Meteorite Flux Research Articles

Survival times of meter-sized rock boulders on the surface of airless bodies

Rock boulders are typical features of the surfaces of many airless bodies, so the possibility of estimating their potential survival times may provide insights into the rates of surface-modification processes. As an opening point of this study we employ estimates of the survival times of meter-sized boulders on the surface of the Moon based on analysis of the spatial density of boulders on the rims of small lunar craters of known absolute age (Basilevsky et al., 2013), and apply them, with necessary corrections, to boulders on other bodies. In this approach the major factor of rock destruction is considered to be impacts of meteorites. However another factor of the rock destruction, thermal fatigue due to day–night cycling, does exist and it was claimed by Delbo et al. (2014) as being more important than meteorite impacts. They concluded this on the basis of known presence of fine material on the surface of small asteroids, claiming that due to extremely low gravity on those bodies, the products of meteorite bombardment should leave these bodies, and thus their presence indicates that the process of thermal fatigue should be much more effective there. Delbo et al. (2014) made laboratory experiments on heating–cooling centimeter-sized samples of chondrites and, applying some assumptions and theoretical modeling concluded that, for example, at 1AU distance from the Sun, the lifetime of 10cm rock fragments on asteroids with period of rotation from 2.2 to 6h should be only ~103 to 104 years (that is ~3.5×106 to 1.5×107 thermal cycles) and the larger the rock, the faster it should be destroyed.In response to those conclusions we assessed the results of earlier laboratory experiments, which show that only a part of comminuted material produced by high-velocity impacts into solid rocks is ejected from the crater while another part is not ejected but stays exposed on the target surface and is present in its subsurface. This means that the presence of granulometrically fine material on the surface of small asteroids does not prove the predominance of thermal stresses over rupture by meteorite impacts as a factor in the comminution of the surface material. We then analyzed images of lunar rocks of decimeters- to meters-size whose lunar surface exposure ages were radiometrically dated. This analysis shows that the presence of the fragment on the lunar surface for a time period 26–400Ma (that is, ~3×108 to 5×109 day–night thermal cycles) did not lead to the formation of any features conclusively supporting rock destruction by thermal cycles. In turn, this means that on the lunar surface as well as on the surface of other bodies at 1AU and further from the Sun, the destruction of rocks by thermal fatigue is secondary compared to rock rupture by the meteorite impacts. The possible implications of the difference in environments on fast spinning asteroids and on the Moon require additional analysisThen utilizing the entire catalog of inner solar system minor planet orbits out to Jupiter as a proxy for the distribution of potential impactors throughout the inner solar system, we calculated the meteorite flux and impact velocities for a number of airless bodies to use them for estimates of survival times of rock boulders on their surfaces (normalized to those for lunar boulders). We found that if the average survival time for meter-size rock boulders on the surface of the Moon is 1, then considering rupture by the meteorite impacts as the major process of rock destruction, for Phobos it is ~0.8, for Deimos ~0.7, for asteroid Itokawa ~1, for Eros ~0.3, for Vesta and Ceres ~0.03 and for the average of the first 150 Trojans discovered is ~12.5. Implications of these findings are that on the surfaces of Vesta and Ceres, compared to the Moon, the regolith layer should generally have a larger thickness and higher maturity, while small craters with rocky ejecta are rare. On the typical Trojans, where impact flux is closer to that on the Moon, but the impact velocities are by factor 4 lower, the situation should be the opposite: thinner layer of regolith, lower maturity and a larger percentage of small craters with rocky ejecta. These predictions and observations can be tested with future robotic and human exploration of the Moon and small bodies.

