Crater Floor Research Articles

The Seismically Active Lobate Scarps and Coseismic Lunar Boulder Avalanches Triggered by 3 January 1975 (MW 4.1) Shallow Moonquake

AbstractOn 3 January 1975, the largest shallow moonquake (MW 4.1) occurred at Laue impact crater on the Moon. The fault responsible for the moonquake and origins of coseismic boulder avalanches are unknown. Our study reveals a set of previously unreported, seismically active, young lobate scarps near the epicenter. In addition, hundreds of boulder falls are observed on the interior walls of two impact craters on either side of the lobate scarps. The varying preservation levels and crater size‐frequency distributions of impact craters superimposed on the boulder falls indicate their episodic origins at 1.6 Ma and during the 1975 shallow moonquake. Our ground motion simulations confirm that the MW 4.1 moonquake along the lobate scarp at 1‐ to 5‐km focal depths produced strong ground shaking that triggered the boulder avalanches. Also, the fault slip along the Lorentz basin wall beneath the Laue crater floor produced the lobate scarps and the shallow moonquakes.

Evidence of thrust faulting and widespread contraction of Ceres

The surface of the dwarf planet Ceres is considered to be dominated by geological processes typical of small bodies or medium-sized icy bodies, such as impact cratering1,2; there are also features of putative cryovolcanic origin3 as well as those related to flow of near-surface ice4. Extensional features4–6 include regional linear troughs, fractures and pit chains, fractures associated with impact craters and with crater floors, and polygonal craters whose walls seem to be structurally controlled. However, no contractional features, which are related to thrust fault activity more typical of large silicate bodies7–11, have been described. Here we report the presence of scarps, ridges and fractures associated with thrust faults, tectonically raised terrains and thrusted craters—all contractional features. These structures closely resemble thrust-fault-related lobate scarps on Mercury7,8 and Mars9,10, albeit with lower displacement. They seem more abundant in high-latitude ancient terrains, perhaps owing to illumination effects that aid identification. The observed deformation implies that the crustal material is stronger than water ice but weaker than silicate rocks, consistent with our current knowledge of crustal composition12 and rheology13. These features suggest that large-scale contraction, possibly related to differentiation processes, occurred in the history of Ceres. Contractional features such as scarps, ridges and fractures from thrust faults are typical of large silicate bodies such as Mercury and Mars. Evidence for similar features on Ceres suggests the possibility of large-scale contraction on this icy body.

Thermophysical Features of Shallow Lunar Crust Demonstrated by Typical Copernican Craters Using CE-2 CELMS Data

Chang'E lunar microwave sounder (CELMS) data provide a potential way to understand the thermophysical features of the shallow lunar crust. In this study, four typical Copernican craters, including Copernicus, Aristarchus, Tycho, and Jackson, have been selected and their brightness temperature $(T_{B})$ performances are evaluated with the CE-2 CELMS data. The results are as follows. First, the hot $T_{B}$ anomaly is reunderstood according to the $T_{B}$ behaviors. The cause to the anomaly is still in doubt, and we rule out the previous explanations as rock abundance or topography. Second, the existence of the cold anomaly indicates the shallow lunar crust is likely much colder than what we knew. Third, the $T_{B}$ performances indicate that the shallow lunar crust is likely homogeneous, and the materials here have very low (FeO+TiO2) abundance and thermal inertia. Also, the difference of the $T_{B}$ performances from the crater floors to far distance will provide a new constraint for the cratering formation study.

