Blocky Ejecta Research Articles

Evolution of Subkilometer-scale Impact Craters on the Lunar Maria as Constrained from Mini-RF Data and Topographic Degradation Model

Physical properties (e.g., ejecta size and distribution) of impact craters are crucial and essential to understanding the ejecta excavation and deposition process, estimating rock breakdown rate, and revealing their evolution characteristics. However, whether these physical properties are scale-dependent and how they evolve in different radial regions needs further studies. In this study, we first investigated the physical properties and evolution of sub-kilometer (D ≤ 800 m) craters on lunar maria based on the radar circular polarization ratio (CPR). In addition, we estimated the periods over which rocks and blocky ejecta are exposed and buried in the shallow subsurface layer (termed as exposure time) in different radial regions and assessed the retention time and degradation states for potential radar anomalous craters. We found that in the central region of craters, the largest median CPR occurs after an 80 Myr delay following crater formation. In the rim region, there is no obvious CPR peak in the first 100 Ma, whereas in the upper wall region, an evident CPR peak occurs beyond 100 Ma and could last over one billion years. In addition, the probable exposure time of rocks and blocky ejecta is estimated to be ∼2.0 Gyr (central region), ∼2.7 Gyr (upper wall region), ∼2.1 Gyr (rim region), and ∼0.6 Gyr (continuous ejecta blanket region). We also propose that the retention time of radar anomalous craters depends on the crater size, whereas their degraded states are independent of crater size.

Small crater relaxation and ice abundance at high northern latitudes on Mars

The cold, northern high-latitudes around the Phoenix landing site feature a population of small and very shallow craters known as palimpsest craters or boulder halos that could give insight into the subsurface ice content of the region. Notably there is also a split in the population by the size of craters that feature blocky ejecta: the smallest crater sizes that are <200–350 m in diameter typically do not feature blocky ejecta, while craters larger than this size range do. This has been interpreted as there being a less consolidated layer of mainly ice that sits over a more consolidated layer containing more silicate material from which the larger craters are able to excavate the blocky ejecta. This work focuses on determining whether these 200 m - 1 km shallow craters could be indicative of topographic relaxation due to substantial ice in the subsurface, with goals to determine the subsurface structure and most importantly the amount of ice present through its abundance and depth extent. We use finite element modeling of axisymmetric simple craters, employing the most recent characterizations of solid-state creep for water ice, silicate-rich ice, and CO2 ice, to determine the timescales and magnitudes of relaxation accommodated by an icy single or multiple material layered subsurface to determine the environment's properties. Our models include a 35 m thick layer of pure ice (water, or water and CO2) over a mobile substrate of particulate rich water ice of variable thicknesses able to accommodate relaxation for larger crater sizes up to 1 km in diameter. Our findings demonstrate that relaxation is able to account for significant flattening of crater topography, though for smaller craters the relaxation is incomplete, leaving about half the depth. The accommodated relaxation of our modeled subsurface does imply associated increases in ice abundances of 4 to 10 times greater than is currently estimated for the region, dependent on the extent of the mobile layer.

Empirical H/V spectral ratios at the InSight landing site and implications for the martian subsurface structure

