Large Meteoroids Research Articles

New method for the study of airborne meteoritic particles trapped on treeresin: some results from the Tunguska region

Abstract After the complete disintegration of large meteoroids or small asteroids or cometary nuclei that have not the structural and compressive strength to survive the aerodynamic pressure and stress during the penetration in the dense part of the atmosphere, only a few methods to reconstruct the chemical composition of the parent meteoroid exist. The described methods of using natural or artificial sticky traps, were tested on two of such events: Tunguska 1908 (natural traps) and Emilia 1993 (artificial traps). Here the preliminary results of the former are described.

Powerful bolide explosion over North Italy

In the night of 19 January 1993 at 00:33:20 ± s UT, North Italy, Solvenia and Croatia were illuminated by powerful terminal flare during fireball disintergation. Some tens of KT were liberated as high. In the region of Emilia the low frequency, very strong roar (thunder) was also heaard, shaking windows and walls, some 80–140 s after the boiled explosion. The servere fragmentation and after that a complete destruction of the fireball body took place over the Italion region of Emilia at the height of 35–40 km. Sticky flim traps were used for the frist time after a bright fireball event and a probable fall down of glassy spherules sizing micron was captured. This is one more case where very large meteoroids, group (type) I, II and III or small asteroids/comments (density 0.2–5 g cm −3 were completely destroyed by aerodynamic pressure and stress in the dense part of the atmosphere, leaving no geological consequencees. The presented data are the best possible to have been collected and studied up until 1 June 1993.

Long-term changes in elemental deposition at the earth's surface

The rate of deposition of elements at a point on the earth's surface can change, quite dramatically, even on relatively short time-scales, as a function of wheather patterns. On a global scale volcanoes (and more rarely large meteors) can overwhelm steadier sources of trace elements. In recent centuries human activities have increased emissions to the atmosphere to a point where they are above those of natural sources for some of the rarer elements. On a longer time-scale climate change can also alter the deposition of elements, although such changes are often slower than those brought about through anthropogenic emissions. Changes in climate can also alter the distribution of deposition, but there are few studies estimating such changes. This paper uses estimates of the balance of natural and anthropogenic sources of a range of elements to examine the likely variation in deposition at the earth's surface. It particularly focuses on those elements regarded as toxic, whose concentrations seem likely to go on increasing in industrial areas.

Dust particle impacts during the Giotto encounter with comet Grigg–Skjellerup

IN the European Space Agency's 1992 Giotto Extended Mission, the Dust Impact Detection System operated successfully during a fly-by that took the spacecraft within about 200 km of the nucleus of comet Grigg–Skjellerup. During the encounter, three meteoroid impacts were detected on Giotto's front shield. The particle masses were found to be lOO+105-50 µg, 2+4-1 µg and 20+25-10 µg, suggesting that the mass distribution of the cometary dust was dominated by larger particles. This is supported by the independent detection of a very large meteoroid (14+40-4 mg) by the Giotto Radio-Science Experiment, and is consistent with data over the same mass range from the 1986 encounter with comet Halley. The results indicate a higher rate of mass loss from the nucleus than previously thought, and hence a higher dust-to-gas mass ratio.

The fragmentation of small asteroids in the atmosphere

The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths

Lidar Observations of the Meteoric Deposition of Mesospheric Metals

The mesospheric sodium and iron layers at an altitude between about 80 and 110 kilometers are routinely monitored by atmospheric physicists using resonance fluorescence lidar techniques because these constituents are excellent tracers of mesopause chemistry and dynamics. The mesospheric metals are the products of meteoric ablation. Existing ablation profiles are model calculations based in part on radar observations of the ionized background atmosphere left in the wake of high-speed (> 20 kilometers per second) meteoroids. Thin trails of neutral metal atoms, ablated from individual meteoroids, are occasionally observed with high-power lidars. The vertical distribution of 101 sodium and 5 iron meteor trails observed during the past 4 years at Urbana, Illinois; Arecibo, Puerto Rico; and near Hawaii is approximately Gaussian in shape with a centroid height of 89.0 (+/- 0.3) kilometers and a root-mean-square width of 3.3 (+/- 0.2) kilometers. This directly measured ablation profile is nearly the same as the mean iron layer profile but is considerably different from existing models and the distribution of ionized meteor trails observed by radars. A lower limit on the influx to the mesopause region from the lidar meteors is approximately 1.6 x 10(3) sodium and 2.7 x 10(4) iron atoms per second per square centimeter, which corresponds to an annual flux of meteoric debris into the mesosphere of about 2.0 (+/-0.6) gigagrams. Because the lidars can detect only the ablation trails left by the larger meteors, the observations suggest that the actual meteoric influx may be larger than the more recently reported values, which range between 16 and 78 gigagrams per year.

