Ejecta Layer Research Articles

Probing the Innermost Ejecta Layers in Supernova Remnant Kes 75: Implications for the Supernova Progenitor

Abstract Supernova remnants (SNRs) that contain pulsar wind nebulae (PWNe) are characterized by distinct evolutionary stages. In very young systems, the PWN drives a shock into the innermost supernova (SN) material, giving rise to low-excitation lines and an infrared (IR) continuum from heated dust grains. These observational signatures make it possible to cleanly measure the properties of the deepest SN ejecta layers that can, in turn, provide constraints on the SN progenitor. We present Herschel Space Observatory far-IR observations of the PWN in the Galactic SNR Kes 75, containing the youngest known pulsar that exhibited magnetar-like activity. We detect highly broadened oxygen and carbon line emission that arises from the SN ejecta encountered by the PWN. We also detect a small amount (a few times 10−3 M ⊙) of shock-heated dust that spatially coincides with the ejecta material and was likely formed in the SN explosion. We use hydrodynamical models to simulate the evolution of Kes 75 and find that the PWN has so far swept up 0.05–0.1 M ⊙ of SN ejecta. Using explosion and nucleosynthesis models for different progenitor masses in combinations with shock models, we compare the predicted far-IR emission with the observed line intensities and find that lower-mass and explosion energy SN progenitors with mildly mixed ejecta profiles and comparable abundance fractions of carbon and oxygen are favored over higher-mass ones. We conclude that Kes 75 likely resulted from an 8 to 12 M ⊙ progenitor, providing further evidence that lower-energy explosions of such progenitors can give rise to magnetars.

Resurge deposits associated with the shallow marine early Cambrian Vakkejokk impact, north Sweden

AbstractThe lower Cambrian Vakkejokk Breccia is a proximal ejecta layer from a shallow marine impact. It is exposed for ~7 km along a steep mountainside in Lapland, northernmost Sweden. In its central parts, the layer is up to ~27 m thick. Here the breccia shows a vertical differentiation into (1) a lower subunit consisting of strongly deformed target sediments mixed with up to decameter size, mainly crystalline basement clasts (i.e., lower polymict breccia [LPB]); (2) a middle subunit consisting of a polymict, blocky to gravelly breccia, commonly graded (i.e., graded polymict breccia [GPB]), that, in turn, is sporadically overlain by (3) a few dm thick, sandy bed (i.e., top sandstone [TS]). Previous work interpreted the graded beds as deposited by resurging water during early crater modification. We made three short (<1.35 m) core drillings through the graded beds. The line‐logging technique previously used on cores from other marine‐target craters was complemented by logging of equal‐sized cells in photos made along the cores. Granulometry and clast lithology determinations provide further evidence for the top beds of the breccia being resurge deposits. However, the magnitude of this resurge can only be assessed by future deep core drilling of the infill of the crater hidden below the mountain.

Bosumtwi impact structure, Ghana: Evidence for fluidized emplacement of the ejecta

AbstractThe about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

AbstractThe circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H2O, <300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε54Cr value in polymict breccia. Two 40Ar/39Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater >10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

Shocked quartz grains in the early Cambrian Vakkejokk Breccia, Sweden—Evidence of a marine impact

AbstractHere we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North‐Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low‐shock semi‐autochthonous bombarded strata. In the overlying, more fine‐grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (<0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.

