Fallback Ejecta Research Articles

Physical properties of the Didymos system before and after the DART impact

Context. On September 26, 2022, the Double Asteroid Redirection Test (DART) successfully changed the trajectory of the asteroid Dimorphos (i.e. 65803 Didymos I), a satellite circling (65803) Didymos. Aims. We aim to characterize the consequence of this collision and derive the physical properties of the ejecta features based on ground-based observations in East Asia. Methods. Filtered photometric observations were made between September 21 2022 (~5 days before DART impact) and January 5 2023 using the Lulin 1-m telescope to identify the taxonomy, size, and rotational period of Didymos. The Finson-Probstein dust dynamical models were used to determine the grain sizes (mm–cm) released after the DART impact and the date of the activity. Results. We report a rapid increase in the brightness by about one order of magnitude after the impact, to be followed by a gradual 0.07 mag decrease over the first two weeks producing a relatively shallow brightness slope at the end of October. The size and rotation period at post-impact were 0.7−0.10+0.12 km and 2.27 h, respectively. The Principal Component Index (PCI), relative reflectance, and colors were all classified as S-complex. The Dydimos system became bluer after the collision before returning to its original color. The formation of a comet-like trail containing debris in the anti-sunward direction can be explained by expansion driven by the pressure of solar radiation. A Finson-Probstein modeling approach led to an estimate of the grain size in the mm-cm range. The splitting of the tail into two components is shown in the image acquired on October 12, which may possibly be interpreted as being due to the secondary impact of fallback ejecta about a week after DART.

Electromagnetic signatures of white dwarf collisions in AGN discs

ABSTRACT In the inner region of the disc of an active galactic nucleus (AGN), the collision of two white dwarfs (WDs) through Jacobi capture might be inevitable, leading to a Type Ia supernova (SN Ia) explosion. This transient event, influenced by the disc gas and the gravity of the supermassive black hole (SMBH), exhibits distinct characteristics compared with normal SNe Ia. The energy of the explosion is mainly stored in the ejecta in the form of kinetic energy. Typically, the ejecta is not decelerated effectively by the AGN disc and rushes rapidly out of the AGN disc. However, under the influence of the SMBH, most of the ejecta falls back toward the AGN disc. As the fallback ejecta becomes more dispersed, it interacts with the disc gas, converting its kinetic energy into thermal energy. This results in a high-energy transient characterized by a rapid initial rise followed by a decay with L∝t−2.8. The time-scale of the transient ranges from hours to weeks, depending on the mass of the SMBH. This process generates high-energy radiation spanning from hard X-rays to the soft γ range. Additionally, the subsequent damage to the disc may result in changing-look AGNs. Moreover, the falling back of SNe Ia ejecta on to the AGN disc significantly increases the metallicity of the AGN and can even generate heavy elements within AGN discs.

Depositional processes of impactites from the YAX‐1 drill core in the Chicxulub impact structure inferred from vertical profiles of PDF orientations and grain size distributions of shocked quartz

AbstractCore samples from the Chicxulub impact structure provide insights into the formation processes of a shallow‐marine‐target, complex crater. Although previous studies investigated the impactites (generally suevitic and polymict breccias) of the Yaxcopoil‐1 (YAX‐1) drill core in the Chicxulub impact structure, the interpretation of its deposition remains controversial. Here, we analyze planar deformation features (PDFs), grain size, and abundance of shocked quartz throughout the YAX‐1 impactite sequence (794–895 m in depth). PDF orientations of most quartz grains in YAX‐1 impactites show a distribution of both low angles ({104}, {103}, {102}) and high angles (orientations higher than 55° to c‐axis), while the lower part of the impactite sequence contains quartz showing only PDF orientations of low angles. High‐abundance, coarse‐grained shocked quartz is found from the lower to middle parts of the impactites, whereas it abruptly changes to low‐abundance, fine‐grained shocked quartz within the upper part. In the uppermost part of the impactites, repeated oscillations in contents of these two components are observed. PDF orientation pattern suggests most of the shocked quartz grains experienced a range of shock pressure, except two samples in the lower part of impactites, which experienced only a high level of shock. We suggest that the base and lower part of the impactite sequence were formed by ejecta curtain and melt surge deposits, respectively. Our results are also consistent with the interpretation that the middle part of the impactite sequence is fallback ejecta from the impact plume. Additionally, we support the contention that massive seawater resurges into the crater occurred during the deposition of the upper and uppermost part of the impactites.

