Ejecta Layer Research Articles

Temporal and spatial variation in subsurface cryosphere in East Coprates Planum, Mars

There is general acceptance among researchers regarding the presence of a cryosphere in the subsurface of Mars. The two main geomorphic indications assumed to support the existence of this cryospheric layer are layered ejecta rampart craters and outflow channels. In essence, a layered ejecta rampart crater is a crater with an ejecta blanket having an asymmetric lobate geometry surrounding it. While there are no outflow channels in the East Coprates Planum, which is located within Mars’ equatorial region, it does have a population of layered ejecta rampart craters. Therefore, these craters are the only avenues to study the local cryospheric level at the time of impacts which likely led to their formation. Identification, mapping and dating of Single Layered Ejecta craters (SLE) and Multiple Layered Ejecta craters (MLE), the two types of layered ejecta rampart crater present in East Coprates Planum, was integral to this study. Excavation depths (with respect to the areoid) and ages for each crater was estimated. Statistical surfaces for SLE and MLE craters were drawn to assess the control of local topography on the local cryosphere. It has been estimated, by comparing excavation depths of SLEs and MLE with respect to their time of formation, that the bottom layer of the Martian cryosphere in the study area was at lower altitudes (from areoid) prior to ∼2.65 Ga, than it was in ∼0.97 Ga. This indicates a change in the Martian cryosphere over time within the study region, and the analysis of the crater excavation depths indicates that the cryosphere became thinner over time. Also, East Coprates Planum being a lower elevated region may have facilitated a greater inflow of groundwater from surrounding regions as compared to the adjacent higher elevated Thaumasia Minor region.

Spatial and Temporal Heterogeneity of Martian Tropical Water Ice Through Analysis of Radial and Layered Ejecta Craters

AbstractMartian layered ejecta craters are theorized to form by impacting into an ice‐rich crust. The inference that some equatorial layered ejecta craters are Amazonian indicates that ice has persisted in the tropics. However, the detailed spatial and temporal distribution and evolution of this ice remain unknown, which is critical to constraining Mars' global water cycle and climate change over eons. Here we estimate absolute model formation ages for layered and radial (ballistic) ejecta craters to constrain the spatial and temporal distribution of equatorial ice. The assumption is that radial ejecta form where volatiles are not present in significant quantities. Ages are derived from the density of smaller craters superposed on the ejecta blankets. We examined 73 craters in a 30° × 30° area centered at 15°S, 355°E, with 44 layered and 29 radial ejecta. Layered and radial ejecta craters are mixed over distances comparable to their diameters, which represents an unreasonably short length scale for ground‐ice emplacement. This, along with the lack of trend with age, supports the suggestion that intermittent low‐latitude surface ice—from excursions to high obliquity—could be responsible. Analysis also suggests an increasing proportion of layered ejecta craters with decreasing diameter for those older than 3.4 Ga. This trend would support the hypothesis of more ice being available in early martian history. Conversely, this could indicate that “armoring” preferentially preserves layered ejecta relative to radial ejecta.

Characterization of Supernovae Interacting with Dense Circumstellar Matter with a Flat Density Profile

Interaction between supernova (SN) ejecta and dense circumstellar medium (CSM) with a flat density structure (ρ ∝ r −s , s < 1.5) was recently proposed as a possible mechanism behind interacting SNe that exhibit exceptionally long rise times exceeding 100 days. In such a configuration, the interaction luminosity keeps rising until the reverse shock propagates into the inner layers of the SN ejecta. We investigate the light curves of SNe interacting with a flatly distributed CSM in detail, incorporating the effects of photon diffusion inside the CSM into the model. We show that three physical processes—the shock breakout, the propagation of the reverse shock into the inner ejecta, and the departure of the shock from the dense CSM—predominantly determine the qualitative behavior of the light curves. Based on the presence and precedence of these processes, the light curves of SNe interacting with flatly distributed CSM can be classified into five distinct morphological classes. We also show that our model can qualitatively reproduce doubly peaked SNe whose peaks are a few tens of days apart, such as SN 2005bf and SN 2022xxf. Our results show that the density distribution of the CSM is an important property of CSM that contributes to the diversity in light curves of interacting SNe.

