Lunar Chronology Research Articles

A Catalogue of Impact Craters and Surface Age Analysis in the Chang’e-6 Landing Area

Chang’e-6 (CE-6) is the first sample-return mission from the lunar farside and will be launched in May of 2024. The landing area is in the south of the Apollo basin inside the South Pole Aitken basin. Statistics and analyses of impact craters in the landing area are essential to support safe landing and geologic studies. In particular, the crater size–frequency distribution information of the landing area is critical to understanding the provenance of the CE-6 lunar samples to be returned and can be used to verify and refine the lunar chronology model by combining with the radioisotope ages of the relevant samples. In this research, a digital orthophoto map (DOM) mosaic with resolution of 3 m/pixel of the CE-6 landing area was generated from the 743 Narrow Angle Camera of the Lunar Reconnaissance Orbiter Camera. Based on the DOM, craters were extracted by an automated method and checked manually. A total of 770,731 craters were extracted in the whole area of 246 km × 135 km, 511,484 craters of which were within the mare area. Systematic analyses of the crater distribution, completeness, spatial density, and depth-to-diameter ratio were conducted. Geologic model age estimation was carried out in the mare area that was divided into three geologic units according to the TiO2 abundance. The result showed that the east part of the mare had the oldest model age of μ3.27−0.045+0.036 Ga, and the middle part of the mare had the youngest model age of μ2.49−0.073+0.072 Ga. The crater catalogue and the surface model age analysis results were used to support topographic and geologic analyses of the pre-selected landing area of the CE-6 mission before the launch and will contribute to further scientific researches after the lunar samples are returned to Earth.

Recalibration of the lunar chronology due to spatial cratering-rate variability

Cratering chronologies are used to derive the history of planetary bodies and assume an isotropic flux of impactors over the entire surface of the Moon. The impactor population is largely dominated by near-Earth-objects (NEOs) since ∼3.5 billion years ago. However, lunar impact probabilities from the currently known NEO population show an excess of impacts close to the poles compared to the equator as well as a latitudinal dependency of the approach angle of impactors. This is accompanied by a variation of the impact flux and speed with the distance from the apex due to the synchronicity of the lunar orbit around the Earth. Here, we compute the spatial dependency of the cratering rate produced by such variabilities and recalibrate the lunar chronology. We show that it allows to reconcile the crater density measured at mid-latitudes around the Chang'e-5 landing site with the age of the samples collected by this mission. Our updated chronology leads to differences in model ages of up to 30% compared to other chronology systems. The modeled cratering rate variability is then compared with the distribution of lunar craters younger than ∼1 Ma, 1 Ga and 4 Ga. The general trend of the cratering distribution is consistent with the one obtained from dynamical models of NEOs, thus potentially reflecting a nonuniform distribution of orbital parameters of ancient impactor populations, beyond 3.5 Ga ago, i.e., planetary leftovers and cometary bodies. If the nonuniformity of the cratering rate could be tested elsewhere in the Solar System, the recalibrated lunar chronology, corrected from spatial variations of the impact flux and approach conditions of impactors, could be extrapolated on other terrestrial bodies such as Mercury and Mars, at least over the last 3.5 billion years. The modeled cratering rate presented here has strong implications for interpreting results of the Artemis program, aiming to explore the South Pole of our satellite, in particular when it will come to link the radiometric age of the samples collected in this region and the crater density of the sampled units.

Geological mapping and chronology of lunar landing sites: Apollo 14

The lunar cratering chronology function is based on the careful and systematic correlation of crater size-frequency distributions (CSFDs) with radioisotopic ages of samples from the Apollo, Luna, and Chang'e landing sites. The link between the samples and the CSFDs requires a detailed understanding of the landing site geology. Therefore, in this study, we have created a new detailed regional geological map of the Apollo 14 landing site using LRO, M3, and SELENE/Kaguya data. The geologic map allows us to define homogeneous areas for new CSFD measurements, from which updated N(1) values (i.e., cumulative number of craters with diameter ≥ 1 km) can be determined for the sampled geologic units. In particular, at the Apollo 14 landing site we measured the N(1) of the Imbrian Fra Mauro Formation (Ifm), which is interpreted to be ejecta from the Imbrium basin, and can be correlated with the Apollo 14 Imbrium ejecta samples. This new calibration point can be compared with the lunar chronology of Neukum (1983). We find that the new calibration point is consistent with the lunar cratering chronology of Neukum (1983). Together with our previous studies, this result supports the validity of this chronology.

