Crater Ejecta Deposits Research Articles

Effect of Target Properties on Regolith Production

Based on the measurements of regolith thicknesses on the lunar maria (basalts), the lunar regolith was determined to have accumulated at a rate of about 1 m/Gyr since the era of the late heavy bombardment. However, regolith production on porous targets (e.g., crater ejecta deposits) is less studied, especially for Copernican units, and how target properties affect regolith production is not well understood. Here, we measured regolith thicknesses on the ejecta blanket of the Copernicus crater, showing that the regolith production rate sensitively depends on the initial target properties. The regolith production rate of the Copernicus ejecta blanket (3.0 ± 0.1 m/Gyr) is significantly larger than that of the Copernicus impact melt, which was previously estimated to be 1.2 ± 0.2 m/Gyr. Although crater production varies with different targets, our observed crater density of the Copernicus impact melt is indistinguishable from that of the Copernicus ejecta because impacts fracture the melt, causing it to resemble the ejecta. However, due to the fact that the formation of crater ejecta had already caused them to undergo fragmentation, ejecta require fewer fragmentation times to become regolith compared to impact melt; thus, the growth of regolith on the ejecta is faster than the melt. This indicates that similar observed size–frequency distributions do not indicate similar regolith production, especially for the targets with significant differences in initial physical properties.

Stratigraphic cross-sections, geologic history, and provenance of material at the candidate landing sites of the Artemis missions

From the scientific perspective, Artemis lunar missions focus on the south circumpolar region (SCR) mainly to investigate the existence and abundance of volatiles and to explore and sample ancient lunar deposits. The volatile distribution is primarily related to the cold traps in permanently shadowed regions, while the availability of ancient material is due to the proximity to the early lunar ∼2300 km diameter South Pole-Aitken (SPA) impact basin. One of the critical factors for future missions will be determining the geological structure and provenance (sources) of material at each candidate landing site, which can be predicted utilizing three-dimensional stratigraphic reconstructions of geological map units and crater ejecta deposits. This type of reconstruction permits a better understanding of candidate material that can be collected and analyzed at each site, and a ranking of landing sites can be formulated on this basis. Here, we present reconstructed geological cross-sections at Artemis landing sites using our recent SCR geological map and numerical modelling of crater ejecta thicknesses and their sequence.

Tesserae: Surface differences across Venus’s “continents”

The heavily deformed upland tesserae are some of the most ancient geologic units on Venus and, as such, record the longest history of surface evolution. Our geologic understanding of these landforms is based largely on radar images from the Magellan mission, in which gross morphology and small-scale properties can be difficult to deconvolve. Here we use Magellan radar backscatter data for ridge slope surfaces in 22 highland areas to understand whether the tesserae can be subdivided in ways that differentiate surface property variations. Significant variations occur in the mean backscatter of ridge slopes, and we divide the tesserae into two groups with echoes lower (n = 15) or higher (n = 7) than an average tessera radar scattering behavior. While both few-kilometers-scale slopes and centimeter-scale roughness can modulate the radar returns, at least seven out of 15 tesserae with lower echoes are correlated with fine-grained impact crater ejecta deposits that smooth the surface. We propose that distal ejecta deposition plays a major role in creating the observed range of tessera radar properties and obscuring aspects of their original formation and in situ weathering. Our twofold classification system provides a new way of assessing the physical characteristics of tesserae from the Magellan data. Upcoming missions must consider both their original morphology and post-emplacement processes if we are to unlock the geologic record preserved in tesserae.

Well‐preserved low thermal inertia ejecta deposits surrounding young secondary impact craters on Mars

AbstractFollowing the most recent updates to the Mars Odyssey Thermal Emission Imaging System daytime and nighttime infrared global mosaics, a colorized global map was produced that combines the thermophysical information from the nighttime infrared global mosaic with the morphologic context of the daytime infrared global mosaic. During the validation of this map, large numbers of low thermal inertia ejecta deposits surrounding small young impact craters were observed. A near‐global survey (60°N–60°S) identified 4024 of these low thermal inertia ejecta deposits, which were then categorized based on their apparent state of degradation. Mapping their locations revealed that they occur almost exclusively in regions with intermediate‐to‐high thermal inertias, with distinct clusters in northern Terra Sirenum, Solis Planum, and southwestern Daedalia Planum. High‐Resolution Imaging Science Experiment images show that the thermophysically distinct facies of the deposits are well correlated with the estimated average ejecta grain sizes, which decrease with radial distance from the crater. Comparisons with recent primary impact craters and secondary impact craters surrounding Zunil Crater show that the low thermal inertia ejecta deposits very closely resemble the secondary craters, but not the primary craters. We conclude that the low thermal inertia ejecta deposits are secondary impact crater ejecta deposits, many of which originated from the rayed crater primary impact events, and are both well preserved and easily identifiable due to the absence of dust cover and aeolian modification that would otherwise reduce the thermal contrast between the ejecta facies and the surrounding terrain.