Variations in the solar fluxes of helium and protons on the long-term scale

A reconstruction of data on ancient (to ∼600 Ma ago) solar fluxes of protons and helium has been performed on the basis of isotopic investigations of helium and neon in lunar soil samples from columns delivered by the automatic stations Luna-16 and Luna-24 in 1970 and 1976. Recent investigations have shown the presence of long-term climate variations, which can be explained in the context of solar-terrestrial links. However, the “space” impact, simultaneously with the Sun’s influence on the Earth, takes place in the form of cosmic ray irradiation and as an influence on the Earth that is exerted by the flux of cosmic dust and meteorites (including the very large ones at the early stage of the Earth evolution). Therefore the existence of long-term variability of solar corpuscular fluxes may serve as direct evidence of the manifestation of solar-terrestrial links. The possibility of finding these links appears on the basis of the revealed variations of solar wind fluxes with an age from the contemporary level to the level of ∼600 Ma ago.

Sulfur isotope mass-independent fractionation in impact deposits of the 3.2 billion-year-old Mapepe Formation, Barberton Greenstone Belt, South Africa

Theoretical and experimental studies have shown that atmospheric SO2 isotopologue self-shielding effects in the 190–220nm region of the solar spectrum are the likely cause for mass independent fractionation of sulfur isotopes (S-MIF). The main products of this photochemical reaction – SO3 and S0 – typically define a compositional array of ca. Δ33S/δ34S=0.06–0.14. This is at odds with the generally observed trend in Archean sulfides, which broadly defines an array of ca. Δ33S/δ34S=0.9. Various explanations have been proposed, including a diminution of δ34S caused by chemical and biogenic mass-dependent fractionation of sulfur isotopes (S-MDF), mixing with photolytic products produced during felsic volcanic events, or partial blocking of the low-wavelength part of the spectrum due to the presence of reduced atmospheric gases or an organic haze. Early in Earth history large meteorite impacts would have ejected dust and gas clouds into the atmosphere that shielded solar radiation and affected global climate. It is thus likely that at certain time intervals of high meteorite flux the atmosphere was significantly perturbed, having an effect on atmospheric photochemistry and possibly leaving anomalous sulfur isotopic signatures in the rock record. Here we describe the sulfur isotopic signatures in sulfides of spherule beds S2, S3 and S4 of the Barberton Greenstone Belt, South Africa. In particular, in spherule bed S3 – and to a lesser extent S4 – a trend of ca. Δ33S/δ34S=0.23 is observed that closely follows the expected trend for SO2-photolysis in the 190–220nm spectral range. This suggests that an impact dust cloud (deposited as spherule beds), which sampled the higher region of the atmosphere, specifically incorporated products of SO2 photolysis in the 190–220nm range, and blocked photochemical reactions at higher wavelengths (250–330nm band). By implication, the generally observed Archean trend appears to be the result of mixing of different MIF-S sources arising from a variety of photochemical reactions that took place in the lower part of the atmosphere.

A fossil winonaite-like meteorite in Ordovician limestone: A piece of the impactor that broke up the L-chondrite parent body?

About a quarter of all meteorites falling on Earth today originate from the breakup of the L-chondrite parent body ∼470 Ma ago, the largest documented breakup in the asteroid belt in the past ∼3 Ga. A window into the flux of meteorites to Earth shortly after this event comes from the recovery of about 100 fossil L chondrites (1–21 cm in diameter) in a quarry of mid-Ordovician limestone in southern Sweden. Here we report on the first non-L-chondritic meteorite from the quarry, an 8 cm large winonaite-related meteorite of a type not known among present-day meteorite falls and finds. The noble gas data for relict spinels recovered from the meteorite show that it may be a remnant of the body that hit and broke up the L-chondrite parent body, creating one of the major asteroid families in the asteroid belt. After two decades of systematic recovery of fossil meteorites and relict extraterrestrial spinel grains from marine limestone, it appears that the meteorite flux to Earth in the mid-Ordovician was very different from that of today.

