Diviner Lunar Radiometer Experiment Research Articles

Temperature Dependence of the Dielectric Constant on the Lunar Surface Based on Mini-RF and Diviner Observations

Radar observation is an effective way to understand subsurface structures in terms of the dielectric constant, whose controlling factors include chemical composition, packing density, and water/ice content. Recently, laboratory measurements have shown that the dielectric constant of lunar regolith simulants also depends on the temperature, which has never been evaluated from remote sensing data. In this study, we estimated the dielectric constant from the Miniature Radio Frequency (Mini-RF) data on a lunar crater floor in the north polar region at two different local times (i.e., different surface temperatures). We calculated the dielectric constant using the inversion method and obtained the bolometric surface temperature from the Diviner Lunar Radiometer Experiment (Diviner) data. The histograms of the estimated dielectric constant values are different between the two local times. This could be interpreted as a result of the temperature dependence of the dielectric constant, while further evaluation of the influence of topography on the incidence angle and small surface roughness is needed. Nevertheless, our result suggests that the temperature dependence of the dielectric constant should be considered when interpreting S-band radar observations of the Moon and other celestial bodies with large surface temperature differences.

Synthesizing lunar regolith-geopolymer emulating lunar positive temperature regime

The development of lunar habitat using lunar regolith is a captivating research area for constructing lunar bases, especially after the discovery of polar ice, and molecular water on the lunar south pole. The aim of this research is to synthesize a robust lunar regolith geopolymer by fine-tuning the concentration and ratios of alkaline activators while implementing curing under lunar positive thermal regime. The geopolymer was synthesized containing lunar highlands simulant (LHS-1) and mare regolith simulant (LMS-1) at the lowest water content using sodium (Na) and potassium (K) based alkaline activators emulating positive temperature regime collected by Diviner Lunar Radiometer Experiment (DLRE). The maximum compressive strength of 41.23 MPa was achieved for highlands regolith simulant-based geopolymer containing a Na-activator with the lowest water-to-precursor ratio of 0.23. The surplus alkali cations in the geopolymer paste matrix resulted in the formation of carbonation products after reacting with atmospheric carbon dioxide. K-activator geopolymer resulted in Kalicinite which has lower thermal stability and dissolution in water, whereas sodium carbonate formed in Na-activator-based geopolymer engrained and embedded precursor particles alongside sodium-calcium aluminosilicate hydrate (N-(C)-A-S-H) gel forming more densified microstructure. Conclusively, lunar regolith geopolymer has the potential for the construction of lunar habitat. However, this study recommends synthesizing geopolymer in vacuum conditions, emulating freeze-thaw cycles for a more precise estimation of microstructural developments and evaluating other critical properties.

A Microphysical Thermal Model for the Lunar Regolith: Investigating the Latitudinal Dependence of Regolith Properties

AbstractThe microphysical structure of the lunar regolith provides information on the geologic history of the Moon. We used remote sensing measurements of thermal emission and a thermophysical model to determine the microphysical properties of the lunar regolith. We expand upon previous investigations by developing a microphysical thermal model, which more directly simulates regolith properties, such as grain size and volume filling factor. The modeled temperatures are matched with surface temperatures measured by the Diviner Lunar Radiometer Experiment on board the Lunar Reconnaissance Orbiter. The maria and highlands are investigated separately and characterized in the model by a difference in albedo and grain density. We find similar regolith temperatures for both terrains, which can be well described by similar volume filling factor profiles and mean grain sizes obtained from returned Apollo samples. We also investigate a significantly lower thermal conductivity for highlands, which formally also gives a very good solution, but in a parameter range that is well outside the Apollo data. We then study the latitudinal dependence of regolith properties up to ±80° latitude. When assuming constant regolith properties, we find that a variation of the solar incidence‐dependent albedo can reduce the initially observed latitudinal gradient between model and Diviner measurements significantly. A better match between measurements and model can be achieved by a variation in intrinsic regolith properties with a decrease in bulk density with increasing latitude. We find that a variation in grain size alone cannot explain the Diviner measurements at higher latitudes.

