Ejecta Blanket Research Articles

The chronostratigraphy of protoplanet Vesta

In this paper we present a time-stratigraphic scheme and geologic time scale for the protoplanet Vesta, based on global geologic mapping and other analyses of NASA Dawn spacecraft data, complemented by insights gained from laboratory studies of howardite–eucrite–diogenite (HED) meteorites and geophysical modeling. On the basis of prominent impact structures and their associated deposits, we propose a time scale for Vesta that consists of four geologic time periods: Pre-Veneneian, Veneneian, Rheasilvian, and Marcian. The Pre-Veneneian Period covers the time from the formation of Vesta up to the Veneneia impact event, from 4.6Ga to >2.1Ga (using the asteroid flux-derived chronology system) or from 4.6Ga to 3.7Ga (under the lunar-derived chronology system). The Veneneian Period covers the time span between the Veneneia and Rheasilvia impact events, from >2.1 to 1Ga (asteroid flux-derived chronology) or from 3.7 to 3.5Ga (lunar-derived chronology), respectively. The Rheasilvian Period covers the time span between the Rheasilvia and Marcia impact events, and the Marcian Period covers the time between the Marcia impact event until the present. The age of the Marcia impact is still uncertain, but our current best estimates from crater counts of the ejecta blanket suggest an age between ∼120 and 390Ma, depending upon choice of chronology system used. Regardless, the Marcia impact represents the youngest major geologic event on Vesta. Our proposed four-period geologic time scale for Vesta is, to a first order, comparable to those developed for other airless terrestrial bodies.

Nano particles as the primary cause for long-term sunlight suppression at high southern latitudes following the Chicxulub impact — evidence from ejecta deposits in Belize and Mexico

Life on Earth was sharply disrupted 66Ma ago as an asteroid hit the sea-floor in what is today Yucatan Peninsula, Mexico. Approximately 600km3 of sedimentary rock were vapourized, ejected into the atmosphere and subsequently deposited globally as an ejecta apron and fallout layer. Proximal ejecta deposits occur in Belize and southern Mexico where the so called Albion island spheroid bed is superimposed on the target rock (the Barton Creek Formation). We analysed the spheroid bed via Mössbauer spectroscopy, petrology, XRD, and palynology at several sites ~350–500km distance from the crater centre. Our results show that the relative concentrations of Fe in nano-phase goethite (α-FeOOH) are very high in the spheroid bed samples from Albion Island (Belize) and from Ramonal South (Mexico), but are low to absent in the spheroid bed at Ramonal North, and in the Cretaceous target rock. Moreover, our study shows that goethite and haematite are the dominant Fe-oxide nano-phases and the XRD results show that the target rock consists of both calcite and dolomite. We suggest that the heterogeneous composition of the spheroid bed between the various sites reflects the different types of target rocks that were dispersed within the rapidly expanding vapour plume and the complex sorting processes involved in the formation of the ejecta blanket. The distribution of the vapourized target rock strongly influenced life on Earth at the close of the Mesozoic. However, the comparatively thin K–Pg boundary clay in high-latitude Gondwanan successions combined with evidence of catastrophic changes to the biota in this region implies that the long-term sunlight suppression in the Southern Hemisphere was mainly governed by the large quantities of hydrous aerosols nucleated around sulphuric acid droplets or nano-sized particles, such as the nano-phase Fe-oxides.