Modeling the microphysics of CO2 ice clouds within wave-induced cold pockets in the martian mesosphere

Mesospheric CO2 ice clouds on Mars are simulated with a 1D microphysical model, which includes a crystal growth rate adapted to high supersaturations encountered in the martian mesosphere. Observational constraints (crystal radius and opacity) exist for these clouds observed during the day around the equator at ∼60–80km altitude. Nighttime mesospheric clouds interpreted as CO2 ice clouds have also been characterized at low southern latitudes, at ∼90–100km altitude. From modeling and observational evidence, it is believed that mesospheric clouds are formed within temperature minima created by thermal tides, where gravity wave propagation allows for the creation of supersaturated layers (cold pockets). Thus, temperature profiles perturbed by gravity waves are used in the model to initiate nucleation and maintain growth of CO2 ice crystals. We show that it is possible to reproduce the observed effective radii for daytime and nighttime clouds. Crystal sizes are mainly governed by the altitude where the cloud forms, and by the amplitude of supersaturation. The temporal and spatial behavior of the cloud is controlled by the extent and lifetime of the cold pocket. The cloud evaporates fast after the cold pocket has vanished, implying a strong correlation between gravity wave activity and CO2 cloud formation. Simulated opacities remain far below the observed ones as long as typical dust conditions are used. In the case of the lower daytime clouds, the enhanced mesospheric dust loading typically reached during dust storm conditions, allows for greater cloud opacities, close to observed values, by supplying the atmosphere with condensation nuclei. However, CO2 ice clouds are not detected during the dust storm season, and, because of fast sedimentation of dust particles, an exogenous supply (meteoritic flux) appears necessary to explain opacities of both daytime and nighttime mesospheric CO2 ice clouds along their whole period of observation.

Stones from Mohave County, Arizona: Multiple falls in the “Franconia strewn field”

AbstractOne of the most productive and well‐sampled dense collection areas for meteorites on Earth is the “Franconia strewn field” in Mohave County, Arizona, which since 2002 has yielded hundreds of meteorites in an ellipsoidal area approximately 5 × 16 km across. Based on petrographic, mineral‐chemical, and terrestrial age data, we conclude that among 14 meteorites examined, there are at least 6 and possibly 8 distinct meteorites represented, which fell over a period of approximately 0–20 kyr ago. These include equilibrated H‐chondrites such as Franconia (H5) and Buck Mountains (BM) 001 (H6); H3–6 breccias such as Buck Mountains Wash and BM 004; and L6 chondrites such as BM 002 and BM 003 (which may be paired), Palo Verde Mine, and BM 005. To confidently pair such meteorites often requires thorough petrographic examination, mineral‐chemical analyses, and terrestrial ages. We estimate that 50 ± 10% of the larger specimens in this area are paired, yielding a relatively high value of approximately 2.3–2.9 distinct meteorites km−2. The meteorite flux estimated for Franconia area is higher than the flux inferred from contemporary fireball data for larger masses. We suggest that one large H3–6 meteoroid fell in the area, most likely that of Buck Mountains Wash approximately 4 kyr ago, which produced an elliptical strewn field with masses generally increasing toward one end, and which raised the meteorite productivity in the recovery area.

Revisiting the radiation history of fossil meteorites from Sweden (Rebuttal to a comment by Wieler et al. On the paper by V.A. Alexeev “Radiation History of Fossil Meteorites from Sweden,” published in solar system research, 44, 311–319)

From the analysis of the data acquired for the chromite grains recovered from fossil meteorites from Sweden (Heck et al., 2004, 2008), the dependence of the exposure age of meteorites on a mass of the examined samples was obtained (Alexeev, 2010). This dependence, whatever its nature, puts into question the validity of the conclusion on a long lasting (over the course of 1–2 Myr) and substantial (by 1–2 orders of magnitude) increase in the intensity of the flux of meteorites (L-chondrites) that fell onto the Earth about 470 Myr ago. All of the fossil meteorites found in southern Sweden could be the fragments of a single meteorite that fell as a meteorite shower in the Thorsberg quarry region ∼470 Myr ago.

The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment

Many precious, 'iron-loving' metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet's deep interior. One explanation of their over-abundance is a 'late veneer'--a flux of meteorites added to the Earth after core formation as a 'terminal' bombardment that culminated in the cratering of the Moon. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio (182)W/(184)W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon (more than four billion years ago) or core-mantle interaction, do not explain the W isotope data well. The decrease in mantle (182)W/(184)W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in (142)Nd/(144)Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.