Lunar Boulder Fields as Indicators of Recent Tectonic Activity

Wrinkle ridges are the predominant tectonic structure on the nearside lunar maria. Although lunar wrinkle ridge formation began as early as ∼3.9–4.0 Ga, recent investigations have identified wrinkle ridges in the lunar maria that were tectonically active as recently as the Copernican period of lunar geologic history. Some of those geologically young wrinkle ridges were identified by the presence of dense fields of meter-scale boulders on their scarps and topographic crests. Other investigations have identified recently active lunar wrinkle ridges that lack the ubiquitous presence of boulder fields, thereby rendering the presence of boulder fields ambiguous in the search for ongoing tectonic activity on the Moon. Here we assess boulder populations associated with 1116 wrinkle ridge segments on the lunar maria that are inferred to be recently active (<1.5 Ga) based on their crisp morphologies and crosscutting relationships with small impact craters. We utilize data from the Lunar Reconnaissance Orbiter Mini-RF and Diviner Lunar Radiometer Experiment instruments to assess surface rock populations across these recently active structures. Our results indicate that, where present, meter-scale boulder fields are likely indicators of fault-slip-induced ground acceleration given the short lifespan of lunar surface boulders. However, elevated boulder populations are not observed on all recently active ridges analyzed here. This latter observation supports the notion that wrinkle ridge boulder fields are a nonunique indicator of recent tectonic activity. Furthermore, the spatial distribution of those boulder fields indicates that variable mare protolith properties may play a role in boulder field formation.

Surface Roughness at the Moon’s South Pole: The Influence of Condensed Volatiles on Surface Roughness at the Moon’s South Pole

Condensed volatiles within lunar permanently shadowed regions are of high scientific and resource utilization importance. Volatiles remain elusive and difficult to observe directly, due to low direct solar illumination. In this work, we investigate correlations between, as well as possible effects of, condensed volatiles and surface roughness. We analyze topographic roughness at 50 m and 30–120 m baselines of the lunar south pole (poleward of 85° S). We focus on six south polar craters of interest and their directly surrounding non-cold-trap areas: Faustini, Shoemaker, Haworth, Cabeus, Nobile, and an unnamed region. We further analyze six analogous equatorial craters to investigate the non-ice smoothing contributions: Morozov F, Rosenberger C, Van Maanen, Fraunhofer E, Brisbane, and Asclepi. Lastly, we compare a sunlit and a permanently shaded portion of the Amundsen crater floor. Utilizing data from the Lunar Reconnaissance Orbiter’s Lunar Orbiter Laser Altimeter (LOLA), Lyman Alpha Mapping Project (LAMP), Lunar Reconnaissance Orbiter Camera, and Diviner Lunar Radiometer Experiment, we find subdued roughness within cold traps but determine that roughness is not a unique identifier of condensed volatiles. However, a correlation between LOLA roughness, LAMP normalized Off-band/On-band albedo, temperature, and water-ice stability suggests possible terrain softening due to condensed volatiles, although we cannot rule out dust ponding and/or fairy castle structure contributions. We conclude that LAMP volatile signatures at the topmost ∼100 nm may be indicative of volatile deposits at depths beyond the LAMP sensing capabilities.

The Variability of Lunar Mare Basalt Properties from Surface Rock Abundance

We use the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment surface rock abundance data set to investigate how rock content changes with surface age in the maria. We find that surface rock abundance decreases with unit age as expected for a thickening regolith, but age alone cannot explain most of the observed regional variability in rock abundance. We propose that this additional variability is primarily caused by differences in the initial mare basalt properties such that some flows break down more readily than others. We calculate the best-fit relationship between the surface rock abundance and unit age and investigate the regional variability in the residual errors of this fit. Relative to this best-fit relationship, we find that Mare Humorum and parts of Oceanus Procellarum are anomalously rocky, and Mare Australe is rock-poor for its age. These regional differences, which cannot be explained by unit age, are likely due to variability in the initial properties of mare basalts.