Lunar cryptomaria: Physical characteristics, distribution, and implications for ancient volcanism

Cryptomaria, lunar volcanic deposits obscured by crater and basin impact ejecta, can provide important information about the thermal and volcanic history of the Moon. The timing of cryptomare deposition has implications for the duration and flux of mare basalt volcanism. In addition, knowing the distribution of cryptomaria can provide information about mantle convection and lunar magma ocean solidification. Here we use multiple datasets (e.g., M3, LOLA, LROC, Diviner) to undertake a global analysis to identify the general characteristics (e.g., topography, surface roughness, rock abundance, albedo, etc.) of lunar light plains in order to better distinguish between ancient volcanic deposits (cryptomaria) and impact basin and crater ejecta deposits. We find 20 discrete regions of cryptomaria, covering approximately 2% of the Moon, which increase the total area covered by mare volcanism to 18% of the lunar surface. Comparisons of light plains deposits indicate that the two deposit types (volcanic and impact-produced) are best distinguished by mineralogic data. On the basis of cryptomaria locations, the distribution of mare volcanism does not appear to have changed in the time prior to its exposed mare basalt distribution. There are several hypotheses explaining the distribution of mare basalts, which include the influence of crustal thickness, mantle convection patterns, asymmetric distribution of source regions, KREEP distribution, and the influence of a proposed Procellarum impact basin. The paucity of farside mare basalts means that multiple factors, such as crustal thickness variations and mantle convection, are likely to play a role in mare basalt emplacement.

How old are young lunar craters?

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.

Atmospheric dust dispersal analyzed by granulometry of the Misers Gold Event

Granulometric analysis of ejecta from Misers Gold high explosive cratering experiment demonstrates that atmospheric dust dispersal can be evaluated by particle‐size distribution data. From size analyses of the Misers Gold preshot test bed alluvium, ejecta, and sweep‐up materials collected out to 35 crater radii (1227 m), we find that approximately 5.9 × 106 kg (∼11% of the total crater ejecta mass) was depleted from the crater ejecta deposits and likely represents the portion of the cratered mass initially lofted into atmospheric suspension. The dominant size range of this lofted dust was 88 to 2000 μm with a mean diameter by mass of 384 μm,. In addition to the dust lofted in the explosion column, dust in the size range of 100–800 μm was swept up from the ground by the explosive air blast and base surge dominantly between ranges of 10 and 18 crater radii (360 and 650 m from ground zero). This sweep‐up dust was convectively drawn into the column and contributed up to 2% of the total mass of lofted. Based on the measured abundance of coarse (>250 μm diameter) dust particles in the estimated lofted dust, it is likely that about 70% of this lofted mass fell out within about 1 hour, such that the remaining dust cloud mass was ∼1.8 × 106 kg, which is equivalent to a dust lofted per unit blast yield of 0.5 kT/kT or about 4% of the crater ejecta mass. This study supports the hypothesis that if initial distributions can be constrained, the volume of dust lofted into atmospheric suspension from large surface explosions can be estimated from analysis of particle‐size distributions of ejecta deposited near the explosion. This result may have particular applications to study of the atmospheric effects of historic and prehistoric volcanic eruptions.

Lunar magnetic anomalies detected by the Apollo substatellite magnetometers

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side. The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 10 9 y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of ∼7 × 10 −2 G cm 3/g (assuming a density of 3 g/cm 3), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced ∼3.9 × 10 9 y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles. The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage. The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data.

Latitudinal variation of wind erosion of crater ejecta deposits on Mars

Wind erosion seems to be the dominant process eroding crater ejecta deposits and sorrounding materials on Mars. In the equatorial zone, ejecta deposits are eroded back by scarp recession, where scarp heights appear to be approximately equivalent to ejecta thickness. In mantled areas, escarpments develop by relatively rapid deflation of sorrounding aeolian debris, leaving the ejecta deposit (continuous deposit and zone of high density of secondary craters) standing high above sorrounding terrain. If the rate of scarp recession is controlled by the rate of aeolian undercutting of escarpment bases, then recession rates may scale roughly as the inverse with respect to scarp height. Thus, preferential preservation of ejecta deposits emplaced in thickest aeolian debris may occur. An empirical model developed for wind erosion of ejecta deposits in nonmantled areas suggests that removal of ejecta materials on the average is exceedingly slow (∼10 −5m/yr for 10m high scarp). On the other hand, rapid deflation of aeolian debris around crater ejecta is implied. Results suggest high differential aeolian erosion rates that are a function of both grain sizes and large-scale surface roughness. Aeolian activity on Mars has probably been dominated by rapid recycling of fine-grained debris, the bulk of which formed under more erosive conditions prevalent in the early history of Mars.