Thermophysical property variations across Dione and Rhea

Maps of the variation in bolometric Bond albedo and thermal inertia across Rhea and Dione have been produced using various day and nighttime observations, taken by Cassini’s Composite Infrared Spectrometer (CIRS). The albedo maps show the same trend that has been previously observed on these satellites from reflected sunlight and thermal observations: a higher albedo on their leading hemispheres. The thermal inertia maps show two previously unknown anomalous high thermal inertia regions: low latitudes on Dione’s leading hemisphere and the bright ejecta blanket of Rhea’s Inktomi crater.The thermal inertia on Dione increases modestly from a background value of 8Jm−2K−1s−1/2 to 11Jm−2K−1s−1/2 in a region preferentially bombarded by high-energy electrons. We believe that this region on Dione is probably analogous to the thermally anomalous regions recently discovered at equivalent locations on Mimas and Tethys, dubbed Pac-Man features. The smaller magnitude of Dione’s thermal anomaly, compared to that of Mimas and Tethys, provides additional evidence that surface alteration by high-energy electrons produces these anomalies, as the high-energy electron flux decreases with increasing distance from Saturn. However, unlike on Mimas and Tethys, the thermally anomalous region on Dione does not display a spatially correlated decrease in the IR/UV (0.930μm/0.338μm) color ratio, implying that the minimum electron energy threshold of the IR/UV ice surface darkening mechanism is not met on Dione. The average of the mapped bolometric Bond albedos on the leading and trailing hemispheres of Dione are 0.49±0.11 and 0.44±0.13 respectively.On Rhea the thermal inertia increases from a background value of 10Jm−2K−1s−1/2 to 19Jm−2K−1s−1/2 on the ejecta blanket of Inktomi crater, probably due to a mixture of high and low thermal inertia material in this region, such as a fine- and large-grain ice mixture. This is the first time a thermal inertia anomaly has been associated with an impact crater on an icy saturnian satellite. On Rhea the average of the mapped bolometric Bond albedos on the leading and trailing hemispheres are 0.59±0.11 and 0.56±0.13 respectively.

Shock and annealing in the amphibole‐ and mica‐bearing R chondrites

AbstractMIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan‐phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite‐plagioclase assemblages, chromite veinlets and blebs, pincer‐shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite‐bleb‐laden cores and opaque‐free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700–900 °C under conditions of moderately high PH2O (perhaps 250–500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock‐stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1–S2 ordinary chondrites of high petrologic type. The pressure–temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende‐ and biotite‐bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.

Progressive deformation of feldspar recording low‐barometry impact processes, Tenoumer impact structure, Mauritania

AbstractThe Tenoumer impact structure is a small, well‐preserved crater within Archean to Paleoproterozoic amphibolite, gneiss, and granite of the Reguibat Shield, north‐central Mauritania. The structure is surrounded by a thin ejecta blanket of crystalline blocks (granitic gneiss, granite, and amphibolite) and impact‐melt rocks. Evidence of shock metamorphism of quartz, most notably planar deformation features (PDFs), occurs exclusively in granitic clasts entrained within small bodies of polymict, glass‐rich breccia. Impact‐related deformation features in oligoclase and microcline grains, on the other hand, occur both within clasts in melt‐breccia deposits, where they co‐occur with quartz PDFs, and also within melt‐free crystalline ejecta, in the absence of co‐occurring quartz PDFs. Feldspar deformation features include multiple orientations of PDFs, enhanced optical relief of grain components, selective disordering of alternate twins, inclined lamellae within alternate twins, and combinations of these individual textures. The distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures, starting below that required for shock deformation of quartz. We suggest that experimental analysis of feldspar behavior, combined with detailed mapping of shock metamorphism of feldspar in natural systems, may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures.

Characterization of melt and ejecta deposits of Kepler crater from remote sensing data

AbstractWe used Moon Mineralogy Mapper (M3), Arecibo and Mini‐RF radar, and Diviner radiometer data with Lunar Reconnaissance Orbiter (LRO) Camera and Kaguya Terrain Camera images to characterize the target, ejecta, and impact melt‐rich lithologies in and around lunar central peak crater Kepler. M3 data indicate the impact melt rocks of crater floor to be high‐Ca pyroxene dominated, distinct from the low‐Ca pyroxene‐dominated crater wall. The central uplift is high‐Ca pyroxene dominated, and has higher albedo. These observations are consistent with thin mare basalts underlain by noritic Imbrium ejecta, underlain by gabbroic crustal material. M3 data reveal an enigmatic, splash‐like feature of melt‐rich material on the southeastern (uprange) crater wall and flank. M3 data also highlight halos around Kepler. In detail the halos are slightly variable, but in broad terms they define a consistent feature, offset to the inferred downrange direction, and interpreted to reflect the distribution of glass‐bearing impact breccia. The radar data sets show most of the proximal ejecta to be radar‐bright. However, Diviner rock abundance data do not indicate the presence of blocks on the surface nor can they be seen using LRO Narrow Angle Camera images. Thus, the blocks giving rise to the enhanced radar signal are buried. Beyond the radar‐bright zone, a subtle radar‐dark halo emerges, coincident with a region of very low rock abundance in Diviner data. This multidisciplinary approach provides a robust analysis of the main characteristics of a lunar complex crater and reveals previously unidentified features related to the distribution of impact melt.