Atmospheric Evolution of the Terrestrial Planets

The major atmospheric gases on Earth, Venus, and Mars were probably CO 2 , H 2 O, and N 2 . Most of the Earth's CO 2 is tied up in minerals such as limestone, and Venus has lost most of its H 2 O, leaving the CO 2 in the atmosphere. Much of Mars' atmosphere may have been eroded in impacts by large meteoroids early in solar-system history. Noble gases are very underabundant everywhere, and must have been lost during an early period; they were probably dragged along during rapid loss of massive amounts of hydrogen. The tenuous atmospheres of Mercury and the moon have lifetimes of a few days or less and must be continuously replenished from internal or external sources.

A search for clustering among the meteoroid impacts detected by the Apollo lunar seismic network

We examined temporal clustering of meteoroid impacts detected by the Apollo lunar seismic network and found a distinct difference between “small” meteoroids (masses smaller than about 1 kg) and “large” meteoroids (masses larger than about 1 kg). Small meteoroids show strong clustering, many of which are identified with showers known from terrestrial meteor studies. In contrast, little clustering is found for large meteoroids, suggesting that they represent meteoroids of type and origin different from those of the small meteoroids. Overall, 28% of the small events and 15% of the large events occur as clusters. The small meteoroids appear to be mostly cometary, while the large meteoroids may be derived from near-Earth asteroids and short-period comets. Two swarms of large meteoroids detected in June 1975 and January 1977 possibly contain high-density meteoritic objects, and thus may represent “meteorite streams.”

The Sfax L6 chondrite: A new fall from Tunisia

— The Sfax meteorite fell on 16 October 1989. Four pieces totaling less than 10 kg were recovered from a much larger meteoroid, according to the cosmogenic gas measurements. It is an L6 chondrite, strongly degassed and shocked, with olivine of composition Fa24.

Ungrouped Iron Meteorites in Antarctica: Origin of Anomalously High Abundance

Eighty-five percent of the iron meteorites collected outside Antarctica are assigned to 13 compositionaily and structurally defined groups; the remaining 15 percent are ungrouped. Of the 31 iron meteorites recovered from Antarctica, 39 percent are ungrouped. This major difference in the two sets is almost certainly not a stochastic variation, a latitudinal effect, or an effect associated with differences in terrestrial ages. It seems to be related to the median mass of Antarctic irons, which is about 1/100 that of non-Antarctic irons. During impacts on asteroids, smaller fragments tend to be ejected into space at higher velocities than larger fragments, and, on average, small meteoroids have undergone more changes in orbital velocity than large ones. As a result, the set of asteroids that contributes small meteoroids to Earth-crossing orbits is larger than the set that contributes large meteoroids. Most small iron meteorites may escape from the asteroid belt as a result of impact-induced changes in velocity that reduce their perihelia to values less than the aphelion of Mars.

Ablation of large meteors during radiative heating

A problem in radiative gas dynamics concerning the removal of mass and the change in shape of a large cosmic body caused by heating due to the emission of the shock layer on entering the earth's atmosphere is solved. The equations of motion of an ablating body are calculated simultaneously with the determination of the aerodynamic forces and the radiative thermal fluxes. The spectral emission is treated in the diffusion approximation and under the assumption of a locally one-dimensional optically planar layer. The results of calculations for three natural cosmic objects belonging to different classes (stony and iron meteorites and a snow-ice comet body) are presented.

