Comet Showers Research Articles

Properties of outer solar system pebbles during planetesimal formation from meteor observations

Observations of proto-planetary disks, as well as theoretical modeling, suggest that in the late stages of accretion leading up to the formation of planetesimals, particles grew to pebbles the size of 1-mm to tens of cm, depending on the location and ambient conditions in the disk. That is the same size range that dominates the present-day comet and primitive asteroid mass loss. Meteoroids that size cause visible meteors on Earth. Here, we hypothesize that the size distribution and the physical and chemical properties of young meteoroid streams still contain information about the conditions in the solar nebula during these late stages of accretion towards planetesimal formation. If so, they constrain where long-period (Oort Cloud) comets, Jupiter-family (Scattered Disk Kuiper Belt) comets, and primitive asteroids (Asteroid Belt) formed. From video and visual observations of 47 young meteor showers, we find that freshly ejected meteoroids from long-period comets tend to have low bulk density and are distributed with equal surface area per log-mass interval (magnitude distribution index χ ∼ 1.85), suggesting gentle accretion conditions. Jupiter-family comets, on the other hand, mostly produce meteoroids twice as dense and distributed with a steeper χ ∼ 2.15 or even χ ∼ 2.5, which implies that those pebbles grew from particles fragmenting in a collisional cascade or by catastrophic collisions, respectively. Some primitive asteroids show χ > 2.5, with most mass in small particles, indicating an even more aggressive fragmentation by processes other than mutual collisions. Both comet populations contain an admixture of compact materials that are sometimes sodium-poor, but Jupiter-family comets show a higher percentage (∼8% on average) than long-period comet showers (∼4%) and a wider range of percentages among comets. While there are exceptions in both groups, the implication is that most long-period comets formed under gentle particle growth conditions, possibly near the 30 AU edge of the Trans Neptunian Disk, while most Jupiter family comets formed closer to the Sun where pebbles reached or passed the fragmentation barrier, and primitive asteroids formed in the region where the cores of the giant planets formed. This is possible if the Scattered Disk represents all objects scattered by Neptune during its migration, while the present-day outer Oort cloud formed only during and after the time of the planet instability, well after the Sun had moved away from sibling stars.

Fine-grained interplanetary dust input during the Turonian (Late Cretaceous): evidence from osmium isotope and platinum group elements

The Turonian age (~ 90–94 Ma) was the hottest geological interval in the Cretaceous and also marked by the K3 event, a pronounced enrichment of 3He in pelagic sediments (i.e., massive input of extraterrestrial materials). Here, we present Os isotopic (187Os/188Os) and platinum group element (PGE) data from Turonian sedimentary records. After a sharp unradiogenic shift during the end-Cenomanian oceanic anoxic event 2, the 187Os/188Os ratios declined continuously throughout the Turonian, which could be ascribed to the formations of several large igneous provinces (LIPs). Because the interval with the most unradiogenic 187Os/188Os ratios (i.e., enhanced LIP volcanism) does not correspond to the warmest interval during the mid-Cretaceous, additional sources of CO2, such as subduction zone volcanism or the kimberlite formation, may explain the Cretaceous Thermal Maximum. As Os isotope ratios do not show any sharp unradiogenic shifts and PGE concentrations do not exhibit a pronounced enrichment, an influx of fine-grained cosmic dust to the Earth’s surface, possibly from the long-period comet showers, can be inferred at the time of the 3He enrichment during the mid-Turonian K3 event. Our findings highlight the different behaviors of 3He and PGE information in the sedimentary rocks during the input of fined-grained extraterrestrial materials.

