Observed Fall Research Articles

Cometary impacts into ocean: their recognition and the threshold constraint for biological extinctions

The Montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied. The impact crater is 45 km in diameter and was formed at 50.8 Ma by a fall of probably an old cometary. nucleus 3.4 km in diameter, into shallow (<600 m) ocean. Comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim. Instead, the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent, return water flow into the excavating cavity. By this process most of the fall-out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land. At a microscopic scale, the shock metamorphism features are about the same as those for land impacts. Geochemically, impacts of comet nucleii, may not leave a recognizable signature at the impact horizon except for a minor increase in iridium. Thus stratigraphic horizons associated with extinctions and/or major changes in biota have to be closely examined for other impact indicators, like the presence of tectites, glass spherules, and quartz grains with shock features. Occurrence of megatsunami wave deposits, extensive erosion on continental margins, margin failures and faunal mixing above erosional unconformities are other potential impact indicators. There is no single indicator that can provide sufficient proof of an impact event. Such interpretations have to be based on multiparameter studies of global extent, since many of the impact indicators are only of regional extent. The lack of extinction of any marine plankton genera, or of bottom dwellers at the Montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus >4 km in diameter. The calculated frequency for a cometary impact which could result in a 10% extinction of marine genera is about 6 × 10 −7 yr −1 and for the K/T boundary type of extinctions about 2 × 10 −8 yr −1. Even allowing for a large degree of uncertainty in these estimates, it is unlikely that the biological extinction events for the last 250 Ma identified by Sepkoski (1990) could have been all caused by meteorite impacts.

On the Bur Gheluai H5 chondrite and other meteorites with complex exposure histories

Abstract— We present the 14C, 26Al, 10Be, 3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, 38Ar, and 40Ar concentrations and the track densities measured in up to 13 samples of the Bur Gheluai (H5) meteorite fall. Only a multi‐stage exposure history can explain the data in a self‐consistent way. Parameters for a model two‐stage history obtained by simultaneous, least‐squares fitting of the concentrations of 14C, 26Al, 10Be, and 21Ne were: first stage duration ∼10 Ma and radius &gt;2 m; second stage duration ∼0.6 Ma and radius 40–100 cm.Nominal one‐stage 21Ne production rates (P21) inferred from 26Al in Bur Gheluai samples exceed those inferred from 10Be as expected for a meteorite with a complex history. Nonetheless, data for other meteorites indicate that multi‐stage irradiations alone do not account for all the high reported values of P21 based on 26A***l: The equations describing the production of cosmogenic nuclides show that uncorrected shielding effects may also play a role.A compilation of ordinary, solar‐gas‐poor chondrites for which two‐stage histories have been proposed includes many with short second stages but none with unambiguously long first stages (&gt;0.2 Ga).

The meteorite of Ensisheim: 1492 to 1992

Abstract— On November 7, 1492, a 127‐kg stony meteorite fell at Ensisheim in Alsace after a fireball explosion that was heard for a distance of 150 km over the upper Rhineland. Today, a 56‐kg specimen of the stone, an LL6 chondrite with large patches of fusion crust, remains on display in the Hotel de Ville at Ensisheim. This was the earliest witnessed meteorite fall in the West from which pieces are preserved. Initially, the stone's survival depended on the presence of a magistrate at Ensisheim who forbade the removal of pieces, which had begun apace as soon as a crowd gathered and pulled the stone out of a 1‐m hole in a wheat field. He ordered the stone brought into the city to await the arrival of King Maximilian, son of the Holy Roman Emperor Friedrich III, who was approaching with his army. In nearby Basel, broadsheets were printed within weeks bearing the story in Latin and German verses by the eminent poet, Sebastian Brant, who turned the sheets into propaganda tracts by claiming the stone as a portent of victory and admonishing Maximilian to make war on the French without delay. Maximilian declared the stone to be a sign of divine favor and ordered it to be preserved in the Ensisheim parish church. The stone grew in fame when Maximilian won his impending battle with the French, but strange new elements entered the story as it was repeated over the years in books and chronicles. Through centuries of battle and political changes, the stone remained in the church until 1793 when French revolutionaries transferred it to a new National Museum in Colmar. There, many pieces were taken for chemical analyses during the birth of the meteoritics at the turn of the 19th century. In 1803 the stone was returned to the Ensisheim church where it outlasted the structure itself which collapsed in 1854. This paper traces the history of the stone itself and people's responses to it through the 500 years since the fall at Ensisheim.