Multi-mission, multi-sensor study of the Shackleton Crater constrained for volatiles with emphasis on albedo distribution of the Lunar South Pole

Shackleton Crater on the Moon is named in honor of a pivotal figure of the Heroic Age of Antarctic Exploration Sir Ernest Henry Shackleton. The Crater was formed about 3.6 billion years ago at the Lunar South Pole. Due to the low (1.5°) tilt of the lunar axis of rotation, the interiors of the crater receive almost no direct sunlight and observations by passive hyperspectral sensors are only possible due to reflected light. Active sensors are however able to illuminate the interior surface of the crater and make observations. Here we present the results of study of the crater using data from multiple sensors onboard two missions of Indian Space Research Organization (ISRO) and National Aeronautics and Space Administration (NASA). Hyperspectral observations from Moon Mineralogy Mapper (M3) on-board Chandrayaan-1, temperature from Diviner Lunar Radiometer Experiment (DLRE), albedo from Lunar Orbiter Laser Altimeter (LOLA) and Off/On band ratios from Lyman-Alpha Mapping Project (LAMP) on board Lunar Reconnaissance Orbiter (LRO) have been used. Permanently Shaded Regions (PSRs) are identified and Shackleton has been found, as reported, to be a PSR. Emphasis has been laid on LOLA albedo analysis of lunar south pole (80°-90° South). It is observed that the Shackleton Crater exhibits significantly higher albedo compared to its surrounding regions. Some regions exhibit albedo even higher than the Shackleton Crater in the lunar south pole region. The findings of high albedo, coupled with high Off/On band ratio of LAMP, extremely low DLRE temperatures in the permanently shaded regions and detection of hydration features in the M3 hyperspectral data suggests higher probability of hydration in the Shackleton Crater.

Effects of Albedo on the MIR Emissivity Spectra of Silicates for Lunar Comparison

AbstractWe used laboratory analysis to investigate the effect of mineral albedo, as defined at 750 nm, on the midinfrared emissivity spectra of silicates under lunar environment conditions. Optical darkening has long been recognized as an effect of space weathering on the visible‐to‐near‐infrared spectra of the Moon. However, space weathering has not been as thoroughly investigated in the mid‐infrared. Because mid‐infrared spectra are strongly influenced by the anisothermality of the lunar surface environment, it is likely that any darkening effects of space weathering would also change the thermal gradient in heavily space weathered lunar regolith. To isolate this variable, we added nanophase carbon to particulate samples of forsterite, augite, and anorthite to achieve a range of albedo samples and measured their midinfrared spectra under lunar environment conditions within the Planetary and Asteroid Regolith Spectroscopy Environment Chamber at Stony Brook University. We observe a shift in the Christiansen Feature maximum to longer wavelengths and decreasing spectral contrast with decreasing albedo. These shifts are well correlated with the observation of space weathering effects on the Diviner Lunar Radiometer Experiment compositional data, and point to the need for further investigation into the effects of space weathering on the midinfrared spectra of airless bodies.

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.

Lunar Surface Temperature and Emissivity Retrieval From SDGSAT-1 Thermal Imager Spectrometer

The lunar surface temperature is one of the important thermophysical parameters of the Moon, which helps to study the radiative properties of the lunar surface. Thermal infrared emission spectra is sensitive to the thermophysical properties of the lunar surface material, and the emissivity data can be used for lunar surface composition inversion. In the past, humans have conducted hundreds of lunar exploration missions, but only a small number have been conducted in the infrared band, typical examples include the Apollo program and the Diviner lunar radiometer experiment of the LRO satellite. Only part of the lunar surface has been explored by these missions. SDGSAT-1 carries out a complete observation of the lunar surface in three infrared wavelength bands (B1: 8-10.5μm; B2: 10.3-11.3μm; B3: 11.5-12.5μm) by its thermal imager as part of its mission. In this study, the temperature-emissivity separation (TES) algorithm is used to retrieve the lunar surface temperature and calculate the thermal infrared band emissivity using the radiometric data obtained by SDGSAT-1 thermal imager, the temperature distribution of full-disk Moon and the emissivities distribution of three bands are also mapped. The temperature retrieval error is verified less than 1K.