Improvement of Lunar Surface Dating Accuracy Utilizing Crater Degradation Model: A Case Study of the Chang’e-5 Sampling Area

Taking the Chang’e-5 (CE-5) sampling area as an example, this study carried out an investigation on improving the crater size-frequency distribution (CSFD) dating accuracy of lunar surface geologic units based on the crater degradation model. We constructed a three-parted crater degradation model, which consists of the diffusion equation describing crater degradation and equations describing the original crater profile for small craters (D < 1 km) and larger craters (D ≥ 1 km). A method that can improve the accuracy of CSFD dating was also proposed in this study, which utilizes the newly constructed degradation model to simulate the degradation process of the craters to help determine the crater degradation process and screen out the craters suitable for CSFD analysis. This method shows a good performance in regional dating. The age determined for the CE-5 sampling area is 2.0 ± 0.2 Ga, very close to the 2.03 ± 0.004 Ga of isotopic dating result of the returned sample. We found that the degradation state of the craters simulated by our constructed degradation model is highly consistent with the real existing state of the craters in terms of their topographic, geomorphological, and compositional (e.g., FeO) features. It fully demonstrates that the degradation model proposed in this study is effective and reliable for describing and distinguishing the degradation state of craters over time due to the cumulative effects of small craters. The proposed method can effectively distinguish between diffusively degraded (which conform to the degradation model) and non-diffusively degraded (which do not conform to the degradation model) craters and improve the CSFD accuracy through the selection of the craters. This not only provides an effective solution to the problem of obtaining a more “exact” frequency distribution of craters, which has long plagued the practical application of the CSFD method in dating the lunar surface but also advances our understanding of the evolutionary history of the geologic units of the study area. The results of this work are important for the in-depth study of the formation and evolution of the moon, especially for lunar chronology.

Martian surface dating model refinement based on Chang'E-5 updated lunar chronology function

Planetary surface ages are extremely important to geologic evolution studies. For extraterrestrial bodies, the crater size-frequency distribution dating method, including a crater production function and a chronology function, was established based on the radiometric ages of Apollo and Luna samples from the Moon. Recently, the radiometric age of a sample from Chang'e-5 mission added a data point where there was previously a gap in sample ages, and an updated lunar chronology function was proposed. This research aims to recalculate the Martian surface dating model from the newly updated lunar chronology function. The calculation takes account of the differences in impact rates, impact velocities, surface gravities, the threshold diameters of craters' transition from the strength to gravity regime and from simple to complex crater structures on the Moon and Mars. We use the Neukum production function, although other production functions could also be implemented with our method. A series of Martian isochrons is derived, and an approximated analytical form production function is provided for convenient applications in future studies. The Martian chronology function is finally obtained through fitting crater frequencies larger than 1.0 km in diameter with the corresponding ages of the isochrons. The resultant Martian PF and CF in this research are largely comparable with previous results. Based on the refined lunar chronology function, our model will enhance the reliability of the estimating Martian surface ages and we suggest it be used in future studies.

The Effects of Terrain Properties Upon the Small Crater Population Distribution at Giordano Bruno: Implications for Lunar Chronology.

The distribution of impact craters on the ejecta of Giordano Bruno, a recent (<10 Ma) 22‐km diameter crater within the lunar highlands, exhibits substantial variations. We surveyed craters D ≥ 10 m across a 1,323 km2 area of Giordano Bruno's ejecta and compared the distribution of craters with variations in thermophysical properties derived from the Lunar Reconnaissance Orbiter Diviner instrument. We used Diviner‐derived rock abundance and nighttime regolith temperatures along with thermal model‐predicted surface temperatures for a diversity of terrains to identify and isolate areas of the ejecta based on thermophysical properties such as bulk density and thermal conductivity. We found that thermophysical properties of the ejecta vary considerably both laterally and vertically, and consistently differ from typical regolith, indicating the presence of higher thermal inertia materials. Crater‐size frequencies are significantly lower in areas with terrain properties exhibiting higher: rock abundance, nighttime temperatures, and/or modeled thermal inertia. This discrepancy in crater distribution increases for craters smaller than ∼25 m. These thermophysical variations indicate changes in the mechanical properties of the target materials. We suggest that these variations—specifically, terrain‐dependent crater scaling variations and impactor‐scale heterogeneities in material properties such as the presence or absence of large boulders—may influence crater diameters or inhibit crater production altogether in Giordano Bruno's ejecta; furthermore, these factors are size‐dependent.