Evidence for ultra-cold traps and surface water ice in the lunar south polar crater Amundsen

The northern floor and wall of Amundsen crater, near the lunar south pole, is a permanently shaded region (PSR). Previous study of this area using data from the Lunar Orbiter Laser Altimeter (LOLA), Diviner and LAMP instruments aboard Lunar Reconnaissance Orbiter (LRO) shows a spatial correlation between brighter 1064 nm albedo, annual maximum surface temperatures low enough to enable persistence of surface water ice (<110 K), and anomalous ultraviolet radiation. We present results using data from Diviner that quantify the differential emissivities observed in the far-IR (near the Planck peak for PSR-relevant temperatures) between the PSR and a nearby non-PSR target in Amundsen Crater.We find features in far-IR emissivity (50–400 μm) could be attributed to either, or a combination, of two effects (i) differential regolith emissive behavior between permanently-shadowed temperature regimes and those of normally illuminated polar terrain, perhaps related to presence of water frost (as indicated in other studies), or (ii) high degrees of anisothermality within observation fields of view caused by doubly-shaded areas within the PSR target that are colder than observed brightness temperatures.The implications in both cases are compelling: The far-IR emissivity curve of lunar cold traps may provide a metric for the abundance of “micro” cold traps that are ultra-cool, i.e. shadowed also from secondary and higher order radiation (absorption and re-radiation or scattering by surrounding terrain), or for emissive properties consistent with the presence of surface water ice.

POTENTIAL GEOLOGIC ISSUES OF VON KÁRMÁN CRATER REVEALED BY MULTISOURCE REMOTE SENSING DATA

Abstract. Von Kármán crater is the landing area for CE-4 mission, which provides a chance to further study the evolution of South-Pole Aitken basin. In this paper, the topography, composition, temperature and deep structures of Von Kármán crater are systematically studied with LRO LOLA data, Clementine UV-VIS data, CE-2 CELMS data, and GRAIL data. Several potential geologic issues are postulated as follows: (1) There exists a difference for the southern and northern parts of the crater floor in topography. The surface topography is influenced by Leibnitz and Finsen events. (2) There exists an FTA-rich material in depth layer of the crater floor, and the later bombardments exposed the original material in the crater floor. And the material in depth is homogeneous. (3) The composition apparently changes with depth, and there exist a warm anomaly in the northern part of the crater floor and a cold anomaly in the southern part. (4) A large amount of magma has been uplifted after Von Kármán M impact event, and the crust has been melted several times and condensed into dense basalt layer. Generally, this is a good place to measure the material from the original SPA basin and the material from the depth layer of the Moon for the CE-4 landing site.

Degradation of Endeavour Crater Based on Orbital and Rover‐Based Observations in Combination With Landscape Evolution Modeling

AbstractExploration of Endeavour crater's Shoemaker formation rim rocks by the Opportunity rover, combined with extensive observations from the Mars Express and Mars Reconnaissance Orbiters, provides unique and quantitative insights into the processes that have degraded this 22‐km diameter Noachian age impact crater. These insights are informed by comparisons between Endeavour and the relatively young, and morphologically fresh appearing, 19‐km diameter Bopolu crater located 65 km to the southwest. Analyses of rover and orbiter data, combined with landscape evolution modeling using Bopolu topography as a starting point, demonstrate that significant weathering and fluvial degradation of Endeavour occurred during the Noachian Period, with ~0.3 km of vertical rim removal, ~0.9‐km backwasting of the rim, and deposition of ~0.5 km of fluvial‐deltaic‐lacustrine sediments on the crater floor, sourced from rim erosion. Pediments formed on external rim segments, with characteristic thin regolith covers over graded bedrock. Late Noachian to Early Hesperian age Grasberg formation draping sediments, and Burns formation sulfate‐rich sandstones, subsequently embayed all but high standing Shoemaker formation rocks. Burns formation strata accumulated in the crater interior up to a depth of ~0.8 km. Subsequent wind erosion of the interior deposits is indicated by an interior mound and moat, together with yardangs carved into Shoemaker formation rocks, and graded topographic profiles from Grasberg and Burns formation rocks up and through the interior rim Shoemaker formation outcrops. Up to ~0.4 km of Burns formation rocks were exhumed from within the crater by wind erosion to produce the mound and moat.