Thermal and Illumination Environments of Lunar Pits and Caves: Models and Observations From the Diviner Lunar Radiometer Experiment

AbstractLunar collapse pits may provide access to subsurface lava tubes of unknown extent. We present Diviner Lunar Radiometer measurements showing that the Mare Tranquillitatis and Mare Ingenii pits exhibit elevated thermal emission during the night, ∼100 K warmer than the surrounding surface. Using these data, along with computational thermophysical models, we characterize the thermal environment inside pits and potential caves. Near the equator, peak day‐time temperatures on regolith‐covered pit floors can potentially reach &gt;420 K, whereas temperatures beyond the opening in permanent shadow would maintain a nearly constant temperature of ∼290 K, similar to that of a blackbody cavity in radiative equilibrium. Thermal IR measurements such as those of Diviner can readily detect pit thermal signatures but would be insensitive to the existence of caves they may host, as the latter would only induce a 0.1 K increase to night‐time temperatures of the overlying surface.

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.

Temperatures of the Lacus Mortis Region of the Moon

AbstractOver 11 years of data acquired by the Diviner Lunar Radiometer Experiment instrument aboard Lunar Reconnaissance Orbiter have been compiled into a comprehensive data set of surface temperatures in the Lacus Mortis region which includes the landing ellipse of the Astrobotic Mission One lander mission. These data provide diurnal brightness temperatures at 128 pixels per degree (ppd) spatial resolution and 0.1 hr of local time resolution. From this data set, we highlight several features that display variations in radiative and thermophysical properties in the Lacus Mortis region and characterize the temperatures of the Astrobotic Mission One landing ellipse. We identify distinctly contrasting properties of materials in the walls of Bürg crater, hummocks of materials on the southeast margin of the mare basalts, and materials exposed or excavated by impacts. Additionally, we describe an exceptionally rocky fault scarp that predates the formation of Bürg crater suggesting the observed boulders are replenished on a timescale &lt;1 Ga. Within the Astrobotic landing ellipse, temperatures are observed to range from ∼88 to ∼359 K with sunrise and sunset local times constrained to 5.8–6.3 hr and 17.8 and 18.1 hr respectively.

On the Solar Climate of the Moon and the Resulting Surface Temperature Distribution

The solar climate of our Moon is analyzed using the results of numerical simulations and the recently released data of the Diviner Lunar Radiometer Experiment (DLRE) to assess (a) the resulting distribution of the surface temperature, (b) the related global mean surface temperature Ts>, and (c) the effective radiation temperature Te often considered as a proxy for Ts> of rocky planets and/or their natural satellites, where Te is based on the global radiation budget of the well-known “thought model” of the Earth in the absence of its atmosphere. Because the Moon consists of similar rocky material like the Earth, it comes close to this thought model. However, the Moon’s astronomical features (e.g., obliquity, angular velocity of rotation, position relative to the disc of the solar system) differ from that of the Earth. Being tidally locked to the Earth, the Moon’s orbit around the Sun shows additional variation as compared to the Earth’s orbit. Since the astronomical parameters affect the solar climate, we predicted the Moon’s orbit coordinates both relative to the Sun and the Earth for a period of 20 lunations starting May 24, 2009, 00:00 UT1 with the planetary and lunar ephemeris DE430 of the Jet Propulsion Laboratory of the California Institute of Technology. The results revealed a mean heliocentric distance for the Moon and Earth of 1.00124279 AU and 1.00166376 AU, respectively. The mean geocentric distance of the Moon was 384792 km. The synodic and draconic months deviated from their respective means in a range of -5.7 h to 6.9 h and ±3.4 h, respectively. The deviations of the anomalistic months from their mean range between -2.83 d and 0.97 d with the largest negative deviations occurring around the points of inflection in the curve that represents the departure of the synodic month from its mean. Based on the two successive passages of the Sun through the ascending node of the lunar equator plane, the time interval between them corresponds to 347.29 days, i.e., it is slightly longer than the mean draconic year of 346.62 days. We computed the local solar insolation as input to the multilayer-force restore method of Kramm et al. (2017) that is based on the local energy budget equation. Due to the need to spin up the distribution of the regolith temperature to equilibrium, analysis of the model results covers only the last 12 lunations starting January 15, 2010, 07:11 UT1. The predicted slab temperatures, Tslab, considered as the realistic surface temperatures, follow the bolometric temperatures, Tbol, acceptably. According to all 24 DLRE datasets related to the subsolar longitude øss, the global averages of the bolometric temperature amounts to Tbol=201.1k± 0.6K. Based on the globally averaged emitted infrared radiation of FIR>=290.5W·m-2± 3.0W·m-2 derived from the 24 DLRE datasets, the effective radiative temperature of the Moon is Te, M>=Tbol>1/4=271.0k± 0.7K so that Tbol>≅0.742Te, M. The DLRE observations suggest that in the case of rocky planets and their natural satellites, the globally averaged surface temperature is notably lower than the effective radiation temperature. They differ by a factor that depends on the astronomical parameters especially on the angular velocity of rotation.