Glaciation on Mercury: Accumulation and flow of ice in permanently shadowed circum-polar crater interiors

Radar-bright deposits that coincide with regions in permanent shadow typically found within impact craters at and near the poles of Mercury have been interpreted as being composed of water ice. We investigate the dynamic properties of these solid water-ice deposits (glaciers) with the goal of constraining their movement, flow rates, possible deformation, and related structures and deposits. As an end-member, we treat the extreme case of maximum ice accumulation where these deposits fill the crater to the shadow line, an event we would only expect following maximum accumulation conditions such as a large cometary impact or a comet shower. We find that, given the extremely cold conditions and the limited thickness of glaciers, even under the most favorable accumulation conditions, glaciation is cold-based, ice flow velocities are very low (4 × 10−8 m/yr) and, with the exception of a sublimation lag deposit, glaciation is unlikely to leave any significant impact on the terrain. An important accelerator of ice flow is found to be the contribution of lateral heat conduction from the surrounding extremely hot surface terrain (∼225 K average with up to 450 K maximums), which for 55 km craters yields velocities of 10−3 m/yr while for smaller 10 km craters velocities may exceed 1 m/yr, enough to leave a detectable ice deformation imprint such as drop moraines. However given that the observed deposits are substantially below their maximum fill capacities these estimates are unlikely to be attained.

An Independent Confirmation of the Future Flyby of Gliese 710 to the Solar System Using Gaia DR2

Gliese 710 is a K7V star located 19 pc from the Sun in the constellation of Serpens Cauda, which is headed straight for the solar system. Berski & Dybczynski (2016) used data from Gaia DR1 to show that this star will be 13366 AU from the Sun in 1.35 Myr from now. Here, we present an independent confirmation of this remarkable result using Gaia DR2. Our approach is first validated using as test case that of the closest known stellar flyby, by the binary WISE J072003.20-084651.2 or Scholz's star. Our results confirm, within errors, those in Berski & Dybczynski (2016), but suggest a somewhat closer, both in terms of distance and time, flyby of Gliese 710 to the solar system. Such an interaction might not significantly affect the region inside 40 au as the gravitational coupling among the known planets against external perturbation can absorb efficiently such a perturbation, but it may trigger a major comet shower that will affect the inner solar system.

THE CLOSEST KNOWN FLYBY OF A STAR TO THE SOLAR SYSTEM

Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 ("Scholz's star") to calculate its past trajectory. Integrating the Galactic orbits of this $\sim$0.15 M$_{\odot}$ binary system and the Sun, we find that the binary passed within only 52$^{+23}_{-14}$ kAU (0.25$^{+0.11}_{-0.07}$ parsec) of the Sun 70$^{+15}_{-10}$ kya (1$\sigma$ uncertainties), i.e. within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently ($\sim$0.1 Myr$^{-1}$). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is $\sim$98%, but the probability of penetrating the dynamically active inner Oort Cloud ($<$20 kAU) is $\sim$10$^{-4}$. While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.

Planetary perturbations for Oort cloud comets: III. Evolution of the cloud and production of centaurs and Halley type comets

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschlé, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1000AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

Search for New Parent Bodies of Meteoroid Streams Among Comets. I. Showers of Comets 126P/1996 P1 and 161P/2004 V2 with Radiants on Southern Sky

We deal with theoretical meteoroid streams the parent bodies of which are two Halley-type comets in orbits situated at a relatively large distance from the orbit of Earth: 126P/1996 P1 and 161P/2004 V2. For two perihelion passages of each comet in the far past, we model the theoretical stream and follow its dynamical evolution until the present. We predict the characteristics of potential meteor showers according to the dynamical properties of theoretical particles currently approaching the orbit of the Earth. Our dynamical study reveals that the comet 161P/2004 V2 could have an associated Earth-observable meteor shower, although no significant number of theoretical particles are identified with real, photographic, video, or radar meteors. However, the mean radiant of the shower is predicted on the southern sky (its declination is about −23°) where a relatively low number of real meteors has been detected and, therefore, recorded in the databases used. The shower of 161P has a compact radiant area and a relatively large geocentric velocity of ∼53 km s−1. A significant fraction of particles assumed to be released from comet 126P also cross the Earth’s orbit and, eventually, could be observed as meteors. However, their radiant area is largely dispersed (declination of radiants spans from about +60° to the south pole) and, therefore, mixed with the sporadic meteor background. An identification with real meteors is practically impossible.

