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The Saturnian system has been explored by four spacecraft: Pioneer 11, Voyager 1 and 2, and Cassini. Only the last three took images suitable for photogeologic analysis of the surfaces of Saturn's moons, and over the decades, several research groups have published data about the crater distributions on the Saturnian satellites. These groups have used those data to draw conclusions about the impactor populations and resurfacing histories of the moons, but no one has examined how well the different data agree between the researchers. We present independent mapping of the crater populations of Saturn's moons Mimas, Rhea, and Iapetus, and compare them with many published crater populations. We found that Mimas data are the most consistent between different researchers, and Rhea data are the least consistent. We attribute these differences to (a) data biases where there are fewer images upon which to map Mimantean craters but a large variety exist for Rhean, and (b) Rhea likely has different terrains with different impact crater populations which have not been generally recognized before. We also found that Iapetus' small craters appear to have a shallow branch, as others have found, and that shallow branch is not attributable to completeness limitations. Other bodies have shallow branches at small diameters, too, but they are not as shallow as Iapetus's, which suggests varying impacting populations as one moves closer to Saturn, in line with others' work on planetocentric impactors. Plain Language Summary: Saturn has 146 known moons. The largest, Titan, has a thick atmosphere and few impact craters on its surface. The six next‐largest moons are Rhea, Iapetus, Tethys, Dione, Enceladus, and Mimas. In this work, we studied craters on Rhea, Iapetus, and Mimas to better understand crater populations on their surfaces, how craters vary from moon to moon, how craters vary based on researcher differences, and what craters can tell us about objects in the outer solar system that made them. After cataloging more than 150,000 craters, we found that different researchers disagree most about craters on Rhea's surface, which suggests there might be some significant differences from location to location on Rhea. We found that smaller craters on Iapetus are the fewest in number relative to large craters of the three moons we studied. Since Iapetus is farthest from Saturn, we interpret this to mean that they more closely resemble the population of bodies orbiting the Sun in the outer solar system hitting these moons. We found that the population from moon to moon appears to change at small sizes, which suggests that impactors encircling Saturn form distinct impacting populations, too. Key Points: We mapped 10,981, 45,817, and 103,054 impact craters across Saturn's moons Mimas, Iapetus, and Rhea, respectivelyWe compare the crater populations to those of other researchers, and we found the best agreement on Mimas and worst on RheaThe satellites are indistinguishable from cratering equilibrium at 10 s km and were impacted by a shallow population with small diameters [ABSTRACT FROM AUTHOR]

Abstract: One important, almost ubiquitous, tool for understanding the surfaces of solid bodies throughout the solar system is the study of impact craters. While measuring a distribution of crater diameters and locations is an important tool for a wide variety of studies, so too is measuring a crater's “depth.” Depth can inform numerous studies including the strength of a surface and modification rates in the local environment. There is, however, no standard data set, definition, or technique to perform this data‐gathering task, and the abundance of different definitions of “depth” and methods for estimating that quantity can lead to misunderstandings in and of the literature. In this review, we describe a wide variety of data sets and methods to analyze those data sets that have been, are currently, or could be used to derive different types of crater depth measurements. We also recommend certain nomenclature in doing so to help standardize practice in the field. We present a review section of all crater depths that have been published on different solar system bodies which shows how the field has evolved through time and how some common assumptions might not be wholly accurate. We conclude with several recommendations for researchers which could help different data sets to be more easily understood and compared. [ABSTRACT FROM AUTHOR]

Abstract: Impact crater populations help us to understand solar system dynamics, planetary surface histories, and surface modification processes. A single previous effort to standardize how crater data are displayed in graphs, tables, and archives was in a 1978 NASA report by the Crater Analysis Techniques Working Group, published in 1979 in Icarus. The report had a significant lasting effect, but later decades brought major advances in statistical and computer sciences while the crater field has remained fairly stagnant. In this new work, we revisit the fundamental techniques for displaying and analyzing crater population data and demonstrate better statistical methods that can be used. Specifically, we address (1) how crater size‐frequency distributions (SFDs) are constructed, (2) how error bars are assigned to SFDs, and (3) how SFDs are fit to power‐laws and other models. We show how the new methods yield results similar to those of previous techniques in that the SFDs have familiar shapes but better account for multiple sources of uncertainty. We also recommend graphic, display, and archiving methods that reflect computers’ capabilities and fulfill NASA's current requirements for Data Management Plans. [ABSTRACT FROM AUTHOR]

Measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces. The calibration for this method, which enables conversion of crater density to absolute age, comes from Apollo and Luna lunar samples for which absolute radiometric ages have been determined. Researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration, correlating crater densities with absolute ages. However, no uniform crater study has been conducted on all calibration terrains, a limitation that was previously unaddressed until this study. The latest lunar images from the Lunar Reconnaissance Orbiter were used here to re-map the eleven main sampling sites and new crater counts of those surfaces were conducted. These show significant differences for many sites' crater counts, in many cases having more craters than previously identified. These results, calibrated to the radiometric ages, show a revised lunar crater chronology that changes previously established crater-based ages by up about 1 billion years: Surfaces younger than ∼ 3.6 Gyr and older than ∼ 3.9 Gyr under the classic chronology are younger in this system, and those in-between are older in this new system. [ABSTRACT FROM AUTHOR]

Abstract: The Tharsis bulge is the largest magmatic/volcanic center on Mars and in the solar system, having a volume of ~3×108 km3, with the majority of its mass emplaced more than 3.7billion years ago. The igneous history of Tharsis has been implicated in the generation of an atmosphere and hydrosphere capable of clement conditions on early Mars. It has been proposed that an early plume migration from the southern highlands of Mars relative to a one-plate lithosphere led to the development of the Tharsis bulge in its current location along the Martian crustal dichotomy. We used geologic mapping, crustal magnetic data, crater age-dating and crater morphometry to detail a previously-unidentified path of putative plume migration. Our results indicate that extensive volcanic resurfacing occurred from a location near the present south pole to the equator around 3.8billion years ago, obliterating older cratered terrains. The resurfacing path is manifest as smooth volcanic plains embaying ancient massifs and infilling large impact craters as well as a lack of a magnetic signature in this portion of the crust. Our results have significant ramifications for mantle dynamics and the early geologic history of Mars. [Copyright &y& Elsevier]

Abstract: Determining absolute surface ages for bodies in the Solar System is, at present, only possible for Earth and Moon with radiometric dating for both bodies and biologic proxies such as fossils for Earth. Relative ages through cratering statistics are recognized as one of the most reliable proxies for relative ages, calibrated by lunar geologic mapping and Apollo program sample returns. In this work, we have utilized the Mars Reconnaissance Orbiter’s ConTeXt Camera’s images which provide the highest resolution wide-scale coverage of Mars to systematically crater-age-date the calderas of 20 of Mars’ largest volcanoes in order to constrain the length of time over which these volcanoes – and major volcanic activity on the planet, by extension – were active. This constitutes the largest uniform and comprehensive research on these features to date, eliminating unknown uncertainties by multiple researchers analyzing different volcanoes with varied data and methods. We confirm previous results that Mars has had active volcanism throughout most of its history although it varied spatially and temporally, with the latest large-scale caldera activity ending approximately 150ma in the Tharsis region. We find a transition from explosive to effusive eruption style occurring in the Hesperian, at approximately 3.5Ga ago, though different regions of the planet transitioned at different times. Since we were statistically complete in our crater counts to sizes as small as ∼60m in most cases, we also used our results to study the importance of secondary cratering and its effects on crater size–frequency distributions within the small regions of volcanic calderas. We found that there is no “golden rule” for the diameters secondaries become important in crater counts of martian surfaces, with one volcano showing a classic field of secondaries ∼2 crater diameters from the center of its primary but not affecting the size–frequency distribution, and another clearly showing an influence but from no obvious primary. [Copyright &y& Elsevier]

Based on the input data from the crater identification process (center latitude/longitude and diameter), four annuli centered on the crater are automatically placed on the DTM of the crater and a surrounding region. This measurement is the most uncertain of the three measurements made because of uncertainties in terrain modification since crater formation, and because it is not possible to know the pre-existing topography before the crater formed. Unlike Charon, Pluto has been geologically active throughout its history, meaning its craters are generally more modified or have been entirely eliminated, and so the number of minimally modified craters identified on Pluto was roughly half that on Charon. [Extracted from the article]

Abstract: Near-horizontal planetary impacts result in elliptical craters. The percentage of elliptical craters on a planet can be used to infer the impact angle at which craters become elliptical. Previous surveys of the Moon, Mars, and Venus indicated that planetary craters become elliptical at more vertical angles than experimental impacts into a strengthless medium, and this was attributed to a higher ratio of crater diameter to projectile diameter. Here we determined the percentage of elliptical craters on the mid-sized saturnian satellites and Mercury, bodies that represent Solar-System extremes of impactor velocity, target density, and target strength. On the saturnian satellites, 7.6% of the craters have ellipticities e (ratio of major to minor axis) greater than 1.2, but only 0.4% have e >1.5, and no craters have e >1.75. On Mercury, 3% of the craters have e >1.2 and 0.5% have e >1.5. The mercurian percentages are slightly lower than the other terrestrial planets, attributable to a higher crater diameter to projectile diameter caused by the higher impact velocities at Mercury. We attribute the high percentage of moderately elliptical craters on the saturnian satellites to the rugged target terrain on those bodies. We interpret enhanced crater collapse on the icy surfaces of the saturnian satellites as preventing craters with extremely high ellipticities like the lunar crater Schiller. Finally, a reexamination of the martian crater population shows its elliptical crater population to be consistent with the other planets, and we see little evidence for a large population of craters formed by inward-spiraling moonlets. [Copyright &y& Elsevier]