A Global Thermal Conductivity Model for Lunar Regolith at Low Temperatures

AbstractAlthough some of the coldest surface temperatures in the entire Solar System are found near the poles of our own Moon, the thermophysical properties of lunar regolith at these ultracold temperatures (i.e., below ∼150 K) are not well understood. Standard lunar thermal models generally match the surface temperatures observed by global orbital remote sensing data but are inconsistent with infrared data collected from ultracold polar terrain. We build upon previous theoretical work on the low‐temperature physics of lunar regolith to introduce a global thermal conductivity model consistent with the temperature trends observed by the Diviner Lunar Radiometer Experiment (Diviner). This updated thermophysical model primarily affects nighttime surface temperatures, subsurface temperatures at high latitudes, and permanently shadowed regions (PSRs). An additional outcome of this thermophysical model is the ability to accommodate the surface temperature trends observed by Diviner both in warm low latitudes and cold high latitudes. Subsurface temperatures in near‐polar craters are ∼5–10 K warmer than previous thermal models, and cooler nighttime surface temperatures are observed globally. Model results of PSRs reveal larger surface temperature amplitudes (as observed by Diviner) and steeper geothermal gradients. A comprehensive understanding of lunar regolith's low‐temperature thermal behavior is an essential step in modeling the potential location and quantity of cold trapped volatiles in the lunar south pole. Here, we hope to provide theoretical support and motivation for more complete low‐temperature thermal conductivity laboratory measurements.

Reconciling the Infrared and Microwave Observations of the Lunar South Pole: A Study on Subsurface Temperature and Regolith Density

AbstractThe Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter and the Microwave Radiometer (MRM) onboard Chang'e‐2 (CE‐2) orbiter performed nearly coincident measurements of the lunar south polar region in 2010–2011. In this study, we reconcile infrared data from Diviner and microwave data from MRM to reveal thermal behavior of lunar regolith at very low temperatures. Assuming a uniform density structure and dielectric properties across the polar region, we retrieve a relative apparent thermal‐gradient map based on radiative transfer model. The result shows apparent thermal gradients in the permanently shadowed regions (PSRs) are much larger than that in non‐PSRs, which implies a systematic difference in thermal conductivity and/or density of lunar regolith in PSRs. We also model surface temperature and microwave brightness temperature over time in PSR and non‐PSR locations. The modeling results at these representative locations and the thermal gradient map confirm that the regolith in the PSRs is consistent with a much lower thermal conductivity and higher porosity than non‐PSRs.