Ries impact crater sedimentary conglomerates: Sedimentary particle 'impact pre-processing', transport distances and provenance, and implications for Gale crater conglomerates, Mars

Terrestrial impact cratering events produce anomalous rock fragmentation and alteration, and form sedimentary basins that are out of equilibrium with the local geomorphological environment. Assessement of their initial deposits, crater erosion and infilling, lacustrine environments and biology can provide important insights into similar features on Mars. Here we report on fluviodeltaic conglomerates associated with the 26 km diameter Miocene Ries impact structure (target: sediments overlying crystalline basement) with respect to shock features, grain size distribution, roundness, and potential parent rocks (provenance). The poorly sorted conglomerates form coarsening-upwards cycles associated with a delta-mouth bar system, with subangular to subrounded pebbles derived from crystalline rock impact breccia deposits. Pebbles comprise a wide range of lithologies and states ranging from fresh and solid to intensively impact-fractured, friable crystalline rocks, including shatter cones. Shocked quartz grains, however, are restricted to the sand fraction. The latter observation points to suevite as an additional, but subordinate source of the conglomerates. Granulometric analysis indicates that, in contrast to being derived from weathering of bedrock, impact brecciation of the parent crystalline rocks generated the initial grain size distribution of the conglomerates (i.e., preprocessing). This concept of “impact preprocessing” helped to defne the provenance, the post-impact sedimentologic history and the sediment transport distances of the conglomerates. Post-impact fluvial transport leads to the outwash of fine fractions into the prodelta subenvironment, and an increase in roundness and mean diameter in the residual conglomerates (i.e. hydraulic sorting). On the basis of the characteristics of the ejecta deposits and the present-day locations and occurrences of crystalline rock impact breccias and blocks in the northwestern Ries crater segment analyzed, a transport distance of less than 3.5 km is derived for these impact-preprocessed conglomerates. Reconstruction of the initial distribution and composition of the primary ejecta layer indicates that the conglomerates composed solely of crystalline rock had a maximum transport distance less than 6.5 km, i.e., within the closed basin of the Ries impact structure. In contrast to traditional interpretations of conglomerates, requiring long transport distances from bedrock source regions to deposition to ensure sufficient rounding and sorting, the rounding and sorting associated with these short transport distances are explained by preprocessing of parent rocks by impact brecciation and enhanced susceptibility to abrasion. Consequently, transport distances of conglomeratic fragments associated with impact craters, such as Gale crater on Mars, are likely to have been only a few kilometers, and derived from nearby ejecta, most likely from the inner crater walls. This Ries crater-based preprocessing and transport model implies that the unusual Gale crater conglomeratic fragments are likely to represent the immediate target material excavated by the Gale cratering event, rather than fragments transported laterally from greater distances outside the crater.

Lunar surface processes inferred from cosmogenic radionuclides in Apollo 16 double drive core 68002/68001

Measurements of cosmogenic 10Be, 26Al and 36Cl in Apollo 16 double drive core 68002/68001 are combined with a high resolution digital surface model of the sampling site to investigate the surface processes on the Moon. We find both a significant deficit of solar cosmic ray (SCR)-produced 26Al and a lack of SCR-produced 36Cl in the top 3–5 g/cm2 of the lunar regolith. The topographic model shows the core was taken from just inside a crater with a rim diameter of 25–30 cm. These observations are consistent with regolith removal and displacement by a shallow impact that occurred on the order of 100 kyr ago, or less. Our findings are also compatible with shallow mixing, or gardening, of the lunar regolith to depths of a few cm, a value often found in other lunar cores over the ∼106 yr averaging times of 26Al and 53Mn measurements. More definitive regolith mixing depth estimates are not possible due to the likelihood of disturbance in the top of the core as a result of sampling and/or handling. Our results support the hypothesis that the lunar surface experiences more frequent disturbances by small primary and secondary impacts than has previously been assumed. Additionally, we find no evidence that fine-grained ejecta from the 2.0 Myr old South Ray Crater impact reached this site. If the layer of fine-grained ejecta that reached the sampling site from the South Ray Crater was no more than a few cm thick, this absence can be explained by the erosion that formed the small, relatively recent crater at the coring location.