SUMMARY The horizontal-to-vertical (H/V) spectral ratio inversion is a traditional technique for deriving the local subsurface structure on Earth. We calculated the H/V from the ambient vibrations at different wind levels at the InSight landing site, on Mars, and also computed the H/V from the S-wave coda of the martian seismic events (marsquakes). Different H/V curves were obtained for different wind periods and from the marsquakes. From the ambient vibrations, the recordings during low-wind periods are close to the instrument self-noise level. During high-wind periods, the seismic recordings are highly contaminated by the interaction of the lander with the wind and the martian ground. Therefore, these recordings are less favourable for traditional H/V analysis. Instead, the recordings of the S-wave coda of marsquakes were preferred to derive the characteristic H/V curve of this site between 0.4 and 10 Hz. The final H/V curve presents a characteristic trough at 2.4 Hz and a strong peak at 8 Hz. Using a full diffuse wavefield approach as the forward computation and the Neighbourhood Algorithm as the sampling technique, we invert for the 1-D shear wave velocity structure at the InSight landing site. Based on our inversion results, we propose a strong site effect at the InSight site to be due to the presence of a shallow high-velocity layer (SHVL) over low-velocity units. The SHVL is likely placed below a layer of coarse blocky ejecta and can be associated with Early Amazonian basaltic lava flows. The units below the SHVL have lower velocities, possibly related to a Late Hesperian or Early Amazonian epoch with a different magmatic regime and/or a greater impact rate and more extensive weathering. An extremely weak buried low velocity layer (bLVL) between these lava flows explains the data around the 2.4 Hz trough, whereas a more competent bLVL would not generate this latter feature. These subsurface models are in good agreement with results from hammering experiment and compliance measurements at the InSight landing site. Finally, this site effect is revealed only by seismic events data and explains the larger horizontal than vertical ground motion recorded for certain type of marsquakes.

Lunar crater ejecta: Physical properties revealed by radar and thermal infrared observations

We investigate the physical properties, and changes through time, of lunar impact ejecta using radar and thermal infrared data. We use data from two instruments on the Lunar Reconnaissance Orbiter (LRO) – the Diviner thermal radiometer and the Miniature Radio Frequency (Mini-RF) radar instrument – together with Earth-based radar observations. We use this multiwavelength intercomparison to constrain block sizes and to distinguish surface from buried rocks in proximal ejecta deposits. We find that radar-detectable rocks buried within the upper meter of regolith can remain undisturbed by surface processes such as micrometeorite bombardment for >3Gyr. We also investigate the thermophysical properties of radar-dark haloes, comprised of fine-grained, rock-poor ejecta distal to the blocky proximal ejecta. Using Diviner data, we confirm that the halo material is depleted in surface rocks, but show that it is otherwise thermophysically indistinct from background regolith. We also find that radar-dark haloes, like the blocky ejecta, remain visible in radar observations for craters with ages >3Ga, indicating that regolith overturn processes cannot replenish their block populations on that timescale.

Improved discrimination of volcanic complexes, tectonic features, and regolith properties in Mare Serenitatis from Earth-based radar mapping

Radar images at 70 cm wavelength show 4–5 dB variations in backscatter strength within regions of relatively uniform spectral reflectance properties in central and northern Mare Serenitatis, delineating features suggesting lava flow margins, channels, and superposition relationships. These backscatter differences are much less pronounced at 12.6 cm wavelength, consistent with a large component of the 70 cm echo arising from the rough or blocky transition zone between the mare regolith and the intact bedrock. Such deep probing is possible because the ilmenite content, which modulates microwave losses, of central Mare Serenitatis is generally low (2–3% by weight). Modeling of the radar returns from a buried interface shows that an average regolith thickness of 10 m could lead to the observed shifts in 70 cm echo power with a change in TiO2 content from 2% to 3%. This thickness is consistent with estimates of regolith depth (10–15 m) based on the smallest diameter for which fresh craters have obvious blocky ejecta. The 70 cm backscatter differences provide a view of mare flow-unit boundaries, channels, and lobes unseen by other remote sensing methods. A localized pyroclastic deposit associated with Rima Calippus is identified based on its low radar echo strength. Radar mapping also improves delineation of units for crater age dating and highlights a 250 km long, east-west trending feature in northern Mare Serenitatis that we suggest is a large graben flooded by late-stage mare flows.

