, but different sizes. This enables the study of how the rate of morphologic evolution of small craters depends on the crater size. Using the high resolution images obtained by Lunar Reconnaissance Orbiter Camera we selected 15 craters of ~350 950 m and 8 craters ~250 650 m in diameter being secondaries of Copernicus and Tycho, respectively, and measured their depth/diameter ratios and maximum angles of crater inner slopes. Analysis of the measurement results showed distinct dependence of craters' morphologic degradation with time on their sizes. Assuming that the pristine craters are geometrically similar, we conclude that larger craters degrade slower. Application of the model of topographic diffusion to our observations showed that the modeling results reasonably agree with observations for the crater relative depths suggesting that the time scale of crater elimination is proportional to the square of the crater diameter. Meanwhile, for the crater inner slope steepness, the agreement of the model to observations is not so good, which demands additional studies.

title: Oblique impact cratering experiments in brittle targets: Implications for elliptical craters on the Moon
abstract: 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 1 MPa, with the exception of ice. A strength on the order of 1 MPa is considered to be the corresponding material strength for the formation of craters in the 100 m 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 3.2 MPa. A spherical nylon projectile (diameter 7.14 mm) was shot into the target surface at a nominal velocity of 2.3 km/s, with an impact angle of 5deg 90deg from horizontal. The threshold angle of this experiment ranges from 15deg to 20deg. 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 100 m size range on the Moon, given their elliptical shape, originated as secondary craters.

title: The Geologic History of the Moon
abstract: More than two decades of study have established the major features of lunar geologic style and history. The most numerous and significant landforms belong to a size morphology series of simple craters, complex craters, and ringed basins that were formed by impacts. Each crater and basin is the source of primary ejecta and secondary craters that, collectively, cover the entire terra. The largest impacts thinned, weakened, and redistributed feldspathic terracrustal material averaging about 75 km in thickness. Relatively small volumes of basalt, generated by partial remelting of mantle material, were erupted through the thin subbasin and subcrater crust to form the maria that cover 16 percent of the lunar surface. Tectonism has modified the various stratigraphic deposits relatively little; most structures are confined to basins and large craters. This general geologic style, basically simple though complex in detail, has persisted longer than 4 aeons (1 aeon 109 yr) Impacts began to leave a visible record about 4.2 aeons ago, after the crust and mantle had differentiated and the crust had solidified. At least 30 basins and 100 times that many craters larger than 30 km in diameter were formed before a massive impact created the Nectaris basin about 3.92 aeons ago. Impacts continued during the ensuing Nectarian Period at a lesser rate, whereas volcanism left more traces than during pre Nectarian time. The latest basin forming impacts created the giant and still conspicuous Imbrium and Orientale basins during the Early Imbrian Epoch, between 3.85 and 3.80 aeons ago. The rate of crater forming impacts continued to decline during the Imbrian Period. Beginning in the Late Imbrian Epoch, mare basalt flows remained exposed because they were no longer obscured by many large impacts. The Eratosthenian Period (3.2 1.1 aeons ago) and the Copernican Period (1.1 aeons ago to present) were times of lesser volcanism and a still lower, probably constant impact rate. Copernican impacts created craters whose surfaces have remained brighter and topographically crisper than those of the more ancient lunar features.

title: Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater
abstract: Impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters. Although theoretical spallation models were developed about three decades ago, only limited geological observations have been made so far to test these models. The 3.8 km Censorinus crater on the Moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact. Using the Lunar Reconnaissance Orbiter Narrow Angle Camera images, we prepared the ejecta boulder distribution map of Censorinus crater and measured the boulder sizes and shapes. Mapping of about 242,000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater. Larger size boulders dominate the crater rim areas, while they become smaller away from the crater. The boulder distribution exhibits a radial asymmetry suggesting Censorinus is a oblique impact, in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim, while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion. The cumulative size frequency distribution (CSFD) of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation. The ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics. There is a general decrease of axial ratios away from the crater rim. Rectangular boulders dominate the crater rim and they become more equant away from the crater. In addition to the boulder sizes, the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact. Small size fresh impact craters (84,000 craters) are abundant on the Censorinus ejecta and post date Censorinus. These craters are found in two morphologic types in which a large majority of craters have subdued ejecta (rayless craters) while some possess bright rayed ejecta (bright rayed craters) The CSFD of rayless craters show a steep power law slope with a b value of 4.0, similar to the secondary craters produced by the impact of ejecta from primary craters. We therefore interpret the rayless craters as the secondary craters of Censorinus. On the other hand, the CSFD of bright rayed craters have smaller power law slope b value 2.7) which is a characteristic of primary craters, and thus provide 3 Ma age for Censorinus crater. When the characteristics of Censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target (lunar highland) the models generally agree with the Censorinus boulders. However, the observed shape and size characteristics of the Censorinus boulders are found to be more complex than the theoretical spallation models. The ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact. The spallation models accounting oblique impacts have not yet been developed. Therefore, our Censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts.

