10 Astronomical implications for the meteoroid and the impact event
Extraterrestrial ferrosilicides in meteorites
Regarding the peculiar matter found in the Chiemgau strewnfield, we notice striking correspondences to special kinds of meteorites and presolar grains. Only recently, different phases of FeXSiY, FeSi, Fe2Si (hapkeite) and FeSi2, have been found as new minerals in Dhofar 280 (Dh-280, Dhofar region, Oman, April 2001), a meteorite probably coming from the Moon (Anand et al. 2003). The meteorite FRO 90036 from Antarctica (Frontier Mountains) contains the mineral gupeiite (Fe3Si) (Wlotzka 1994), which we were able to detect also in the Chiemgau strewnfield matter [9].
Another unique found verifies gupeiite and xifengite as substantial parts of a meteorite (type CV) that was discovered 1984 in the Yanshan Mountains (Hebei, China) (Yu Zuxiang 1984). The low percentage of Ni is striking and reminds of the low Ni content in the Chiemgau strewnfield matter. Moreover, TiC together with gupeiite (Fe3Si) is the essential part of the centre of the spherules in the Yanshan meteorite (Yu Zuxiang 1984). A very similar composition is given in the Chiemgau impact samples [10].
α-Fe, FexSiy, TiC, and peculiar elements in presolar dust grains
We suggest that the peculiar material we found in the Chiemgau strewnfieled may be related with presolar matter. Quite recently, research work showed that α-Fe, FeXSiY, TiC, and certain peculiar elements are represented in grains of stardust coming from the primordial solar nebula and detected as well in primitive unaltered meteorites as in micrometeorites of the interplanetary dust, which had been collected in the stratosphere, in the ice sheet at the Earth’s poles and in deep-sea sediments. The grain sizes range from 1 μm to 1 mm. (Ferrarotti et al. 2000, Gordon et al. 2000, Ferrarotti & Gail 2002, Clayton & Nittler 2003, Chigai et al. 1999, Rubin 1997, Kimura & Kaito 2002, Bernatowicz 1996 a,b; Croat et al. 2002, Nittler 2003, Jessberger et al., and many others)
The Chiemgau strewnfield is widely scattered with ferromonosilicide (FeSi). Now, FeSi was detected in grains of circumstellar dust clouds. Stars with initial masses < 8 M (M = mass of the sun) go through a special evolution at the so-called Asymptotic Giant Branch (AGB) of the Hertzsprung-Russel diagram (HRD). During their transition from M-class to C-class stars, these S-class stars lose most of their originally mass by strong stellar winds (WN stars) or eruptions (LBV stars) into very thick dust shells. Among other materials, the dust condensates of these stars contain solid metallic iron (α-Fe) and FeSi. An important component assumed to trigger the nucleation of the dust is titanium carbide (TiC). In addition, dust grains consisting of β-FeSi2, which had been discovered in the nebula NGC7023, support the idea that FeXSiY is a part of interstellar matter.
The analysis of the material in the Chiemgau strewnfield showed a high percentage of TiC. This is understandable regarding its formation during the final stages of post-AGB stars and its property to trigger the condensation of metallic iron (α-Fe) and FeSi. From an examination of the Murchison meteorite it is known that within graphite spherules, which are associated to supernova (SN) ejecta, pure metallic iron (α -Fe) is joined to cubic TiC in the grains (Croat et al. 2002; Stadermann et al. 2002). We found that in the Chiemgau impact material cubic TiC had been embedded in a FeXSiY-matrix, which contains additional pure metallic iron (α-Fe) and higher phases of ferrosilicides (Fe3Si [gupeiite] and Fe5Si3 [xifengite] (analysis by Dr. Raeymaekers, InfraServ Gendorf). This is very similar to the composition of the matter associated with the star dust ejected by S-class stars and supernovae. We also observed significant values for V, Co, Zr, Nb, Mo, Ta, W in the material of the Chiemgau strewnfield. It is very interesting to note that the spherules embedded in interstellar dust grains contain Zr, Mo, Ru, which are related to the fractionating of elements during the condensation of the carbides (Bernatowicz 1991; Lodders 1996). V, Nb, Mo, Ru, Ta, W, Re are significantly present also in some types of carbonaceous chondrites (CV-class) (Rubin 1997).