The supposed meteorite fall in 1912 at Tingsås, revisited

The supposed meteorite fall in 1912 at Tingsås, revisited

Exposure histories of the lunar meteorites: MAC88104, MAC88105, Y791197, and Y86032

Four lunar meteorites, MacAlpine Hills (MAC) 88104, MacAlpine Hills 88105, Yamato (Y) 791197, and Yamato 86032 were analyzed for the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca. From these and published data, histories of exposure to cosmic rays were modelled in terms of two-stage irradiations each with a long first stage on the Moon lasting a time T 2 π > 5 Ma at a burial depth d 2π[ g cm 2 ] followed by a second stage in space, i.e., the transit time between the Moon and the Earth, lasting a time T 4π [Ma] in a body of typical meteoroidal size. The terrestrial age T t [Ma]gives the time elapsed between meteorite fall and recovery in Antarctica. The following sets of parameters were obtained: MAC88104/5, 390 ≤ d 2 π ≤ 500, 0.04 ≤ T 4 π ≤ 0.11, 0.10 ≤ T t ≤ 0.19; Y791197, d 2 π < 80, T 4π < 0.1, T t < 0.1; Y86032, d 2 π > 1000, T 4 π = 10 ± 2, 0.08 < T t < 0.12. From the number and exposure histories of lunar meteorites we infer a production rate on the order of 5 Ma −1 and an arrival rate worldwide of about 3 × 10 6 meteorites Ma −1. These results suggest that each impact event large enough to produce lunar meteorites sends a large number of them to the Earth.

The supposed meteorite fall in 1912 at Tingsås, Sweden

Abstract The meteor that appeared on October 8, 1912 at 6:55 p.m. over Tingsås (=Tingsryd) was thought to have started a fire in the thatch-roofed economy buildings of the vicarage. Through the efforts of David Stenquist, it was shown that the meteorite exploded at least 63 km above the ground. No fall of a meteorite can therefore be expected at this time at Tingsryd, no meteorite has been found and no fire can hardly be attributed to its fall.

The accumulation rate of meteorite falls at the Earth's surface: The view from Roosevelt County, New Mexico

Abstract— The discovery of 154 meteorite fragments within an 11 km2 area of wind‐excavated basins in Roosevelt County, New Mexico, permits a new calculation of the accumulation rate of meteorite falls at the Earth's surface.Thermoluminescence dating of the coversand unit comprising the prime recovery surface suggests the maximum terrestrial age of the meteorites to be about 16.0 ka. The 68 meteorite fragments subjected to petrological analyses represent a minimum of 49 individual falls. Collection bias has largely excluded carbonaceous chondrites and achondrites, requiring the accumulation rate derived from the recovered samples to be increased by a factor of 1.25. Terrestrial weathering destroying ordinary chondrites can be modelled as a first‐order decay process with an estimated half‐life of 3.5 ± 1.9 ka on the semiarid American High Plains. Having accounted for the age of the recovery surface, area of field searches, pairing of finds, collection bias and weathering half‐life, we calculate an accumulation rate of 9.4 × 102 falls/a per 106 km2 for falls &gt; 10 g total mass. This figure exceeds the best‐constrained previous estimate by more than an order of magnitude. One possible reason for this disparity may be the extraordinary length of the fall record preserved in the surficial geology of Roosevelt County. The high accumulation rate determined for the past 16 ka may point to the existence of periods when the meteorite fall rate was significantly greater than at present.

A contemporary account of the Ensisheim meteorite, 1492

Abstract— A report of the 1492 fall of a large stone meteorite near Ensisheim, Alsace, reached the Italian parish priest Sigismondo Tizio, who included this report and a picture of the event in his manuscript History of Siena. The colored picture presents some variations on the woodcut illustration which accompanied Sebastian Brant's printed broadsheets issued immediately after the appearance of the “thunderstone,” while Tizio's report demonstrates how such information travelled quickly along European trade routes.