Type Ia Supernovae in the First Few Days: Signatures of Helium Detonation versus Interaction

Abstract The mechanism for the blue and excessive early-phase emission reported within the first few days of several Type Ia supernovae (SNe Ia) has been suggested to be the interaction of the SN ejecta with either a non-degenerate companion star or circumstellar media (CSM). Recently, another mechanism has been suggested within the context of the He-detonation-triggered SN scenario (i.e., the double-detonation scenario or He-ignited violent merger), in which the radioactive isotopes in the outermost layer of the SN ejecta produce the early emission. In this paper, we investigate the properties of the excessive early-phase emission predicted by these different scenarios. The early flash in the He-detonation scenario shows behaviors different from those in the companion/CSM interaction scenarios. Especially clear diagnostics are provided once the behaviors in the UV and in the optical are combined. The spectra synthesized for the He-detonation scenario are characterized by absorptions due to He-detonation products, which develop in the decay phase. We further expect a relation between the properties of the early-phase flash and those of the maximum SN emission, in such a way that the brighter and slower initial flash is accompanied by a more substantial effect from the additional absorptions (and reddening). This relation, however, should be considered together with the maximum luminosity of the SN, since the larger luminosity suppresses the effect of the additional absorption. With these expected features, we address the possible origins of the excessive early-phase emission observed in several SNe.

Thermodynamic properties of ZrSiO4 zircon and reidite and of cotunnite-type ZrO2 with application to high-pressure high-temperature phase relations in ZrSiO4

Enthalpies of high-pressure phase transitons in ZrSiO4 were measured by high-temperature drop-solution calorimetry. The enthalpies for the zircon-reidite transition in ZrSiO4 and for dissociation of reidite to cotunnite-type ZrO2 + SiO2 stishovite were obtained to be 27.5 ± 3.3 and 50.3 ± 3.0 kJ/mol, respetively, at 298 K. By thermal relaxation method, low-temperature isobaric heat capacities of ZrSiO4 reidite and cotunnite-type ZrO2 were measured, giving standard entropies at 298 K of 77.29 ± 0.04 and 49.10 ± 0.04 J/mol·K, respectively. Combining the measured enthalpies and entropies with other available thermophysical data, the transition boundaries in ZrSiO4 were calculated. The calculated zircon-reidite transiton boundary is consistent with that by in situ high-pressure expreiments by Ono et al. (2004a) within the errors. The calculated dissociation boundary of reidite has a smaller slope than that by high-pressure quench expreiments by Tange and Takahashi (2004). The equilibrium zircon-reidite transition pressure is calculated to be 8 GPa at 298 K, which is by 12–15 GPa lower than the transition pressures observed by room-temperature static compression experiments. Our results suggest that zircon transforms to reidite and dissociates into the two phases at 330 and 610 km depths, respectively, along the normal mantle geotherm. Combining the calculated phase relations with kinetics estimated from the first principles calculation may provide some insights to P, T conditions which reidites found in impact ejecta layers and craters experienced.

REMOTE SENSING OBSERVATIONS AND NUMERICAL SIMULATION FOR MARTIAN LAYERED EJECTA CRATERS

Abstract. To understand past Martian climates, it is important to know the distribution and nature of water ice on Mars. Impact craters are widely used ubiquitous indicators for the presence of subsurface water or ice on Mars. Remote sensing observations and numerical simulation are powerful tools for investigating morphological and topographic features on planetary surfaces, and we can use the morphology of layered ejecta craters and hydrocode modeling to constrain possible layering and impact environments. The approach of this work consists of three stages：Firstly, the morphological characteristics of the Martian layered ejecta craters are performed based on Martian images and DEM data. Secondly, numerical modeling layered ejecta are performed through the hydrocode iSALE (impact-SALE). We present hydrocode modeling of impacts onto targets with a single icy layer within an otherwise uniform basalt crust to quantify the effects of subsurface H2O on observable layered ejecta morphologies. The model setup is based on a layered target made up of a regolithic layer (described by the basalt ANEOS), on top an ice layer (described by ANEOS equation of H2O ice), in turn on top of an underlying basaltic crust. The bolide is a 0.8 km diameter basaltic asteroid hitting the Martian surface vertically at a velocity of 12.8 km/s. Finally, the numerical results are compared with the MOLA DEM profile in order to analyze the formation mechanism of Martian layered ejecta craters. Our simulations suggest that the presence of an icy layer significantly modifies the cratering mechanics, and many of the unusual features of SLE craters may be explained by the presence of icy layers. Impact cratering on icy satellites is significantly affected by the presence of subsurface H2O.

