Lunar Reconnaissance Orbiter Research Articles

High alumina basalts identification and their feature analysis in Mare Fecunditatis

High alumina (HA) mare basalts play unique roles in understanding the heterogeneity of lunar mantle. Their presence was confirmed by the Apollo and Luna samples, and their remote sensing identification was implemented using HA sample FeO, TiO2 and Th concentration constraints. This study selected the surfaces with ~0.5% rock abundance as windows into HA basalts identification. The lithology of these rock pixels was first classified based on thorium maps from the Lunar Prospector and major element oxide products from Diviner data onboard the Lunar Reconnaissance Orbiter (LRO). Then, the LRO Diviner Al2O3 (~11 wt%) concentration constraint was applied in the mare basalt rock pixels across the Moon. The mare-highland mixtures were distinguished from HA basalt rocks based on the positive linear relationships between Al2O3 and Mg# in the adjacent pixels for four impact vector directions away from each candidate HA pixel. These HA basalts rock pixels identified by this study indicate that HA basalts are concentrated locally in South Pole-Aitken (SPA) basin, Schiller-Schickard region and 13 maria such as southern and northern Oceanus Procellarum, central Humorum, Tranquillitatis, Fecunditatis and Serenitatis, northern Imbrium and southern Nubium, but are seldom found in Mare Moscoviense and Orientale regions on the farside. Detailed investigations in Mare Fecunditatis found that fifteen HA basalt units or patches could be confidently identified. These HA basalts have the total area and volume of <77,658 km2 and <54,301 km3, and the maximum depth and thickness of 1147 m and 1062 m respectively. In addition, analyses of the HA rocks indicated that the HA basalts are discontinuous and have variable thicknesses.

High‐Resolution Nighttime Temperature and Rock Abundance Mapping of the Moon Using the Diviner Lunar Radiometer Experiment With a Model for Topographic Removal

AbstractThe Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter (LRO) has been mapping the surface temperatures of the Moon since 5 July 2009. Diviner has since collected over 500 billion radiometric measurements with excellent spatial and local time coverage. However, the most recently published high‐resolution Diviner global maps only use data collected from 2009 to 2016. In this work, we compile ∼13 years of Diviner data to produce improved global maps of nighttime brightness temperature, bolometric temperature, regolith temperature, and rock abundance (RA). Errors in Diviner's pointing have been corrected and past effective field of view modeling has been optimized to improve data georeferencing without spatial interpolation. We estimate an effective resolution of ∼330 m longitudinally and ∼700 m latitudinally at the equator, which corresponds to an improvement of ∼3.5× longitudinally and ∼1.3× latitudinally. In addition, we develop a thermal model that accounts for indirect scattering and emission from surrounding topography. The resulting temperature anomaly maps better highlight variations in temperature caused by thermophysical properties by removing most topographic effects. These improvements allow for the identification of smaller and fainter thermal features than was previously possible. The improved effective resolution of Diviner maps allows for excellent spatial correlation with other high‐resolution data sets. To demonstrate this, we compare Diviner RA to a manual survey of boulders in the Apollo 17 landing site region. We show that Diviner RA correlates well with the areal fraction of rocks larger than ∼1–2 m in diameter visible in LRO Camera imagery.

Coarse-to-Fine Crater Matching From Heterogeneous Surfaces of LROC NAC and Chang’e-2 DOM Images

The centers of matching craters can be beneficial additions to the control point database. Crater matching on heterogeneous surfaces is helpful in testing its applicability to the entire moon. Therefore, we propose a coarse-to-fine crater matching method for heterogenous surfaces on images acquired from the narrow angle camera (NAC) of the lunar reconnaissance orbiter camera (LROC) and the Chang’e-2 digital orthophoto map (DOM). First, we perform coarse matching based on the Hausdorff distance using the area and coordinates of the crater. Then, the mismatched craters are removed by using the affine transform model fitted by corresponding points of mutual information matching. Finally, we use the retained matched crater centers to fit the affine transformation model between the images, predict the corresponding position, and obtain the corresponding crater around it to achieve fine matching. The results show that the proposed method obtains numerous crater matches on images covering different terrains and solar altitude angles compared to the Hausdorff distance-based crater matching method. For the five experimental scenes registered using matched craters, the mean values of the checkpoints are approximately 2 and 3 pixels for scenes with small and large differences from Chang’e-2 solar altitude angles, respectively, and the standard deviations (STDs) for both are approximately 1 pixel. In addition, the highlands have lower accuracy than the maria, with a variance of less than 1 pixel. Furthermore, the registration accuracy is related to the diameter and number of craters.