A record of the extraterrestrial 3He flux through the Late Cretaceous

Late Cretaceous (100–73Ma) pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3He as a tracer. In the Bottaccione section near Gubbio, Italy, three intervals of elevated 3He concentration were detected: K1 in the Campanian stage at ∼79Ma, K2 in the Santonian stage at ∼85Ma, and K3 in the Turonian stage at ∼91Ma. All three of these episodes are associated with high 3He/4He and 3He/non-carbonate ratios, consistent with their derivation from an enhanced extraterrestrial 3He flux rather than decreased carbonate sedimentation or dissolution. While K2 is modest in magnitude and duration and thus is of limited significance, K1 and K3 are each identified by a few myr interval with an ∼4-fold enhancement in mean 3He flux compared with pre-event levels. Samples from ODP Hole 762C in the Indian Ocean spanning both K2 and K3 (93–83Ma) confirm the presence of a peak in the Turonian stage, suggesting that K3 is a global event. The K1 and K3 3He events are similar in most respects to the two peaks previously detected in the Cenozoic, suggesting a similar origin. These have been attributed to a major asteroid collision in the Late Miocene and to a shower of either comets or asteroids in the Late Eocene. Based on the age and temporal evolution of K1, we suggest that it most likely records the collision which produced the Baptistina asteroid family independently dated at ∼80Ma. The K3 event is less easily explained. It is characterized by an unusually spiky and erratic temporal progression, suggesting an unusual abundance of very 3He rich particles not previously seen in the sedimentary 3He record. We suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust-producing lunar impacts.

Evaluating the accretion of meteoritic debris and interplanetary dust particles in the GPC-3 sediment core using noble gas and mineralogical tracers

Extraterrestrial (ET) noble gases (helium and neon) in 35 sediment samples from Central Pacific core LL-44 GPC-3 demonstrate the variable flux of interplanetary dust particles (IDPs) and major meteorite impacts over the past 70Ma (21–72Ma). Spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of IDPs, including the well-established Cretaceous/Tertiary (K/T) and late Eocene (E/O) impact boundaries. No spinel grains with chemical or mineralogical evidence of a distinctly ET origin were found in an extensive survey of ‘background’ samples (i.e. non E/O or K/T boundary) suggesting that either the carrier grains for ET noble gas occur within the Fe–Ti oxide mineral fraction observed in this study (found to include ilmenite and ulvospinel) or are too small for identification by SEM. The presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for ET noble gas-rich particles. Noble gas analysis on both the EMF (extractable magnetic fraction) and the Bulk minus EMF (Bulk−EMF) show that the He and Ne compositions are consistent with partially degassed noble gas signatures of zero-age magnetic grains (Z-MAG) and stratospheric interplanetary dust particles (IDPs). Conclusive evidence for a ‘planetary’ (Ne-A) noble gas signature is found only in the bulk sediments at the K/T boundary, although all GPC-3K/T fractions (Bulk, EMF, and HF Digestion) plot along a mixing line between planetary (Ne-A) and solar wind (SW). Spinels from major impact boundaries (K/T; E/O) exhibit dendritic texture and elevated [Ni], consistent with previous reports. In contrast to the otherwise consistent [3He] signal from IDPs, the [3He] at the known impact boundaries (K/T and E/O) actually decreases. These anomalously low [3He] are accompanied by significantly elevated [Ne] and significantly lower (3He/20Ne)solar ratios (∼10× lower) produced by both preferentially degassing of He relative to Ne at times of increased flux of larger ET material. Degassed (“degassed-He/enriched-Ne profile”) noble gas characteristics occur in two sample intervals that do not correspond to any known impact events (47 and 71Ma), explained by an influx of larger particles. SEM analysis of the 47Ma sample shows spinels with dendritic textures, but without distinctive markers of large meteorite impacts (e.g. elevated Ni). Particle size increases and degassed signatures may be caused by major bolides, micrometeorites, comet showers; or simply a flux of larger IDPs, potentially with a different source.

