The Experts below are selected from a list of 186 Experts worldwide ranked by ideXlab platform
H J Melosh - One of the best experts on this subject based on the ideXlab platform.
-
detection and characterization of buried Lunar Craters with grail data
Icarus, 2017Co-Authors: H J Melosh, C Milbury, D M Blair, Rohan Sood, L Chappaz, Kathleen C Howell, Maria T ZuberAbstract:Abstract We used gravity mapping observations from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) to detect, characterize and validate the presence of large impact Craters buried beneath the Lunar maria. In this paper we focus on two prominent anomalies detected in the GRAIL data using the gravity gradiometry technique. Our detection strategy is applied to both free-air and Bouguer gravity field observations to identify gravitational signatures that are similar to those observed over buried Craters. The presence of buried Craters is further supported by individual analysis of regional free-air gravity anomalies, Bouguer gravity anomaly maps, and forward modeling. Our best candidate, for which we propose the informal name of Earhart Crater, is approximately 200 km in diameter and forms part of the northwestern rim of Lacus Somniorum, The other candidate, for which we propose the informal name of Ashoka Anomaly, is approximately 160 km in diameter and lies completely buried beneath Mare Tranquillitatis. Other large, still unrecognized, Craters undoubtedly underlie other portions of the Moon’s vast mare lavas.
-
preimpact porosity controls the gravity signature of Lunar Craters
Geophysical Research Letters, 2015Co-Authors: C Milbury, H J Melosh, B C Johnson, G S Collins, D M Blair, J M Soderblom, F Nimmo, C J Bierson, Roger J. PhillipsAbstract:We model the formation of Lunar complex Craters and investigate the effect of preimpact porosity on their gravity signatures. We find that while preimpact target porosities less than ~7% produce negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction of pore space due to shock wave compression. The central BA of Craters larger than ~215 km in diameter, however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking differences between the gravity signatures of Craters on the Earth and Moon are the result of the higher average porosity and variable porosity of the Lunar crust.
-
projectile remnants in central peaks of Lunar impact Craters
Nature Geoscience, 2013Co-Authors: Zongyu Yue, H J Melosh, B C Johnson, David A Minton, Yanbing LiuAbstract:Unusual minerals observed in Lunar Craters were thought to originate from beneath the Moon’s surface. Numerical simulations show that rather than being vaporized, much of the impactor material can survive in the crater, implying that the unusual minerals come from the impactor and may not be indigenous to the Moon.
-
distributions of boulders ejected from Lunar Craters
Icarus, 2010Co-Authors: G D Bart, H J MeloshAbstract:We investigate the spatial distributions of boulders ejected from 18 Lunar impact Craters that are hundreds of meters in diameter. To accomplish this goal, we measured the diameters of 13,955 ejected boulders and the distance of each boulder from the crater center. Using the boulder distances, we calculated ejection velocities for the boulders. We compare these data with previously published data on larger Craters and use this information to determine how boulder ejection velocity scales with crater diameter. We also measured regolith depths in the areas surrounding many of the Craters, for comparison with the boulder distributions. These results contribute to understanding boulder ejection velocities, to determining whether there is a relationship between the quantity of ejected boulders and Lunar regolith depths, and to understanding the distributions of secondary Craters in the Solar System. Understanding distributions of blocky ejecta is an important consideration for landing site selection on both the Moon and Mars.
Richard A. F. Grieve - One of the best experts on this subject based on the ideXlab platform.
-
Effect of impact velocity and acoustic fluidization on the simple‐to‐complex transition of Lunar Craters
Journal of Geophysical Research: Planets, 2017Co-Authors: E A Silber, Gordon R Osinski, B C Johnson, Richard A. F. GrieveAbstract:We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on Lunar Craters in the simple-to-complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block-Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce Craters of the same diameter within a suite of simulations, ranging in diameter from 10-26 km, which straddles the simple-to-complex crater transition on Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block-Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of Craters in the simple-to-complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple-to-complex Craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of Lunar Craters. This scaling suggests that the simple-to-complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple-to-complex transition occurs at larger sizes on Mercury than Mars.