Possible relationship between the Farmington meteorite and a seismically detected swarm of meteoroids impacting the Moon

Abstract— The Farmington ordinary L5 chondrite with its uniquely short cosmic‐ray exposure age of less than 25 000 years may have been a member of a large meteoroid swarm which was detected by the Apollo seismic network when it encountered the Moon in June 1975. The association implies that the parent body of the Farmington meteorite was in an Earth‐crossing orbit at the time the swarm was formed. This supports the idea that at least some meteorites are derived from the observable population of Earth‐crossing asteroids.

The origin of dust in the Solar System

The assumption that the Zodiacal Cloud is a predominantly meteoritic rather than a meteoroidal complex is questioned. On the basis of (i) the observed exposure ages of interplanetary dust particles collected from the stratosphere, (ii) the compressive strength of the commonest fireballs, (iii) the existence of a broad ecliptic stream centred on the Taurids and (iv) the observation of substantial short-lived meteoroid swarms therein, a suitably consistent replenishment model is constructed in which the Zodiacal Cloud appears to derive from a now defunct large comet that arrived in an Earth-crossing orbit ca. 10-100 ka ago. A corollary of this model is that the latter’s remnant, a surviving large meteoroid, may be reactivated as a comet at intervals of ca. 1 ka giving rise to a variety of observable effects such as Zodiacal Cloud enhancements and rare multiple bombardments of the Earth by many bodies with masses at least 10 11 g, which typify a general process throughout Earth history responsible for climatic excursions and extinction events. It is recommended that a search be conducted for the large meteoroid or minor planet responsible for the dust now in the Solar System, to place our understanding of the latter’s evolution on a secure quantitative basis. If verified, this model would have profound implications so far as our understanding of the origin of comets is concerned because most of the cometary mass would apparently be contained in large differentiated bodies.

COSMIC DUST: COLLECTION AND RESEARCH

The term as used here refers to particulate material that exists or has existed in the interplanetary medium as bodies smaller than 1 mm. The particles can be collected both in space and in the terrestrial environment, and they are a valuable resource of meteoritic material. The dust samples are complementary to the traditional meteorites that were much larger meteoroids in space. Because of their size, the analysis of collected dust particles is more difficult and limited than studies of meteorites. Nevertheless, collected dust samples are being vigorously investigated for two fundamentally important reasons. The first is that the most friable extraterrestrial materials can only be collected in the form of dust. Highly fragile materials cannot survive hyper velocity entry into the atmosphere in chunks as large as conventional meteorites. Most cometary meteoroids are known to be fragile, and dust collection is the only Earth­ based technique for obtaining typical cometary solids. The second reason why dust is important is that it is very abundant. Meteorites are exceedingly rare, but cosmic dust is so common that quite literally every footstep a person takes contacts a fragment of cosmic dust. Dust is so abundant that, with appropriate techniques, it can be recovered from historical deposits in deep-sea sediments and collected in real time in space and in the stratosphere. Stratigraphic layers in sediments record a continuous history of the terrestrial accretion of space debris that extends beyond 108 yr ago. This record can be searched for temporal changes in the meteoroid complex, such as might occur during fluctuations in the number of comets in the inner solar system or during passage of the solar system through an interstellar cloud. Effects of isolated events, such as the terrestrial impact of a large body, are also contained in sediments. The flux