Some Studies of Terrestrial Impact Cratering Rate

Abstract In 1984, a 28.4 Myr periodicity was detected in the ages of terrestrial impact craters and a 26 Myr periodicity in the epochs of mass extinctions of species. Periodic comet showers from the Oort cloud seemed to cause catastrophic events linked to mass extinctions of species. Our first study revealed that the only significant detected periodicity is the “human signal” caused by the rounding of these data into integer numbers. The second study confirmed that the original 28.4 Myr periodicity detection was not significant. The third study revealed that the quality and the quantity of the currently available data would allow detection of real periodicity only if all impacts have been periodic, which cannot be the case. The detection of a periodic signal, if present, requires that more craters should be discovered and the accuracy of age estimates improved. If we sometimes will be able to find the difference between the craters caused by asteroid and comet impacts, the aperiodic component could be removed. The lunar impact craters may eventually provide the required supplementary data.

The key role of massive stars in Oort cloud comet dynamics

The effects of a sample of 1300 individual stellar encounters spanning a wide range of parameter values (mass, velocity and encounter distance) are investigated. Power law fits for the number of injected comets demonstrate the long range effect of massive stars, whereas light stars affect comets mainly along their tracks. Similarly, we show that the efficiency of a star to fill the phase space region of the Oort cloud where the Galactic tides are able to inject comets into the observable region – the so-called “tidally active zone” (TAZ) – is also strongly dependent on the stellar mass. Power laws similar to those for direct injection are obtained for the efficiency of stars to fill the TAZ. This filling of the tidally active zone is crucial for the long term flux of comets from the Oort cloud. Based on long-term Monte Carlo simulations using a constant Galactic tide and a constant flux of stellar encounters, but neglecting the detailed effects of planetary perturbations, we show that this flux essentially results from a two step mechanism: (i) the stellar injection of comets into the TAZ; and (ii) the tidal injection of TAZ comets into the loss cone. We find that single massive stars are able to induce “comet drizzles” – corresponding to an increase of the cometary flux of about 40% – which may last for more than 100 Myr by filling the TAZ to a higher degree than normal. It appears that the stars involved in this process are the same that cause comet showers.

Chromium-isotopes in Late Eocene impact spherules indicate a likely asteroid belt provenance

Spherules and magnetic fractions extracted from two late Eocene impact deposits — Massignano, Italy and Ocean Drilling Program Hole 709C, western Indian Ocean, have anomalous concentrations of Ir, and high concentrations of Cr, Ni, Co, and Fe. An excess of 53Cr, relative to 53Cr/ 52Cr ratios in terrestrial samples, shows that much of the Cr in these samples is extraterrestrial. The absence of relatively large 54Cr excesses, as found in carbonaceous chondrites, precludes a carbonaceous chondrite source and the data best fit ordinary chondrites. The two localities produced samples with somewhat different elemental and isotopic compositions, but these are likely different components of the same impact, rather than from two distinct impacts. The multiple impacts and dust deposition in the late Eocene were more likely caused by disruption of a large asteroid in the inner asteroid belt, than by a comet shower. The Brangäne asteroid family may be the result of this disruption, the source of the dust and impactors in the Late Eocene, and its members may be composed of H chondrite lithologies based upon the coeval ~ 35 Ma peak in H-chondrite cosmic ray exposure ages.

Nemesis reconsidered

Abstract The hypothesis of a companion object (Nemesis) orbiting the Sun was motivated by the claim of a terrestrial extinction periodicity, thought to be mediated by comet showers. The orbit of a distant companion to the Sun is expected to be perturbed by the Galactic tidal field and encounters with passing stars, which will induce variation in the period. We examine the evidence for the previously proposed periodicity, using two modern, greatly improved paleontological data sets of fossil biodiversity. We find that there is a narrow peak at 27 Myr in the cross-spectrum of extinction intensity time series between these independent data sets. This periodicity extends over a time period nearly twice that for which it was originally noted. An excess of extinction events is associated with this periodicity at 99 per cent confidence. In this sense we confirm the originally noted feature in the time series for extinction. However, we find that it displays extremely regular timing for about 0.5 Gyr. The regularity of the timing compared with earlier calculations of orbital perturbation would seem to exclude the Nemesis hypothesis as a causal factor.