Crustal-mantle layering deposits in the Von Kármán crater, South Pole-Aitken basin

Comprehending the properties of the Moon's crust and mantle materials is essential for decoding its interior. Whether the South Pole-Aitken (SPA) basin potentially excavated the upper mantle materials remains mysterious. The Chang'E Four Lunar Penetrating Radar (LPR) has provided an unprecedented opportunity to investigate the SPA basin in the Von Kármán (VK) crater. Utilizing a data-driven generative deep learning inversion algorithm, the subsurface physical properties along the Yutu-2 path were determined. 4 distinct deposit zones were discovered beneath the surface. All zones feature an upper low-permittivity (paleo-)regolith layer overlaying on a lower high-permittivity layer of rockier ejecta or basal basalt. Zone 1, an ejecta layer from the Finsen crater within the SPA Compositional Anomaly, has a bulk density (2.81±0.22g/cm3) denser than that of highland crust represented by materials originating from VKL and VKL' craters in the Mg-Pyroxene Annulus, which are present in Zones 2 and 3. This suggests the possible presence of mantle materials in Zone 1, providing the first diagnostic evidence that the SPA impact may have penetrated the upper lunar mantle.

Intense Changes in the Main Source of Organic Carbon to the Gulf Coastal Plain Following the Cretaceous‐Paleogene Boundary

AbstractTo explore both environmental change and the response of non‐fossilizing phytoplankton across the Cretaceous‐Paleogene (K‐Pg) boundary mass extinction event, we determined changes in organic matter (OM) sources using a range of apolar (n‐alkanes, acyclic isoprenoids, steranes, and hopanes) and polar (BIT index) biomarkers. We analyzed two K‐Pg proximal sections, located in the Mississippi Embayment, Gulf Coastal Plain (USA), covering ∼300 kyrs prior to and ∼3 myrs after the K‐Pg event. The OM abundance and composition changed dramatically across the boundary. The post‐impact ejecta layer and burrowed unit are characterized by an increase in the mass accumulation rate (MAR) of plant and soil biomarkers, including high‐molecular‐weight n‐alkanes and C29 steranes as well as the BIT index, related to an erosive period which transported terrestrial OM to the ocean in the aftermath of the impact event. At the same time, MARs of putative aquatic biomarkers decrease (low‐molecular‐weight n‐alkanes, C27 steranes and pristane and phytane), which suggests a collapse of the marine phytoplankton community. The increase of terrestrial OM to the ocean, during the first 280 kyrs after the Chicxulub impact event, is a combination of reworked kerogen, soil and some plant material. Crucially, within the latter part of this erosion period, only ∼160 kyrs after the K‐Pg do biomarkers return to distributions similar to those in the upper Cretaceous, although not to pre‐impact MARs. Thus, our results suggest a long‐term interval for the full sedimentary and ecological recovery of the non‐fossilizing phytoplankton community after this event.

A Hubble Space Telescope Search for r-Process Nucleosynthesis in Gamma-Ray Burst Supernovae

The existence of a secondary (in addition to compact object mergers) source of heavy element (r-process) nucleosynthesis, the core-collapse of rapidly rotating and highly magnetized massive stars, has been suggested by both simulations and indirect observational evidence. Here, we probe a predicted signature of r-process enrichment, a late-time (≳40 days post-burst) distinct red color, in observations of gamma-ray burst supernovae (GRB-SNe), which are linked to these massive star progenitors. We present optical to near-IR color measurements of four GRB-SNe at z ≲ 0.4, extending out to >500 days post-burst, obtained with the Hubble Space Telescope and large-aperture ground-based telescopes. Comparison of our observations to models indicates that GRBs 030329, 100316D, and 130427A are consistent with both no enrichment and producing 0.01–0.15 M ⊙ of r-process material if there is a low amount of mixing between the inner r-process ejecta and outer supernova (SN) layers. GRB 190829A is not consistent with any models with r-process enrichment ≥0.01 M ⊙. Taken together the sample of GRB-SNe indicates color diversity at late times. Our derived yields from GRB-SNe may be underestimated due to r-process material hidden in the SN ejecta (potentially due to low mixing fractions) or the limits of current models in measuring r-process mass. We conclude with recommendations for future search strategies to observe and probe the full distribution of r-process produced by GRB-SNe.

