Sciency Thoughts: Hunting for fragments of the Benešov Superbolide.

At three minute past eleven pm on 7 May 1991 the brightest fireball (large meteor) ever recorded was observed over the Czech Republic. This was recorded by four all-sky and two spectral cameras at three observatories belonging to the European Fireball Network, enabling detailed recording of its trajectory, spectrographic analysis of its chemistry and reconstruction of its orbit, and enabling a very detailed estimate of the area where any surviving fragments might have fallen. As this was only the third instrumentally recorded meteor fall (after Příbram in 1961, Lost City in 1971 and Innisfree in 1978) and was clearly a particularly large object, there were great hopes for material from the Benešov Superbolide being recovered. However several extensive searches failed to recover any such material.

In a paper published in the journal Astronomy & Astrophysics on 13 October 2014, Pavel Spurný of the Astronomical Institute of the Academy of Sciences of the Czech Republic, Jakub Haloda of the Czech Geological Survey and of Oxford Instruments NanoAnalysis, Jiří Borovička and Lukáš Shrbený, also of the Astronomical Institute of the Academy of Sciences of the Czech Republic and Patricie Halodová, also of the Czech Geological Survey re-examine the data obtained on the Benešov Superbolide with modern methods, in order to gain a better insight to the area where any fragments might have fallen, with the aim of recovering such fragments and studying the nature of the object which produced them.

Detail of the Benešov Superbolide recorded by the fixed all-sky camera at Telč station showing the main terminal flare at a height of 24 km from where a cloud of small fragments originated. Spurný et al. (2014).

Since the Benešov a number of such bolides have been observed and tracked in order to locate debris which have reached the Earth’s surface (notably Morávka in 2000, Neuschwanstein in 2002, Villalbeto de la Peña in 2004, Bunburra Rockhole in 2007, Jesenice in 2009, Košice in 2010 and Mason Gully in 2010), enabling scientists to considerably improve the methods involved.

The Benešov was recorded from three observation stations, Ondřejov, Telč, and Přimda, equipped with fixed high resolution fish-eye cameras which recorded all-sky hemisphere images. These cameras had rotating shutters which caused 12.5 interruptions per second. The Ondřejov station also had a guided camera used to determine the time of the fireball, though images from both Ondřejov stations were badly overexposed. The event was also observed with two spectrographic cameras, producing the most detailed spectrographic recordings of a fireball event ever recorded.

Modern GPS technology has enabled more precise positioning of the three observatories than was possible in 1991. At the time the positions of the observatories were calculated using 1:25 000 scale topographic maps; the units have now been placed with GPS units with a precision of 10 cm, leading to the precise longitude and latitude of each station being adjusted by as much as 20 m. Improvements in computer technology have enabled considerably improved calculations of bolide trajectories to be made from recorded images, and (importantly) an error made in the recording of the time on the Telč images (which at the time was done by hand) was detected by analysis of the position of the stars using modern software and corrected.

This led to a revision of the calculated location of the meteorite fall by about 385 m to the southwest, placing any such material in a ploughed field rather than coppiced woodland, and explaining the inability of searchers in the 1990s to find any meteorite material.

Detail of the terminal part of the Benešov bolide, where the projection of the original trajectory solution (dashed line) is plotted along with the new solution from 2011(filled line). Spurný et al. (2014).

This data also enabled Spurný et al. to recalculate the orbital properties of the Benešov Superbolide, which they calculate had a 1429 day orbital period and a highly eccentric orbit tilted at an angle of 24˚ to the plane of the Solar System, which took it from 0.92 AU from the Sun (i.e. 0.92 times the average distance at which the Earth orbit the Sun) to 4.04 AU from the Sun (4.04 times the average distance at which the Earth orbits the Sun, and considerably more than twice the distance at which the planet Mars orbit the Sun). This would make the Benešov Superbolide an Apollo Group Asteroid, i.e. an asteroid which spends most of it time outside the orbit of Earth, but which does occasionally pass closer to the Sun than us.