Bimodal Long-lasting Components in Short Gamma-Ray Bursts: Promising Electromagnetic Counterparts to Neutron Star Binary Mergers

Abstract Long-lasting emission of short gamma-ray bursts (GRBs) is crucial to reveal the physical origin of the central engine as well as to detect electromagnetic (EM) counterparts to gravitational waves (GWs) from neutron star binary mergers. We investigate 65 X-ray light curves of short GRBs, which is six times more than previous studies, by combining both Swift/BAT and XRT data. The light curves are found to consist of two distinct components at >5σ with bimodal distributions of luminosity and duration, i.e., extended (with a timescale of ≲103 s) and plateau emission (with a timescale of ≳103 s), which are likely the central engine activities, but not afterglows. The extended emission has an isotropic energy comparable to the prompt emission, while the plateau emission has ∼0.01–1 times this energy. Half (50%) of our sample has both components, while the other half is consistent with having both components. This leads us to conjecture that almost all short GRBs have both the extended and plateau emission. The long-lasting emission can be explained by the jets from black holes with fallback ejecta, and could power macronovae (or kilonovae) like GRB 130603B and GRB 160821B. Based on the observed properties, we quantify the detectability of EM counterparts to GWs, including the plateau emission scattered to the off-axis angle, with CALET/HXM, INTEGRAL/SPI-ACS, Fermi/GBM, MAXI/GSC, Swift/BAT, XRT, the future ISS-Lobster/WFI, Einstein Probe/WXT, and eROSITA.

Zhamanshin astrobleme provides evidence for carbonaceous chondrite and post-impact exchange between ejecta and Earth\u2019s atmosphere

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ17O values as low as −0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed 17O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ17O = −0.47‰) following material ejection. In contrast, combined Δ17O–ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.

Geochemistry of impact glasses and target rocks from the Zhamanshin impact structure, Kazakhstan: Implications for mixing of target and impactor matter

Internal structure and element chemistry including contents of highly siderophile elements (HSE) and Os isotope ratios have been studied in target rocks and several groups of impact glasses of the Zhamanshin impact structure, Kazakhstan. These include larger irregularly-shaped fragments and blocks of impact glass (zhamanshinites), and three types of tektite-like splash-form glasses, part of fallback ejecta. These glassy objects typically are up to 30mm large and are shaped as teardrops, irregularly bent and curved glass rods and fibers. They can be subdivided into acidic types (irghizites; typically 69–76wt.% SiO2), basic splash-forms (typically 53–56wt.% SiO2), and rarely occurring highly inhomogeneous composites with abundant mineral inclusions. A comparison with the target rocks shows that zhamanshinites and basic splash-forms usually have no detectable admixture of the projectile matter, indicated by major and trace elements as well as highly siderophile element contents, with the exception of one sample containing Fe-, Cr-, Ni- and Ti-enriched particles and elevated HSE contents. In contrast, irghizites exhibit clear admixture of the projectile matter, which was incorporated by complex processes accompanied by strong element fractionations. Microscopic investigations confirm that irghizites were formed mainly by coalescence of smaller molten glass droplets sized typically below 1mm. Irghizites exhibit significant enrichments in Ni, Co and Cr, whose concentrations are locally elevated in the rims of the original small droplets. A portion of these elements and also part of Fe and Mn and other elements were derived from the impactor, most likely a Ni-rich carbonaceous chondrite. The contents of HSE are low and strongly fractionated, with moderate depletions of Pt and Pd and strong depletions of other HSE with respect to chondritic element ratios. Osmium shows the strongest depletion, likely related to the presence of oxygen in the post-impact atmosphere causing strong Os loss through volatilization. One composite splash-form contains Fe–Ni–S inclusions and exhibits a less fractionated HSE pattern suggesting the lowest degree of melting, volatilization and condensation. The observed structural and microchemical features of irghizites are interpreted to reflect variable proportions of the uppermost target sediments and the projectile matter, with HSE element ratios influenced by evaporation and condensation processes, and differences in volatility of individual HSE elements and/or their compounds. Two possible pathways of incorporation of the projectile matter into the irghizites include either re-condensation of evaporated projectile matter on the surface of glass droplets, or incorporation of less chemically fractionated microparticles dispersed by the explosion.