The densest meteorite collection area in hot deserts: The San Juan meteorite field (Atacama Desert, Chile)

Abstract– We describe the geological, morphological, and climatic setting of the San Juan meteorite collection area in the Central Depression of the Atacama Desert (Chile). Our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3.88 km2 area. The recovery density is in the range 9–12 falls km−2 depending on pairing, making it the densest among meteorite collection areas in hot deserts. This high meteorite concentration is linked to the long‐standing hyperaridity of the area, the stability of the surface pebbles (> Ma), and very low erosion rates of surface pebbles (approximately 30 cm Ma−1 maximum). The San Juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka, and limited weathering compared with other dense collection areas in hot desert. Chemical weathering in San Juan is slow and mainly controlled by the initial porosity of meteorites. As in the Antarctic and other hot deserts, there is an overabundance of H chondrites and a shortage of LL chondrites compared with the modern falls population, suggesting a recent (< few ka) change in the composition of the meteorite flux to Earth.

A 3‐D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X‐ray tomographic microscopy—Method and applications

Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.

Long-Time Data Storage: Relevant Time Scales

Dynamic processes relevant for long-time storage of information about human kind are discussed, ranging from biological and geological processes to the lifecycle of stars and the expansion of the universe. Major results are that life will end ultimately and the remaining time that the earth is habitable for complex life is about half a billion years. A system retrieved within the next million years will be read by beings very closely related to Homo sapiens. During this time the surface of the earth will change making it risky to place a small number of large memory systems on earth; the option to place it on the moon might be more favorable. For much longer timescales both options do not seem feasible because of geological processes on the earth and the flux of small meteorites to the moon.

Regolith depth growth on an icy body orbiting Saturn and evolution of bidirectional reflectance due to surface composition changes

Using a Markov chain model, we consider the regolith growth on a small body in orbit around Saturn, subject to meteoritic bombardment, and assuming all impact ejecta are re-collected. We calculate the growth of regolith and the fractional pollution, assuming an initial pure ice body and amorphous carbon as a pollutant. We extend the meteorite flux of Cuzzi and Estrada (Cuzzi, J., Estrada, P. [1998]. Icarus 132, 1–35) to larger sizes to consider the effect of disruption of the moonlet on other moonlets in the ensemble. This is a relatively small effect, completely negligible for moonlets of 1m radius. For the given impact model, fractional pollution reaches 22% for 1m bodies, but only 3% for 10m bodies, 1.7% for 20m bodies, and 1% for 30m bodies after 4byr. By considering an ensemble of moonlets, which have identical cross-sections for releasing and capturing ejecta, this analysis can be extended to a model of particles in Saturn’s rings, where the calculated spectra can be compared to observed ring spectra. The measured spectral reflectance of Saturn’s rings from Cassini observations therefore constrains the size and age of the ring particles. The comparison between 1m, 10m, 20m, and 30m particles confirms that for larger ring mass, the current rings would be less polluted; for the largest particles, we expect negligible changes in the UV spectrum after 4byr of meteoritic bombardment. We consider two end members for mixing of the meteoritic material: areal and intimate. Given the uncertainties in the actual mixing of the meteoritic infall and in its composition (as a worst case, we assume the meteoritic material is 100% amorphous carbon, intimately mixed) initially pure ice 30m ring particles would darken after 4byr of exposure by 15%.