Ground penetrating radar geologic field studies of the ejecta of Barringer Meteorite Crater, Arizona, as a planetary analog

AbstractGround penetrating radar (GPR) has been a useful geophysical tool in investigating a variety of shallow subsurface geological environments on Earth. Here we investigate the capabilities of GPR to provide useful geologic information in one of the most common geologic settings of planetary surfaces, impact crater ejecta. Three types of ejecta are surveyed with GPR at two wavelengths (400 MHz, 200 MHz) at Meteor Crater, Arizona, with the goal of capturing the GPR signature of the subsurface rock population. In order to “ground truth” the GPR characterization, subsurface rocks are visually counted and measured in preexisting subsurface exposures immediately adjacent to and below the GPR transect. The rock size‐frequency distribution from 10 to 50 cm based on visual counts is well described by both power law and exponential functions, the former slightly better, reflecting the control of fragmentation processes during the impact‐ejection event. GPR counts are found to overestimate the number of subsurface rocks in the upper meter (by a factor of 2–3x) and underestimate in the second meter of depth (0.6–1.0x), results attributable to the highly scattering nature of blocky ejecta. Overturned ejecta that is fractured yet in which fragments are minimally displaced from their complement fragments produces fewer GPR returns than well‐mixed ejecta. The use of two wavelengths and division of results into multiple depth zones provides multiple aspects by which to characterize the ejecta block population. Remote GPR measurement of subsurface ejecta in future planetary situations with no subsurface exposure can be used to characterize those rock populations relative to that of Meteor Crater.

Impacts into non-polar ice-rich paleodeposits on Mars: Excess ejecta craters, perched craters and pedestal craters as clues to Amazonian climate history

We compare three previously independently studied crater morphologies – excess ejecta craters, perched craters, and pedestal craters – each of which has been proposed to form from impacts into an ice-rich surface layer. Our analysis identifies the specific similarities and differences between the crater types; the commonalities provide significant evidence for a genetic relationship among the morphologies. We use new surveys of excess ejecta and perched craters in the southern hemisphere in conjunction with prior studies of all of the morphologies to create a comprehensive overview of their geographic distributions and physical characteristics. From these analyses, we conclude that excess ejecta craters and perched craters are likely to have formed from the same mechanism, with excess ejecta craters appearing fresh while perched craters have experienced post-impact modification and infilling. Impacts that led to these two morphologies overwhelmed the ice-rich layer, penetrating into the underlying martian regolith, resulting in the excavation of rock that formed the blocky ejecta necessary to armor the surface and preserve the ice-rich deposits. Pedestal craters, which tend to be smaller in diameter, have the same average deposit thickness as excess ejecta and perched craters, and form in the same geographic regions. They rarely have ejecta around their crater rims, instead exhibiting a smooth pedestal surface. We interpret this to mean that they form from impacts into the same type of ice-rich paleodeposit, but that they do not penetrate through the icy surface layer, and thus do not generate a blocky ejecta covering. Instead, a process related to the impact event appears to produce a thin, indurated surface lag deposit that serves to preserve the ice-rich material. These results provide a new basis to identify the presence of Amazonian non-polar ice-rich deposits, to map their distribution in space and time, and to assess Amazonian climate history. Specifically, the ages, distribution and physical attributes of the crater types suggest that tens to hundreds of meters of ice-rich material has been episodically emplaced at mid latitudes in both hemispheres throughout the Amazonian due to obliquity-driven climate variations. These deposits likely accumulated more frequently in the northern lowlands, resulting in a larger population of all three crater morphologies in the northern hemisphere.

Distributions of boulders ejected from lunar craters

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.