Supernovae from massive stars with extended tenuous envelopes

Massive stars with a core-halo structure are interesting objects for stellar physics and hydrodynamics. Using simulations for stellar evolution, radiation hydrodynamics, and radiative transfer, we study the explosion of stars with an extended and tenuous envelope (i.e. stars in which 95% of the mass is contained within 10% or less of the surface radius). We consider both H-rich supergiant and He-giant progenitors resulting from close-binary evolution and dying with a final mass of 2.8–5 M⊙. An extended envelope causes the supernova (SN) shock to brake and a reverse shock to form, sweeping core material into a dense shell. The shock-deposited energy, which suffers little degradation from expansion, is trapped in ejecta layers of moderate optical depth, thereby enhancing the SN luminosity at early times. With the delayed 56Ni heating, we find that the resulting optical and near-IR light curves all exhibit a double-peak morphology. We show how an extended progenitor can explain the blue and featureless optical spectra of some Type IIb and Ib SNe. The dense shell formed by the reverse shock leads to line profiles with a smaller and near-constant width. This ejecta property can explain the statistically narrower profiles of Type IIb compared to Type Ib SNe, as well as the peculiar Hα profile seen in SN 1993J. At early times, our He-giant star explosion model shows a high luminosity, a blue colour, and featureless spectra reminiscent of the Type Ib SN 2008D, suggesting a low-mass progenitor.

Global Scale Analysis of Martian Landslide Mobility and Paleoenvironmental Clues

AbstractThe mobility of landslides on Mars is studied based on a database of 3,118 events. To establish the volume of the landslides for the whole data set based on the deposit area, a new volume‐area relationship based on a representative data set of 222 landslides is used. By plotting the H/L ratio between fall height H and runout L versus volume, the landslide mobility is analyzed and compared with existing empirical relationships for Martian and terrestrial landslides. By analyzing the mobility in terms of normalized residuals, that is, the relative deviation of the H/L ratio from the data set best‐fit line, mobility is found to depend on both the landslide location on Mars and the landslide typology. This allows us to identify four different types of high‐mobility (hypermobile) landslides. Three classes of high‐mobility landslides are associated respectively to meteoroid impact, the Olympus Mons aureoles, and landslides with Toreva‐block failure style, and their mobility can be explained by the peculiar flow mechanics. The fourth class includes landslides associated with isolated craters, those in the regions wetted by the putative Oceanus Borealis, and the ones at high latitudes. We suggest that the common factor behind all the hypermobile landslides of this fourth kind is the presence of ice. This is confirmed by our data showing that landslides increase in mobility with latitude. The latitudinal trend mirrors the distribution of ice as detected by radar, neutron probes, and the presence of glacial and layered ejecta morphologies. Because the overall landslide distribution supports the presence of ice‐lubricated conditions, two ice lubrication models are presented showing how ice melting within or underneath the landslides could enhance mobility. By proper analysis in terms of apparent friction residuals, we find that the mobility of landslides in Valles Marineris with the largest landslide concentration is lower than average. We explain this circumstance partly from the smaller role of ice in equatorial Valles Marineris and partly because the collapses from high slope relief imply high‐speed impact with the floor valley confinement, loss of momentum, and decrease in mobility. Environmental consequences imply that the present subsurface ice distribution may have been persistent throughout the Amazonian period.

