Search

We present systematic measurements of the frequency‐dependent complex dielectric permittivity of lunar regolith analogue samples with increasing amounts of the mineral ilmenite along with Bayesian model fits using a one‐pole Cole‐Cole model. We use these results to calculate a lower bound for the attenuation of radar signals in dB/m based on ilmenite content. We compare our measurement results with previous efforts to use Earth‐based radar maps to calculate the effect of ilmenite on radar attenuation and find that they are in agreement. We also revisit the ilmenite‐dependent loss tangent relationships of Carrier III et al. (1991) and demonstrate the significant frequency‐dependent effect of ilmenite content on signal attenuation as well as the effect of minor variations in loss tangent for depth‐to‐feature determinations. The results presented here are the first systematic laboratory measurements investigating the effect of ilmenite on radar attenuation and show future promise for the application of dielectric spectroscopy, or the identification of materials based on their electromagnetic properties in the radar and microwave range. Plain Language Summary: We measure and model the electrical parameters that are responsible for absorbing and reflecting radar signals in powdered rocks that have a similar composition to the surface of the Moon. We show that knowing the abundance of the mineral ilmenite is important for knowing how much of the signal gets absorbed. We also show that the frequency (or wavelength) of the signal is important. We do this by measuring samples with more and more ilmenite at different frequencies. We find that adding more ilmenite causes the material to absorb more signal. By knowing how much more signal at specific frequencies gets absorbed with increasing ilmenite, we can better understand both the surface and below the surface of the Moon using radar data. For example, if we see in radar data that something might be buried beneath the surface of the Moon, we can use this information to try to find out how deep it is. Key Points: We present systematic measurement results of the frequency‐dependent complex permittivity of ilmenite‐bearing lunar regolith analogsWe make our measurements over a broad range of frequencies between 100 MHz and 8.5 GHzThese results provide new frequency‐dependent constraints on the attenuation of radar in the lunar regolith [ABSTRACT FROM AUTHOR]

We propose a Bayesian Markov Chain Monte Carlo parameter estimation technique to determine frequency‐dependent complex dielectric permittivity and permeability of powdered planetary regolith analog materials from coaxial transmission line measurements. The technique uses either a flat (non‐dispersive) or a generalized dielectric response model to simultaneously determine the complex permittivity and permeability from measured scattering parameters. By exploring the parameter space, the technique reduces the dependence on good initial model parameters required by typical optimization methods. The technique can be used with either two‐port measurements or shorted measurements, where an additional one‐port shorted sample is measured. We find the technique is able to accurately determine electromagnetic parameters of low‐loss, high dielectric, and magnetic materials. The technique is also robust to half‐wavelength resonances in the sample as well as the presence of higher order modes. Plain Language Summary: We propose a method to find the electrical and magnetic parameters responsible for absorbing and reflecting radar signals in powdered rocks. This is useful because we find powdered rock material on the surfaces of most rocky planetary bodies. Since radar is often used to look at planetary bodies in the solar system, it is important to understand how different types of powdered rocks affect radar signals. Other methods like ours already exist to measure these types of parameters but we are proposing one that doesn't need a good guess of the right answer to work. We measure three types of powders to see if our method works: Powders that don't absorb much radar signal, a powder that absorbs a lot of signal and is not magnetic, and a powder that absorbs a lot of signal that is magnetic. We find that our method works for all of these types of powdered rocks. Key Points: We present methods to determine the frequency‐dependent complex permittivity and permeability of powdered geologic materialsWe use a Bayesian Markov Chain Monte Carlo parameter estimation methodWe measure low‐loss, high‐loss, and magnetic materials between 100 MHz and 8.5 GHz [ABSTRACT FROM AUTHOR]