The fall of the Skåne-Tranås meteorite

Abstract The fall of a meteorite in Skåne-Tranås, SE Scania, Sweden, took place on February 12, 1922 at 12.45 local time and was witnessed by many people. The circumstances of the fall are reviewed and a translation of an article in the local newspaper is given. Two pictures taken of the mill-pond in which the meteorite fell are also reproduced. The search for the meteorite was fruitless.

Age of Allan Hills 82102, a meteorite found inside the ice

THE recovery of thousands of meteorites in the Antarctic since 1969 has not only greatly increased knowledge of the meteorites themselves but has also provided a new tool for glaciology. Most Antarctic meteorites are found on blue ice areas where old ice is continuously ablated. A measurement of the age of this ice helps us understand meteorite accumulation mechanisms and the dynamics of ice movement. The terrestrial age of a meteorite, the time period since the date of meteorite fall, can be determined from the reduction in concentration of cosmogenic radionuclides during the time the meteorite has been shielded by the Earth's atmosphere. Here we report the terrestrial age of a meteorite that was recovered from below the surface of the ice and argue that this represents a measurement of the age of the ice itself. We measured the cosmogenic radionuclides 10Be (half-life = 1.5 Myr), 14C (5,730 yr), 26A1 (0.71 Myr), 36C1 (0.30 Myr) and 53Mn (3.7 Myr) in the meteorite and 10Be and 36C1 in the ice. We obtained a terrestrial age of 11,000 years for the meteorite which suggests that the snow accumulation area where it fell was only a few tens of kilometres away.

Geminid fireballs and the peculiar asteroid 3200 Phaethon

Twelve long-enduring Geminid fireballs from the camera network in western Canada are studied in an attempt to infer properties of their parent object, 3200 Phaethon. The Geminids decelerate from entry velocities of 36 km sec −1 to lower s limit near 15 km sec −1 and disappear at heights above 38 km. No fragments larger than dust particles appear capable of surviving as meteorites. Dynamic masses are derived for the Geminids from observed decelerations using an assumed shape based on data from the recovered fragments of the Innisfree meteorite fall. The luminosity of each fireball is used to obtain a photometric mass, assuming a luminous efficiency, τ, that is independent of velocity. Since Geminids resist crumbling and fragmentation, the dynamic and photometric masses should agree. It is shown that for reasonable values of τ, between 0.02 and 0.06, the masses agree for particle densities between 0.7 and 1.3 g cm −3. A best value of ρ m = 1.0 with τ = 0.034 is suggested. Geminids commonly exhibit flickering in their light, interpreted as an oscillation of the particle in flight. This may indicate the development of a relatively thin meteoroid during ablation and, in this case, the upper limit for the density could be raised to about 1.7. Eleven other fireballs, including four known or suspected meteorites and two fragile members of the Taurid complex, were studied for comparison with the Geminids. The meteorites penetrate the atmosphere considerably deeper than the Geminids, which in turn survive to lower heights than the more fragile objects. The ablation coefficients, σ, are comparable for Geminids and the meteorites, although σ may become meaningless for the low velocities observed in some meteorite events, due to neglect of the energy loss due to deceleration. Classification parameters suggest the Geminids are not as “tough” as meteorites but are more cohesive than many fireballs. It is concluded that Geminid meteoroids have too low a density to associate them with meteorites or normal asteroids. Frequent exposure to intense solar radiation near perihelion (0.14 AU) may lead to a loss of volatiles and may produce relatively tough meteoroids that resemble the crust of a comet rather than its interior.

Cosmic induced radioactivity in the meteorite Trebbin

7 days after the fall of a stony meteorite its γ-spectrum has been measured on a Ge/Li/-detector installed in an underground laboratory. The activity of22Na,26Al,40K,48V, and54Mn has been determined.