Application of laser ablation-ICP-MS to determine high-resolution elemental profiles across the Cretaceous/Paleogene boundary at Agost (Spain)

A high-resolution analysis of the distribution of major and trace elements across a Cretaceous/Paleogene boundary (KPgB) was done using Laser Ablation-Inductivity Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and was compared with traditional distinct sampling and analysis. At the Agost site (SE Spain) a 22-cm-long core containing the KPgB was recovered using a Rolatec RL-48L drill. Within this interval, the lowermost 5 cm correspond to the Maastrichtian and the uppermost 17 cm to the Danian. The core section was resin-embedded under O2-free conditions, cut and polished for LA-ICP-MS continuous measurements with 10 μm increments and a laser-beam of 80 μm. Elemental concentrations in discrete samples taken prior to embedding from the same core interval were determined by Inductivity Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The LA-ICP-MS analyses in continuous mode considerably improve the resolution of geochemical profiles, allowing the compositional variability at a micrometer scale within the ejecta layer to be detected. In this study, we obtained profiles with 255 data points for the ejecta layer interval compared to 3 data points obtained by traditional manual sampling and ICP-OES analyses. Yet our recovered core section showed a rather limited preservation of the ejecta layer. This paper focuses on the presentation of LA-ICP-MS analysis as a particularly useful tool to investigate paleoenvironmental changes associated with bio-events. Additionally, the high-resolution of major and trace elemental distribution made it possible to study remobilization across thin but distinct boundaries such as the KPgB.

Associations between sediment architecture and liquefaction susceptibility in fluvial settings: The 2010–2011 Canterbury Earthquake Sequence, New Zealand

This study focuses on the sedimentary architecture of sites in the Halswell floodplain (Christchurch, New Zealand) that experienced liquefaction during the 2010–2011 Canterbury Earthquake Sequence. We investigated landforms where ejecta were and were not observed, and define their 3D geometry and sediment characteristics. The first site straddles an abandoned channel of the Halswell River that is overlain by a crevasse splay deposit. The second site is located on the inside bend of an active meander of the river. Both sites were severely affected by liquefaction during the Darfield and Christchurch earthquakes. A suite of techniques was used in the characterization of both sites: detailed geomorphic and liquefaction mapping with DEMs derived from LIDAR; Ground-Penetrating Radar (GPR); paleoseismic trenches; hand-piston cores, radiocarbon dating, grain-size analysis, and cone-penetration tests. Potentially liquefiable sands, based on their grain characteristics and water saturation, were found in many environments within the alluvial setting studied (river channel, point bar deposits and flood plain). The surficial geology of the two study sites is mapped as crevasse splay and point bar deposits. However, the layers that liquefied were at depth within buried abandoned channel and point bar deposits at the Hardwick site, and within buried point bar deposits at the Marchand site. The location of the surface manifestation of liquefaction was characterized by specific geometry of the sedimentary architecture of the alluvial floodplain that facilitated or hindered flow of the slurry of water and entrained sediment towards the surface. On the one hand, the inclined contacts of paleochannel and point bar deposits created pathways for the liquefied material to travel to the surface. On the other hand, in the floodplain, with no ejecta, sand and silty sand layers interfingered with clay lenses, lack the specific geometry that facilitates the ejecta at the surface. The CPT analysis showed that liquefaction ejecta occurred above a thick homogeneous layer of sand, whereas liquefaction ejecta did not occur (or sand blows did not form) where clay lenses were interfingered with sand. At both sites the surface manifestation of liquefaction was mainly located on slightly higher topographic areas, suggesting the topography is an additional controlling factor on the surface distribution of liquefaction ejecta. In summary, the surface manifestation of liquefaction is a result of a specific sediment type that was saturated, sedimentary setting that facilitated flow of fluidized sediment to the surface, and topographic relief prone to cracking.