Heliocentric orbit of the Benešov meteoroid projected onto the plane of the ecliptic along with the orbits of all inner planets and Jupiter and the direction to the vernal equinoctial point. Spurný et al. (2014).

The Benešov Superbolide is calculated to have been between one and two meters in diameter, with a mass of about 4100 kg (4.1 tons). It is known to have undergone a final bright flare at an altitude of 24.4 km, interpreted as an airburst in which the bolide reached a sufficiently high temperature to explode. In 1991 it was believed that such an explosion would leave only a few large fragments intact, which would continue along the original path of the bolide until impacting the ground. Since then it has been realized that a much larger proportion of such objects will survive as smaller fragmentary material, with the Benešov Superbolide likely to produce around 250 000 fragments in the 1-10 g range including around 40 000 fragments larger than 5 g, for a total mass of 800-1000 kg, most of which would reach the ground.

Such small fragments would be heavily influenced by wind-speeds (also not fully appreciated in 1991). On 7 May 1991 the highest winds in the area were found at altitudes of between 5 and 12 km, and were blowing from the west and southwest, shifting the likely impact area for any small fragments roughly 2.5 km to the east-north-east.

New trajectory of the fireball with marked position of the main flare and the corresponding impact area for small pieces that originated in this flare. Spurný et al. (2014).

The area where any meteorite remains is calculated to have fallen lies in a field which has been ploughed at least 20 times since the Benešov event, and which is subject to winter frosts reaching a depth of 30 cm. This is not conducive to the preservation of meteorite remains at the surface, making it likely that any meteorite remains will have been buried at depth of 30-40 cm, and that they will have undergone considerable surface alteration, making them hard to distinguish from other rocks found locally.

Such conditions are far from ideal for searching for meteorite remains, however spectrographic analysis of the Benešov fireball suggested that the meteorite was chondritic in nature, and likely to have a very high iron content. This suggested that it might be possible to search for meteorites with metal detectors. Spurný et al. therefore assembled a team of about 20 searchers, and having gained permission from the landowner (searching the land without such permission would be illegal in the Czech Republic) made a series of transverse scans of the field a few hundred meters long and about fifteen meters wide, centred on the calculated line of highest probability for meteorite finds.

Details on meteorite finds and their positions with respect to the predicted impact line and impact area. Spurný et al. (2014).

The initial search took place on 9 April 2011, when several tens of samples were located, had their positions recorded with portable GPS units, were collected, weighed and labelled. These samples were then returned to the lab where they were cleaned, weighed, photographed and more carefully inspected, resulting in all but eight being rejected as possible meteorites. These remaining samples were further cleaned by ultrasound, then had part of their surfaces brushed and examined by microscope, eventually determining that two samples were genuine meteorites (a better result than was expected). Further visits to the site on 21 April 2011 and 25 April 2012 (when a method involving sieving topsoil from close to the line was employed) yielded two more such meteorites.

First three Benešov meteorites found by metal detectors in April 2011. From left to right: 1.56 g H5 chondrite (M1), 7.72 g LL3.5 chondrite with achondrite clast (M2), and 1.99 g LL3.5 chondrite (M3). Spurný et al. (2014).

There is a faint possibility that these meteorite could have come from some event other than the Benešov Superbolide. However the meteorites do not appear to be more than a few decades old and the area in question has been scanned for meteors photographically on every clear night since 1951, and photoelectrically on every night, clear or otherwise, since 1999. The area also has a reasonably high population density, with a high level of public interest in such events, making it unlikely that any such events would fail to be recorded. As the meteorites found were of a size which implies a parent body in excess of a meter in diameter, and only about 40 such objects strike the entire surface of the Earth each year, seldom going un-noticed in populated areas, the chances of tow such events happening in the same area within a few decades and one of them not being recorded are considered negligible.