-
effect of impact velocity and acoustic fluidization on the simple to complex transition of Lunar Craters
Journal of Geophysical Research, 2017Co-Authors: Gordon R Osinski, E A Silber, B C Johnson, Richard A. F. GrieveAbstract:We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on Lunar Craters in the simple-to-complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block-Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce Craters of the same diameter within a suite of simulations, ranging in diameter from 10-26 km, which straddles the simple-to-complex crater transition on Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block-Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of Craters in the simple-to-complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple-to-complex Craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of Lunar Craters. This scaling suggests that the simple-to-complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple-to-complex transition occurs at larger sizes on Mercury than Mars.
E A Silber - One of the best experts on this subject based on the ideXlab platform.
-
Effect of impact velocity and acoustic fluidization on the simple‐to‐complex transition of Lunar Craters
Journal of Geophysical Research: Planets, 2017Co-Authors: E A Silber, Gordon R Osinski, B C Johnson, Richard A. F. GrieveAbstract:We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on Lunar Craters in the simple-to-complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block-Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce Craters of the same diameter within a suite of simulations, ranging in diameter from 10-26 km, which straddles the simple-to-complex crater transition on Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block-Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of Craters in the simple-to-complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple-to-complex Craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of Lunar Craters. This scaling suggests that the simple-to-complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple-to-complex transition occurs at larger sizes on Mercury than Mars.
-
effect of impact velocity and acoustic fluidization on the simple to complex transition of Lunar Craters
Journal of Geophysical Research, 2017Co-Authors: Gordon R Osinski, E A Silber, B C Johnson, Richard A. F. GrieveAbstract:We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on Lunar Craters in the simple-to-complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block-Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce Craters of the same diameter within a suite of simulations, ranging in diameter from 10-26 km, which straddles the simple-to-complex crater transition on Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block-Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of Craters in the simple-to-complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple-to-complex Craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of Lunar Craters. This scaling suggests that the simple-to-complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple-to-complex transition occurs at larger sizes on Mercury than Mars.
M S Robinson - One of the best experts on this subject based on the ideXlab platform.
-
small Lunar Craters at the apollo 16 and 17 landing sites morphology and degradation
Icarus, 2018Co-Authors: Prasun Mahanti, M S Robinson, T J Thompson, M R HenriksenAbstract:Abstract New analysis and modeling approaches are applied to high-resolution images and topography of the Apollo 16 and 17 landing sites to investigate the morphology and estimate degradation of small Lunar Craters (SLCs; 35 to 250 m diameter). We find SLCs at the two sites are mostly degraded with an average depth-diameter ratio ( d D ) 0.1 , resulting in a landscape dominated by shallow, inverted cone-shaped Craters. An improved standardized morphological classification and a novel set of quantitative shape indicators are defined and used to compare SLCs between the two sites. Our classification methodology allows morphological class populations to be designated with minimal (and measurable) ambiguity simplifying the study of SLC degradation at different target regions. SLC shape indicators are computationally obtained from topography, further facilitating a quantitative and repeatable comparison across study areas. Our results indicate that the interior slopes of SLCs evolve faster and through different processes relative to larger Craters ( > 500 m). Assuming SLCs are formed with large initial depth-to-diameter ratio ( d D ≥ 0.2 ), our observation that even the fresher SLCs are relatively shallow imply that a faster mass wasting process post-formation stabilizes the crater walls and eventually slows down degradation. We also found that the Apollo 16 Cayley plains have a higher percentage of fresh Craters than the Apollo 17 Taurus Littrow (TL) plains. A combination of a less-cohesive target material and/or seismic shaking resulting from moonquakes or the impact of Tycho crater secondaries was likely responsible for a higher degradation rate in the TL-plains compared to the Cayley plains. This study explores the relationship between the symmetry and probability densities of key morphological traits like d D , mean wall slope and rate of degradation. We show that the shape of d D probability density function of SLCs in a study area encodes their rate of degradation. Comparison of power-law fitting and probabilistic modeling of depth-diameter relations shows that probabilistic methods complement regression models and are necessary for robust prediction of SLC depths from diameter (and vice versa) for different geological targets.
