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Kratery Morasko w świetle wiedzy na temat ukośnych impaktów

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Walesiak T.

Acta Societatis Metheoriticae Polonorum

2017

vol. 8

149-168

It is considered that The Morasko meteorite shower happened about 5400 years BP with a trajectory from NE to SW (Bronikowska et al. 2015). Recent studies not only prove that this fall could be observed in Februrary 14, 1271, but also that location of most findings in the area to the east side of the craters, should not be taken as the main indicator of impact direction. The proper answer can be given base on knowledge related to oblique impacts, supported by understanding of complicated fragmentation process. The largest crater in Morasko Meteorite Restricted Area has main ejecta plume located in SE, side walls in SW and NE, and free of ejecta Zone of Avoidance (ZoA) in NW. This clearly shows that impact trajectory was approximatelly from NW to SE. There were also discovered other structures, 2 km North from Restricted Area, having common features. Their ejecta plumes as well as few raised ducts (possible traces of underground penetration) are directed to SE, pointing the largest structure – Umultowskie Lake, located approx. 2 km ESE from known main Morasko crater. Shapes of cavities, their walls and ejecta asymmetry allow estimating trajectories and defining new Morasko strewnfield, which matches not only location of impact strucures and discovered meteorite fragments, but also explains smaller (3–4 kg) findings in the neighborhood of Oborniki Wielkopolskie.

Kratery Morasko – obrona hipotezy ukośnego impaktu w kontekście dostępnej wiedzy i wyników badań

100%

Walesiak T.

Acta Societatis Metheoriticae Polonorum

2021

vol. 12

108-128

Over 2 years ago there was published an article (Bronikowska 2018a) challenging some results of research related to Morasko meteorite fall. Those results are huge problem for scientists who are not able to explain them using simplified models and maybe do not want to admit that some assumptions should be verified and redefined. One of hypothesis discussed in the paper was article manifesting that Morasko craters were created during oblique impact (Walesiak 2017) and that initial trajectory can be estimated by elongation of almost all cavities, their bilateral symmetries according to longer axis and observed asymmetries of rims. By comparison to Campo del Cielo craters Walesiak suggested that impact angle could be very shallow (10–20°), as ellipticity of all smaller Morasko structures is approx. 1,3. Further analysis of topography in neighborhood of Morasko revealed that there may exist more impact craters around this area, which may explain discovery of two iron meteorites near Oborniki village (17 km NW from Meteorite Morasko Restricted Area), unfortunately lost during World War II. In fact, Bronikowska confused definitions “oblique impact” and “elliptical craters”, what can be supported by given references (Elbeshausen et al. 2009) confirming Walesiak hypothesis. Unfortunately, this misunderstanding touches also another article of this author (Bronikowska et al. 2017), where obliquity was neglected. However, estimated during that modeling pre-impact angle (30–43°), according to known definitions written in many publications, should be classified as oblique impact. All elongated craters, as well as morphology of the largest structure in Meteorite Morasko Restricted Area suggest impact from NW while during reconstruction of meteoroid parameters Bronikowska assumed impactor coming from NE (based on existing strewn field). Considering not clear relation between meteorites and craters (e.g., lack of findings in cavities, relatively poor number of shrapnel fragments around structures and possibility that craters may be much older than fall of meteorites), it may be not enough reliable justification. Even if relation exists, example of Whitecourt (with distribution of meteorites downrange), compared with abundance of Morasko shrapnel fragments collected hundreds of meters NE from Restricted Area, undermines parameter used in modelling. Also, single fragmentation is doubtful, concluded after unreasonable omitting known craters outside Restricted Area (e.g., crater no 8 described by Pokrzywnicki (1964) and structure no 9 mentioned by Hodge (1994)). Finally, use of iSALE-2D shock physics code (valid for vertical impacts only) for modelling of elliptical craters formed during highly oblique impact (angle lower than 12°), even considering vertical component (which approximation is only applicable for impact into materials with a friction coefficient of about f=0,7 with no or very little cohesion) (Elbeshausen et al. 2009), also should not be considered as proper applied method leading to get valuable results. In this article there is proposed new strewn field definition for Morasko meteorite, based on elongation of all known and unconfirmed (yet) craters. Surprisingly, estimated (redefined) pre-impact trajectory gives convincing explanation for bilaterally symmetrical distribution of documented findings.

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2 Acta Societatis Metheoriticae Polonorum

1 2021

1 2017

2 Walesiak T.

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Kratery Morasko w świetle wiedzy na temat ukośnych impaktów

Kratery Morasko – obrona hipotezy ukośnego impaktu w kontekście dostępnej wiedzy i wyników badań