Caloris impact basin: Exterior geomorphology, stratigraphy, morphometry, radial sculpture, and smooth plains deposits

The Mariner 10 flybys revealed the eastern third of the Caloris basin and demonstrated its importance as a chronostratigraphic marker for the geological history of Mercury. Uncertain after that mission were the size of the basin, the full distribution of materials deposited or modified during the impact process, and the stratigraphic and age relationships between Caloris and surrounding terrain. Images obtained during MESSENGER's January 2008 flyby of Mercury revealed that the Caloris basin is ∼ 15% larger than previously estimated and moderately elliptical (∼ 1525 by ∼ 1315 km). Basin-related sculpture and secondary craters are dispersed widely in areas surrounding the basin, confirming the widespread significance of this event as a stratigraphic marker. Units mapped around Caloris on the basis of Mariner 10 data are generally recognized in regions observed for the first time by MESSENGER, including most facies of the Caloris Group (the Caloris Montes, Odin Formation, and Van Eyck Formation). The only unit without obvious exposures in MESSENGER images is the Nervo Formation, which was originally interpreted as impact melt or fallback ejecta. New measurements of the size-frequency distribution of impact craters both within the Caloris basin and on surrounding units, particularly the Odin Formation, support the hypothesis that the vast majority of circum-Caloris plains postdate the basin and are likely volcanic. The MESSENGER observations re-emphasize the importance of understanding the exterior stratigraphy of the basin in order to utilize the Caloris event as a discrete time horizon in the geological history of Mercury.

Argillic alteration in the Novarupta Vent Region, Katmai National Park, Alaska

Secondary mineralogy in the 1912 vent region at Novarupta illustrates an alteration history that evolved from high‐temperature degassing of ejecta through surficial argillic alteration to near‐neutral, wet‐steam alteration as the posteruptive volcanic system cooled. The locallized distribution of surficial argillization appears to have been controlled mainly by fractures related to slumping and compaction in the near‐surface fallback ejecta. The fractures guiding the surviving warm, near‐neutral fumarolic fluids, however, must be connected with deeper vent structures that reach a subsurface heat source, perhaps the boiling zone of a hydrothermal system. Trace metals, including Mo, Pb, Cu, Cd, and Zn, concentrated by high‐temperature, vapor‐phase mobilization, have subsequently been partly removed by argillization processes; in contrast, Hg, Sb, Tl, Bi, and As remain high even though the fumaroles now emit only low‐temperature, near‐neutral wet steam.

Fossil and active fumaroles in the 1912 eruptive deposits, Valley of ten thousand smokes, Alaska