title: The morphology of small fresh craters on Mars and the Moon
abstract: The depth/diameter ratio for new meter to decameter scale Martian craters formed in the last ~20 years averages 0.23, only slightly deeper than that expected for simple primary craters on rocky surfaces. Large variations in depth/diameter (d/D) between impact sites indicate that differences between the sites such as target material properties, impact velocity, angle, and physical state of the bolide(s) are important in determining the depth of small craters in the strength regime. On the Moon, the d/D of random fresh small craters with similar diameters averages only 0.10, indicating that either the majority of them are unrecognized secondaries or some proportion are degraded primaries. Older craters such as these may be shallower due to erosional infilling, which is probably not linear over time but more effective over recently disturbed and steeper surfaces, processes that are not yet acting on the new Martian craters. Brand new meter to decameter scale craters such as the Martian ones studied here are statistically easily distinguishable as primaries, but the origins of older craters of the same size, such as the lunar ones in this study, are ambiguous.

title: Explosive volcanism in complex impact craters on Mercury and the Moon: Influence of tectonic regime on depth of magmatic intrusion
abstract: Vents and deposits attributed to explosive volcanism occur within numerous impact craters on both the Moon and Mercury. Given the similarities between the two bodies it is probable that similar processes control this spatial association on both. However, the precise morphology and localization of the activity differs on the two bodies, indicating that the nature of structures beneath impact craters and/or volcanic activity may also be different. To explore this, we analyze sites of explosive volcanism within complex impact craters on the Moon and Mercury, comparing the scale and localization of volcanic activity and evidence for post formation modification of the host crater. We show that the scale of vents and deposits is consistently greater on Mercury than on the Moon, indicating greater eruption energy, powered by a higher concentration of volatiles. Additionally, while the floors of lunar craters hosting explosive volcanism are commonly fractured, those on Mercury are not. The most probable explanation for these differences is that the state of regional compression acting on Mercury's crust through most of the planet's history results in deeper magma storage beneath craters on Mercury than on the Moon. The probable role of the regional stress regime in dictating the depth of intrusion on Mercury suggests that it may also play a role in the depth of sub crater intrusion on the Moon and on other planetary bodies. Examples on the Moon (and also on Mars) commonly occur at locations where flexural extension may facilitate shallower intrusion than would be driven by the buoyancy of the magma alone.

title: Comparisons of fresh complex impact craters on Mercury and the Moon: Implications for controlling factors in impact excavation processes
abstract: The impact cratering process is usually divided into the coupling, excavation, and modification stages, where each stage is controlled by a combination of different factors. Although recognized as the main factors governing impact processes on airless bodies, the relative importance of gravity, target and projectile properties, and impact velocity in each stage is not well understood. We focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater exterior structures for similar sized fresh complex craters on the Moon and Mercury. We find that the ratios of continuous ejecta deposits, continuous secondaries facies, and the largest secondary craters on the continuous secondaries facies between same sized mercurian and lunar craters are consistent with predictions from gravity regime crater scaling laws. Our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the Moon and Mercury. On the other hand, similar sized craters with identical background terrains on Mercury have different spatial densities of secondaries on the continuous secondaries facies, suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities. Moreover, some craters on Mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on Mercury or the Moon. This morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on Mercury. A possible interpretation is that at some places on Mercury, the target material might have unique properties causing larger ejection angles during the impact excavation stage. We conclude that gravity is the major controlling factor on the impact excavation stage of complex craters, while impact velocity and target properties also affect the excavation stage but to a lesser extent than gravity.

title: Large rock slides in impact craters on the Moon and Mercury
abstract: Impact craters are the most common surface features on the Moon and Mercury. On these two bodies, we recognized and mapped large landslides on the walls of impact craters. Through visual inspection of high resolution imagery, we compiled an inventory of 60 landslides on the Moon and a second inventory of 58 landslides on Mercury. Adopting categories used to catalog terrestrial mass movements, we classified the landslides on the Moon and Mercury as rock slides. We determined the probability density distribution of their planimetric area, and we compared the distributions with similar distributions for terrestrial and martian landslides using data from the literature. We found that rock slides mapped in impact craters on the Moon are, on average, larger than analogous rock slides on Mercury. The relationship between the area of the individual rock slides and the area of the hosting crater suggests that rock slides on Mercury initiate in smaller craters. We hypothesize that the above findings are an effect of the weaker surface gravity of the Moon compared to that of Mercury and/or an effect of the rock material properties.