Lunar craters with radar bright ejecta

A small fraction of the lunar impact craters with diameters of 1 km and greater have extensive enhanced 3.8-cm radar echoes associated with their ejecta deposits. The physical properties of these ejecta deposits and the ages of the central craters have been characterized via various infrared, radar, and optical signatures. Most of these ejecta deposits are radar bright at the 3.8-cm wavelength but are not radar bright at the 70-cm wavelength. Some ejecta have large infrared signatures, others do not. Although most of these ejecta have bright albedos in full-moon photographs, a significant fraction of the bright albedo markings do not extend beyond the crater. This mix of remote-sensing signatures indicates that craters with 3.8-cm radar bright halos are young and have ejecta deposits containing an excess of surface or near-surface rocks relative to the surrounding terrain. Abundant centimeter-sized rocks are inferred from the high 3.8-cm radar and infrared signatures. The low 70-cm radar signatures indicate that larger blocks are much less numerous. The population of craters with 3.8-cm radar bright craters on the Moon is much smaller than the population of craters in a similar size range on a young mare (Oceanus Procellarum) and has a different slope. This population is interpreted as a steady-state population reflecting a balance between the production of fresh craters and the destruction of the high infrared and radar signatures by small-scale cratering. The slope of difference between visual and radar craters is attributed to more rapid destruction of the radar signatures in smaller craters. Relative densities of 3.8-cm radar bright craters and mare craters are estimated to be 0.04 in−0.042 +0.016 at a 4-km diameter and 0.100 0.07 0.007 at a 32-km diameter. Assigning ages on the basis of these relative densities raises the question of whether the 4- to 32-km-diameter visual crater population is truly representative of a 3.3-by age. If it is, and if crater rates between 3.3 by and the present have been uniform, than the 3.8-cm radar crater lifetimes are estimated to be 0.13 ∮0.04 +0.25 by and 0.03 −0.13 +0.23 by at diameters of 4 and 32 km, respectively. Similarly, lifetimes of the infrared signatures of 4-km-diameter craters may be as short as 10 7 years. However, some data suggest that these estimates may be in error by a factor of 5 too small. Comminution of blocky ejecta material and the smoothing of slopes by lunar surface processes could account for the elimination of radar signatures on these time scales and the development of a steady-state crater population. An altenative interpretation, which we do not fsvor, is that the 3.8-cm radar bright crater population is formed by a subpopulation of primary bodies or by secondary cratering.

Geologic Setting of the Apollo 14 Samples

The Apollo 14 lunar module landed in a region of the lunar highlands that is part of a widespread blanket of ejecta surrounding the mare Imbrium basin. Samples were collected from the regolith developed on a nearly level plain, a ridge 100 meters high, and a blocky ejecta deposit around a young crater. Large boulders in the vicinity of the landing site are coherent fragmental rocks as are some of the returned samples.

Geology: Craters

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.

Genetic implications of Lunar regolith thickness variations

Results of the analyses of the distribution of thickness of the regolith in 12 Lunar Orbiter sites are presented. Each of the sites analyzed has one of four type of thickness distributions characterized by approximate median values of 3.3, 4.6, 7.5, and 16 meters. The regolith thickness correlates directly with cratering density, and the average regolith thickness computed from volumes of all craters in an area is in excellent agreement with the measired average thickness determined through use of normal and concentric craters. The form of the distributions of thickness determined from crater ejecta distribution maps are also in excellent agreement with the forms of measured distributions. The results indicate that most of the craters considered and the regolith must be of impact origin. Variations in thickness of the regolith in different areas of the lunar surface reflect differences in elapsed time since the production of new rock surfaces. An idealized history of the evolution of the regolith suggests that the regolith changes with time from an initially coarse-grained, poorly sorted thin deposit of nearly uniform thickness to a thick, fine-grained deposit with nearly equal areas of each thickness interval. Evidence is presented which supports the view that the substrate rocks are of volcanic flow origin, and that flow activity occured repeatedly in any one place but came to a close at different times in different regions of the Moon. Apollo sampling procedures are reviewed and it is concluded that the most readily available samples which can be related to specific selenological events with the highest degree of confidence would be those collected from the blocky ejecta on the rims of concentric craters. The Lunar Orbiter III P11 potential Apollo landing site should be considered as a prime target because of the abundance of craters with blocky rims and because it is the only Apollo site which differs in age from the other potential Apollo sites.