Geological characterization of the Chang'e-4 landing area on the lunar farside

On January 3, 2019, China's Chang'e-4 probe, accomplishes the first-ever soft landing on Moon's farside in the Von Kármán crater floor mare volcanism plain (177.59°E, 45.46°S) within the South Pole-Aitken (SPA) basin. Its surface in-situ measurements would provide the first “ground truth” for lunar farside volcanism and SPA materials. We present a detailed geologic context, topographical, morphological, quantitative geochemical and mineralogical characterization of a 20 × 20 km area centered at the Chang'e-4 landing site, using the latest multi-source high-resolution orbital data sets. The landing area is very flat at a baseline of 30 m, with 90% of surface having slope <5°, and deficient of surface boulder exposures. The landing area mare is superposed by abundant sub-kilometer, block-poor circular impact craters and secondary craters in chains and clusters. Mare basalts at the landing site is dominated by low-titanium basalts, and appear to be less mafic and iron-depleted than all typical kinds of retuned lunar basalt samples and may represent a new kind of mare basalt. The landing site mare is heavily sculptured by a set of linear parallel ridged and furrowed textures formed by ballistic ejecta from the Finsen crater, which are characterized by a more feldspathic composition, while with elevated orthopyroxene abundance, and may represent compositional properties of the primitive SPA basin floor materials. We propose the Finsen ejecta and mare basalt outcrops at the landing area as prioritized targets for Chang'e-4 surface geological and spectroscopic explorations. Follow-on mission data interpretations should consider the mixing effect of local mare basalts and distant feldspathic materials.

Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures

Abstract The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO4) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.

Evidence for a lava lake on Mt. Michael volcano, Saunders Island (South Sandwich Islands) from Landsat, Sentinel-2 and ASTER satellite imagery

Mt. Michael is an active stratovolcano on Saunders Island in the South Sandwich Islands; a remote, oceanic island arc in the southern Atlantic Ocean, bordering the Southern Ocean. The arc contains the only active volcanoes in the South Georgia and South Sandwich Islands British Overseas Territory, yet little is known of their activity. Despite lava lakes being extremely rare with only a few global examples, previous analyses of satellite AVHRR imagery of Mt. Michael in the 1990s showed persistent thermal anomalies not associated with magma overflowing the crater. This suggested the existence of a lava lake inside Mt. Michael's crater. However, their study relied on 1 km resolution imagery, and there have been no long-term investigations to determine if this is a persistent feature.In-situ observations of Mt. Michael are extremely difficult given the location. We used Landsat, Sentinel-2 and ASTER satellite data to monitor activity and detect thermal anomalies within the crater. Persistent volcanic plumes and eruptions were identified throughout the thirty-year period studied. On all 15 occasions that a suitable satellite image was available a thermal anomaly within the crater was detected in the high transmissivity shortwave infrared bands (SWIR) centred on 1.65 μm and 2.2 μm. Pixel-integrated surface temperatures over the crater floor yield temperatures of 284–419 °C; comparable to lava lakes elsewhere. Unmixing these temperatures using the dual-band method reveals a molten lava component at basaltic temperatures (989–1279 °C) among a cooler crust. This is the first evidence for a lava lake within Mt. Michael from data that can resolve the crater floor, and the first evidence for magmatic temperatures. As thermal anomalies were detected in all images showing the crater, we suggest that the lava lake is a common and persistent feature.

Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

AbstractGranophyre dykes in the central part of the Vredefort impact structure are believed to be the remnants of the impact melt sheet, which intruded downwards along the fractures in the crater floor. Little is known about their original penetration depth, dip, structural relationships with the host rocks, and their general geophysical characteristics. This information is critical to understand the emplacement history of the granophyre dykes, as it relates to the formation and modification of large impact structures. We conducted magnetic and resistivity surveys across the Daskop granophyre dyke (DGD), one of the impact melt dykes in the structure's core. The magnetic survey revealed that the DGD gives a strong magnetic response at positions where the dyke outcrop exceeds the surface topography, but a very weak response where the outcrop is nearly at the same elevation as the surrounding topography. The magnetic anomaly is thus predominantly due to the outcrop protruding above ground level, suggesting a limited volume of dyke material in the subsurface and a small penetration depth. The resistivity survey performed on two profiles, set perpendicularly across the DGD, indicated a shallow penetration depth (&lt;3 m), consistent with the magnetic interpretation. Thus, our geophysical study demonstrates that the DGD is currently at the very bottom of its original emplacement. This may either be an erosional coincidence, or it may be controlled by a fundamental process of impact cratering. Further studies are warranted to determine if other granophyre dykes at Vredefort are similarly at their lowermost terminations.