Nuclear track study of Jilin chondrite

Twenty-one specimens of Jilin, sampling the whole spectrum of 21Ne concentrations and coming from the main mass as well as from various locations in the strewn field, were studied for their nuclear track record in olivines. Exceedingly low track densities were found in 19 of these specimens (< 300 cm −2), including 10 in which the low 60Co(T 1/2 = 5.3 years) activities were seemingly indicative of a low shielding during the second (4π) irradiation stage. In two specimens (VI-42-04 and VI-21-057, which have high 21Ne contents and the largest 40Ar (and 4He) losses, track densities of ∼ 750 and ∼ 1500 cm −2 clearly emerge from the background. These tracks are attributed to very heavy cosmic ray nuclei. (1)They were produced during the first- (2π) stage exposure. It follows that the two samples were: (a) the least-shielded during the first irradiation period (< 28 cm of chondritic matter); (b) not subjected to a last environmental temperature higher than 350°C for a few days—in contradiction with the thermal model of Müller and Jessberger, otherwise the cosmic ray tracks would have been totally erased; (c) located at a depth of at least 20 cm within the meteoroid during the second (4π) irradiation episode of 0.4 Myr, otherwise CRTDs produced during this time would have been detected. This depth implies that the 60Co activities of these two specimens (not known until now) should be around 130–140 dpm/kg. It remains then to explain why 10 other specimens, with 60Co activities in the range of 50–100 dpm/kg, defining depths of 6–15 cm within the meteoroid, show no evidence at all of CRTDs above the track density background of 150 ± 100 cm −2. (2)The CRTDs were registered during the second (4π) irradiation stage of 0.4 Myr. This requires the samples to have been located at depths of ∼ 15 and 12 cm from the pre-atmospheric surfaces (within a meteoroid with a radius of 85 cm). At such depths the predicted 60Co activities for specimens VI-21-057 and VI-42-04 would range between ∼ 70 and ∼ 100 dpm/kg. The major problem with our results is to understand why in 10 locations with low (50–100 dpm/kg) 60Co activities no CRTDs have been registered during the second irradiation stage. A possible answer would be that depths inferred from 60Co activities are in error, at least in the case of large meteoroids and for depths < 15–20 cm. This has indeed been found to be the case in Allende where at shallow locations (< 15 cm), depths of burial derived from CRTDs are larger by factors of 2–5 than those inferred from 60Co activities, while at larger depths the agreement between the two methods appears to be good. The ubiquitous presence of a track density background found in most samples is best explained by the spontaneous fission of 238U since 4.0 Gyr. This explanation allows one to define a strict upper limit to the temperature of the last thermal event responsible for the losses of radiogenic and spallogenic gases: a temperature of 500°C (1 hour) or 465°C (24 hours) would have annealed the internal fission tracks in olivines. These limiting temperatures are not in real disagreement with the thermal model of Müller and Jessberger. Provided the CRTDs found in the key specimens VI-21-057 and VI-42-04 were registered during the second (4π) irradiation stage and that shielding depths derived from CRTDs are right, the Jilin meteoroid had an original mass of about 15 tons ( R 0 ≅ 1 m ), of which about 4 tons were recovered.

Optical Detection of Large Meteoroids in Space

AbstractCCD sensors placed across the focal plane of large Schmidt telescopes have great potential for detecting and measuring the very low flux in space of meteoroids with diameters larger than 1 meter. With the Palomar “Big Schmidt”, a detection rate of 1.4 per hour is obtained for meteoroids between 0.6 and 200 meters in diameter. For the Baker-Nunn “Satellite Tracking Camera”, the corresponding rate is about 0.8 per hour. The key to obtaining such high detection rates derives from approximately setting the sensor integration time equal to the time it takes a meteoroid to cross a pixel field of view. This minimizes signal to noise problems and is accomplished, in practice, by multiple summing of short integration time data records to obtain data records of longer effective integration times.

The oblique impact hypothesis and relative probabilities of lunar and Martian meteorites

The author has previously suggested that shergottites and possibly other SNC meteorites were launched from the Martian surface by the oblique impact of large meteoroids. This hypothesis is further evaluated in the context of the subsequent discovery of a lunar meteorite. The oblique impact hypothesis is consistent with observations concerning the lunar and putative Martian meteorites. Oblique impacts are comparatively rare, consistent with the observation that ejecta from only one lunar event and one or two putative Martian events have been recognized. The hypothesis is also consistent with the observation of ejecta from a volatile‐poor object, the moon, as well as the probable existence of ejecta from a more volatile‐enriched object, Mars. Furthermore, in spite of the comparative rarity of oblique impacts, there exists at least one candidate source crater for the lunar meteorite in the lunar highlands adjacent to a source of the VLT basalt fragment discovered in it. There also exist at least two candidate source craters on Martian terrain of appropriate age to be the source terrain of the SNC meteorites. The candidate source craters are of appropriate size to be representative of events that have contributed to the current inventory of a continuous background of space stored lunar and Martian ejecta. Finally, it is shown that within calculational uncertainties and the current poorly known fall statistics of lunar and probable Martian meteorites, the oblique impact hypothesis gives a satisfactory explanation of the relative abundances of lunar, Martian, and other meteorites. These results support the oblique‐impact‐on‐Mars origin of SNC meteorites and give a satisfactory explanation for the occurrence of a lunar meteorite.