Determination of the Spatial Extent of the Engine Exhaust-Disturbed Region of the Chang'E-4 Landing Site Using LROC NAC Images

The regolith of the Chang'E-4 landing site was disturbed by its engine exhaust. To explore the interaction between the engine exhaust and the regolith, it was necessary to identify the exhaust-disturbed region. This article focuses on determining the extent of the disturbed region by using lunar reconnaissance orbiter camera narrow angle camera (LROC NAC) images. For this purpose, the tools of temporal-ratio images, phase-ratio images, reflectance profiles, and reflectance isoline graphs are employed. The reflectance profiles and isoline graphs derived from the temporal-ratio images reveal the reflectance changes before and after landing. Compared with the reflectance profiles, isoline graphs further include the spatial information of isolines, thus more robust to noise. Based on the magnitudes of changed reflectance around the lander, the engine exhaust-disturbed region was further divided into the focus disturbed region (FDR) and the diffuse disturbed region (DDR). The final estimated spatial extent along the north–south and east–west directions of the FDR were ∼9.6 and ∼10.8 m, and those of the DDR were ∼75 and ∼80 m. As compared with the estimated spatial extent of the Chang'E-3 landing site, the DDR of the Chang'E-4 landing site was larger, but the FDR was smaller. We attributed this to geological and topography factors. The reflectance changes between the FDR and the undisturbed region increased by ∼10±1%. This indicates similar processes causing the variations in the regolith properties, likely including the smoothing of the surface from microscopic to macroscopic by destroying fine-grained regolith components, or changing of the surface maturity.

The Distribution and Accessibility of Geologic Targets near the Lunar South Pole and Candidate Artemis Landing Sites

Transformative lunar science will be driven by the accessibility, recovery, and return to Earth of geological specimens. Isolated boulders, rock exposures, and rocky craters at the lunar south pole all provide opportunities for geologic characterization and sampling of the lunar crust. Here, we present the results of orbital geological mapping of the region surrounding the south pole using Lunar Reconnaissance Orbiter Narrow Angle Camera images (0.5–2 m pixel−1). We mapped the distribution of isolated boulders (86,896), rocky craters (3556), and rock exposures (7553) around potential Artemis landing sites 001 and 004 (NASA 2020), which are within or near one of the Artemis III candidate landing regions. We found that boulder abundance decreases with increasing distance from the rim of Shackleton crater. From that correlation, we infer that most of the boulders and rock exposures near Shackleton were deposited as ejecta by the Shackleton impact, and by later the reworking of that material during smaller impact events. We additionally assessed the accessibility of the mapped features, and documented geologic targets located on shallow (<15°) slopes, including those on the Shackleton crater rim and on the “Connecting Ridge” between Shackleton and Henson crater. Such targets could be sampled by a future mission to the lunar south pole region. Our catalog of mapped features is made available to the lunar community.