Phenomenological constraints on Lemaître-Tolman-Bondi cosmological inhomogeneities from solar system dynamics

We, first, analytically work out the long-term, i.e. averaged over one orbital revolution, perturbations on the orbit of a test particle moving in a local Fermi frame induced therein by the cosmological tidal effects of the inhomogeneous Lemaître-Tolman-Bondi (LTB) model. The LTB solution has recently attracted attention, among other things, as a possible explanation of the observed cosmic acceleration without resorting to dark energy.Then, we phenomenologically constrain both the parameters and of the LTB metric in the Fermi frame by using different kinds of solar system data. The corrections Δϖ̇ to the standard Newtonian/Einsteinian precessions of the perihelia of the inner planets recently estimated with the EPM ephemerides, compared to our predictions for them, yield preliminarily K1 = (4±8) × 10−26 s−2, K2 = (3±7) × 10−23 s−2. The residuals of the Cassini-based Earth-Saturn range, compared with the numerically integrated LTB range signature, allow to preliminarily obtain K1 ≈ K2 ≈ 10−27 s−2. Actually, the LTB effects should be explicitly modeled in the ephemerides softwares, so that the entire planetary and spacecraft data sets should be accordingly re-processed. The LTB-induced distortions of the orbit of a typical object of the Oort cloud with respect to the commonly accepted Newtonian picture, based on the observations of the comet showers from that remote region of the solar system, point towards K1 ≈ K2≲10−30−10−32 s−2. Such figures have to be compared with those inferred from cosmological data which are of the order of K1 ≈ K2 = −4 × 10−36 s−2.

Late evolution of planetary systems

This chapter discusses some of the main effects of the interaction of planets with remnant planetesimal disks, after the disappearance of the gas. It focuses on planet migration and its possible outcomes. In particular, we discuss the possibility that the migration of the planets leads them into an unstable configuration which changes drastically the structure of the system. The late heavy bombardment (LHB) of the terrestrial planets, occurring 650 Myr after planet formation, is a strong indication that this kind of evolution occurred in our solar system. Other stars show evidence of intense comet showers, which may indicate that LHB-analogs are ongoing in those systems at the current time.

Void for Vagueness

When law regulates a profession, where does it get its standards? Largely from the profession. Members of professions acquire esoteric and abstract knowledge through formal education and the experience of practice. They use professional judgment in applying this knowledge to each case. Because legislatures and courts lack this expertise, they adopt the standards of the experts. Thus in a malpractice suit, juries are instructed to determine whether the doctor met medicine's standard of care. Furthermore, physicians must be called as expert witnesses to guide juries in that work. Even when lawmakers contemplated intensifying their regulation of medicine by creating the duty of consent, they could consult a literature to which doctors and medical ethicists contributed crucially. In some jurisdictions, even the scope of the duty is determined by using the medical standard of disclosure (although in other jurisdictions the standard is the degree of disclosure sufficient to permit the ordinary patient to make a sound decision). Nor have lawmakers striven to extend the reach of consent beyond the norms of medicine. They might have done so in two ways. First, they might have broadened the legal standard of disclosure. This seems to have happened only sporadically and tentatively. Second, fact-finders (juries and trial-court judges) might have interpreted the legal standard as demanding elaborate or unusual disclosures. This too has apparently not much happened. Indeed, plaintiffs rarely bring consent actions (except as appendages to malpractice suits), rarely win them, and rarely obtain large verdicts. In short, lawmakers have essentially established rules intended to hold medicine to its own standards and then mostly left the system to work unmolested. What lawmakers have not noticed, however, is that the status of consent in the medical literature has become parlous. Two developments particularly matter. First, a torrent of empirical evidence now suggests that consent does not work as intended: Doctors generally tell patients too little and patients generally understand too little for patients to make the choices that lawmakers had imagined. Second (and relatedly?), the literature seems to be deserting the term informed consent. And what instead? A comet shower of novel terms. [T]here is now a profusion of competing models that attempt to convey subtle differences in the sharing of information and power between clinician and patient. (1) A smattering of the latest models: evidence-based patient choice, informed decision-making, informed medical decision-making, informed treatment decision-making, physician as perfect agent, decision-making, clinical decision-making, medical decision-making, and treatment decision-making. From this welter of multiplying, mystifying distinctions, one term has emerged most stoutly--shared decision-making. So, what is shared decision-making? Would that I knew. Or that anyone did. Makoul and Clayman heroically slogged through the literature and concluded that there is no shared definition of shared decision making. They identified 31 separate concepts used to explicate SDM, only two of which appeared in more than half of the conceptual definitions. In fact, 60% of articles that purport to focus on shared decision-making failed to include any conceptual definition at all. (2) As this suggests, many proponents of shared decision-making seem to regard its meaning as self-evident. And no doubt most people suppose that they understand the term. Sharing does the real rhetorical work here. Who doesn't understand sharing? Who could oppose it? Yet the skimpiest reflection reveals that the slogan is ambiguous unto incoherence. For example, what makes a decision shared? It is logical, plausible, reasonable to say, as many advocates of shared decision-making seem to say, that anyone who helps shape a decision, helps give the decision meaning, helps give it effect, shares in it. …