Investigation of tectonic processes in the lunar South Polar Region using Mini-SAR and other data

Studies focusing on the geological processes on the moon have found various tectonic features suggesting similar geological activity occurring on the Earth (Shwarz, 1928). Also, it can be inferred that meteoritic impact, cosmic and solar wind flux variability coupled with the interior changes of the Moon may have manifested in the form of tectonic activity on the moon. Lobate scarps which are generated from thrust faulting on the moon and are indicative of recent tectonic activity have also been identified in the sub-polar and equatorial regions (Watters et al., 2010). Alternatively, “grabens” which are indicative of extensional tectonics have also been identified at various locations in the South Polar Region (Kramer et al., 2012). Previous studies investigating tectonic activity have used optical data primarily to study lunar tectonics. But areas such as partially and completely shadowed crater interiors present in the polar regions can also contain unique features indicative of tectonic activity. The study therefore focuses on the identification of features indicating the presence of faults/lineaments/ fractures on the lunar topography using mini-SAR data from CHANDRAYAAN-1 mission as well as optical data from Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) from LROC mission. Mini-SAR imaging radar onboard CHANDRAYAAN-1 mission was the first mono-static lunar orbiting synthetic aperture radar (Spudis et al., 2009). MiniSAR data is primarily used to identify signatures suggesting the possibility of the presence of water ice within the completely shadowed regions of the lunar poles (Fa and Cai, 2013). Further, this data has been used to view various other geological features such as melt flows, crater ejecta blankets, secondary craters etc. (Saran et al., in press). Therefore, identification of distinct morphological features in shadowed as well as illuminated regions of lunar surfaces can be easily done using mini-SAR data (Spudis et al., 2009). The SAR technology can also penetrate the surface and therefore is an effective tool to look for hidden fault lines in the shadowed portions on the lunar regolith. Alternatively, other distinct surface and subsurface geological features have also been identified which can further help to study the process of impact cratering as well as understand the formation of secondary features developed post impact. Digital image processing of mini-SAR data using m-χ decomposition technique was done to achieve single, double, and multiple/diffuse backscattering contributions to view tectonically derived morphological features in detail (Saran et al., 2012). Fault lines overlying possible subsurface faults depicting presence of past or present seismic activity can be seen well after using m-χ decomposition technique. Other features such as rock/ debris avalanches within the inner crater walls were also identified lying perpendicular to the inferred fault lines on m-χ decomposition images within the South Polar Region. Slumping activity on the shadowed inner crater walls forming terraced structures can also be seen using m-χ decomposition images. These could have formed due to nearby impact shaking or seismic activity from underlying dislocation along fault planes making the material to slide down in single or multiple blocks. Circular polarization ratio (CPR) was another parameter calculated to infer surface roughness in the regions having the tectonically derived features. Circular polarization ratio gives the ratio of the same sense circular polarization backscatter power received as emitted over the opposite sense backscatter power received as emitted (Calla et al., 2013). The inferred rock/ debris avalanche associated with tectonic activity have high values of CPR ranging between 0.1–2.7, which indicates the presence of highly rough surfaces. Such type of mass wasting activity can also be seen on terrestrial hilly terrains being formed after seismic shaking of unconsolidated material. Microwave remote sensing therefore helps to reveal a number of geological features on varying terrains otherwise not visible on optical data. Mini-SAR data makes it easy to view several of such unique features over the lunar surface both in the illuminated and the shadowed regions. The identification of such features can help to further understand the past and present geological activity taking place resulting in the formation of such features. It can also explain the physics behind the process of impact catering and the subsequent formation of other secondary structures. Further the results obtained can be correlated and compared with similar features seen on terrestrial surfaces. High CPR values derived from Stokes vectors and degree of polarization can primarily be attributed to the rough and rugged surfaces mainly found on the inner walls of impact craters and on the outer

The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure?

Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2-D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks.

Small fresh impact craters on asteroid 4 Vesta: A compositional and geological fingerprint

AbstractSmall morphologically fresh impact craters (<10 km in diameter) on Vesta's surface with a photometrically distinct ejecta blanket are expected to represent fresh surface material and thus provide the opportunity to study the composition of the unweathered surface. Dawn‐Framing Camera and Visual and Infrared Spectrometer (VIR) data reveal impact craters with bright, dark, and mixed, i.e., partly bright and dark, ejecta existing on Vesta's surface, which not only differ in the visible albedo from their surroundings but also in their composition. Differences in the composition are related to the visible albedo and/or the geographic location of the impact craters. Bright ejecta, only seen in the southern Vestan hemisphere, are dominated by howardite/eucrite‐like material as expected for Vesta's upper crust. Dark ejecta associated with dark impact craters are dominated by a strongly absorbing, spectrally neutral compound, supporting an origin from carbon‐rich impactors. Few impact craters of intermediate albedo in Vesta's southern hemisphere contain material resembling diogenites, which are expected to exist in the deeper parts of Vesta's interior. The geological settings suggest that the diogenite‐like material represents a part of a layer of diogenitic material surrounding the Rheasilvia basin or local concentrations of diogenitic material as part of the ejecta excavated during the latter stage of the Rheasilvia impact event. The spectral differences between eucrite‐ and diogenite‐dominated materials also could be verified due to spin‐forbidden absorptions in the visible spectral range, which are known from laboratory spectra of pyroxenes, but, which have been identified in the VIR spectra of Vesta for the first time.

The production of small primary craters on Mars and the Moon

We model the primary crater production of small (D<100m) primary craters on Mars and the Moon using the observed annual flux of terrestrial fireballs. From the size–frequency distribution (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both slope and magnitude. We include an atmospheric model for Mars that accounts for the deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere or the fragmentation of the meteors do not strongly influence our results. The downturn in the crater SFD from atmospheric filtering is predicted to occur at D∼10–20cm, well below the downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). Crater counts are conducted on the ejecta blanket of Zunil crater and the interior of Pangboche crater on Mars and North Ray and Cone craters on the Moon. Our model isochrons produce a similar slope and age estimate for the formation of Zunil crater as the Hartmann production function (∼1Ma). We derive an age of 35.1Ma for Pangboche when accounting for the higher elevation (>20km higher than Zunil), a factor ∼2 younger than estimated using the Hartmann production function which assumes 6mbar surface pressure. We estimate ages of 52.3Ma and 23.9Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

The geology of the Marcia quadrangle of asteroid Vesta: Assessing the effects of large, young craters

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (∼63km diam.) and Calpurnia (∼53km diam.) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between ∼40 and 60Ma (depending upon choice of chronology system), and Marcia’s ejecta blanket ranges in age between ∼120 and 390Ma (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of ∼280–990Ma based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta’s ancient crust preserved on Vestalia Terra.

Impact-induced compositional variations on Mercury

Remote sensing data suggest Mercury's surface has compositional variations spatially associated with crater and basin ejecta, the so-called “Low-Reflectance Material” (LRM), which has been suggested to be enriched in a subsurface native darkening agent that is excavated and redeposited onto the surface. This unit may record the evidence of impact-induced mixing of Mercury's outer layers during its early history. Here, we develop a fully three-dimensional Monte Carlo model of impact cratering, excavation, and ejecta blanket deposition on a global scale for Mercury.New dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional asteroids in a region interior to the present-day belt, known as the “E-belt”. We use Monte Carlo methods to show that the predicted bombardment from this population matches the observed spatial crater densities on Mercury. Impacts large enough to pierce through the crust create surface ejecta deposits rich in mantle material. Later impacts onto enriched ejecta deposits redistribute mantle material away from the basins. For the suggested average mercurian crustal thickness of 50 km, the surface has, on average, ∼0.4% mantle material by volume; the most enriched areas have ∼30% mantle by volume.The regional coverage of impact-induced compositional changes is strongly dependent on the thickness to the subsurface source. Because observations indicate LRM covers ∼15% of Mercury's surface, our model suggests the darkening agent is ∼30 km deep. Considering the current estimated average mercurian crustal thickness of 50 km, this implies the darkening agent is likely located within a chemically distinct lower crust.