Cratering Records in the Chang'e‐5 Mare Unit: Filling the “Age Gap” of the Lunar Crater Chronology and Preparation for Its Recalibration

AbstractThe Moon is the cornerstone for dating planetary surfaces as it is the only planetary body that has been sampled multiple times from multiple places. The major deficiencies of the lunar crater chronology function are the somewhat limited number of calibration points and data gaps, especially in the ∼1–3 Ga segment. China's Chang'e‐5 mission recently returned samples from a young nearside mare, whose age is estimated to be in that age gap, providing a long‐awaited opportunity for recalibrating the lunar chronology. Here, we report a direct density measurement of impact craters ≥1 km in diameter of Chang'e‐5 mare unit, as (1.696 ± 0.221) × 10−3 km−2. The Chang'e‐5 mare deposits are estimated as ∼1.3–2.7 Ga old in various lunar chronologies, younger than all collected Apollo/Luna basalts. Accurate radio‐isotopic ages of Chang'e‐5 samples can be combined with our crater density measurement for a recalibration of lunar chronology.

Absolute model ages of three craters in the vicinity of the Chang'E-5 landing site and their geologic implications

The Chang'E-5 (CE-5) mission has successfully collected lunar regolith samples from the northern Oceanus Procellarum. The Eratosthenian-aged mare units where the CE-5 landed on, might have been contaminated by distant ejecta delivered by several craters, such as Pythagoras, Sharp B, Harpalus, Copernicus, and other craters. Laboratory dating of distant ejecta in the CE-5 regolith can provide the formation ages of these craters. Therefore, these ejecta materials, the same as local young mare basalts, would serve as new calibration points for lunar chronology curve. Among above craters, the absolute model ages (AMAs) of Pythagoras, Sharp B and Harpalus are not well constrained. Here we used the Crater Size Frequency distribution (CSFD) method to yield their formation ages and the crater density values (N(1)/(km−2)). Using high resolution images, craters on the selected counting areas of their floors, ejecta blankets were identified. Our results indicate that Pythagoras is an Eratosthenian-age crater with an AMA of 2.13 ± 0.16 Ga and a N(1) =1.79 × 10−3 km−2. The Sharp B and Harpalus are both Copernican. The AMA of Sharp B crater is 1.01 ± 0.12 Ga with a N(1) of 8.46 × 10−4 km−2 while the AMA of Harpalus is 1.10 ± 0.04 Ga with a N(1) of 9.22 × 10−4 km−2. The ejecta from Sharp B and Harpalus as well as local mare basalt of the CE-5 site may be served as calibration points for lunar chronology curve, which will fill the age gaps of lunar chronology curve. Based on the estimated ages, we interpreted that Pythagoras ejecta might be beneath the young CE-5 mare unit, while the Sharp B and Harpalus ejecta should overlie the surface of the CE-5 site.

Detailed Morphologic Mapping and Traverse Planning for a Rover-based Lunar Sample Return Mission to Schrödinger Basin

Abstract Schrödinger basin, a 312 km diameter complex impact structure located near the lunar south pole, has been widely cited as a prime target for future lunar exploration. In 2020 NASA identified Schrödinger as a high-priority landing site for a 2024 mission supported by the Payloads and Research Investigations on the Surface of the Moon solicitation and the Commercial Lunar Payload Services program. Schrödinger basin hosts an uplifted peak ring that would provide a surface mission with access to materials that originated deep within the lunar crust, as well as material ejected from the larger South Pole–Aitken basin. Schrödinger basin also hosts well-preserved mare and pyroclastic deposits that could provide valuable insight into volcanic processes on the Moon. This study used high-resolution Wide Angle Camera and Narrow Angle Camera images from the Lunar Reconnaissance Orbiter, elevation data from the Lunar Orbiter Laser Altimeter, and spectral data from the Clementine mission to produce a high-resolution morphologic map of the basin center consisting of 10 distinct morphologic units. This new map was used to plan traverse paths for a rover mission to the region. The design requirements for this traverse were based on those originally developed for the multiagency Human-Enhanced Robotic Architecture and Capability for Lunar Exploration and Science (HERACLES) mission and the Canadian Space Agency Precursor to Humans And Science Rover concept. The proposed traverse path includes up to 20 sample collection stops with the goal of better understanding lunar chronology, lunar volcanism, and the impact cratering process.