Timing and Distribution of Single‐Layered Ejecta Craters Imply Sporadic Preservation of Tropical Subsurface Ice on Mars

AbstractDetermining the evolution of tropical subsurface ice is a key component to understanding Mars's climate and geologic history. Study of an intriguing crater type on Mars—layered ejecta craters, which likely form by tapping subsurface ice—may provide constraints on this evolution. Layered ejecta craters have a continuous ejecta deposit with a fluidized‐flow appearance. Single‐layered ejecta (SLE) craters are the most common and dominate at tropical latitudes and therefore offer the best opportunity to derive new constraints on the temporal evolution of low‐latitude subsurface ice. We estimate model formation ages of 54 SLE craters with diameter (D) ≥ 5 km using the density of small, superposed craters with D < 1 km on their continuous ejecta deposits. These model ages indicate that SLE craters have formed throughout the Amazonian and at a similar rate expected for all Martian craters. This suggests that tropical ice has remained at relatively shallow depths at least where these craters formed. In particular, the presence of equatorial SLE craters with D ~ 1 km indicates that ice could be preserved as shallow as ~100 m or less at those locations. Finally, there is a striking spatial mixing in an area of highlands near the equator of layered and radial (lunar‐like ballistic) ejecta craters; the latter form where there are insufficient concentrations of subsurface ice. This implies strong spatial heterogeneity in the concentration of tropical subsurface ice.

Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland

Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to find unmelted crystals from the actual impactor within an ejecta layer. Here we detail the first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identified as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni)2P], baddeleyite (ZrO2), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through field relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the first recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province.

Millisecond Magnetar Birth Connects FRB 121102 to Superluminous Supernovae and Long-duration Gamma-Ray Bursts

Abstract Subarcsecond localization of the repeating fast radio burst FRB 121102 revealed its coincidence with a dwarf host galaxy and a steady (“quiescent”) nonthermal radio source. We show that the properties of the host galaxy are consistent with those of long-duration gamma-ray bursts (LGRB) and hydrogen-poor superluminous supernovae (SLSNe-I). Both LGRBs and SLSNe-I were previously hypothesized to be powered by the electromagnetic spin-down of newly formed, strongly magnetized neutron stars with millisecond birth rotation periods (“millisecond magnetars”). This motivates considering a scenario whereby the repeated bursts from FRB 121102 originate from a young magnetar remnant embedded within a young hydrogen-poor supernova (SN) remnant. Requirements on the gigahertz free–free optical depth through the expanding SN ejecta (accounting for photoionization by the rotationally powered magnetar nebula), energetic constraints on the bursts, and constraints on the size of the quiescent source all point to an age of less than a few decades. The quiescent radio source can be attributed to synchrotron emission from the shock interaction between the fast outer layer of the supernova ejecta with the surrounding wind of the progenitor star, or the radio source can from deeper within the magnetar wind nebula as outlined in Metzger et al. Alternatively, the radio emission could be an orphan afterglow from an initially off-axis LGRB jet, though this might require the source to be too young. The young age of the source can be tested by searching for a time derivative of the dispersion measure and the predicted fading of the quiescent radio source. We propose future tests of the SLSNe-I/LGRB/FRB connection, such as searches for FRBs from nearby SLSNe-I/LGRBs on timescales of decades after their explosions.

Raman Microspectroscopic Mapping with Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS) Applied to the High-Pressure Polymorph of Titanium Dioxide, TiO2-II