Geologic map of the northern hemisphere of Vesta based on Dawn Framing Camera (FC) images

The Dawn Framing Camera (FC) has imaged the northern hemisphere of the Asteroid (4) Vesta at high spatial resolution and coverage. This study represents the first investigation of the overall geology of the northern hemisphere (22–90°N, quadrangles Av-1, 2, 3, 4 and 5) using these unique Dawn mission observations. We have compiled a morphologic map and performed crater size–frequency distribution (CSFD) measurements to date the geologic units. The hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to ∼200km in diameter. The most widespread unit is a plateau (cratered highland unit), similar to, although of lower elevation than the equatorial Vestalia Terra plateau. Large-scale troughs and ridges have regionally affected the surface. Between ∼180°E and ∼270°E, these tectonic features are well developed and related to the south pole Veneneia impact (Saturnalia Fossae trough unit), elsewhere on the hemisphere they are rare and subdued (Saturnalia Fossae cratered unit). In these pre-Rheasilvia units we observed an unexpectedly high frequency of impact craters up to ∼10km in diameter, whose formation could in part be related to the Rheasilvia basin-forming event. The Rheasilvia impact has potentially affected the northern hemisphere also with S–N small-scale lineations, but without covering it with an ejecta blanket. Post-Rheasilvia impact craters are small (<60km in diameter) and show a wide range of degradation states due to impact gardening and mass wasting processes. Where fresh, they display an ejecta blanket, bright rays and slope movements on walls. In places, crater rims have dark material ejecta and some crater floors are covered by ponded material interpreted as impact melt.

ESTIMATING THE SIZE OF LATE VENEER IMPACTORS FROM IMPACT-INDUCED MIXING ON MERCURY

Late accretion of a of compositionally diverse planetesimals may introduce chemical heterogeneity in the mantles of the terrestrial planets. The size of the late veneer objects is an important control on the angular momenta, eccentricities, and inclinations of the terrestrial planets, but current estimates range from meter-scale bodies to objects with diameters of thousands of kilometers. We use a three-dimensional global Monte Carlo model of impact cratering, excavation, and ejecta blanket formation to show that evidence of mantle heterogeneity can be preserved within ejecta blankets of mantle-exhuming impacts on terrestrial planets. Compositionally distinct provinces implanted at the time of the late veneer are most likely to be preserved in bodies whose subsequent geodynamical evolution is limited. Mercury may have avoided intensive mixing by solid-state convection during much of its history. Its subsequent bombardment may have then excavated evidence of primordial mantle heterogeneity introduced by the late veneer. Simple geometric arguments can predict the amount of mantle material in the ejecta blanket of mantle-exhuming impacts, and deviations in composition relative to geometric predictions can constrain the length-scale of chemical heterogeneities in the subsurface. A marked change in the relationship between mantle and ejecta composition occurs when chemically distinct provinces are ~250 km in diameter; thus, evidence of bombardment by thousand-kilometer-sized objects should be readily apparent from the variation in compositions of ejecta blankets in Mercury's ancient cratered terrains.

Automatic detection of wrinkle ridges in Venusian Magellan imagery

Abstract Wrinkle ridges constitute one of the most abundant tectonic features on terrestrial planetary surfaces. On Venus, evidence suggests a connection between wrinkle ridges and the climatic evolution of the planet. However, like other planets and moons that experience more active surface geological processes, such as Earth, Mars, Europa, Io and Titan, visible impact craters on the Venusian surface are less common because they are eroded, buried or transformed by tectonics or other geological processes over time. It is of great importance to identify and understand some characteristics of those surface morphologies, such as orientation, length, spacing, original dimension and topography. Nevertheless, these parameters can only be computed on remotely sensed images after their segmentation. Until now, the manual identification of these features has been focused on those of major geological significance, leaving many more to be identified, mapped and studied. The main aim of this paper is to provide a method for automatic detection of wrinkle ridges from Magellan Synthetic Aperture Radar (SAR) imagery at different scales. The proposed algorithm, based on a combination of fractal dimension and morphological operators, identifies regions of interest to this study, namely those of anisotropic behaviour, but also impact craters and their ejecta blankets. The high performances achieved in a variety of situations demonstrate that its robustness can be applied to an automated procedure.