A late Miocene dust shower from the break-up of an asteroid in the main belt

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter approximately 1-1,000 microm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales. Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of approximately 4 times pre-event levels, and dissipated over approximately 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past approximately 10(8) yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower or asteroid collision remains uncertain.

An ordinary chondrite impactor for the Popigai crater, Siberia

With a diameter of ∼100 km, Popigai in Northern Siberia is the largest crater known in the Cenozoic. The concentrations in platinum group elements (PGE) were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile. The method selected was preconcentration by NiS fire assay followed by inductively coupled plasma-mass spectrometry (ICP-MS). This technique measures all the PGE (except Os) and by using aliquots >10g, the results are highly reproducible. The major and trace element composition of the impact melt resembles that of gneissic lithologies of the Anabar shield, which are representative of the target rock. The PGE are enriched in the melt by factors of 3 to 14 compared to the main target lithology, but the meteoritic contamination is only around 0.2 wt.%. Using plots of elemental ratios such as Ru/Rh vs. Pt/Pd or Ru/Rh vs. Pd/Ir, the Popigai impactor is clearly identified as an ordinary chondrite and most likely l-chondrite. This study indicates that PGE elemental ratios allow discrimination of the type of impactor, even in the case of low meteoritic contamination. This study confirms that a significant fraction of the crater-forming projectiles presently documented could have an ordinary chondrite composition. Their probable source, the S-type asteroids, appears to form the majority of the bodies in the main asteroid belt and among Near Earth Objects (NEOs). The ordinary chondrite origin of the Popigai projectile supports an asteroidal origin for the late Eocene impacts as a plausible alternative to the comet shower scenario proposed by Farley et al. (1998).

A very public fireball

An appeal for witnesses to a fireball on 24 September produced an excellent response from the public; 55 eyewitnesses sent accounts. From their observations we calculated the radiant azimuth as 320degrees, and altitude,angle less than or equal to 20degrees. Without video or CCTV footage for control on the fireball's velocity or pre-entry orbit, we used software developed for dust impact experiments, to assess the most likely orbital trajectory. The highest probability solutions have a semimajor axis between 1.6 and 2.0 AU and an eccentricity of 0.4 to 0.5, corresponding to a typical near-Earth asteroid orbit. Of possible comet showers, the kappa Aquarids are within the calculated constraints. No fragments were found, despite considerable public interest, consistent with the absence of reports of a dust trail. Public response to this fireball demonstrates the great interest in meteoritic phenomena, particularly when, as in this case, participation in the scientific enquiry is actively encouraged.

Comet or asteroid shower in the late Eocene?

The passage of a comet shower approximately 35 million years ago is generally advocated to explain the coincidence during Earth's late Eocene of an unusually high flux of interplanetary dust particles and the formation of the two largest craters in the Cenozoic, Popigai and the Chesapeake Bay. However, new platinum-group element analyses indicate that Popigai was formed by the impact of an L-chondrite meteorite. Such an asteroidal projectile is difficult to reconcile with a cometary origin. Perhaps instead the higher delivery rate of extraterrestrial matter, dust, and large objects was caused by a major collision in the asteroid belt.