China's Chang'e-5 landing site: Geology, stratigraphy, and provenance of materials

China's Chang'e-5 (CE-5) mission, the first lunar sample return mission since 1976, landed at 43.06°N, 51.92°W on Dec. 1, 2020, in Northern Oceanus Procellarum. CE-5 targeted a mare plain (Em4/P58) composed of distinctive young (∼1.6-1.7 Ga) moderate-Ti mare basalts, with elevated Th abundance (inherent or extraneous). Thus, the regolith and rock fragments sampled by CE-5 come from some of the youngest mare basalts on the Moon, near Rima Sharp, and from the center of the globally anomalous Procellarum KREEP Terrane (PKT), hypothesized to be responsible for the generation of the young volcanism. To provide context for the analysis and interpretation of the returned samples and in-situ measurements of the regolith substructure with penetrating radar, we constructed a detailed geologic map and stratigraphic assessment of the site. The stratigraphy consists of ancient highland materials (PKT crust and ejecta from Iridum and Imbrium basins), local silica-rich volcanism, overlain by a sequence of mare basalts, capped with Em4/P58. A ∼4-7 m thick regolith layer developed by post-mare bombardment overlies the Em4/P58 protolith and contains admixed impact ejecta from distant sources, mainly from Harpalus (∼6 wt.%), followed by Copernicus (∼2 wt.%) and Aristarchus (∼1 wt.%). New crater size-frequency measurements of Em4/P58 provide the necessary crater spatial density reference for calibration of the lunar cratering chronology with radiometric ages of the returned samples. The geological map and assessment of regolith provenance indicate that samples returned by CE-5 will address fundamental questions in lunar chronology, thermal evolution, basalt petrogenesis, and the nature of PKT, as well as provide key calibration for lunar and planetary chronologies and remote sensing data.

Suggestion that recent (≤ 3 Ga) flux of kilometer and larger impactors in the Earth-Moon system has not been constant

Lunar impact crater chronologies have been developed by combining carefully measured crater densities of lunar terrains with radiometric ages provided by returned samples. However, due to the sparse coverage of the samples during the last three billion years, this part of the chronologies is not well constrained. Lunar crater chronologies generally assume that the bombardment rate has been relatively constant during this epoch for all impactor sizes. Nevertheless, evidence has been gathering that this may not be the case for impactors larger than several hundred meters; however, there may be biases related to some of this evidence. The break-up of large asteroids in the Main Asteroid Belt to make asteroid families could be a source of a changing impact flux in the Earth-Moon system, especially when the families form near strong orbital resonances with the gas giants. In order to further explore the state of the impact flux from today to three billion years ago for impactors larger than ~5 km, we calculate crater retention model ages of 43 lunar craters 50 km and larger in diameter (D). Selected craters were initially suggested by United States Geological Survey geological maps of the Moon and Wilhems (1987) to have formed during the Copernican and Eratosthenian. We use the density of small (D < a few km) craters superposed on their floors, along with Model Production Function (MPF) lunar chronology (Marchi et al., 2009). For this purpose, we assume that the smaller impactors forming the superposed craters follow a constant flux as indicated by the MPF and supported by models of the dynamical evolution of the impactor population. We use the model ages as a proxy for the impact flux of larger impactors and test whether their distribution in time is consistent with a constant flux using two statistical analyses. Our results suggest that the flux of these larger impactors could be variable during the last three billion years, with hints of a relative increase in flux occurring ~2 billion years ago and a decrease in flux ~1 billion years ago. Thus, our results support the evidence that the impact flux has varied on the Moon for impactors over a few kilometers in diameter.