Latest Evolution of the X-Ray Remnant of SN 1987A: Beyond the Inner Ring

Based on our Chandra imaging-spectroscopic observations, we present the latest evolution of the X-ray remnant of SN 1987A. Recent changes in the electron temperatures and volume emission measures suggest that the blast wave in SN 1987A is moving out of the dense inner ring structure, also called the equatorial ring (ER). The 0.5–2.0 keV X-ray light curve shows a linearly declining trend (by ∼4.5% yr−1) between 2016 and 2020 as the blast wave heats the hitherto unknown circumstellar medium (CSM) outside the ER. While the peak X-ray emission in the latest 0.3–8.0 keV image is still within the ER, the radial expansion rate in the 3.0–8.0 keV images suggests an increasing contribution of the X-ray emission from less dense CSM since 2012, at least partly from beyond the ER. It is remarkable that, since 2020, the declining soft X-ray flux has stabilized around ∼7 × 10−12 erg s−1 cm−2, which may signal a contribution from the reverse-shocked outer layers of ejecta as predicted by the three-dimensional magnetohydrodynamic models. In the latest ACIS spectrum of supernova remnant 1987A in 2022 we report a significant detection of the Fe K line at ∼6.7 keV, which may be due to changing thermal conditions of the X-ray emitting CSM and/or the onset of reverse shock interactions with the Fe ejecta.

Paleoenvironment implications of layered ejecta craters in the Chryse Planitia, Mars

The formation of martian layered ejecta craters (LECs), which have one or multiple distinctive fluidized layered ejecta deposits, is generally hypothesized to be directly related to subsurface volatiles. Using a high-resolution (5 m/pixel) context camera (CTX) mosaic as a base map, 525 LECs (including 157 newly identified) were catalogued in the Chryse Planitia and divided into three types according to the number of layered ejecta, i.e., single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE). It is found that the minimum crater diameter (here called the onset diameter) decreases with increasing latitude, and could be as small as 1 km. The average ejecta mobility (EM) values usually increase from the low to the high latitude interval for each LEC type and gradually increase with the number of layered ejecta within each latitude interval. The lobateness values of the SLE craters have a decreasing trend with increasing latitude, however, such a trend is not obvious for the DLE and MLE craters. The absolute model ages (AMAs) of 135 LECs were determined from the crater size-frequency distribution (CSFD) method to constrain the timing of the fluvial activities of the circum-Chryse outflow channels and to investigate the local temporal-spatial evolution of the subsurface volatile-rich layer. The superposition relationship between the LECs and circum-Chryse channels suggests fluvial activity might have extended into the early Amazonian. Floods that carved the Ares, Tiu, and Simud Valles likely ended before ∼μ2.7 Ga, while those relating to theVedra, Manmee, and Maja Valles ceased between ∼μ2.2 Ga and ∼ μ1.9 Ga. The fluvial resurfacing activities in the Kasei Valles likely did not last past ∼μ2.8 Ga. The dating results and excavation depths of LECs (diam. < 6 km) suggest that the minimal roof depth of the volatile-rich layer has remained relatively stable at least around 560– 590 m since the early Amazonian, and regional differences indicate that it could be as shallow as ∼440 m at high latitudes and ∼540 m at low latitudes.

On the nature of slowly rising interaction-powered supernovae

ABSTRACT Some interaction-powered supernovae have long rise times of more than 100 d. We show that such long rise times are naturally expected if cirumstellar matters (CSM) have a flat density structure (s ≲ 1.5, where ρCSM∝r−s). In such cases, bolometric luminosities from the CSM interaction keep increasing as long as the CSM interacts with the outer layers of the SN ejecta. Thus, the rise time is determined by the dynamical time-scale in which the reverse shock propagates the outer layers of the SN ejecta, not by the time-scales in which photons diffuse in the CSM as often considered. Interaction-powered supernovae with very long rise times can be an important probe of extensive non-steady mass-loss in massive stars.