Noble gases in lunar meteorites Yamato-82192 and -82193 and history of the meteorites from the moon

We analyzed the isotope abundances of He, Ne, Ar, Kr, and Xe in bulk samples and grain size separates of the lunar meteorites Yamato-82192 and -82193 collected in Antarctica. The history of exposure to cosmic rays and the terrestrial age is the same for both meteorites, confirming earlier suggestions that the two stones are fragments of the same meteorite fall. The terrestrial ages were determined using the radioisotope 81Kr in combination with stable cosmic-ray-produced noble gases. For Y-82192 and Y-82193 we obtain 83,000 ± 35,000y and 75,000 ± 30,000y, respectively. The Moon-Earth transit time lasted about 8 ± 3My. The upper limit of this time, 10.7 ± 0.6My, is obtained if we assume that Y-82192/3 was not exposed to cosmic rays in the lunar regolith, i.e. that it was excavated from a depth completely shielded from cosmic rays by the same impact that launched it into Earth orbit. The lower limit for the Moon-Earth transit time, about 5 My, is given by the fact that 1.6 My 10Be is present in equilibrium activity for a 4π exposure. If the Y-82192/3 material was ever exposed to cosmic rays in the lunar regolith this exposure must have lasted less than 11 My at shallow shielding of ⩽ 25 g/cm 2. Abundances of trapped solar wind particles are extremely low. Only solar wind Ne and Ar could definitely be detected. Comparison of the histories of lunar meteorites Allan Hills A-81005 and Yamato-791197 studied earlier with the data for Yamato-82192/3 shows that at least two impacts on the Moon are responsible for ejecting the lunar meteorites which have so far been found on Earth.

The Salem, Oregon L6 Chondrite

Abstract— The Salem, Oregon meteorite fall of 1:05 a.m. (07:05 GMT) May 13, 1981 (lat. 44°58′45″N., long. 123°58′10″W) was heard by two observers. A 22.2 g fragment was recovered immediately from a total recovery of 61.4 g from a single individual. No other fall related phenomena were observed. It is a heavily fusion‐crusted, shock‐veined, L6 chondrite.

Yale contributions to meteoric astronomy

Until 1910 Yale University was more active in research on meteors than any other institution in America. A brief introduction describes the work carried out at other observatories in the United States. Then follow accounts of the Yale activities, starting with speculations on a spectacular fireball of 1742, the Weston fall of meteorites in 1802, and numerous investigations on “shooting stars,” especially by D. Olmsted, E. C. Herrick, H. A. Newton, and W. L. Elkin. These astronomers contributed significantly to the evolution of understanding of meteoric phenomena, from early beliefs that meteors originated in the earth's atmosphere, to theories that fireballs were earth circling satellites, to our current conceptions of the relation between comets, asteroids, and meteors. Incidentally it may be noted that four of the investigators also became presidents of Yale College: Clap (1739–1766), Stiles (1778–1795), Day (1817–1847), and Hadley (1899–1921).

Detection of a meteorite “stream”: Observations of a second meteorite fall from the orbit of the Innisfree chondrite

A meteorite-dropping event was observed by the camera network in western Canada on February 6, 1980, precisely 3 years after the fall of the Innisfree meteorite. The orbits of the two objects are essentially identical, making this the first observational evidence for multiple falls from the same orbit. Minor differences in the argument of perihelion and longitude of the node were presumably caused by Earth perturbations at times of previous close encounters. An impact location near the town of Ridgedale, Saskatchewan, is indicated with a predicted mass of 1.8 kg for the object. The Ridgedale event was much less luminous than Innisfree, with a peak absolute panchromatic magnitude near −7.5 with no evidence for the multiple fragmentation observed for Innisfree. Considerations of the probability of detecting two related objects with the meteorite camera networks suggest that Innisfree (a brecciated LL chondrite) was a near-surface fragment from a recent parent object with a radius of several tens of meters. A campaign to locate the Ridgedale object is planned since its recovery would allow various comparisons of great interest with the Innisfree chondrite.

Cosmic-ray records in the LL-chondrite Dhurmsala

The radionuclides 10Be, 26Al and 53Mn, the noble gas isotopic composition of He, Ne and Xe and cosmic-ray particle track densities were determined in six documented samples of a large fragment of the Dhurmsala meteorite fall. Using the 22Ne21Ne ratios and the 21Ne concentrations, the cosmic-ray exposure age of Dhurmsala is deduced to be 13.0 ± 1.0 My. The results suggest a simple exposure history. The six samples covering a distance of ~ 13 cm do not show any significant isotopic variations due to depth effects, as expected in a large meteorite. Consistent with the model calculations of Reedy (1985) and Bhandari and Potdar (1982), the results obtained in this study indicate that the Dhurmsala samples investigated here originate at a depth of ~20 to 24 cm in a meteoroid with a preatmospheric radius of R − 50 cm.