Modeling the Chronologies and Size Distributions of Ceres and Vesta Craters

Abstract We infer the crater chronologies of Ceres and Vesta from a self-consistent dynamical model of asteroid impactors. The model accounts for planetary migration/instability early in the history of our solar system and tracks asteroid orbits over 4.56 Gyr. It is calibrated on the current population of the asteroid belt. The model provides the number of asteroid impacts on different worlds at any time throughout the solar system's history. We combine the results with an impactor-crater scaling relationship to determine the crater distribution of Ceres and Vesta and compare these theoretical predictions with observations. We find that: (i) The Ceres and Vesta chronologies are similar, whereas they significantly differ from the lunar chronology. Therefore, using the lunar chronology for main belt asteroids, as often done in previous publications, is incorrect. (ii) The model results match the number and size distribution of large (diameter &gt;90 km) craters observed on Vesta, but overestimate the number of large craters on Ceres. This implies that large crater erasure is required for Ceres. (iii) In a model where planetary migration/instability happens early, the probability to form the Rheasilvia basin on Vesta during the last 1 Gyr is 10%, a factor of ∼1.5 higher than for the late instability case and ∼2.5 times higher than found in previous studies. Thus, while the formation of the Rheasilvia at ∼1 Gyr ago (Ga) would be somewhat unusual, it cannot be ruled out at more than ≃1.5σ. In broader context, our work provides a self-consistent framework for modeling asteroid crater records.

Geological mapping and chronology of lunar landing sites: Apollo 12

The lunar cratering chronology is fundamentally important, because it is not just used to determine ages for unsampled lunar surfaces, but is commonly applied to other planetary bodies. As part of a systematic study of the calibration of the lunar cratering chronology at the Apollo landing sites, we produced a new detailed geological map of the Apollo 12 landing site, using recent orbital data including Lunar Reconnaissance orbiter (LRO) Narrow Angle (NAC) and Wide Angle (WAC) images, and a Lunar Orbiter Laser Altimeter (LOLA)/SELENE merged digital elevation model (DEM), as well as Chandrayaan M3 spectral data. Using high-resolution LRO NAC images and NAC-derived DTMs, new crater size-frequency distributions (CSFDs) were measured on the updated geological units to determine crater retention ages or N(1) values (i.e., the cumulative number of craters with diameters ≥1 km). Since the Apollo 12 landing site is located in Oceanus Procellarum on a ray of Copernicus crater, it provides two possible calibration points for the lunar cratering chronology: (1) a mare basalt surface age, and (2) an approximate age for Copernicus crater via its ray material. We calibrated our new N(1) values with recently determined radiometric ages of Apollo 12 basalt samples, and compare our results with work done by Hiesinger et al. (2012) for Copernicus crater, and the Neukum (1983) chronology. The updated calibration points are consistent with the lunar chronology of Neukum (1983), which indicates that no updates to this function are currently necessary.

A catalogue of impact craters larger than 200 m and surface age analysis in the Chang'e-5 landing area

Chang'e-5 (CE-5) is the first sample-return mission of China's lunar exploration and will be launched in 2020. The planned landing area is near Mons Rümker in Oceanus Procellarum. It is important to have a detailed geological analysis of the landing area prior to the lunar sample collection. The crater size-frequency distribution curve, combined with radioisotope dating of components of the returned sample by CE-5, may allow a refinement of the lunar chronology function. Based on a digital orthophoto map with a pixel size of 1.5 m generated from more than 700 Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images of the CE-5 landing area, all impact craters larger than 200 m in diameter were extracted. Using the resultant crater catalogue of 32,277 craters, a systematic analysis of the landing area including the crater distribution, morphology characteristics, and geologic model ages is conducted. The results show that craters smaller than 1,000 m are denser in the east and the west parts of the region, while those larger than 1,000 m tend to be more concentrated in the central part, particularly within the range of 61.5°W–64.5°W. Within nine geologic units divided using the spectral characteristics, the geologic unit of Em1 (Eratosthenian-aged mare, Em) near the Mons Rümker is the youngest with a model age of 2.02−0.16+0.16 Ga. The three Rümker plateau (IR) units are the oldest parts of the landing area with the largest model age of 3.49−0.12+0.073 Ga. The crater catalogue and the surface model age analysis results can contribute to the CE-5 mission as well as further scientific research.