Irregular 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 <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. Plain Language Summary: Irregular mare patches (IMPs) are small volcanic features on the Moon that are suggested by some to have formed relatively recently (within the past 100 Myr) compared with the majority of the lunar surface. Other studies suggest that IMPs are of a similar age to their surrounding maria, but appear young due to their unique physical properties. In this work, we investigate the thermophysical properties of eight of the largest lunar IMPs using data from the Diviner Lunar Radiometer Experiment, an instrument onboard the Lunar Reconnaissance Orbiter. We find that the regolith at a number of the IMPs have relatively low thermal inertia, indicating that the material there is less consolidated or contains fewer small rocks than typical lunar regolith. One IMP in particular (named Ina) has noticeably lower thermal inertia than the other IMPs, which we suggest could be due to the large size and more distinct edges of the units within Ina compared with the other IMPs. Our results are consistent with a number of the proposed formation hypotheses; however, given observational limitations due to the small size of many of the IMPs we cannot definitively constrain the formation mechanism. Key Points: Irregular mare patches (IMPs) generally have higher rock abundance (RA) than typical maria, but lower RA than rocky crater ejectaThe regolith fines component of IMPs generally has lower thermal inertia than expected based on their RAIna has lower thermal inertia than the other IMPs, displaying similar H‐parameter values to a nearby pyroclastic deposit but with higher RA [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Our understanding of the lunar chronology relies heavily on comparing crater counts on the ejecta of impact craters with laboratory dating methods of samples returned by the Apollo and Luna missions. However, discrepancies in crater populations on crater ejecta blankets have been observed and may present a source of uncertainty in the modeling of the lunar chronology. In this study, we show that variations in the distribution of craters on the ejecta of crater Giordano Bruno correspond with variations in terrain properties. Nighttime temperatures are sensitive to variations in density and thermal conductivity. We identified the variations in these properties using Diviner nighttime temperature observations and show that these properties correlate with variations in crater frequencies. Our results strongly support the idea that target properties are likely influencing the production of craters and therefore must be accounted for when estimating crater count‐derived absolute ages. Key Points: The distribution of craters on Giordano Bruno varies with thermophysical propertiesThermal modeling of the ejecta shows lateral and vertical variations in thermal propertiesVariations in target properties influence the production of craters on the ejecta, resulting in variable absolute model age estimates [ABSTRACT FROM AUTHOR]

Space weathering processes form submicroscopic metallic iron particles that are optically active, darkening and reddening the lunar surface over time. The optical effects of these particles depend on their size; nanophase iron darkens and reddens, while microphase iron darkens without reddening. Using available Kaguya Multiband Imager parameter maps believed to estimate submicroscopic iron abundance, we investigate trends that may be associated with abundance of nanophase and microphase iron near dated lunar craters. We observe that nanophase iron is strongly correlated with crater age, while microphase iron exhibits a weaker correlation. We present models for the highlands nanophase and microphase iron accumulation rates from 100 ka to 1 Ga. Our observations suggest that highlands nanophase iron abundance is a direct result of space weathering exposure, while highlands microphase iron abundance is likely influenced by lunar source materials or stochastic impact‐delivery mechanisms. Plain Language Summary: The Moon, Mercury, and asteroids lack protective atmospheres and slowly darken over time due to their exposure to space. We call this darkening process space weathering. Apollo samples returned from the Moon show that space weathering produces tiny iron particles that range in size from nanometers to micrometers. We use orbital lunar maps of nanophase and microphase iron content to better understand how quickly space weathering occurs on the Moon. We first find the iron content of young lunar soils deposited by large impact events, then use the age of each impact crater to see how much iron accumulates in different lengths of time. We find that nanophase iron accumulates predictably over time due to space weathering. In contrast, microphase iron accumulates less predictably, possibly because it exists in the lunar soil initially, or is delivered to the Moon by crater‐forming impacts. This work helps us understand how the surface of the Moon weathers over time and how space weathering might work elsewhere in the Solar System. Key Points: We model nanophase and microphase iron accumulation rates using published parameter maps and crater agesHighlands nanophase iron is negligible at 100 ka and accumulates at ∼0.05 wt% ln (Ma)−1 until ∼1 GaHighlands microphase iron is enhanced by 100 ka and less correlated with age, possibly due to variable abundances in lunar source materials [ABSTRACT FROM AUTHOR]