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.

Scheila’s scar: Direct evidence of impact surface alteration on a primitive asteroid

Asteroid (596) Scheila was the first object for which the immediate aftermath of an inter-asteroidal collision was observed. In December 2010, the 113km-sized asteroid was impacted by a smaller asteroid of less than 100m in diameter. The scale of the impactor was established by observations of fading ejecta plumes. Comparison of the lightcurves obtained before and after the impact allowed us to assess how much of Scheila’s surface was altered. Cratering physics based on the impactor size suggests that the size of the affected area is larger than expected, (effective radii of 3.5–10km depending on the change in the albedo of the surface). Similar but more localized albedo changes have been observed on Vesta and the martian moons, but are not understood. Empirical laws describing ejecta blankets however indicate that at distances between 3.5 and 10km from the crater, Scheila’s surface would be covered by a thin layer 2mm to 2cm thick. This dusting, possibly mixed with bright impactor material may be enough to explain to observed brightness increase. Our results show that sub-critical impacts may play a significant role in processing the surfaces of asteroids. The large effect of small impacts on asteroidal lightcurves complicate shape modeling.

Synergetic use of SAR and Thermal Infrared data to study the physical properties of the lunar surface

The surface layer of the Moon preserves vital evidences of lunar impact and cratering processes due to the absence of any Aeolian and fluvial erosion processes acting on it. By examining these evidences, which are recorded throughout the evolutionary history of the Moon, several basic aspects of lunar science can be understood, and this has direct relevance to the surfaces of other airless bodies within the solar system. In this study, rock abundance data obtained from Thermal Infrared (TIR) observations and radar Circular Polarization Ratio (CPR) data sets obtained from polarimetric SAR observations were correlated at some sample sites on the lunar surface. Preliminary results yielded qualitative and quantitative estimates for surface rock abundances. Except at distal ejecta deposits of young, bright craters a general correlation was observed between the two datasets. Mixed results were observed from the impact melt flows where the situation is complex due to the possible subsurface-volume and volume-subsurface interactions of the radar waves. But the flow features were clearly separated from the interior and ejecta regions of their parent craters in terms of CPR and rock abundances. The extent and distributions of pyroclastic deposits and dark haloed regions could not be distinctly identified at the resolution of datasets utilized. Near Gerasimovich D crater, the Diviner Radiometer has provided the first TIR observations of a newly discovered impact melt flow which was not visible in the optical imagery. This facilitated the first ever quantitative comparisons of the radar CPR and rock abundance values near such a region. Also, significant differences in spatial patterns between the radar and rock concentration data sets were observed, owing to the differences in the sensitivity of the two observations.

Neutron absorption constraints on the composition of 4 Vesta

AbstractGlobal maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock‐forming elements. From a circular, polar low‐altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole‐rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg‐rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine‐rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.

Relative rates of optical maturation of regolith on Mercury and the Moon

AbstractQuantifying relative differences in regolith optical maturation rates is critical to interpreting multispectral reflectance measurements of airless bodies. In this study, normalized reflectance measurements of crater ejecta blankets and rays are used as indicators of the relative state of regolith maturation on Mercury and the Moon, as well as for a comparison of surface reflectance. Characterization of craters with high‐reflectance ejecta from Lunar Reconnaissance Orbiter Camera and Mercury Dual Imaging System orbital mosaics indicates that the optical maturation rate is up to 4 times faster on Mercury than on the Moon. Observations also show that there are fewer immature craters (per unit area) on Mercury than on the Moon and suggest a younger average age for mercurian rayed craters than for lunar rayed craters. A comparison of crustal reflectance of immature material yields a ratio of 1.9 ± 0.4 for the average photometrically normalized reflectance of lunar material to that of mercurian material. The difference in reflectance is attributed primarily to differences in composition between the lunar highlands and average surface material on Mercury. The new observations of immature craters per unit area and surface reflectance of immature materials are consistent with previous proposals that regolith maturation rates are faster on Mercury than the Moon, and but here we quantify the relative rate from empirical observations for the first time.