Correction of the paper “A study of meteorite falls”

Correction of the paper “A study of meteorite falls”

Unmelted meteoritic debris in the Late Pliocene iridium anomaly: Evidence for the ocean impact of a nonchondritic asteroid

Ir-bearing particles have been recovered from 2 piston cores in the Antarctic Basin in the southeastern Pacific. In core E13-3 the particles closely correspond to the Late Pliocene Ir anomaly and have a fluence of ~100 mg cm −2. In core E13-4, 120 km to the southwest, the particle fluence is ~4 mg/cm −2. Particles with diameters from 0.5 to 4 mm contain at least 35% of the Ir in this horizon. Three types of particles have been identified: 1) vesicular, 2) basaltic, and 3) metal. The vesicular particles appear to be shock-melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component. These particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of Na, K, Cl, and radiogenic Sr from the ocean water target. The basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact. Combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud. Estimates of the diameter of the projectile range from 100 to 500 m. By many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall.

The Wave That Paved Hawaii 100,000 Years Ago

iiA _1q About 100,000 years ago, a giant wave surged up the southern slope of the Hawaiian island of Lanai, blanketing the island's flanks with coral fragments and limestone gravels that the churning water had ripped from the seafloor. As the wave receded it stripped off the rich soil. And on its second and final sweep it dropped pieces of island volcanic rocks from its clutches. This was the picture painted last week at the American Geophysical Union meeting in San Francisco in a presentation given by James G. Moore, a volcanologist, and his brother George W Moore, a marine geologist, both of the U.S. Geological Survey (USGS) in Menlo Park, Calif. The prevailing view before the Moores began their work was that the gravels found on Lanai were deposited at several different times during the Pleistocene epoch, when the sea level rose, creating a series of ancient shorelines above the present coast. Since the melting and growth of ice sheets alone could not account for the implied large and rapid sea level changes, other ideas, including the uplift of the island due to the intrusion of magma, were proposed and debated over the past 50 years. However, based on the ages of offshore coral reefs and other measurements, the Moores decided that the southeastern Hawaiian islands are actually sinking so fast that any past shorelines would now lie below the present coast, and hence should not be seen on dry land. Indeed, when the researchers went to Lanai to have a closer look at the marine deposits, they discovered, instead of a series of ancient shorelines, a single gravel layer that went from the coastline up to an altitude of 326 meters. Above 326 meters, the researchers found 3 meters of soil in which pineapples were growing, while at lower altitudes there was no soil at all. Skeletons of corals and other reef organisms were found in the gravel as disordered fragments rather than as remnants of old reef systems. The researchers also discovered that the gravel layer became thinner and the u rocks and coral pieces finer with increasc ing altitude. A notable exception was the Kaluakapo Crater. that trapped larger fragments than those found in the surrounding gravel. Moreover, uranium dating of the coral by Barney J. Szabo at the USGS in Denver, Colo., revealed that samples from different parts of the island are all about 100,000 years old. All of this geologic evidence, added to the finding of similar gravels spread at lower altitudes on Oahu, Molokai, Maui and Hawaii, led the researchers to conclude that a massivre wave surged through the Pacific Basin sometime just before 100,000 years ago. Because of the wave's large size, the Moores have ruled out an underwater earthquake as the cause, since the highest tsunami, recorded in 1946 on Hawaii, reached only 17 meters above the shoreline. A meteorite fall in the nearby sea or a subsea volcanic eruption could have triggered the wave, but the scientists think a more likely explanation is an underwater landslide over an area of about 25 square kilometers south of Lanai on the Hawaiian Ridge. Subsea landslides had previously been observed to produce large waves; in 1958 an underwater slide in an Alaskan fjord generated the tallest recorded wave, running up 524 meters on land. And the Hawaiian Ridge, which is among the steepest and highest landforms on earth, is believed to have suffered a number of major subsea landslides in the past. The Lanai wave might have been created when a massive chunk of land rapidly fell, sending water surging into the void and up the island slope. According to the researchers, whose work also appears in the Dec. 14 SCIENCE, if such a wave did occur, it is probably not unique. "Now we have a classic place to go," says James Moore, "to look and see what deposits like this actually look like, so they can be recognized in other places as well." --S. W?isburd