We model impact gardening on Ceres and find that it is orders of magnitude less intense than impact gardening on Earth's Moon. At Ceres' equator, gardening is too shallow to compete with sublimation as a control on the depth to ice. At high latitudes, impact gardening will disturb the top centimeter of ice over 1 Ga but could not contribute to the disruption of thick ice deposits. Because impact gardening is weak on Ceres compared to the Moon, we also predict that the meter and smaller scale surface texture of Ceres may be significantly different from the lunar surface. Regolith may be much thinner on Ceres and the top meter may be significantly less dominated by fine‐grained regolith material. Plain Language Summary: Ceres is a water‐rich dwarf planet located in the asteroid belt. Impact gardening, or the process by which frequent small impacts physically churn the uppermost surface of planetary bodies, is much less intense on Ceres than it is on Earth's Moon. Because of the difference in impact gardening, the meter and smaller scale surface texture of Ceres may be significantly different from the lunar surface. Also because of the relatively shallow effects of impact gardening, the presence or absence of surface ice on Ceres is dominantly controlled by temperature. However, at the poles impact gardening may have some effect of the evolution of surface ice. Key Points: Impact gardening is at least an order of magnitude less intense on Ceres than it is on the MoonOn Ceres, impact gardening may control the depth to ice at high latitudes and in regions of persistent shadowThe meter and smaller scale surface texture of Ceres may be significantly different from that of the Moon [ABSTRACT FROM AUTHOR]

Planetary radar has provided a growing number of data sets on the inner planets and near‐Earth and main belt asteroid populations in the solar system. Physical interpretation of radar data for inference of surface properties requires constraints on the constitutive parameters of the material making up a given surface. In this study, the complex permittivity of seven minerals as a function of frequency and porosity is measured using the coaxial transmission line method to determine the mixing equation that best describes the relationship between the real part of the complex permittivity of single mineral crystals and granular mineral powders. We find the Looyenga‐Landau‐Lifshitz and Bruggeman symmetric mixing equations to describe our experimental results with the highest accuracy. The variation in the real part of the permittivity of solid mineral crystals between different minerals is shown to depend on the grain density and the chemical composition of the minerals. These mixing relationships are incorporated into an asteroid radar model and used to calculate the porosity in the near‐surface of seven asteroids visited by robotic spacecraft using Earth‐based radar observations. The results of the asteroid radar model support the presence of significant porosity in the boulders on the surface of asteroid 101955 Bennu. This research highlights the ability of radar to measure the porosity on asteroid surfaces and provides theoretical and experimental justification for the inversion of permittivity to bulk density assumed by the asteroid radar model. Plain Language Summary: Radar observations of planets, the Moon, and asteroids provide information about the upper surface, within a few tens of centimeters to meters, such as the electrical properties of this material. In this study, these electrical properties are measured in the laboratory for seven mineral samples under environmental conditions expected for planetary surfaces. Several well‐known mixing equations are tested for their accuracy in modeling the change in the measured electrical properties with the change in the porosity, or amount of empty space, in the samples. We incorporate two of these mixing equations into an asteroid radar model that converts the measured electrical properties of radar‐observed asteroids to the density of the surface material. Our model predicts low surface density for seven asteroids observed by Earth‐based radar telescopes, such as Arecibo Observatory. Space missions such as the National Aeronautics and Space Administration's OSIRIS‐REx and the Japanese Aerospace Exploration Agency's Hayabusa2 missions visiting asteroids will provide ground‐truth observations of these asteroids' surface properties, such as density, for validating our models. The density of the surfaces of asteroids provides information about the possible structure and evolutionary history of asteroids, and the early solar system. Key Points: New permittivity measurements support the use of the Looyenga‐Landau‐Lifshitz mixing equation for modeling planetary radar dataWe provide new estimates of near‐surface porosity for seven spacecraft‐visited asteroids using Earth‐based radar observationsPorous boulders on the surface of 101955 Bennu are likely one of the reasons for the low CPR measured by Earth‐based radar [ABSTRACT FROM AUTHOR]