LRO‐LAMP Survey of Lunar South Pole Cold Traps: Implication for the Presence of Condensed H2O

AbstractWe employ nighttime observations from the Lyman Alpha Mapping Project (LAMP) ultraviolet (UV) spectrograph onboard the Lunar Reconnaissance Orbiter to investigate the presence of condensed water ice within lunar south pole cold traps. This study incorporates LAMP observations between 2009 and 2016, which more than doubles the number of observations from previous LAMP Permanently Shadowed Regions (PSRs) studies. In this study, we focus on cold traps within five south polar craters: Faustini, Shoemaker, Haworth, Cabeus, and Amundsen. An unnamed cold trap region adjacent to Haworth and Shoemaker was also investigated. We present On‐band (148–162 nm), Edge‐band (164–173 nm), and Off‐band (175–190 nm) albedo maps at 4‐km spatial resolution and far‐UV albedo spectra of cold traps at 2‐nm spectral resolution. We find Off‐band to On‐band albedo ratios (the UV wavelength ranges with high and low H2O ice reflectivity, respectively) within cold traps that are consistent with previous PSR studies. Spectral intimate mixture models considering 5 μm H2O grains and a range of regolith grain sizes (20–74 μm) with porosities between 0.3 and 0.75 yields abundances of ∼0.9–4.9 wt.%, where increasing the range of H2O and regolith grain sizes may increase the range of water abundance. We further find generally lower albedos within the cold trap regions than at the surrounding non‐cold trap regions across all wavelengths, consistent with increased regolith porosity within cold traps as suggested by Gladstone et al. (2012), https://doi.org/10.1029/2011JE003913.

A high-precision horizon-based illumination modeling method for the lunar surface using pyramidal LOLA data

Precise illumination prediction is a necessary preparatory task for exploration activities on the lunar surface, while altimetry data obtained by the Lunar Orbiter Laser Altimeter (LOLA) is a common basis for illumination modeling. In this paper, a new high-precision horizon-based illumination modeling method is proposed. The method owns the following features: (1) pyramidal LOLA data in a unified Cartesian space is used as the basis for modeling, where each layer has a different accuracy and coverage, allowing the laser terrain to be described with an accurate triangular network and several interpolated rasters; (2) a database describing the horizon elevation angles is pre-calculated and stored, enabling high efficiency in the simulation at each time step; and (3) the Sun is treated as a 2-dimensional disc, the horizon is treated as line segments, and the solar disc's visible area is calculated by integration.Several simulations are performed at a site near the lunar south pole to verify the effectiveness and performance of the method. Under the test environment and precise horizon parameters, the average time cost per frame in a single thread is ∼0.0076 s for ∼15.7 k query cells (covering a 2 × 2 km area). These simulations are cross-validated with a Lunar Reconnaissance Orbiter Narrow-Angle Camera image and a previous study. The effects of interpolating the LOLA point clouds to a topographic grid, as well as adopting a coarser horizon, are discussed quantitatively with respect to illumination modeling. Finally, we applied the proposed method to perform a brief analysis of illumination conditions for some regions of interest near the Shackleton and Shoemaker craters.

Topographic Degradation Processes of Lunar Crater Walls Inferred From Boulder Falls

AbstractRecent explorations by lunar orbiters have shown that boulder falls are distributed over the entire lunar surface. To quantitatively evaluate the effects of moonquakes and meteorite impacts on boulder falls, we performed detailed surveys at two sites: One in the southern part of the Schrödinger basin (Site 1) and the other in Laue crater (Site 2). Using images and topography data from the Lunar Reconnaissance Orbiter and KAGUYA, we measured the detailed distributions of boulder falls, small craters, slope angles, and the optical maturity parameter (OMAT) and modeled maximum ground acceleration due to impacts at these sites. In steeply sloping areas at both sites, we found that the density of small craters was low and areas with high OMAT values corresponded to boulder sources, where many boulders exist. At Site 1, the starting points of boulder falls and ground acceleration due to impacts were correlated. In addition, craters with boulder falls at and around Site 2 were distributed independently of the presumable epicentral distance from a shallow moonquake that occurred in 1975 near Site 2, which was previously inferred to have triggered boulder falls at the site. Our results suggest that boulder falls at these sites were triggered not by moonquakes but by meteorite impacts. We propose a model for the generation and transport of boulders and regolith on slopes by meteorite impacts, which may be directly related to the degradation of crater slopes on the Moon.