The high-pressure, α-PbO2-structured polymorph of titanium dioxide (TiO2-II) was recently identified in micrometer-sized grains recovered from four Neoarchean spherule layers deposited between ∼2.65 and ∼2.54 billion years ago. Several lines of evidence support the interpretation that these layers represent distal impact ejecta layers. The presence of shock-induced TiO2-II provides physical evidence to further support an impact origin for these spherule layers. Detailed characterization of the distribution of TiO2-II in these grains may be useful for correlating the layers, estimating the paleodistances of the layers from their source craters, and providing insight into the formation of the TiO2-II. Here we report the investigation of TiO2-II-bearing grains from these four spherule layers using multivariate curve resolution-alternating least squares (MCR-ALS) applied to Raman microspectroscopic mapping. Raman spectra provide evidence of grains consisting primarily of rutile (TiO2) and TiO2-II, as shown by Raman bands at 174 cm-1 (TiO2-II), 426 cm-1 (TiO2-II), 443 cm-1 (rutile), and 610 cm-1 (rutile). Principal component analysis (PCA) yielded a predominantly three-phase system comprised of rutile, TiO2-II, and substrate-adhesive epoxy. Scanning electron microscopy (SEM) suggests heterogeneous grains containing polydispersed micrometer- and submicrometer-sized particles. Multivariate curve resolution-alternating least squares applied to the Raman microspectroscopic mapping yielded up to five distinct chemical components: three phases of TiO2 (rutile, TiO2-II, and anatase), quartz (SiO2), and substrate-adhesive epoxy. Spectral profiles and spatially resolved chemical maps of the pure chemical components were generated using MCR-ALS applied to the Raman microspectroscopic maps. The spatial resolution of the Raman microspectroscopic maps was enhanced in comparable, cost-effective analysis times by limiting spectral resolution and optimizing spectral acquisition parameters. Using the resolved spectra of TiO2-II generated from MCR-ALS analysis, a Raman spectrum for pure TiO2-II was estimated to further facilitate its identification.

The Vakkejokk Breccia: An Early Cambrian proximal impact ejecta layer in the North‐Swedish Caledonides

AbstractThe ≤27 m thick Vakkejokk Breccia is intercalated in autochthon Lower Cambrian along the Caledonian front north of Lake Torneträsk, Lapland, Sweden. The spectacular breccia is here interpreted as a proximal ejecta layer associated with an impact crater, probably ~2–3 km in size, located below Caledonian overthrusts immediately north of the main breccia section. The impact would have taken place in a shallow‐marine environment ~520 Ma ago. The breccia comprises i) a strongly disturbed lower polymict subunit with occasional, in themselves brecciated, crystalline mega‐clasts locally exceeding 50 m surrounded by contorted sediments; ii) a middle, commonly normally graded, crystalline‐rich, polymict subunit, in turn locally overlain by iii) a thin fine‐grained quartz sandstone, <30 cm thick. The upper sandstone is sporadically either overlain, or replaced, by a conglomerate. In progressively more distal parts of the ejecta layer, the lower subunit is better described as only slightly disturbed strata. The lower subunit is suggested to have formed by ejecta bombardment of the strata surrounding the impact crater, even causing some net outwards mobilization of the sediments. The middle subunit and the uppermost quartz sandstone are considered resurge deposits. The top conglomerate may be caused by subsequent wave reworking and slumping of material from the elevated rim. Quartz grains showing planar deformation features are present in the graded polymict subunit and the upper sandstone, that is, the inferred resurge deposits.

Supernova ejecta with a relativistic wind from a central compact object: a unified picture for extraordinary supernovae

The hydrodynamical interaction between freely expanding supernova ejecta and a relativistic wind injected from the central region is studied in analytic and numerical ways. As a result of the collision between the ejecta and the wind, a geometrically thin shell surrounding a hot bubble forms and expands in the ejecta. We use a self-similar solution to describe the early dynamical evolution of the shell and carry out a two-dimensional special relativistic hydrodynamic simulation to follow further evolution. The Rayleigh–Taylor instability inevitably develops at the contact surface separating the shocked wind and ejecta, leading to the complete destruction of the shell and the leakage of hot gas from the hot bubble. The leaking hot materials immediately catch up with the outermost layer of the supernova ejecta and thus different layers of the ejecta are mixed. We present the spatial profiles of hydrodynamical variables and the kinetic energy distributions of the ejecta. We stop the energy injection when a total energy of 1052 erg, which is 10 times larger than the initial kinetic energy of the supernova ejecta, is deposited into the ejecta and follow the subsequent evolution. From the results of our simulations, we consider expected emission from supernova ejecta powered by the energy injection at the centre and discuss the possibility that superluminous supernovae and broad-lined Ic supernovae could be produced by similar mechanisms.