We update an analytic impact gardening model (Costello et al., 2018, https://doi.org/10.1016/j.icarus.2018.05.023) to calculate the depth gardened by impactors on the Moon and Mercury and assess the implications of our results for the age, extent, and source of water ice deposits on both planetary bodies. We show that if the water presently on the Moon has a primordial origin, it may have been 4–15 m thick. If ice deposits are buried, they may be as shallow as 3 cm or as deep as 10 m and provide a gradient of probability for ice gardened into a column. Our calculations for gardening on Mercury show that thermal lag deposits will be reworked into the background over 200 Myr, and, thus, the most recent large‐scale deposition of ice on Mercury must have occurred no more than 200 Myr ago. We also find that gardening mixes incremental layers of ice with underlying regolith and prevents the growth of pure ice deposits by continuous supply. We conclude that ice deposits on the Moon and Mercury are likely the result of sudden and voluminous deposition. Plain Language Summary: The Moon and Mercury have water ice in permanently shadowed regions at their poles; however, while Mercury's poles are rich in water ice, the Moon's are relatively sparse. Impact cratering mixes surface material, bringing buried ice upward to a relatively hostile surface in a process called "impact gardening." We apply a revised impact gardening model to investigate the depth from which ice is mixed upward by impacts as a function of time and discover the following: (1) Ice on the Moon may have been mixed with underlying regolith or buried under many meters of dry regolith; (2) the most recent large‐scale delivery of water to Mercury happened no more than 200 Myr ago; (3) what ice exists at the poles of the Moon and Mercury was probably delivered suddenly and voluminously; and (4) the differences between the abundance of water ice on the Moon and Mercury will only become more pronounced with time as impact gardening destroys surface ice on the Moon and only scratches the surface of Mercury's extensive deposits. Key Points: Secondary cratering dominates impact gardening in the top few metersWater ice on the Moon may be billions of years oldWater ice on Mercury is no more than 200 million years old [ABSTRACT FROM AUTHOR]

A recently identified class of young lunar craters, cold‐spot craters, provides an optimal data set for probing the subsurface rock content on the Moon. Rocks, or lack of rocks, in crater ejecta have long been used as indicators of regolith thickness. However, the rockiness of crater ejecta depends on the subsurface rock content, crater excavation depth, and crater age. Cold‐spot craters are surrounded by a low thermal inertia signature that fades within 1 Myr, so any rocks in the proximal ejecta blanket have not yet been broken down or buried. Thus, the rock abundance in the proximal ejecta blankets of cold‐spot craters is affected only by the subsurface rock content of the target and the crater excavation depth, not the age of the crater. We show that an abrupt transition between fine‐grained regolith and underlying coherent rock cannot explain the observed rock abundance. Instead, we assume that the volume fraction of coherent rock (in contrast to fine‐grained regolith) increases exponentially in the lunar subsurface and use the observed Diviner rock abundance in cold‐spot crater ejecta to solve for the "e‐folding" depth over which the volume fraction of rock increases. In general, the subsurface rock content is higher in the maria than in the highlands consistent with more recent resurfacing of the maria. However, the highlands show a wider range of subsurface rock content values overall including some overlapping those of the maria. Some of this variability appears to be associated with the Orientale basin and possibly with several cryptomare deposits. Key Points: Cold‐spot craters are ideal for probing the subsurface rock content on the Moon, because they formed recentlyThe subsurface rock content is higher in the maria than in the highlands, as expectedThe highlands show a wider range of subsurface rock content than the maria [ABSTRACT FROM AUTHOR]

We use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (<1 km), young (<200 Ma) impact craters near spacecraft landing sites. The Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate. Plain Language Summary: Many of the boulders found on the surface of the Moon are located around impact craters. Over time, these boulders break down, primarily as a result of being bombarded with small meteorites. Correlating the abundance of boulders with the age of the crater they are located around can inform how long it takes for boulders to break down and become part of the regolith (the Moon's soil). We used high‐resolution images from the Lunar Reconnaissance Orbiter Camera to measure and count boulders around six young lunar impact craters. We found that it takes a few hundred million years for boulders to begin disappearing due to degradation. We also analyzed how boulder distributions change with increasing distance from the crater rim, and we find that more small boulders occur at greater distances than large boulders. Understanding how boulder distributions vary around craters of different sizes and ages is important in planning for the safety of future landed missions to the lunar surface. Key Points: Few boulders remain at craters older than a few hundred million yearsDetermining boulder size distributions as a function of distance from a crater is key in assessing boulder hazards for future missionsLunar Reconnaissance Orbiter Camera Narrow Angle Camera boulder counts are consistent with Diviner rock abundance estimates [ABSTRACT FROM AUTHOR]