Surface scattering model for dual-polarization planetary radars

This work presents a surface scattering model to interpret radar polarimetry derived from dual-polarization planetary radar systems, such as Arecibo Observatory and the Goldstone Solar System Radar. This model divides surface scattering contributions within a radar echo into quasi-specular and diffuse components, and further divides the quasi-specular component into single- and double-bounce scattering. In particular, the increase in the degree of linear polarization resulting from dihedral double-bounce scattering is emphasized, in addition to the increase in the circular polarization ratio. This mechanism is illustrated in radar observations of young craters on the Moon by the Mini-RF radar on board the Lunar Reconnaissance Orbiter. Arecibo radar observations of 8 near-Earth asteroids are compared to this model and the implications for the physical properties of these objects are discussed. This work provides new insight in interpreting radar polarimetry from dual-polarization planetary radar systems and is broadly applicable to all solar system targets.

New Insights into Surface Deposits in the Balmer-Kapteyn Cryptomare Region Provided by Chang’E-2 Microwave Radiometer Data

Lunar cryptomare records both early-stage mare volcanisms and large-scale impact cratering, which can provide important information about the thermal evolution of the Moon. We built a mixing dielectric constant model to represent the cryptomare deposits mixed by highland debris and mare deposits, and the proper radiative transfer simulation was constructed to evaluate the thermal emission features of surface deposits in the cryptomare region. The microwave radiometer (MRM) data in the Balmer-Kapteyn region were extracted, and the linear interpolation method was used to generate brightness temperature (TB) maps at noon and at night. To enhance the correlation between cryptomare deposits and TB performances, normalized TB (nTB) and TB difference (dTB) maps were also generated. Combined with the datasets, including Lunar Reconnaissance Orbiter Wide Angle Camera, Lunar Orbiter Laser Altimeter, and Diviner and Clementine UV–VIS, the main findings are as follows: (1) The mare-like cryptomare deposits were discovered and identified according to the nTB and dTB performances. Combined with the surface compositions, at least two kinds of buried mare deposits were identified in the B-K region, which erupted during different episodes. (2) A construct-like volcanic feature was suggested by the nTB and dTB performances. (3) The results of our analysis indicated the presence of materials with low dTB anomalies in the northern and southwestern parts of the cryptomare region and in the mare unit within the Vendelinus crater, which illustrates the heterogeneity of the lunar crust in the vertical direction.

High-resolution topography from planetary images and laser altimetry

Mapping landforms on the Moon is of great interest and importance for future human settlements and resources exploration. One of the first steps is to map the topography in great detail and resolution. However, data from the Lunar Orbiter Laser Altimeter (LOLA) provide low-resolution elevation maps in comparison to the size of detailed geological features. To improve resolution, we developed a new method to upscale topographic maps to a higher resolution using images from the Lunar Reconnaissance Orbiter Camera (LROC). Our method exploits the relation between topographic gradients and degrees of shading of incoming sunlight. In contrast to earlier published methods, our approach is based on probabilistic, linear inverse theory, and its computational efficiency is very high due to its formulation through the Sylvester Equation. The method operates on multiple images and incorporates albedo variations. A further advantage of the method is that we avoid/reduce the use of arbitrary tuning parameters through a probabilistic formulation where all weighting of data and model parameters is based on prior information about data uncertainties and reasonable bounds on the model. Our results increase the resolution of the topography from ∼60 ​m per pixel to 0.9 ​m per pixel, bringing it to the same pixel resolution as the optical images from LROC, allowing in some cases detection of craters as small as ∼3 ​m of diameter. We estimate uncertainties of the topographic model due to noise in the images, and in the low-resolution (LOLA) model. • Mapping planetary landforms in detailed resolution is hampered by the poor resolution of Laser Altimeter instruments. • Method computes high-resolution topography from the relation between topographic slopes and degrees of shading of sunlight. • The method uses probabilistic inverse theory. It is computationally efficient and avoids arbitrary tuning parameters. • We obtain DEMs of 0.9 m/pixel resolution (same as used optical images), showing surface features as small as ~3m diameter.