Pattern Analyses of Lunar Penetrating Radar Images at Chang'E‐3, 4, and 5 Landing Sites: A New Insight Into the Evolution of Lunar Regolith

AbstractAn “atlas” of the Lunar penetrating radar (LPR) patterns is organized and a comprehensive interpretation of the lunar subsurface layers in the areas of the Chang'E (CE) mission is proposed. There are three layers inferred by the LPR high‐frequency profiles of the CE‐3 and CE‐4. The first two layers (i.e., the fine regolith and the ejecta layer) at CE‐3 and CE‐4 sites have the same strata type but differ in internal structure and thickness. The third layer at the CE‐3 site is inferred as the paleoregolith, while at the CE‐4 site, it is inferred as the alternating layer with fine materials, ejecta, and fragments. Due to the limited penetration depth of the CE‐5 LPR, the data disclose only the fine regolith, with support from the empirical ejecta distribution model. The thicknesses, physical properties, and equivalent growth rates of lunar regolith at three landing sites are estimated and discussed in detail. The lunar regolith thicknesses inferred by the CE‐3 and CE‐5 LPR data are both approximately 1.0 m. In contrast, the thickness of the lunar regolith at the CE‐4 site is 10.0 m, which is greater than that at the other two landing sites. However, the equivalent growth rate of the regolith at the CE‐3 and CE‐5 landing sites is four times greater than that at the CE‐4 landing site. This illustrates that with the increase in lunar regolith thickness, the influence of small impact gardening events on the lunar regolith reconstruction gradually weakened, and the self‐buffering effect avoids the fast growth of lunar regolith.

Multidisciplinary analysis of pit craters at Hale Crater, Mars

Pit craters are circular to subcircular depressions that lack a rim and ejecta layer and typically have a conical shape. There are several mechanisms that can explain the formation of such depressions and they are associated with collapse due to the removal of subsurface material. Possible origins of pit craters include: volcanic processes (collapse of lava tubes, magmatic chambers, intrusion of dikes), karstic dissolution, extensional faulting or volatile processes.In this work, we study pit craters in the north-estern external slope of Hale Crater. We identified >40 pit craters that present unique morphological features in association with different landforms associated with volatile activity in the area. We identified moraine like ridges, gullies and aprons together with pits that represent an interesting suite of landforms. After producing a detailed geomorphic map, we classified the pits into: i) single pit craters, ii) chain pit craters, iii) complex pit craters and iv) elongated pit craters.We used DTMs to determine orientation and slopes of the pit craters-hosting terrains and to analyze elevation profiles of the different types of pits and their relation with the gullies and other features. In addition, we performed a spectral analysis using CRISM multispectral dataset in order to characterize the mineralogy of the sediments in the region and to identify any distinctive properties that could promote the formation of these depressions.Here, we propose that pit craters are stratigraphically on top of the ice-related landforms and present complex relationships with the gullies. The spatial relationship between the pits and these structures, along with the absence of evidence of present or past volcanic activity and the lack of evidence of any extensional faulting allows us to propose that the origin of the pit craters in the study area might be related to some volatile process. We propose here that these particular pit craters at Hale crater, are morphologically similar to Icelandic depressions located in a glacial environment. We conclude that the landforms found in the area are in close relation with glacial or periglacial conditions and pit craters might be formed by sublimation/melting of ground ice.

Understanding the mineralogy of a compositional anomaly north of the Korolev basin using Moon mineralogy mapper

The work attempts to understand the mineralogy of the reported geochemical anomaly located in the north – northeast region of the Korolev basin using Moon Mineralogy Mapper (M3) onboard Chandrayaan −1 and other lunar datasets. To understand the mineralogy, colour composite images using integrated band depth parameters and mineral indices were prepared, and the M3 spectral signatures corresponding to the unique colours of these colours composites were investigated. Further, Modified Gaussian Model (MGM) deconvolution was applied to these spectra. The results of spectra studies reveal that the area is made up of heterogeneous lithologies, predominantly composed of anorthosite along with minor occurrences of pyroxene-bearing (both low-calcium and high-calcium variety) and spinel-bearing lithologies. Correlation of spectral studies with the morphology revealed that pyroxene was typically associated with fresh craters and their ejecta. Spinel was found to be ubiquitous and is well-dispersed, possibly distributed along with the ejecta blanket of large impact craters. This association hints that the pyroxene-hosted layer most likely occurs beneath the spinel-bearing layer. Such observed assemblages may have resulted from physical mixing during the impact cratering events. This mixed lithology could arise due to the mafic mineral-bearing ejecta of the South-Pole Aitken (SPA) basin and spinel-bearing Orientale ejecta. Korolev most likely impacted on a thick layer of SPA ejecta, and impact basins such as Hertzsprung and Orientale have occurred post-Korolev formation. Orientale being the youngest of the large impact basins, its ejecta carrying the light plain material would have overprinted the older SPA ejecta. Smaller impact craters would have churned the ejecta so that presently we observe a composite lithology with a variable abundance of pyroxene and spinel.