Fumaroles in the ash-flow sheet emplaced during the 1912 eruption of Novarupta were intensely active throughout the Valley of Ten Thousand Smokes (VTTS) when first studied in 1917. Fumarole temperatures recorded in 1919 were as hot as 645°C. Influx of surface waters into the hot ash-flow sheet provided the fluid flow to sustain the fumaroles but also enhanced cooling so that by the mid-1930's vigorous activity survived only in the vent region. Configuration and distribution of high-temperature fissure fumaroles tens of meters long, that are prevalent in the middle and upper VTTS, were controlled largely by sintering and degree of welding, which in turn controlled fracturing and permeability of the ash-flow tuff. One fracture type developed parallel to the enclosing valley walls during compaction of the ash-flow sheet. Another type extends across the VTTS nearly perpendicular to the flow direction. A third type of randomly oriented fractures developed as cooling contraction cracks during vapor-phase devitrification. In distal parts of the ash-flow sheet where the tuff is nonwelded, prominent fumaroles have irregular funnel-shaped morphologies. Fumarole distribution in the nonwelded part of the ash-flow sheet is concentrated above pre-emplacement river channels. The hottest, longest-lived fumaroles occurred in the upper VTTS near the 1912 vent where the ash-flow sheet is thicker, more indurated, and on average more mafic (richer in dacite and andesite) in contrast to the thinner, nonwelded rhyolitic tuff in the distal part of the sheet. Fumarolic activity was less intense in the distal part of the tuff because of lower emplacement temperatures, more diffuse fumarole conduits in the nonwelded tuff, and the thinness of the ash-flow sheet. Chemical leaching of ash-flow tuff by hot rising fluids took place adjacent to fumarolic conduits in deep parts of the fumaroles. Deposition of incrustation minerals, the components of which were carried upward by fumarolic gases, took place in the upper part of the ejecta, mostly in the fallout layers. The permeability difference between the ash-flow tuff and the overlying coarse dacite fallout was a critical factor in promoting the abrupt gradients in temperature, pressure, and fO 2 that resulted in deposition of minerals from the fumarolic gases. The permeability difference between nonwelded ash-flow tuff and overlying fine-grained fall layers in the lower VTTS is less pronounced. The total mass of fumarolically deposited minerals appears large at first glance owing to the conspicuous coloration by Fe minerals; the mass is appreciably less than is apparent, however, because most incrustations are composed largely of ejecta coated or cemented by fine-grained fumarolic minerals. A large mass of unstable incrustation minerals, mainly chlorides and sulfates, reported during the 1917–1919 studies have since been removed by dissolution and weathering. In the vent region, argillic alteration that followed high-temperature degassing is localized along arcuate subsidence fractures in fallback ejecta. At widely scattered residual orifices, fumarolic gases presently are near-neutral steam, and temperatures are as hot as 90°C.

The role of fallback ejecta in the modification of impact craters

Fragments of target material ejected from an impact crater's transient cavity may be deposited within the cavity as fallback due to in-flight particle collisions and atmospheric deceleration. The characteristics of aerodynamically decelerated fallback ejecta are inferred by determining the size of the smallest fragment that is aerodynamically capable of traveling beyond the transient cavity rim at successive stages of cavity excavation. This technique has been applied to Meteror Crater, Arizona (MCA), a 1.2-km diameter crater that formed within a group of flat--lying, sedimentary rock units. Model results predict that: (i) ∼5% of the primary ejecta excavated from the crater was deposited as fallback (volume percent), (ii) average fallback fragment size is 1.4 cm, (iii) fallback ejecta is derived from a region encompassing all sedimentary formations intersected by the crater, and (iv) portions of the fallback source region were subjected to large shock stresses, on the order of several hundred kilobars. These results are in excellent agreement with empirical studies of the MCA fallback deposit. This agreement is interpreted to be an indication that atmospheric deceleration is the principal mechanism responsible for fallback deposition on the Earth. Model results for larger terrestrial craters indicate that as excavation cavity diameter ( D e) increases from 1 to 100 km, the proportion of primary ejecta deposited as fallback increases from 5 to 11% (average fragment size increases from 1.5 cm to 1.3 m over the same cavity diameter range). Fallback deposition is inferred to be a major cavity modification process during Venusian cratering events. Model estimates suggest that more than half of the primary ejecta excavated from large scale Venusian cavities ( D e > 10 km) should be deposited as fallback. In contrast, relatively small quantities of fallback are anticipated within lunar and Martian craters formed in coherent target media.