Radiation history of fossil meteorites from Sweden

We present the results of the analysis of the data obtained by Heck et al. (2004; 2008) regarding the contents of 4He and 20,21,22Ne and the exposure age of chromite grains recovered from fossil meteorites (L-chondrites) found in marine limestone—mid-Ordovician sediments—in the Thorsberg and Gullhogen quarries of southern Sweden. It has been shown that the increase of the content of noble gases in the chromite grains (by 1–2 orders of magnitude) and their exposure age with a decreasing mass of the samples (by approximately an order of magnitude) can be caused by an increase of the portion of small grains and by their poor preservation in the samples of small mass. The revealed relationships, as well as the fact that practically all fossil meteorites were found in a small area (∼6000 m2), can be explained by the following assumption: a meteorite shower, caused by a single meteorite, fell ∼470 Myr; this occurred less than 0.2 Myr after the catastrophic destruction of the L-chondrite parent body. The time of this fall corresponds to that of the formation of the oldest sedimentation stratum, containing the meteorite fragments, in the Thorsberg quarry. Meteorite fragments in the younger strata are most likely the result of the subsequent redeposition that occurred in a shallow sea, when the sediment mass was forming. In this case, to explain the distribution of fossil meteorites in the mid-Ordovician sediments in Sweden, there is no necessity to hypothesize that an intense flux of meteorites was falling onto the Earth over the course of ∼1–2 Myr about 470 million years ago.

Scientific Exploration of the Moon

The Moon is a geochemically differentiated object. It has a feldspathic crust (highlands regions) composed of three petrological suites. Underlying that crust is a compositionally heterogeneous upper mantle from which ferrobasalts and picrites (mare regions) were generated. Lunar samples retain a memory of the time-dependent flux of meteorites and comets, which has implications for the origin of sustainable life on Earth and the orbital evolution of the outer planets. Permanently shadowed regions at the lunar poles may contain reservoirs of volatile ices, which would have important resource potential for scientific bases. Geophysical data show that the Moon has a thick, seismically active lithosphere, a partially molten zone beneath that lithosphere, and a small metallic core. The pace of scientific exploration has quickened since 2003 with the successes of spacecraft from Europe, Japan, the People's Republic of China, and India. Upcoming launches of spacecraft from these same nations and the United States herald a new era of lunar discoveries.

The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid‐Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)

Abstract— The Brunflo fossil meteorite was found in the 1950s in mid‐Ordovician marine limestone in the Gärde quarry in Jämtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L‐chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule‐size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent‐body breakup. Chromite maximum diameters and well‐defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Gärde quarry is relatively rich (about 0.45 grain kg−1) in sediment‐dispersed extraterrestrial chromite grains (>63 μm) with chemical composition similar to those in L chondrites and the limestone (1–3 grains kg−1) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.

Platinum-Group Elements: A New Set of Key Tracers for the Earth's Interior

Due to their “iron-loving” properties, platinum-group elements (PGE) are expected to be stored in the Earth’s core. Although very low, at a few parts per billion, PGE concentrations measured in mantle-derived rocks are too high to be in chemical equilibrium with the core. The “late veneer” model offers the best explanation for this paradox—it postulates that a flux of primitive meteorites hit the early Earth after core formation had ceased. However, the inferred PGE composition of the hypothetical primitive mantle exhibits slight positive excesses of Ru, Rh, and Pd compared to the canonical chondritic signature. Such deviations have triggered considerable debate about the composition of the late veneer and the extent of reworking of PGE signatures by igneous processes within the Earth’s mantle.

Disruption of the L chondrite parent body: New oxygen isotope evidence from Ordovician relict chromite grains

Mid-Ordovician fossil meteorites found in the Thorsberg quarry, southern Sweden, are believed to have been deposited during a period of enhanced meteorite flux following the fragmentation of the L chondrite parent body. During diagenesis, the fossil meteorites were largely replaced by a secondary mineral assemblage. However, primary chromite grains have been preserved. High-precision oxygen isotope analysis by laser-assisted fluorination has been undertaken in order to confirm the chemical group (H, L or LL) to which the fossil meteorites belong. To test our methodology, chromites extracted from recent ordinary chondrite falls (Holbrook L6, Appley Bridge LL6 and Kernouve H6) have been analyzed and these show that ordinary chondrites can be classified into their respective groups (H, L, or LL) using the oxygen isotopic composition of chromite alone. Results from the Golvsten 001 meteorite demonstrate that this sample is an equilibrated L chondrite. The uniform major and minor element composition of chromites throughout the southern Swedish fossil meteorite section means that it is highly probable that all are L chondrites. High-precision oxygen isotope analysis of relict chromites thus further strengthens the link between the fossil meteorites and the disruption of the L chondrite parent body. The evidence presented here demonstrates that relict chromite grains survive diagenesis and can be used to classify ancient meteoritic material. Analysis of such fossil grains may prove to be a powerful tool, not only in the case of the mid-Ordovician event, but also in examining changes in the relative distribution of meteorite groups throughout geological time.