Phreatic eruption dynamics derived from deposit analysis: a case study from a small, phreatic eruption from Whak\u0101ri/White Island, New Zealand

On 27 April 2016, White Island erupted in a multi-pulse, phreatic event that lasted for ~ 40 min. Six, variably sized pulses generated three ballistic ejections and at least one pyroclastic surge out of the inner crater and onto the main crater floor. Deposit mapping of the pyroclastic surge and directed ballistic ejecta, combined with numerical modelling, is used to constrain the volume of the ejecta and the energetics of the eruption. Vent locations and directionality of the eruption are constrained by the ballistic modelling, suggesting that the vent/s were angled towards the east. Using these data, a model is developed that comports with the field and geophysical data. One of the main factors modifying the dispersal of the eruption deposits is the inner crater wall, which is ~ 20 m high. This wall prevents some of the pyroclastic surge and ballistic ejecta from being deposited onto the main crater floor but also promotes significant inflation of the surge, generating a semi-buoyant plume that deposits ash high on the crater walls. While the eruption is small volume, the complexity determined from the deposits provides a case study with which to assess the relatively frequent hazards posed by active volcanoes that host hydrothermal systems. The deposits of this and similar eruptions are readily eroded, and for complete understanding of volcanic hazards, it is necessary to make observations and collect samples soon after these events.

Vertical and horizontal heterogeneity of atmospheric dust loading in northern Gale Crater, Mars

This paper updates the record of atmospheric dust loading within northern Gale Crater, Mars, by providing line-of-sight extinction (LOS-Ext) measurements of the intervening dust between the rover and the crater rim. These measurements are derived from images taken with the Navigation Cameras (Navcam) onboard the Mars Science Laboratory (MSL) rover, Curiosity. The observations span 2.44 Mars years, from Mars Year (MY) 31 at a solar longitude (LS) of 208° to t LS = 7° of MY34, sols 100–1701 of the MSL surface mission. This work examines the dataset for seasonal trends of the LOS-Ext in addition to horizontal variations and the vertical structure of LOS-Ext. The LOS-Ext has a repetitive pattern with a single peak in the latter half of the Mars year. The atmosphere in the crater is well mixed horizontally but not vertically as larger LOS-Ext is seen nearer the crater floor than at higher altitudes within the crater. The results allow a discussion on whether or not Gale Crater is a sink for atmospheric dust or a source of atmospheric dust in the current era.

Geological Characterization of the Ina Shield Volcano Summit Pit Crater on the Moon: Evidence for Extrusion of Waning‐Stage Lava Lake Magmatic Foams and Anomalously Young Crater Retention Ages

AbstractIna, a distinctive ~2 × 3 km D‐shaped depression, is composed of unusual bulbous‐shaped mounds surrounded by optically immature hummocky/blocky floor units. The crisp appearance, optical immaturity, and low number of superposed impact craters combine to strongly suggest a geologically recent formation for Ina, but the specific formation mechanism remains controversial. We reconfirm that Ina is a summit pit crater/vent on a small shield volcano ~3.5 billion years old. Following detailed characterization, we interpret the range of Ina characteristics to be consistent with a two‐component model of origin during the waning stages of summit pit eruption activities. The Ina pit crater floor is interpreted to be dominated by the products of late‐stage, low‐rise rate magmatic dike emplacement. Magma in the dike underwent significant shallow degassing and vesicle formation, followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust, resulting in cracking of the crust and extrusion of gas‐rich magmatic foams onto the lava lake crust to form the mounds. These unique substrate characteristics (highly porous aerogel‐like foam mounds and floor terrains with large vesicles and void space) exert important effects on subsequent impact crater characteristics and populations, influencing (1) optical maturation processes, (2) regolith development, and (3) landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages. Accounting for these effects results in a shift of crater size‐frequency distribution model ages from &lt;100 million years to ~3.5 billion years, contemporaneous with the underlying ancient shield volcano.