-
occurrence and mechanisms of impact melt emplacement at small Lunar Craters
Icarus, 2014Co-Authors: J D Stopar, Ray B Hawke, M S Robinson, B W Denevi, T A Giguere, Steven D KoeberAbstract:Abstract Using observations from the Lunar Reconnaissance Orbiter Camera (LROC), we assess the frequency and occurrence of impact melt at simple Craters less than 5 km in diameter. Nine-hundred-and-fifty fresh, randomly distributed impact Craters were identified for study based on their maturity, albedo, and preservation state. The occurrence, frequency, and distribution of impact melt deposits associated with these Craters, particularly ponded melt and lobate flows, are diagnostic of melt emplacement mechanisms. Like larger Craters, those smaller than a few kilometers in diameter often exhibit ponded melt on the crater floor as well as lobate flows near the crater rim crest. The morphologies of these deposits suggest gravity-driven flow while the melt was molten. Impact melt deposits emplaced as veneers and “sprays”, thin layers of ejecta that drape other crater materials, indicate deposition late in the cratering process; the deposits of fine sprays are particularly sensitive to degradation. Exterior melt deposits found near the rims of a few dozen Craters are distributed asymmetrically around the crater and are rare at Craters less than 2 km in diameter. Pre-existing topography plays a role in the occurrence and distribution of these melt deposits, particularly for Craters smaller than 1 km in diameter, but does not account for all observed asymmetries in impact melt distribution. The observed relative abundance and frequency of ponded melt and flows in and around simple Lunar Craters increases with crater diameter, as was previously predicted from models. However, impact melt deposits are found more commonly at simple Lunar Craters (i.e., those less than a few kilometers in diameter) than previously expected. Ponded melt deposits are observed in roughly 15% of fresh Craters smaller than 300 m in diameter and 80% of fresh Craters between 600 m and 5 km in diameter. Furthermore, melt deposits are observed at roughly twice as many non-mare Craters than at mare Craters. We infer that the distributions and occurrences of impact melt are strongly influenced by impact velocity and angle, target porosity, pre-existing topography, and degradation. Additionally, areally small and volumetrically thin melt deposits are sensitive to mixing with solid debris and/or burial during the modification stage of impact cratering as well as post-cratering degradation. Thus, the production of melt at Craters less than ∼800 m in diameter is likely greater than inferred from the present occurrence of melt deposits, which is rapidly affected by ongoing degradation processes.
Mark S. Robinson - One of the best experts on this subject based on the ideXlab platform.
-
How old are young Lunar Craters
Journal of Geophysical Research, 2012Co-Authors: Harald Hiesinger, C. H. Van Der Bogert, Jan Hendrik Pasckert, L. Funcke, Lorenza Giacomini, L. R. Ostrach, Mark S. RobinsonAbstract:[1] The accurate definition of the Lunar cratering chronology is important for deriving absolute model ages across the Lunar surface and throughout the Solar System. Images from the Lunar Reconnaissance Orbiter Narrow Angle Cameras and Wide-Angle Camera and the SELENE/Kaguya Terrain Camera provide new opportunities to investigate crater size-frequency distributions (CSFDs) on individual geological units at Lunar impact Craters. We report new CSFD measurements for the Copernican-aged Craters North Ray, Tycho, and Copernicus, which are crucial anchor points for the Lunar cratering chronology. We also discuss possible reasons for an age discrepancy observed between the impact melt and ejecta units. Our CSFDs for North Ray and Tycho crater ejecta deposits are consistent with earlier measurements. However, for Copernicus crater and one of its rays, we find significantly lower cumulative crater frequencies than previous studies. Our new results for Copernicus crater fit the existing Lunar absolute chronologies significantly better than the previous counts. Our derived model ages of the ejecta blankets of North Ray, Tycho, and Copernicus agree well with radiometric and exposure ages of the Apollo 16, 17, and 12 landing sites, respectively, and are generally consistent with a constant impact rate over the last 3 Ga. However, small variations of the impact rate cannot be resolved in our data and require further investigations.