We investigate what can be learned about a population of distant Kuiper Belt Objects (KBOs) by studying the statistical properties of their light curves. Whereas others have successfully inferred the properties of individual, highly variable KBOs, we show that the fraction of KBOs with low amplitudes also provides fundamental information about a population. Each light curve is primarily the result of two factors: shape and orientation. We consider contact binaries and ellipsoidal shapes, with and without flattening. After developing the mathematical framework, we apply it to the existing body of KBO light curve data. Principal conclusions are as follows. (1) When using absolute magnitude H as a proxy for the sizes of KBOs, it is more accurate to use the maximum of the light curve (minimum H) rather than the mean. (2) Previous investigators have noted that smaller KBOs tend to have higher-amplitude light curves, and have interpreted this as evidence that they are systematically more irregular in shape than larger KBOs; we show that a population of flattened bodies with uniform proportions, independent of size, could also explain this result. (3) Our method of analysis indicates that prior assessments of the fraction of contact binaries in the Kuiper Belt may be artificially low. (4) The pole orientations of some KBOs can be inferred from observed changes in their light curves over time scales of decades; however, we show that these KBOs constitute a biased sample, whose pole orientations are not representative of the population overall. (5) Although surface topography, albedo patterns, limb darkening, and other surface properties can affect individual light curves, they do not have a strong influence on the statistics overall. (6) Photometry from the Outer Solar System Origins Survey (OSSOS) survey is incompatible with previous results and its statistical properties defy easy interpretation. We also discuss the promise of this approach for the analysis of future, much larger data sets such as the one anticipated from the upcoming Vera C. Rubin Observatory. • A statistical review of light curves provides untapped information about Kuiper Belt objects. • Contact binaries have systematically lower-amplitude light curves than equivalent ellipsoids. • A KBO's minimum (brightest) absolute magnitude, not its mean, is the best proxy for its size. • Smaller KBOs may not be systematically more irregular in shape than large ones. • Every KBO data set should contain a large fraction of low-amplitude light curves. [ABSTRACT FROM AUTHOR]

The dominant topographic features on two-lobed Kuiper Belt object (486958) Arrokoth (provisionally designated 2014 MU 69) are scattered, small, circular depressions or pits up to ~1.0 km across and curvilinear troughs observed near the terminator during the New Horizons encounter of 01 January 2019. With important exceptions, evidence for an endogenic origin for pits is lacking and impact remains the most likely origin. Pit depths relative to the local surface are shallower than hypervelocity simple craters on icy moons and consistent with low velocity (~300 m/s) impacts in an icy target (the role of porosity and cohesion in cratering on Arrokoth being uncertain). There is, however, a large scatter in observed d/D, with some pits too shallow to measure reliably. The range of preservation states observed for both pits and troughs is consistent with slow but persistent degradation on the surface of this (and perhaps other) small primitive planetesimal(s) in the Kuiper Belt by agents such as micrometeorite bombardment or volatile loss, indicating that degradation does indeed occur on such bodies. Arrokoth pits could also be related to circular depressions on comet 9P/Tempel (though comparisons of surface features on Arrokoth and comets are intriguing they are limited by resolution differences and the evolved state of cometary surfaces). Two of the curvilinear troughs occur along the terminator of the Large Lobe. One trough is a shallow depression devoid of resolvable structures, the other features several linear scarps a few 10s of meters high, suggesting that coherent failure can occur on the surfaces of small Kuiper Belt objects such as Arrokoth. Several of the deepest small pits are also clustered within this walled trough, similar to tectonically associated pit chains on other planets. These pits and an obliquely viewed linear chain of elongate depressions remain the best candidates for endogenic features on Arrokoth. Subtle undulations and linear features no more than a few 10's of meters in amplitude (and even a few candidate positive relief features) are also evident in the terminator regions, indicating that the long-term evolution of this body may have been complex. • Circular pits on the KBO Arrokoth are shallower than simple craters on icy moons. • Range of observed pit depths suggests slow degradation of Arrokoth over much of Solar System's age. • Linear troughs on the terminator are shallow, <45 m deep. • Linear pit chains are best candidates for endogenic activity. • Whether these are related to linear and circular features on comets is unknown [ABSTRACT FROM AUTHOR]