Lunar megaregolith mixing by impacts: Spatial diffusion of basin melt and its implications for sample interpretation

The formation ages of lunar impact basins are critical to understanding the late accretion history of the inner solar system. Furthermore, the correct interpretation of the provenance and isotopic dates of basin-derived impact melt (‘basin melt’) is essential for the calibration of lunar chronology function. However, abundances of basin melt in the lunar near-surface are not well understood. Basin melt has been gardened by a long sequence of subsequent impact events, altering its abundance and size distribution. We developed a numerical model to investigate this process by means of the Monte Carlo method in a spatially resolved model. The fraction of melt in ejecta was tracked globally and at the Apollo 14–17 and Luna 20 sampling sites and was compared with K-Ar age distributions of lunar impact melt breccias. It was found that melt produced by the very large SPA basin as well as the relatively late-forming Imbrium basin should be dominant in the near-surface (top one meter). The simulation shows that the melt component at the Apollo 14–17 and Luna 20 sites is strongly affected by nearby mid- to late-forming basins. Imbrium melt should be abundant in Apollo 14–17 samples; Crisium melt is the most significant component of basin-sourced melt in Luna 20 samples; all the Apollo 14–17 and Luna 20 samples could include melt from Serenitatis and the SPA basin; Nectaris melt should occur in Apollo 16, Apollo 17 and Luna 20 samples; and Orientale melt has no significant mixing in the Apollo 14–17 and Luna 20 sampling sites. In general, besides a prominent age peak at 3.9 Ga (related to the Imbrium basin), the model predicts pronounced abundance peaks of older basin melt (>3.9 Ga) which tend to be absent from distributions of K-Ar ages of impactites from landing sites. The diffusion characteristics of basin melt suggest that for future sampling aimed at collecting early basin melt, the re-excavation zones of late impact craters larger than tens of kilometer in diameter inside basins may provide the highest abundances of melt from early basins.

Geological mapping and chronology of lunar landing sites: Apollo 11

Crater size-frequency distribution (CSFDs) measurements allow the derivation of absolute model ages (AMAs) for geological units across various terrestrial bodies in the Solar System based on body-specific adjustments to the lunar chronology (e.g., Hartmann, 1970; Neukum et al., 1975, 1983, 2001; Stöffler and Ryder, 2001; Stöffler et al., 2006; Hiesinger et al., 2012; Robbins, 2014). Thus, it is important to revisit and test the accuracy of the lunar chronology using data from recent lunar missions (e.g., Hiesinger et al., 2000, 2012, 2015; Rajmon and Spudis, 2004; Stöffler et al., 2006), as well as newer analyses of lunar samples (e.g., Gaffney et al., 2011, Meyer, 2012; Snape et al., 2016; Welsh et al., 2018). We generated a new detailed geological map of the Apollo 11 landing region based on spectral characteristics, topography, and albedo maps, which shows several mare units adjacent to the lunar module. Lunar Reconnaissance Orbiter Camera (LROC) images were used to measure new CSFDs and derive the cumulative number of craters with diameters ≥1 km or N(1) for the Apollo 11 landing site. The newly derived N(1) values are consistent with the presence of only one surficial unit at the landing site: the Group A, High-K (high potassium) “young” mare basalt as described in Meyer (2012). We reviewed the radiometric ages for Apollo 11 samples that have been determined since the calibration of the lunar cratering chronology, used our new geological map to reinterpret their provenance, and correlated them with the new N(1) values. These are plotted and compared with the lunar chronology of Neukum et al. (1983). Our calibration point for the Apollo 11 landing site is consistent with the earlier values, thus, confirming Neukum's 1983 and Neukum and Ivanov's 2001 lunar chronology curves.

Impact bombardment on the regular satellites of Jupiter and Uranus during an episode of giant planet migration