Burial Depths of Extensive Shallow Cryptomaria in the Lunar Schiller–Schickard Region

Quantifying the volumes and geologic nature of lunar volcanic eruptions is important for constraining the thermal and geologic evolution of the Moon. Cryptomaria are effusive, basaltic lava flows on the Moon that were subsequently buried, and therefore hidden, by higher-albedo basin and crater ejecta. Radar offers the ability to probe the subsurface for geologic units not otherwise apparent at the surface. We use Arecibo/Green Bank Observatory and Lunar Reconnaissance Orbiter Mini-RF radar data sets to characterize maria and cryptomaria within the Schiller–Schickard region. We find significant variability in the radar backscatter across the region that does not correspond to previously mapped boundaries of maria and cryptomaria in the literature. We use the characteristic low backscatter (due to the attenuating nature in radio waves of some basaltic minerals) to analyze the distribution of cryptomaria. We use the reduction in radar backscatter to estimate burial depths of cryptomaria across the area. We present a new map of Schiller–Schickard cryptomaria and the local variability in the thicknesses of the light plains that bury the basalts. We find burial depths ranging from >100 m in the deepest areas to just a few to tens of meters in areas with shallow cryptomaria (particularly prominent in the southeast). These areas are generally contiguous with maria, allowing us to track mare lava flow units into the subsurface at mare/highland margins. We propose that ∼67% of the region contains surface or buried basaltic volcanism, which represents over twice (2.7× increase) the areal extent of cryptomaria reported in previous studies.

Cryogeomorphic Characterization of Shadowed Regions in the Artemis Exploration Zone

AbstractThe Artemis program will send crew to explore the south polar region of the Moon, preceded by and integrated with robotic missions. One of the main scientific goals of future exploration is the characterization of polar volatiles, which are concentrated in and near regions of permanent shadow. The meter‐scale cryogeomorphology of shadowed regions remains unknown, posing a potential risk to missions that plan to traverse or land in them. Here, we deploy a physics‐based, deep learning‐driven post‐processing tool to produce high‐signal and high‐resolution Lunar Reconnaissance Orbiter Narrow Angle Camera images of 44 shadowed regions larger than ∼40 m across in the Artemis exploration zone around potential landing sites 001 and 004. We use these images to map previously unknown, shadowed meter‐scale (cryo)geomorphic features, assign relative shadowed region ages, and recommend promising sites for future exploration. We freely release our data and a detailed catalog of all shadowed regions studied.

Displacement-length scaling relations of nearside graben: Evidence of restricted normal faults on the Moon

Evidence of significant extension on the Moon is associated with nearside mare. The load induced by the mare basalts resulted in lithospheric flexure and graben forming extension. We use high-resolution topographic and image data from the Lunar Reconnaissance Orbiter (LRO) and the SELenological and ENngineering Explorer (SELENE) to generate detailed fault displacement-length (D/L) scaling observations for graben bounding normal faults. Specifically, we focus on the Dmax/L relations of graben within three geologic settings: lunar highlands, mare, and mixed highlands-mare terrains. The population scaling factor γ for the measured lunar graben is 4.5 × 10−3 (n = 50), falling within the range for terrestrial normal faults. The scatter in the Dmax/L data for mare graben (γ = 4.9 × 10–3,n = 25) is significantly higher than that for graben in highlands (γ = 6.3 × 10−3, n = 16) and mixed terrains (γ = 3.0 × 10−3, n = 9). The shape of a displacement profiles for faults is indicative of the mode of growth (i.e., restricted or unrestricted). Displacement plots show 52% of mare graben exhibit plateau-shaped profiles, indicating restricted fault growth, compared to 19% and 33% for highlands and mixed terrain graben, respectively. We attribute the large percentage of plateau-shaped profiles and the scatter in mare graben Dmax/L data to restricted fault growth controlled by the local thickness of the mare basalts. Finally, we consider whether observationally based estimates of the distribution of mare basalt thicknesses is consistent with the width-depth relation of the inferred restricted graben.