Lunar Surface Temperature and Emissivity Retrieval From Diviner Lunar Radiometer Experiment Sensor

AbstractThe lunar surface temperature (LST) derived from thermal infrared (TIR) measurements can aid in understanding the physical properties of the lunar surface. The Diviner Lunar Radiometer Experiment (herein, Diviner) sensor provides global lunar surface observation in seven TIR channels. However, its retrieval of LST constantly uses a single emissivity value (i.e., 0.95) by ignoring the spatial variation of lunar surface, thereby reducing the accuracy of temperature and day–night temperature difference. To overcome this problem, this study developed a physical method called temperature–emissivity separation (TES) algorithm to retrieve LST and lunar surface emissivity from the daytime observation in three Christiansen Feature (CF) channels (7.55–8.05, 8.10–8.40, and 8.38–8.60 μm) of the Diviner, and then used the emissivity from daytime observation to inverse LST at nighttime observation. Findings showed that the TES algorithm could retrieve LST and emissivity with an error of less than 0.8 K and 0.008, respectively. However, observation noise significantly affected the retrieval accuracy, particularly for the low‐temperature pixels; moreover, high retrieval accuracy requires a surface temperature higher than 240 K. The new algorithm was applied to obtain the daytime and nighttime LST and emissivity from the Diviner images. Results showed that the LST retrieved from the algorithm differed approximately 3.9 K from that calculated from a single emissivity 0.95. Finally, an example of global surface temperature and emissivity were obtained. Consequently, the CF pixels were found to distribute in the latitude range from −60° to 60°; however, they did not have a large distribution in high‐latitude and near‐polar regions.

Unsupervised Learning for Thermophysical Analysis on the Lunar Surface

Abstract We investigate the use of unsupervised machine learning to understand and extract valuable information from thermal measurements of the lunar surface. We train a variational autoencoder (VAE) to reconstruct observed variations in lunar surface temperature from over 9 yr of Diviner Lunar Radiometer Experiment data and in doing so learn a fully data-driven thermophysical model of the lunar surface. The VAE defines a probabilistic latent model that assumes the observed surface temperature variations can be described by a small set of independent latent variables and uses a deep convolutional neural network to infer these latent variables and to reconstruct surface temperature variations from them. We find it is able to disentangle five different thermophysical processes from the data, including (1) the solar thermal onset delay caused by slope aspect, (2) effective albedo, (3) surface thermal conductivity, (4) topography and cumulative illumination, and (5) extreme thermal anomalies. Compared to traditional physics-based modeling and inversion, our method is extremely efficient, requiring orders of magnitude less computational power to invert for underlying model parameters. Furthermore our method is physics-agnostic and could therefore be applied to other space exploration data sets, immediately after the data is collected and without needing to wait for physical models to be developed. We compare our approach to traditional physics-based thermophysical inversion and generate new, VAE-derived global thermal anomaly maps. Our method demonstrates the potential of artificial intelligence-driven techniques to complement existing physical models as well as for accelerating lunar and space exploration in general.

Temperature‐Dependent Changes in the Normal Albedo of the Lunar Surface at 1,064 nm

AbstractOver the extreme temperature variations experienced in a single lunar day (∆T ≈ 300 K), particular minerals common to the lunar surface show spectral changes at discrete near‐infrared wavelengths in laboratory settings (Roush &amp; Singer, 1986, 1987, https://10.1111/10.1029/JB091iB10p10301, https://10.1111/10.1016/0019-1035(87)90026-1; Singer &amp; Roush, 1985, https://10.1111/10.1029/JB090iB14p12434). Variations in temperature can cause variations in the size and shape of crystallographic sites, which control the position, shape, and depth of crystal field absorptions. At an observation wavelength of 1,064 nm, the Lunar Orbiter Laser Altimeter (LOLA) should be highly sensitive to temperature‐dependent changes of orthopyroxene. Here we analyze temperature‐dependent spectral changes of the lunar surface as measured from orbit by LOLA. We couple LOLA measurements of normal albedo with measurements of surface temperature from the Diviner Lunar Radiometer Experiment, analyzing the maria and highlands between ±50°. We provide the first evidence of temperature‐dependent spectral changes on the lunar surface from orbital observations, finding that the majority of the surface between ±50° demonstrates a small, yet measurable, negative change in 1,064‐nm albedo with temperature (−∆R/∆T). The measurable effect is on the order of a few percent change in reflectance per ~80 K, indicating that temperature changes do not have a large effect on measurements of albedo at the sensitivity of the LOLA instrument. Stronger −∆R/∆T values tend to be associated with regions with elevated orthopyroxene, and the maria typically have higher levels of orthopyroxene and stronger −∆R/∆T values than the highlands. Our results suggest that single‐wavelength lasers may be powerful tools for understanding the distribution of particular minerals on the lunar surface.