Crater Statistics on the Dark‐Toned, Mafic Floor Unit in Jezero Crater, Mars

AbstractJezero crater is a ~45‐km impact crater on the margin of Isidis basin on Mars. Jezero is the landing site for NASA's Mars 2020 rover mission. The crater hosts a paleolake, and fluvio‐lacustrine deposits formed in this lake remain accessible to exploration. A dark‐toned deposit on the crater floor overlies light‐toned carbonate‐bearing deposits and has been interpreted as a lava flow. We determined the average thickness of this deposit at the margins to be ~13.0 ± 0.8 m. We analyzed the statistics of impact craters superposed on this deposit and estimated its model age as 2.6 ± 0.5 Ga in the Hartmann system, placing it most likely in the Early Amazonian. The error estimate here includes an estimate of the uncertainty associated with the crater counts. Acquisition, caching, and eventual return of a sample from this unit could provide an important calibration point for Mars crater chronology.

Analyses of Lunar Orbiter Laser Altimeter 1,064‐nm Albedo in Permanently Shadowed Regions of Polar Crater Flat Floors: Implications for Surface Water Ice Occurrence and Future In Situ Exploration

AbstractPotential water ice concentrated within the permanently shadowed regions (PSRs) near lunar poles is both scientifically significant and of value for future explorations. However, after decades of observations, the existence and characteristics of PSR water ice remain controversial. The 1,064‐nm laser reflectance measurements collected by the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO) provide a unique opportunity to detect and characterize PSR water ice. In this work, we focus on all major PSRs on the flat floors of lunar polar craters and analyze their detailed LOLA 1,064‐nm albedo and then compare this with the adjacent flat non‐PSRs. We find that the LOLA albedo of the majority of these PSRs is systematically higher than their adjacent non‐PSRs. Potential contributions of various factors to the observed LOLA albedo are individually quantitatively evaluated; we show that each of them is unable to account for the observed LOLA albedo anomalies and that the presence of surface water ice is the most likely explanation. Combined characterization of LOLA albedo and substrate impact cratering records (crater populations and depths) reveals that the inferred PSR water ices are in very small quantity (probably in the form of a surface frost layer or admixture with regolith) and are laterally heterogeneous in model ice concentration, ranging from negligible to ~6%. We recommend that these PSRs as priority targets for future surface in situ exploration endeavors, and a case assessment of Amundsen crater is presented.

The thickness of radar-bright deposits in Mercury's northern hemisphere from individual Mercury Laser Altimeter tracks

The discovery of Mercury's radar-bright deposits has expanded our understanding of volatiles in the solar system. Key to deciphering the history and origin of the radar-bright deposits is an estimate of the volume of radar-bright material that in turn requires a measure of the average thickness of the deposits. In this study we investigate changes in topography across radar-bright deposits hosted in flat-floored, complex craters using individual edited Mercury Laser Altimeter (MLA) tracks. We compare the difference in heights of radar-bright regions and non-radar-bright regions of the crater floor and the difference of similarly sized and located regions in non-radar-bright craters and show that the two populations cannot be distinguished. The similarity of topography in these two sets of craters allows an upper limit of 15 m to be placed on the thickness of the radar-bright deposits.