Do oblique impacts produce Martian meteorites?

Geochronological and geochemical characteristics of several achondritic meteorites match those expected of Martian rocks. Several authors have suggested that these meteorites might have originated on Mars, but no satisfactory explanation has been given of how they may have been ejected from the Martian surface. It is suggested here that the oblique impact of large meteoroids may produce ejecta which is entrained with the ricocheting projectile and accelerated to velocities in excess of Martian escape velocity. This suggestion is based on earlier experimental studies of oblique impacts and on the observation of several large Martian craters with the characteristic ‘butterfly’ ejecta pattern produced by low angle impacts. Several acceleration mechanisms may act on the Martian ejecta. At high impact velocities, the ricocheting projectile should be vaporized and fluid dynamic drag should act on the entrained ejecta. The drag equation can be integrated for an idealized representation of ricochet and explosion of the projectile. It is shown that large ejecta fragments, on the order of 1–10% of the initial projectile radius, could be drag accelerated to velocities in excess of the Martian escape velocity. Fragments greater than or equal to about a meter in size will escape the Martian atmosphere if they are launched at velocities significantly in excess of the Martian escape velocity. Impacts capable of launching sizable ejecta are expected to occur at a maximum rate of about 4×10−8 yr−1 on Mars and at a comparable or lower rate on the moon. The long transit time between Mars and earth would lead to establishment of a steady state population of potential Martian meteorites. The much shorter transit time from the moon to earth would cause any lunar ejecta to arrive in spurts which are short in comparison to their repetition rate. Thus the absence of lunar meteorites from our collection could be a simple observational effect due to the short terrestrial lifetime of meteorites. These considerations are preliminary in the sense that several simplifications and assumptions are made. However, they suggest that a Martian origin of the shergottite meteorites is dynamically possible.

Atmospheric waves from large meteors

The body of airwave data taken by the U.S. Government for the purpose of atmospheric monitoring of distant nuclear explosions, contains fortuitously waves of natural origin as well. We have recently received a portion of this original data base which contains only those signals which are presumably of meteoric origin; other natural signals such as from aurora, volcanic eruptions, meteorological phenomena, etc. having been previously removed from the sample. The ten meteoroid related events have amplitudes of about 0.5 to 10 dynes/cm2 and periods at maximum signal amplitude of about 3 to 45 s at horizontal ranges of about 800 to 14 000 km from the source. Using appropriate signal characteristics with models for the source and for propagation of these waves, source energies can be inferred. For these events energy estimates range from about 0.1 kT to as much as 550 kT (TNT equivalent) assuming that a low altitude point source explosion model is applicable. This detailed data set, having been recorded at several widely spaced arrays around the globe, could be used for a new attempt at modeling the propagation of such waves perhaps using methods suggested by Pierce and Kinney. [AFGL-TR-76-0056, Final Report, 13 March 1976.]

A predictive macroscopic integral radiation efficiency model

A procedure is developed which unites the photometric and dynamic methods of calculating large meteoroid properties from their observed behavior in the atmosphere. By utilizing a linear separation of variables solution for the kinetic energy depletion factor in the classical single‐body entry model the relationship between this factor and the various efficiencies of heat, radiation, ionization, compressional waves, etc. is obtained. By using this equivalency the radiation efficiency (integral value of the fractional ‘light’ production) can be calculated immediately without recourse to the complex quantum mechanical considerations first introduced by Öpik if only the ratios of the various efficiencies to one another are known. By using the free molecular flow value of these ratios the radiation efficiency is predicted via this macroscopic approach without using further meteor data. If other nonintegral values of the radiation efficiency, such as those of ReVelle and Rajan (1979), are converted to the present definition, good agreement between the methods is obtained. Such agreement is also found for integral radiation efficiency values computed recently by Ceplecha for a number of the Prairie Network fireballs. These results suggest that the above ratios do not vary greatly between the extremes of free molecular and continuum flow. This conclusion is also consistent with the results of earlier workers.