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.

Light-curve Model for Luminous Red Novae and Inferences about the Ejecta of Stellar Mergers

The process of unstable mass transfer in a stellar binary can result in either a complete merger of the stars or successful removal of the donor envelope leaving a surviving more compact binary. Luminous red novae (LRNe) are the class of optical transients believed to accompany such merger/common envelope events. Past works typically model LRNe using analytic formulae for supernova light curves that make assumptions (e.g., radiation-dominated ejecta, neglect of hydrogen recombination energy) not justified in stellar mergers due to the lower velocities and specific thermal energy of the ejecta. We present a one-dimensional model of LRN light curves that accounts for these effects. Consistent with observations, we find that LRNe typically possess two light-curve peaks, an early phase powered by initial thermal energy of the hot, fastest ejecta layers and a later peak powered by hydrogen recombination from the bulk of the ejecta. We apply our model to a sample of LRNe to infer their ejecta properties (mass, velocity, and launching radius) and compare them to the progenitor donor star properties from pretransient imaging. We define the maximum luminosity achievable for a given donor star in the limit that the entire envelope is ejected, finding that several LRNe violate this limit. Shock interaction between the ejecta and predynamical mass loss may provide an additional luminosity source to alleviate this tension. Our model can also be applied to the merger of planets with stars or stars with compact objects.

Subsurface structure of the proposed Sirente meteorite crater: insights from ERT synthetic modelling

The Sirente main crater is a ≈ 130 m wide, in plan view droplet-shaped depression with an elevated rim, surrounded by 30 smaller depressions. It was proposed to be of meteorite impact origin. Given the age of formation in the 3rd to 5th centuries A.D., the inferred catastrophic origin was related to the celestial sign (“Chi Rho”) said to have been seen by Emperor Constantine in 312 A.D. and suggested to have changed the course of both Roman and Christian history. However, the meteoritic origin is not yet confirmed. This paper presents new results from synthetic modelling of Electric Resistivity Tomography field data collected at the Sirente main crater which provide further clues around the controversy of its origin. This study arises from the need to validate the observed structural features which include possible upturned strata (i.e., overturning of strata below impact crater rims) and compaction-fissure-like features below and just outside the crater rim, well-developed “breccia lens”, as well as an ejecta layer, and provide key indicators for objective and quantitative interpretation of the measured resistivity pattern. The results from this study are consistent with the hypothesis of a small impact crater in a low-strength target, with a relatively shallow apparent crater and do not support other proposed mechanisms of formation such as karst, mud volcano or merely anthropogenic origin.

A Revision of the Formation Conditions of the Vredefort Crater

AbstractThe Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.

X-Ray Studies of the Inverted Ejecta Layers in the Southeast Area of Cassiopeia A

The central strong activities in core-collapse supernovae are expected to produce the overturning of the Fe- and Si/O-rich ejecta during the supernova explosion based on multidimensional simulations. X-ray observations of the supernova remnant Cassiopeia A have indicated that the Fe-rich ejecta lies outside the Si-rich materials in the southeastern region, which is consistent with the hypothesis on the inversion of the ejecta. We investigate the kinematic and nucleosynthetic properties of the inverted ejecta layers in detail to understand its formation process using the data taken by the Chandra X-Ray Observatory. Three-dimensional velocities of Fe- and Si/O-rich ejecta are obtained as >4500 km s−1 and ∼2000–3000 km s−1, respectively, by combining proper motion and line-of-sight velocities, indicating that the velocity of the Si/O-rich ejecta is slower than that of the Fe-rich ejecta from the early stages of the explosion. To constrain their burning regime, the Cr/Fe mass ratios are evaluated as % in the outermost Fe-rich region and % in the inner Fe/Si-rich region, suggesting that the complete Si burning layer is invertedly located to the incomplete Si burning layer. All the results support the ejecta overturning at the early stages of the remnant’s evolution or during the supernova explosion of Cassiopeia A.