LCNS Positioning of a Lunar Surface Rover Using a DEM-Based Altitude Constraint

With the renewed interest in lunar surface exploration, the European Space Agency envisions to stimulate the creation of lunar communications and navigation services (LCNS) to enable, among others, autonomous navigation capabilities for lunar rovers. As the number of satellites foreseen in such a service is much smaller compared to Earth-based global navigation satellite systems, different complementary technologies are pursued to improve the attainable navigation accuracy for lunar rovers. One way to improve the position accuracy provided by the LCNS satellites is to constrain their vertical position using a high resolution digital elevation model (DEM). This article presents the results of a variance covariance analysis of an extended Kalman filter implementation in which the LCNS ranging measurements are used together with the altitude provided by a DEM from the Lunar Orbiter Laser Altimeter instrument of the Lunar Reconnaissance Orbiter. Assuming a realistic orbit determination and time synchronization accuracy of the LCNS satellites, the usage of a navigation-grade inertial measurement unit and an oven-controlled crystal oscillator, a 3-sigma position accuracy of less than 10 m can be obtained. Furthermore, the availability is substantially improved as the DEM-aided solution enables a position solution in case of only 3 visible satellites.

Lunar Pit Morphology: Implications for Exploration

AbstractLunar pits are small (∼10–300 m wide) collapse features with vertical walls and sometimes overhangs. We have identified almost 300 pits, mostly in ponds of cooled impact melt inside large craters younger than ∼1 billion years. Several of these pits may provide access to lava tubes or other caves, and those in the maria expose the layering record of the top 20–100 m of basaltic lava flows. We investigated the 21 known pits outside of impact melt ponds to determine possible origins, ages, and present‐day access to the lunar subsurface. We used Lunar Reconnaissance Orbiter Narrow Angle Camera images (&lt;2 m per pixel) to produce detailed 3D reconstructions of six pit interiors. We evaluated the general morphology and geologic context of all twenty‐one. While four pits have contexts suggestive of lava tubes, the majority are ambiguous, although all occur in or near the maria. We also propose that pit formation is an ongoing process, as the degree of degradation of a pit is unrelated to the age of the host terrain. Much of the original volume of most pits is now filled with debris, but some exhibit significant overhangs and may have present‐day cave access. Viewing pit walls and floors, even from the rims, requires navigating steep slopes of loose material. Since the floors are also covered with rough debris piles, we recommend simple flying vehicles for initial reconnaissance.

Utilization of a meteorological satellite as a space telescope: the lunar mid-infrared spectrum as seen by Himawari-8

The Japanese meteorological satellite Himawari-8 has captured the Earth’s atmosphere and stars, planets, and the Moon in its field of view, enabling us to capture their spectroscopy with 16 bands from visible to mid-infrared wavelengths. The nine infrared bands in the Advanced Himawari Imager (AHI) onboard Himawari-8 are unique in spaceborne observations and are potentially useful for lunar science. In addition, infrared bands of AHI cover wavelengths similar to those of other interplanetary instruments and thus are useful for calibrations. However, infrared AHI data have not yet been investigated in planetary science. To confirm the utility of AHI for planetary science, we develop a procedure to retrieve the lunar infrared spectrum and compare it with thermal conduction simulations. Our analysis shows that lunar brightness temperature curves can be obtained in the morning, evening, and nighttime for all nine bands. Particularly at 8.5 μm, they show a good agreement with previous observations by the Diviner radiometer onboard NASA’s Lunar Reconnaissance Orbiter. As pointed out previously, the brightness temperatures differ between the bands, indicating temperature mixing within a pixel. Our simulation suggests that surface roughness as steep as those measured at the Apollo landing sites significantly contributes to the observed brightness temperature differences in the morning and evening; however, nighttime brightness temperatures are greatly affected by rocks with higher thermal inertia than the regolith. The rock abundances are estimated to be 0.18–0.48% and 6.1–10.3% at the equator and within Tycho crater, respectively. Our estimations from AHI data are consistent with those of Diviner. These results support the idea that AHI potentially serves as a space telescope for future lunar and planetary sciences, for example, for constraining water content on the lunar surface.Graphical