The role of rim slumping in the modification of lunar impact craters

Wall failure has significantly altered the structure of virtually all large, fresh‐appearing lunar craters. Terrace blocks exposed upon a crater's interior walls are interpreted to be sections of the transient cavity rim that slumped into the cavity during the terminal stages of crater formation. Impact excavation cavities have been reconstructed by restoring the innermost terrace block exposed within a crater to its inferred original position at the cavity rim and accounting for the volume of material that slumped into the cavity. Critical model assumptions include (1) the radial variation of topography near the initial cavity rim crest, (2) the structure of the failure surface along which terrace blocks slumped into the cavity, and (3) the geometric shape of the initial cavity. This terrace restoration model has been applied to 12 fresh lunar craters with observed rim crest diameters D0 ranging from 19 to 137 km. Up to an initial cavity diameter Di of 30 km, reconstructed cavity depths are comparable to or greater than cavity depths extrapolated from nonterraced craters of less than 15 km in diameter. For a wide range of model parameters the reconstructed depths of impact cavities 15–30 km in diameter are significantly greater than depths predicted by small‐crater morphometry, implying that cavity depths inferred from depth/diameter ratios observed for small craters may substantially underestimate the depth of excavation of impact cratering events in this size class. Reconstructed cavity depths for Di > 70 km, however, are consistently less than cavity depths extrapolated from smaller craters. This indicates that the morphometric transition from small, relatively unmodified, bowl‐shaped craters (D0 < 15 km) to large, terraced, saucer‐shaped craters (D0 > 80–90 km) cannot be solely attributed to rim‐slumping modification. Ejecta fallback and basement rebound also play a role in modifying impact crater cavities; however, the manner in which the volume of fallback ejecta and basement rebound material varies with increasing crater size is unknown. The discrepancy between cavity depths extrapolated from small craters and those obtained from the terrace restoration model suggests that impact excavation cavities become relatively shallower at larger diameters. However, this cannot be conclusively demonstrated until the effects of rebound and ejecta fallback are quantitatively accounted for.

Phobos: Photometry and origin of dark markings on crater floors

Viking Orbiter 1 close encounter pictures of Phobos reveal unusual dark patches on the floors of many craters. Photometry of these features indicates that they have a similar normal reflectance but a phase coefficient 30% larger than the average surface. These facts suggest that the "dark" material has a similar composition but a much rougher texture than the average surface of Phobos. By analogy with terrestrial impact, explosion and laboratory craters, the dark patches are interpreted as impact generated melt pools which remain visible on Phobos due to the low value of g and consequent small amount of fallback ejecta and slumping. Since blanketing by ejecta from subsequent impacts and micrometeoroid erosion should eventually obliterate such features, they should be more conspicuous in the fresher craters. Similar features should be visible on small asteroids and other low g bodies. However, they are not prominent on the surface of Deimos—possibly because most of the craters imaged at high resolution on the outer satellite appear to have been blanketed by several meters of fine‐grained material.

Volume, energy and cyclicity of eruptions of Arenal volcano, Costa Rica

The 1968–73 (and continuing) eruption of Arenal Volcano, Costa Rica, a small 1633 m strato-volcano with long periods of repose, defines an eruptive cycle which is typical of Arenal’s pre-historic eruptions. An intense, short explosive phase (July 29–31, 1968) grades into an effusive phase, and is followed by a block lava flow. The eruptive rocks become increasingly less differentiated with time in a given cycle, ranging from andesite to basaltic andesite. Nuees ardentes are a characteristic of the initial explosions, and are caused by fall-back ejecta on slopes around the main crater — an explosion crater in the 1968 eruption — which coalesce into hot avalanches and descend major drainage channels. Total volume of pyroclastic flows was small, about 1.8 ± 0.5 × 10n m3, in the July 29–31 explosions, and are block and ash flows, with much accidental material. Overpressures, ranging up to perhaps 5 kilobars just prior to major explosions, were estimated from velocities of large ejected blocks, which had velocities of up to 600 m/sec. Total kinetic energy and volume of ejecta of all explosions are an estimated 3 × 1022 ergs and 0.03 km3, respectively. The block lava flow, emitted from Sept., 1968 to 1973 (and continuing) has a volume greater than 0.06 km3, and covers 2.7 km2 at thicknesses ranging from 15 to over 100 m. The total volumes of the explosive and effusive phases for the 1968–73 eruption are about 0.05 km3 and 0.06 km3, respectively. The last eruption of Arenal occurred about 1500 AD. based on radiocarbon dating and archaeological means, and was about twice as voluminous as the current one (0.17 km3 versus 0.09 km3). The total thermal energies for this pre-historic eruption and the current one are 8 × 1023 and 18 × 1023, respectively. The total volume of Arenal’s cone is about 6 km3 from 1633 m (summit) to 500 m, and, estimates of age based on the average rate of cone growth from these two eruptions, suggest an age between 20,000 to 200,000 years.