Following these considerations we suggest that the meteoroid responsible for the Chiemgau strewnfield contained primordial matter from the time the solar system originated. We wouldn’t be surprised if future research would reveal the existence of diamonds in the material, because these seem to play an important role in the nucleation process in a dust cloud and had been confirmed in carbonaceous meteorites, which represent a clue for presolar matter.
Modeling the impact
Together with the considerations mentioned before and available computer programs (http://www.lpl.arizona.edu/~marcus/crater2.html [Earth Impact Effects Program]; http://www.lpl.arizona.edu/tekton/crater.html [Crater]; http://keith.aa.washington.edu/craterdata/scaling/index.htm [Crater Sizes from Explosions or Impacts]; http://janus.astro.umd.edu/astro/impact/ [Solar System Collsions]; software "Tunguska" (2002) by D. Neisius, based on Hills et al. 1993) we can try to get some impressions of the size, structure, and mass of the meteoroid. Within the limits of available data it is possible to describe the impact event itself .
From the high quantity of larger craters in the strewnfield and the widespread distribution of heavy ferrosilicides we conclude that the original meteoroid must have contained large solid lumps of higher density embedded in matter of very low density, possibly the so-called methane ice. Because of its content of methane gas, methane ice becomes instable and is flammable above 18 °C. Upon atmospheric entry of the meteoroid, changes in temperature and pressure cause the methane to be set free in a giant explosion heating up the atmosphere along the entry channel. Thus the effects of high temperatures and pressures including thermal and mechanical shock we observe in the materials from the Chiemgau strewnfield are easily understood.
The meteoroid responsible of this scenario could have been a planetoid or a comet of very low density. A candidate for an underdense body is a C-class (253) Mathilda type planetoid (density 1.3 + 0.2 g/cm³) thought to be completely shattered however reassembled to form an aggregate rather than a solid. However, with respect to the unusual matter we found associated with the Chiemgau strewnfield and to the extension of the scattering ellipse, we suggest a cometary impactor that mainly consisted of ice (methane, ammonia, water) and a relatively small part of solid stony and iron material.
Whereas the impact cratering of individual asteroidal projectiles is largely understood (e.g., Melosh 1989), the behaviour of a comet and its nucleus on their passage through the Earth's atmosphere and the crash with the ground are still disputed. For the Chiemgau strewnfield, a rough approach to the suggested cometary impact cratering is given by some computations. To fit our observations, we estimate that the projectile had a diameter of about 1.100 m and a mean density of 1.3 g/cm³. It passed the Earth's atmosphere under an entry angle of about 7° at an entry velocity of ca 12 km/s implying a start of the breakup at an altitude of 70 km. The major mass of the projectile hits the ground at a velocity of 0.99 km/s, and the impact energy is calculated to be roughly 106 megatons equivalent. The fragmentation results in a scattering ellipse of 59.7 km by 7.3 km, and the largest fragment is estimated to produce a simple bowl-shaped crater with a final 832 m diameter and a final 177 m depth. Based on the size distribution of the craters - the larger ones in the southern, the smaller ones in the northern part - we conclude that the meteoroid moved from northeast to southwest keeping an azimuth of about 43 degree to north. These figures should be taken as a glimpse on the event only taking into consideration that the effects of a giant methane ice explosion are not well known yet. A multiple fragmentation of the impactor before its striking the Earth cannot be excluded either, possibly explaining the very long and broad strewnfield for the larger craters.