Thermophysical Properties of Lunar Irregular Mare Patches From LRO Diviner Radiometer Data

AbstractIrregular mare patches (IMPs) are enigmatic features in the lunar maria that are characterized as having smooth mounds surrounded by uneven or blocky terrain. The IMPs appear anomalously young compared to surrounding maria, with crater counting methods estimating IMPs to be &lt;100 Myr old and established measures of maturity showing that IMPs are relatively immature. Some studies propose that IMPs are the result of recent basaltic volcanism or episodic outgassing of volatiles, while others propose that the IMPs are of a similar age to the surrounding maria and appear geologically young due to unique physical properties resulting from late‐stage eruption dynamics. Here we present observations from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment for eight of the largest IMPs. We find that IMPs are slightly rockier than typical mare surfaces, and the fines components of a number of IMPs have relatively low thermal inertia. The IMP Ina has noticeably lower thermal inertia than the other IMPs. We suggest that the other IMPs (which in many cases are smaller and have ambiguous or gradational contacts with the surrounding maria) could contain similarly low thermal inertia material as Ina, but that it is not fully resolvable with Diviner. We interpret lower‐thermal inertia material as being less‐consolidated or containing fewer small rocks than typical regolith. This would be consistent with elements of a number of proposed formation hypotheses; however, higher spatial resolution is needed for a full comparison of the thermophysical properties of the different IMPs and different terrains within each IMP.

A Global Perspective on Lunar Granular Flows

AbstractDry granular flows are ubiquitous, yet poorly understood mass wasting features on the Moon. Above all, their global distribution, relation to the physical environment, and drivers are poorly understood. Here, we build and deploy a convolutional neural network and map 28,101 flow features between 60°N and S by scanning through ∼150,000 Lunar Reconnaissance Orbiter images. We observe that flows are heterogeneously distributed over the Moon, where all major hotspots are located in craters and almost all hotspots are located in the nearside maria. We further observe that younger surfaces feature higher flow feature densities, while pre‐Nectarian terranes can still host flows, remaining subject to active erosion billions of years after their formation. Our observations suggest that impacts at various scales have been—and likely still are—acting as the main, global‐scale, long‐ and short‐term driver of flow occurrence, strongly influenced by the properties of the target rock material.

COMPARISON OF DIGITAL TERRAIN MODELS FROM TWO PHOTOCLINOMETRY METHODS

Abstract. We evaluate the horizontal resolution and vertical precision for digital topographic models (DTMs) of the Moon derived from image radiance information, a process known as photoclinometry (PC) or shape-from-shading (SfS). We use the implementations in two available planetary image processing software systems, single image PC in the U.S. Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) system, and multi-image SfS in the Ames Stereo Pipeline (ASP), and test results obtained with and without use of a starting solution from stereo, with single and multiple images, and for varying illumination conditions. To obtain the higher quality reference DTMs against which the products can be evaluated, we derived DTMs by stereoanalysis of Lunar Reconnaissance Orbiter Narrow-Angle Camera (LROC NAC) images at their native pixel spacing of ∼0.5 m, then produced a 16-m/post stereo DTM from images downsampled to 4 m/pixel and refined it with images at 16 m/pixel. When used with a single image, both algorithms improved resolution (by a factor of 1.4 for PC and 2.4 for SfS compared to stereo). An albedo map produced in ISIS by ratioing the image to a simulation based on the stereo DTM was well correlated with one output by SfS. The albedo correction was crucial for PC with ∼60° incidence but not at ∼80°. DTMs produced by PC and SfS without a starting stereo DTM had larger errors but good detail, and could be useful for many applications. In SfS, it was necessary to increase smoothing to get a usable DTM when the weighting on an a priori DTM was reduced. Multi-image SfS including modeling of spatially varying albedo reduced vertical errors by factors of 1.5 or more compared to single-image SfS.