Evidence for widely-separated binary asteroids recorded by craters on Mars

Over the last decades, a significant number of small asteroids (diameter <10km) having a satellite in orbit around them have been discovered. This population of binary asteroids has very specific properties (secondary-to-primary diameter ratio of about 0.3, semi-major axis to primary diameter ratio around 2 and an obliquity of the system close to either 0∘ or 180∘) pointing at formation by YORP-induced spin-up and rotational fission. When impacting the surface of terrestrial bodies, those exotic objects lead to the formation of binary craters, exhibiting various morphologies depending on the configuration of the system at the moment of the impact. Planetary surfaces constitutes therefore the best (if not the only one) record of binary asteroid population through time. In contrast to the Moon or Mercury, a large fraction of impact craters on Mars exhibits thick ejecta layers due to the presence of volatile material at the moment of the impact (e.g., water ice). The martian surface represents thus the ideal case to survey for the existence of binary craters, as the ejecta morphology can attest of a synchronous impact. From a survey of 87% of Mars surface, we identify 150 binary craters (0.5% of the total), likely formed by the impact of binary asteroids. The properties of these craters contrast with those of the population of binary asteroids: size ratio close to unity, large separation, and isotropic orientation on the surface. We run numerical simulations of impacts to test whether tidal effects on the impact trajectory can explain these discrepancies. Our results suggest that a population of similarly-sized and well-separated binary asteroids with non-zero obliquity remains to be observed.

Layered Ejecta Craters in the Candidate Landing Areas of China's First Mars Mission (Tianwen‐1): Implications for Subsurface Volatile Concentrations

AbstractOne important scientific objective of China's first Mars mission (Tianwen‐1) is to investigate the martian surface soil characteristics and water‐ice distribution. Layered ejecta craters (LECs) on Mars are characterized by their fluidized ejecta deposits which have been interpreted to indicate volatiles in the substrate where the impact occurred. Thus, they have potential to serve as important planetary exploration sites/targets of geological significance. In this study, we estimated the absolute model ages (AMAs) of 68 LECs with diameters &gt;4 km using the Context Camera (an instrument onboard the Mars Reconnaissance Obiter, MRO) images in the two candidate landing areas of Tianwen‐1 mission, aiming to reveal temporal‐spatial variation of the regional subsurface volatiles, in particular water ice. The results from a combination of the derived AMAs and the ejecta mobility (EM) values for the southern Utopia‐Isidis Planitia (SUIP) and southern Chryse Planitia (SCP) show that the LECs in both regions plausibly have formed throughout the Amazonian and Hesperian periods. However, the subsurface volatile concentrations might have undergone different evolution histories: SUIP shows a relatively stable trend with time, while there seems a general downward trend in SCP. The presence of the fresh ∼1.5‐km‐diameter LECs suggests that volatile concentrations might have been at depth less than 150 m. We conclude with a discussion of recommendations for Tianwen‐1 and future explorations that have the ability to effectively detect any signs of past and present water/ice on Mars, which contain a wealth of information of past and recent climate.

Hargraves Crater, Mars: Insights into the internal structure of layered ejecta deposits

Impact craters and their ejecta deposits offer insights into the structure and composition of planetary crusts. The 60 km diameter Hargraves Crater, Mars, demonstrates an unusual balance of exposure and preservation in its ejecta deposits. We investigated the morphologic, morphometric, thermophysical, and stratigraphic characteristics of the Hargraves Crater ejecta blanket. Our work shows that the crater produced two distinct ejecta units. The lower unit, He1, interpreted as a lithic impact breccia, is unsorted and composed of sub-angular clasts. These clasts in the breccia unit have a mean diameter of ~10–12 m but range in size from m to km in size. The overlying unit, He2, is smooth, dark toned, comprises fewer and much smaller clasts (~1 m diameter on average at image resolution) and exhibits polygonal fracturing. We interpret this unit as an impact melt-bearing breccia or impact melt rock unit. A sharp contact is visible between the two ejecta units. These orbital observations are markedly similar to field observations of ejecta deposits on Earth. With the context of both in-situ and orbital impact studies, we interpret these two layers as a ballistic ejecta layer overlain by impact melt-rich flows and ponds. The two distinct layers, separated by a sharp contact, indicate two separate phases of ejecta emplacement. Hargraves Crater thus represents a unique opportunity to observe what may be a common ejecta blanket structure as other craters across the surface of Mars show similar characteristics. Future study of this “Hargraves-type” ejecta will improve our understanding of ejecta and ejecta emplacement processes.