Foreign meteoritic material of howardites and polymict eucrites

Howardites and polymict eucrites are fragments of regolith breccias ejected from the surface of a differentiated (eucritic) parent body, perhaps, of the asteroid Vesta. The first data are presented demonstrating that howardites contain, along with foreign fragments of carbonaceous chondrites, also fragments of ordinary chondrites, enstatite meteorites, ureilites, and mesosiderites. The proportions of these types of foreign meteoritic fragments in howardites and polymict eucrites are the same as in the population of cosmic dust particles obtained from Antarctic and Greenland ice. The concentrations of siderophile elements in howardites and polymict eucrites are not correlated with the contents of foreign meteoritic particles. It is reasonable to believe that cosmogenic siderophile elements are concentrated in howardites and polymict eucrites mostly in submicrometer-sized particles that cannot be examined mineralogically. The analysis of the crater population of the asteroid Vesta indicates that the flux of chondritic material to the surface of this asteroid should have been three orders of magnitude higher than the modern meteoritic flux and have been comparable with the flux to the moon’s surface during its intense meteoritic bombardment. This provides support for the earlier idea about a higher meteoritic activity in the solar system as a whole at approximately 4 Ga. The lithification of the regolith (into regolith breccia) of the asteroid Vesta occurred then under the effect of thermal metamorphism in the blanket of crater ejecta. Thus, meteorite fragments included in howardites provide record of the qualitative composition of the ancient meteorite flux, which was analogous to that of the modern flux at the Earth surface.

The first fossil meteorite from the mid-Ordovician of the Gullhögen quarry, Billingen, southern Sweden

A fossil meteorite, c. 1 cm in diameter, has been found in a loose limestone block in the Gullhögen quarry at Billingen, southern Sweden. The meteorite, named Gullhögen 001, is almost completely pseudomorphosed by clay minerals and calcite, but contains abundant relict chromite grains and high platinum group element concentrations. Chromite element chemistry indicates that Gullhögen 001 most likely belongs to the L chondrite group, like most or all of the c. fifty mid-Ordovician fossil meteorites previously found in the Thorsberg quarry at Kinnekulle, 35 km to the northwest. Conodont studies show that the limestone surrounding Gullhögen 001 belongs to the Yangtzeplacognathus crassus Zone, i.e. the meteorite fell at the same time as those in the middle to upper part of the meteorite-yielding section in the Thorsberg quarry. The find of Gullhögen 001 supports the idea that the meteorite flux to Earth was strongly enhanced during a part of the mid-Ordovician, directly following a major asteroid disruption event in the asteroid belt.

The Flux of Lunar Meteorites onto the Earth

Numerous new finds of lunar meteorites in Oman allow detailed constraints to be obtained on the intensity of the transfer of lunar matter to the Earth. Our estimates show that the annual flux of lunar meteorites in the mass interval from 10 to 1000 g to the entire Earth's surface should not be less than several tenths of a kilogram and is more likely equal to tens or even a few hundred kilograms, i.e., a few percent of the total meteorite flux. This corresponds to several hundred or few thousand falls of lunar meteorites on all of Earth per year. Even small impact events, which produce smaller than craters on the Moon smaller than 10 km in diameter, are capable of transferring lunar matter to the Earth. In this case, the Earth may capture between 10 to 100% of the mass of high-velocity crater ejecta leaving the Moon. Our estimates for the lunar flux imply rather optimistic prospects for the discovery of new lunar meteorites and, consequently, for the analyses of the lunar crust composition. However, the meteorite-driven flux of lunar matter did not play any significant role in the formation of the material composition of the Earth's crust, even during the stage of intense meteorite bombardment.