The intensity and effects of early impact bombardment on the major satellites of the giant planets during an episode of giant planet migration is still poorly known. We use a combination of dynamical N-body and Monte Carlo simulations to determine impact probabilities, impact velocities, and expected masses that collide with these satellites to determine the chronology of impacts during the migration. Volatile loss through bombardment is typically 20% for Miranda, a few percent for the larger Uranian satellites and negligible for the Galilean satellites. Due to its small size and the high impact velocity there is a >99% chance that Miranda suffered a catastrophic impact that shattered the satellite. Subsequent re-accretion from a circum-Uranian ring could account for its peculiar surface morphology and low density. The probability to destroy Ariel and Umbriel is 15% and 1% for Titania and Oberon. Approximately 90% of the mass in planetesimals that passes through the Jovian and Uranian satellite systems (about 4 M⊕ and 2 M⊕ respectively) does so in about 15 Myr. This extremely rapid and intense bombardment causes repeated local crustal melting on all satellites. The combination of these effects results in an entirely different impact chronology than that of the inner solar system. We conclude that the simple extrapolation of the lunar chronology to the outer solar system satellites is not correct. The tail end (after 25 Myr) of the chronology function has an e-folding time of 100 Myr at Jupiter, but follows a cumulative Weibull distribution at Uranus, making direct comparisons between the gas and ice giant planets difficult. Based on our results the surfaces of the Uranian satellites, Callisto, and possibly Ganymede, are all about the same age, and are roughly 150 Myr younger than the timing of the dynamical instability.

Reexamination of Early Lunar Chronology With GRAIL Data: Terranes, Basins, and Impact Fluxes

AbstractFlooding of the lunar surface by ancient mare basalts has rendered uncertain the ages of lunar geochemical terranes and several impact basins. Here we combine craters having recognizable surface expressions with craters identified only by their gravitational signatures in Gravity Recovery and Interior Laboratory data to reassess the chronological sequence of lunar impact basins and the ages of major lunar geochemical terranes. Our results indicate that although volcanically flooded regions are deficient in craters with diameters greater than 20 km by more than 50% relative to unflooded regions, craters with diameters greater than 90 km can be readily recognized either by topography or by gravity anomaly. On the basis of the areal density of craters with diameters greater than 90 km we conclude that (1) the Serenitatis basin could be as young as the Imbrium basin; (2) the areal density of craters within the Procellarum KREEP Terrane is significantly lower than that for the South Pole‐Aitken basin and the Feldspathic Highlands Terrane; (3) if the youngest age of final crystallization of the lunar magma ocean is adopted as a lower bound on the age of the Procellarum KREEP Terrane, a minimum age of approximately 4.3 Ga is inferred for ~40% of lunar impact basins, including South Pole‐Aitken; and (4) the flux of impactors capable of forming craters with diameters of at least 90 km decreased substantially through the Nectarian and Imbrian periods.

Fundamental Problems of Lunar Research, Technical Solutions, and Priority Lunar Regions for Research

In this article, we discuss four fundamental scientific problems of lunar research: (1) lunar chronology, (2) the internal structure of the Moon, (3) the lunar polar regions, and (4) lunar volcanism. After formulating the scientific problems and their components, we proceed to outlining a list of technical solutions and priority lunar regions for research. Solving the listed problems requires investigations on the lunar surface using lunar rovers, which can deliver a set of analytical equipment to places where geological conditions are known from a detailed analysis of orbital information. The most critical research methods, which can answer some of the key questions, are analysis of local geological conditions from panoramic photographs, determination of the chemical, isotopic, and mineral composition of the soil, and deep seismic sounding. A preliminary list is given of lunar regions with high scientific priority.

Roosting patterns of the Edible-Nest Swiftlet (Aerodramus fuciphagus) of the Andaman Islands: effects of lunar phase and breeding chronology

ABSTRACTEnvironmental conditions and biological rhythms can affect the behavioural routines of animals. However, the effect of lunar phase on individual roosting behaviour remains uninvestigated in most species. Here, we monitored the effect of lunar phase, sunset time, temperature, humidity and the breeding chronology on the roosting patterns of the Edible-Nest Swiftlet (Aerodramus fuciphagus) of the Andaman Islands, across breeding stages. Counts revealed that more than 98% of the population returned to the roosting caves during peak roosting hours, i.e. 1700–2000h. The proportion of birds roosting in caves was highest during the ‘new moon’ phase and when birds were at the nest-building and fledging stage of their breeding cycle. We confirmed that the variation in the roosting behaviour of the Edible-Nest Swiftlet is linked both to the stages in the breeding cycle and to the lunar phase. We hypothesise that the cause for the lunarphilic roosting pattern is an anti-predator strategy. Feeding habits and food requirements during different breeding stages are presumed to influence the roosting pattern of the species. An improved understanding of the association of the behaviour, physiology and the environmental conditions which influence these traits can only help us improve conservation outcomes for this economically important species.