The first meteorite discovered (M1) is an H chondrite (High Iron Chondrite) which weighed 2.91 g when found and 1.54 g after cleaning. It lacks a fusion crust (the outer layer of a meteorite formed by melting of its surface by the friction with the atmosphere) and it outer surface was heavily weathered. A section of the meteorite examined under the petrographic microscope revealed that it had a fairly homologous composition, with some chondrules (large distinct clasts of different material within the matrix) which appear to have been recrystallized as a result of thermal metamorphism. The rock contains olivine, low- and high-calcium pyroxene and plagioclase silicate minerals, with extensive shock-fracture features within the olivine. Heavy weathering of the sample has led to oxidation of about 80% of the iron and nickel minerals.

Backscattered electron images documenting the texture of H5 lithology of meteorite M1 – designated as Benešov (b). All Fe-Ni phases and troilite are strongly oxidized from weathering processes, and weathering products also fill visible microfractures. Spurný et al. (2014).

The second meteorite examined (M2) is an LL chondrite (Low Iron Low Total Metal Chondrite) which weighed 12.93 g when discovered and 7.72 g after weighing. This meteorite was also heavily weathered, and lacked a fusion crust. Microscopic examination of a thick section revealed a fine-grained matrix with well-defined chondrules. The silicate minerals were dominated by olivine, low-calcium pyroxene and plagioclase, as well as weathered alcalic glass. Iron and nickel minerals were again predominantly oxidised. The chodrules are 0.2-1.9 mm across and chemically distinct from the matrix, being dominated by olivins and low calcium pyroxene, with alkaline glasses with a variety of chemical compositions also present. Shock features are present in the olivine, pyroxene and plagioclase minerals, suggesting shock pressures in the range of 15-20 giga-Pascals.

Backscattered electron images documenting the texture of the LL3.5 lithology of meteorite M2 – designated as Benešov (a). Well-defined chondrules and fine-grained matrix are disrupted by a network of microfractures filled by weathering products. Spurný et al. (2014).

The meteorite also contained a large clast of achondritic material measuring approximately 4.8 by 2.6 mm. thi achondritic clast is cemented to the chondritic material by an irregular vein of impact melt, and has a composition dominated by olivine and low-calcium pyroxene, with anorthitic plagioclase and high calcium pyroxene also present.

Contact between LL3.5 lithology and achondritic clast of meteorite M2 – Benešov (a). The achondritic clast is cemented to LL3.5 lithology by an irregular vein of impact melt. Spurný et al. (2014).

The third meteorite collected (M3) weighed 2.29 g when found and 1.99 g after cleaning. This also lacked a fusion crust and was heavily weathered, and petrographically resembled the LL3.5 chondrite material from the second meteorite.

Backscattered electron images documenting the texture of the LL3.5 lithology of meteorite M3 – designated as Benešov (a). Well-defined chondrules and fine-grained matrix are disrupted by a network of microfractures filled by weathering products. Spurný et al. (2014).

The fourth meteorite weighed 0.50 g when collected and 0.38 grams after cleaning. This meteorite was not examined petrographically.

Spurný et al. believe that the meteorites all share a common parent body, which they propose had a brecciated composition (i.e. was made up of large pieces of material with different mineral compositions), which would account for the different mineralogy of meteorite M1 compared to M2 and M3, and for the large clast of mineralogically distinct material within M2. As such they wished to name all the meteorite as ‘Benešov Meteorites’. However the Nomenclature Committee of the Meteoritical Society did not accept this, due to the distinctive mineralogy of M1 (this is not entirely unreasonable, as such formal name and descriptions are used to compare meteorites to other meteorites, and a formal designation which includes meteorites with different compositions could be problematic). Spurný et al. therefore classed M2 and M3 together as ‘Benešov (a) Meteorites’, while M1 is classed as a ‘Benešov (b) Meteorite’.

Brecciated compositions in meteorites and asteroids are a relatively new idea, and a few years ago would have proved highly controversial. However the Almahata Sitta meteorite fall of 2009(?) has also been shown to contain brecciated material, as has asteroid (21) Lutetia, confirming that such lithologies are possible and do occur in asteroids.

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Tuesday, 3 March 2015

Hunting for fragments of the Benešov Superbolide.