Teaching Curiosity to do gravimetry

Martian Geology![Figure][1] Self-portrait of Curiosity rover in Gale crater created using images taken by Curiosity’s Mars Hands Lens Imager, June 2018 PHOTO: NASA/JPL-CALTECH/MSSS Gravimetry—the measurement of tiny changes in gravitational fields—can be used to weigh mountains. Large-scale gravimetric mapping can be done from orbit, but examining small details requires a vehicle on the ground. The Curiosity rover on Mars carries several accelerometers used for routine navigation. Lewis et al. recalibrated these accelerometers to allow them to be used for gravimetry. They measured how the local gravitational field changed as the rover moved through Gale crater and began to climb Aeolis Mons (Mount Sharp). The resulting density of material under Gale crater shows that it is relativity porous, disproving a theory that the crater floor was once buried under several kilometers of rock. Science , this issue p. [535][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aat0738

Remnants of paleoflora in impact melt rocks of the El'gygytgyn crater (Chukotka, Russia)

AbstractRemnants of paleoflora were discovered in impact melt rocks from the El'gygytgyn crater, Chukotka, Russia. El'gygytgyn is a 3.58 Ma, 18 km diameter impact structure in Chukotka, northeastern Russia. A circular crater basin is surrounded by an uplifted rim. The crater floor is occupied by the El'gygytgyn Lake, 12 km in diameter, surrounded by lacustrine terraces up to 80 m in height. Impactites found at the El'gygytgyn crater include impact melt rocks, glass bombs, and shock metamorphosed volcanic rocks. Most impact melt rocks occur only in redeposited state in the terrace lake deposits. Floral remnants were discovered in impact melt rocks from various locations in the terrace deposits. The floral remnants include fragments of leaves, cell tissue, and undetermined organic matter that occur in vesicles within glassy melt rocks and impact melt breccias. After the discovery of floral remnants in impact melt breccias from upper Miocene strata in Argentina, and the description of floral imprints in the Dakhleh Glass of proposed impact origin in Egypt, the detection of paleoflora remnants in impact melt rocks of the El'gygytgyn structure is the first such occurrence in a confirmed impact crater on Earth.

Semi-automated crater depth measurements

Impact cratering is a major process driving planetary landscape evolution. Statistics of craters spatial density is extensively used to date planetary surfaces. Their degradation state and morphometry are also key parameters to understand surface processes.To exploit the increasing coverage of digital terrain models (DEM) on Mars at high spatial resolution, we propose a semi-automated pipeline for crater depth measurement based on coupled optical images and DEM. From a craters map shapefile coupled with a co-registered DEM, we propose to measure crater depth as the difference between the 60th percentile of elevation values on the edge of the crater and the 3rd percentile value of the elevations within the crater. We present here this method and its calibration.•Aside to this paper, we provide a simple python code of this pipeline.•This method can rapidly produce crater depth dataset big enough to be interpreted statistically.•We provide solid tests on the precision of measured crater depth. Especially, we show that minimal elevation value within a crater, sometime used as crater floor elevation, is a far less precise approximation than a low percentile of elevation.

Non-Impact Origin of the Baisha Structure in Hainan Province, China

The Baisha Structure, with a rim-to-rim diameter of ~3.7 km, in the center of the Hainan Province, southern China has been considered to be an impact crater. Field investigation and petrological study are presented in this paper to investigate the impact hypothesis for this structure. The ~600-m-thick Lower Cretaceous feldspathic quartz sandstones from the Lumuwan Formation are the major outcrops both within and outside of the structure. The amphitheater-shaped rim of the structure is composed of granite porphyries that are intruded in the Lumuwan Formation. Previously interpreted impact breccia and impact melt rocks are actually granite porphyries different cooling rates and weathering status. Rocks from locations that most likely have recorded shock metamorphic signatures are sampled, but petrographic analyses reveal no indications of shock metamorphism. While subtle structural deformation occurs at the contact boundary between the granite porphyries and the feldspathic quartz sandstones, the feldspathic quartz sandstones exhibit uniformed dipping strata across the crater floor and walls. All the evidence suggests that the Baisha Structure was not formed by impact cratering. It most likely has been shaped by a combination of magmatic intrusion and long-term differential erosion.