We develop a full methodology to measure the complex relative permittivity of planetary regolith analog materials in vacuum and up to 120 °C over a broad range of frequencies (400 MHz to 8.5 GHz or wavelengths 18.8–3.5 cm). We demonstrate our method with measurements of analog regolith materials appropriate for asteroid (101955) Bennu, the target of National Aeronautics and Space Administration's OSIRIS‐REx mission: individual and mixed components of UCF/DSI‐CI‐2, a new carbonaceous asteroid regolith simulant produced by Deep Space Industries based on CI chondrite meteorite mineralogy. We measure, for the first time, the effect of carbonaceous material on the complex relative permittivity of asteroid regolith analogs by measuring the powdered serpentine component of the simulant mixed with varying amounts of carbonaceous material in vacuum at 25 and 40 °C. We find that at a bulk density of 1.60 g/cm3 and wavelength of 12.6 cm, serpentine with 5 wt% carbonaceous material has εr′ = 3.30 ± 0.01 and tanδ = 0.016 ± 0.003 and that carbonaceous material increases the attenuation of electromagnetic energy in our samples. Ground‐based radar (at 12.6‐ and 3.5‐cm wavelengths) has previously been used to investigate carbonaceous asteroid (101955) Bennu. Our measurements provide new constraints on the attenuation of radar energy in granular carbonaceous materials. Plain Language Summary: We measure the electrical parameters that are responsible for absorbing and reflecting radar waves in powders that have been mixed to resemble the composition of the regolith of the asteroid Bennu. Regolith is powdered rock material that covers the surfaces of most rocky planetary bodies. Bennu is the target of National Aeronautics and Space Administration's OSIRIS‐REx mission, which is a spacecraft that will return regolith from the surface of Bennu to Earth. Radar has been used from Earth to look at Bennu, so it is important to understand how the electrical parameters of the regolith on Bennu affect radar signals. Bennu's regolith is thought to contain organic carbon. We have measured the effect of adding organic carbon to our samples and found that adding carbon increases absorption of radar waves. Key Points: We present methods for measuring the complex relative permittivity of regolith analog materialsWe measure carbonaceous asteroid analog materials in vacuum between 400 MHz and 8.5 GHz at 25 and 40 °CThese measurements provide new constraints on the attenuation of radar waves in granular carbonaceous materials [ABSTRACT FROM AUTHOR]

Mapping of lunar nighttime surface temperatures has revealed anomalously low nighttime temperatures around recently formed impact craters on the Moon. The thermophysically distinct "cold spots" provide a way of identifying the most recently formed impact craters. Over 2,000 cold spot source craters were measured with diameters ranging from 43 m to 2.3 km. Comparison of the crater size‐frequency distribution with crater chronology models and crater counts of superposed craters on the ejecta of the largest cold spot craters constrains the retention time of the cold spots to no more than ~0.5–1.0 Myr with smaller cold spots possibly retained for only few hundred kyr. This would suggest a relatively rapid impact gardening rate with regolith overturn depths exceeding ~5 cm over this time scale. We observe a longitudinal heterogeneity in the cold spot distribution that reflects the Moon's synchronous rotation with a higher density of cold spots at the apex of motion. The magnitude of the asymmetry indicates the craters formed from a population of objects with low mean encounter velocities ~8.4 km/s. The larger cold spots (D > 800 m) do not follow this trend, and are concentrated on the trailing farside. This could result from a shorter retention time for larger cold spots on the leading hemisphere due to the greater number of smaller, superposed impacts. Alternatively, the abundance of large cold spots on the trailing farside resulted from a swarm of 100‐m‐scale impactors striking the Moon within the last ~0.5 Myr. Plain Language Summary: Impact craters on the Moon modify the surfaces around them, resulting in patches of colder nighttime surface temperatures. These "cold spots" fade over time. Using the cold spots as markers to identify the most recent impacts that have occurred on the Moon, we measured the diameters of all the craters with cold spots. Comparing the population of these craters with the expected impact rate, we estimate that the cold spots fade over a few hundred thousand years. The cold spots are also concentrated on the western hemisphere due to the Moon's synchronous rotation where the western half of the Moon always faces toward the direction of motion of the Moon orbiting the Earth. This suggests that a relatively slow population of objects impacted the Moon in the last few hundred thousand years. The largest cold spots, however, are concentrated on the trailing hemisphere. This could result either from more small impacts on the leading hemisphere destroying larger cold spots and leaving a higher number of large cold spots on the trailing hemisphere, or a swarm of 100‐m‐sized objects colliding on the trailing side during this time period. Key Points: We measure diameters of craters associated with cold spots. Their size‐frequency distribution indicates cold spots survive a few hundred kyrThe distribution of cold spots reflects the Moon's synchronous rotation with cold spots focused on the apex of motionThe largest cold spots with source craters larger than 800 m are concentrated on the trailing side of the moon [ABSTRACT FROM AUTHOR]