Calibration of a multichannel millimeter wave radiometer of FY-4M based on the real-time brightness temperature along the lunar equator

Chinese FY-4M is a new-generation geosynchronous satellite, which will be launched in the near future. It is equipped with multichannel millimeter wave radiometers, which operate in the range of 50–430 GHz. This satellite will increase revisit time and significantly improve the monitoring and weather variation forecasting capabilities. Calibration is an important issue for the quantitative analysis and application of brightness temperature observations. Calibration is first conducted in a thermal/vacuum chamber prior to the launch. A hot target onboard the satellite is used to provide relative irradiance for onboard calibration, and the cold cosmos is used as a cold target. However, complicated space conditions may result in many uncertainties and instabilities during calibration. A new method is required to evaluate the performance of the instruments during long-term operation. Because of the non-existence of atmospheric and other environmental interferences, the lunar surface maintains its physical and chemical stability. Thus, it is an ideal source for thermal calibration. The earliest ground-based observation of thermal microwave emission of the Moon dates back to the early 1940s. At that time, the temperature profile and dielectric constant of the lunar surface had not been studied in detail. Thus, interpreting the observed microwave brightness temperature was difficult. In recent years, many studies of the lunar surface have been performed owing to the advances in satellite-borne remote sensing programs. The Diviner Lunar Radiometer Experiment onboard the Lunar Reconnaissance Orbiter investigated the global solar reflectance and mid-IR irradiance. These infrared measurements contribute to the quantitative description of the thermophysical properties of the superficial lunar regolith media. Chinese probes, Chang’e-1 and Chang’e-2, carried four-channel microwave radiometers to observe the microwave thermal emission of the lunar regolith media, which may provide information on superficial dielectric properties of the lunar surface. These advances created a foundation for the millimeter channel observation of the lunar surface. In this study, a real-time model of the sub-solar position and total irradiance of the lunar surface was developed. The altitude of FY-4M is 35800 km. The distance between the Moon and the Earth is between 363400 km (perigee) and 405400 km (apogee). This large distance enables nadir observation from the FY-4M radiometer toward the center region of the lunar equator. The observation beam angle ranges from 0.097° to 0.027°. The diameters of the observation regions at different channels are 750 km (55 GHz), 480 km (89, 118 GHz), 300 km (166, 183 GHz), and 210 km (425 GHz) at apogee. Based on the surface albedo derived from the Clementine data and the constrained thermophysical parameters of the lunar regolith media obtained from the Diviner infrared brightness temperature, the one-dimensional thermal transfer equation is solved to obtain the physical temperature profile of the lunar regolith media. Using the 37-GHz brightness temperature from Chang’e-2 observations, the loss tangent of the regolith media is inverted. Using the fluctuation dissipation theorem, the brightness temperature of the lunar surface in FY-4M millimeter channels is calculated. The thermophysical profile of the regolith can be validated using the Diviner infrared brightness temperature data. The calculated temperature at a depth of 1.3 m is consistent with the measurements obtained in the Apollo heat flow experiment. The surface temperature and millimeter brightness temperature on the lunar equator (0°N, 0°E) during the entire year of 2010 are simulated. The peak value of the brightness temperature is attained after that of the surface physical temperature peak value. In the low frequency channel, the penetration depth increases, its peak value is typically small, and the peak appears later. This study thus presents an effective model for FY-4M calibration and data analysis.