The age of the event and historical implications
Upper and lower limits for the age of the event are given by the dating of trees from within the craters and especially from archaeological finds indicating a recent event in historical time. Trees rooting in craters and on their walls are 120 years old on average, and in one case we found a tree that is about 400 - 500 years old. The upper limit of the impact event is further lowered by the find of the peculiar matter described above, embedded in an undisturbed layer of a 1000-years-old forest (information forestry office; also see Rugner 1956). The peculiar material has also been found beneath the big retaining walls of the Burghausen castle dated to 15th c. AD. At another place, we dug out a treasure of coins (dated to 1540 - 1572 AD) located above the layer containing the ferrosilicides.
A lower limit is given by archaeological observations. The excavation of one of the craters has clearly affected a large artificial dam (11 km long), which according to archaeologists has been constructed in the High Middle Age about 10th - 12th c. AD or may be Graeco-Roman, between 50 BC and 50 AD (Stechele 1911, Harlander 1983). Other scientists suggest an earlier construction of the dam, some centuries BC. At another site, finds from the Celtic culture together with the peculiar impact material exhibit strange surfaces pointing to sudden heating on one side only (Fig. 41). Thus, the late Latène period (480 BC to 30 BC) may be the earliest date for the impact. To get a more precise age, we took ash samples from the layers associated with the impact event in several craters for radiocarbon dating the result of which will be available soon.
Fig. 41. Celtic archaeological finds give evidence of a short but strong heat pulse experienced upon ejection from a crater
In any case, the insignificant erosion of the craters, with exception of their destruction by human influence, is remarkable once more pointing to a recent impact event in historical time. Thus we think that a date fore the impact event can be set between the 5th c. BC and the 9th c. AD. We also checked the historical and climatic records for exceptional events falling in this period. Quite recently (Rigby et al. 2004) a cometary meteoroid with a size about 500 m was claimed to be responsible for the AD 536 event. The dendrochronology of Irish oaks shows that the tree-rings had been strongly influenced by a climatic change between 536 and 545 but also at around 207 BC. Regarding the possible earlier date, is it only casual that at about 205/204 Roman authors handed down stories about several stone showers falling from the sky and terrifying the people? Because of those remarkable events, the Roman senate decided to bring back the conical shaped Needle of Cybele, a meteorite, which was recognized as the Great Mother Godess Cybele from Asia Minor to Rome. At present we can only speculate about an association with these both dates. Taking into account the sample of thermally shocked coins from the late Latène period, we tend to set the Chiemgau impact event into that period. At present we favor the early date, because of the fact that archeologists had already found roman relics at the rim of the big “Tüttensee” crater, which date about 200 AD.
In any case the effects on nature and people in the Altoetting-Chiemgau area, and probably in nearby regions must have been very strong. The big size of the meteoroid, which caused the Chiemgau strewnfield of craters, let us think that the area had been devastated for some decades. Therefore people should have avoided to settle down in this region over some decades. We are currently looking for gaps in the cultural tradition at the proposed earlier and later date. There are several blanks in the archaeological records which must be further evaluated, aided by the results of radiocarbon dating.
The effects of the impact
At a distance of 10 km people will feel the major seismic shaking as a quake of magnitude 6.0 on the Richter scale, two seconds after the fragments of the impactor hit the ground. It is expected that the damage is moderate regarding well-built structures, but is considerable with regard to poorly built objects. At 10 km distance, the air blast will arrive ca 30 s after the impact with a wind velocity of about 225 km/h. The excess pressure is estimated to have a peak at 142,000 Pa. This will cause collapsing of buildings, in particular wooden ones. Up to 90 % of the trees will be blown down, and 10 % should loose their branches and leaves.
But the effects could have been much stronger regarding the assumed explosion of methane ice in the atmosphere. The thermal shock, which we have observed in our material, supports such an expectation. A great radiant fireball should have been seen by the people. From this, the thin layer of ash is easily understood, which we found in and between the craters: The forest must have been ignited suddenly until the air blast had shut down the fires. In addition, we estimate that dust was blown up some kilometers high and transported around the world. Thus it will be possible to trace the event in the ice records of Greenland or Antarctica. People should have seen a big toroidal cloud. Finally, we checked what people would have heard in a distance of 10 km: The sound intensity would have reached about 103 dB, enough to cause strong ear pain.
