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Gibeon Iron Meteorites
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Gibeon Iron Meteorites. Their Discovery, History and Research. By Svend Buhl
Maltahöhe solid proof for a mid air break up
Another feature described by Buchwald is structural alteration
in many Gibeons which indicates a
violent mid air break up: Many specimens show tensile-torsional
shear fractures with extensive necking, twisting, faulting and
plucking.
Because other researchers repeatedly argued that these
features may as well be evidence for impact deformations
Buchwald made clear: It is still widely accepted that these
plastic deformations so common to Gibeon are due to impact with
the earth. This cannot, however, be true since the distortions
often occur in numerous places along the periphery and almost always,
moreover, display necking and other
effects of tensile and shear deformation, rather than compression
from impact with the ground.
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The shield shaped 29.9 kg Maltahöhe mass displays several shear planes along its edges
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Buchwald also mentioned evidence for extensive reheating of
several Gibeon specimens for a short period of time, probably
as a result of local energy release during the violent atmospheric
rupturing. It may be more than a coincidence that the most
reheated specimens seem to come from the periphery
of the strewn field and thus represent material thrown 100
200 km away from the central area.
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Extension crack caved out through ablation |
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During the research for this article the author had the
opportunity to acquire a previously undiscribed specimen of the Gibeon meteorite
which provides additional evidence for a violent atmospherical break up of the Gibeon body.
The specimen discussed here was found by Manfred Gollub on
his farm between Maltahöhe and Gibeon in 1985 and sent to Germany under
the name of its find location. With dimensions of 42 x 36 x 6.6 cm the
meteorite forms a flat oval shield narrowing towards frayed edges. Due to the thin
profile of the shield shape the Maltahöhe mass
weighs only 29.96 kg.
One slightly concave surface is completely covered with small regmaglypts
which show an average extension of 2 cm and an average depth of ~1.2 cm.
The regmaglypts are distributed in a random pattern rather than in a radial
orientation. Their depth increases towards the centre of the concavity,
which is located ~ 8 cm
off the geometrical centre and on the expanded end of the shield.
The other surface is slightly convex, and apart from a 15 cm long groove, shows
little plastic sculpting. Opposite the concavity from the regmaglypted side the
convex surface shows a narrow apex which also constitutes the centre of
gravity of the mass. No obvious flight marks are visible on this surface other than the groove.
Though no fusion crust is preserved the overall erosion on the Maltahöhe mass
can be considered as very moderate. Regmaglypts are still clearly defined,
extension cracks show only superficial caving or smoothing and the common
exaggeration of features as well as any sharp edges or spikes on
either surface, which are usually attributed to soil etching, are absent.
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Maltahöhe shows well preserved regmaglypts indicating a very modest degree of soil etching
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One third of the roughly oval fringe of the shield shows vaulting
towards the regmaglypted side, most likely due to atmospheric ablation.
The other two thirds are jagged shear surfaces clearly indicating a
rupture of the fringe across a plane that was at least 4 5 cm thick.
Short expansion cracks, wedge shaped clefts and two twisted peninsula
shaped protrusions account for a violent fragmentation. Despite some
degree of ablation on their edges all these features still show the
jagged shrapnel like appearances which are characteristic to shear
surfaces. All textures are only lightly overformed by flight ablation.
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L-shaped shear plane along the edge of the meteorite overformed by athmospheric ablation |
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It is interesting to note that along parts of the edge the rupture
left a staircase shaped shear profile. Namely the parts which sheared
off tore more material from the regmaglypted side of the shield than
from the opposite flat side. This is perhaps an indication of an oriented
flight at the time of the break up and may be treated as evidence for
a temperature gradient and differences in the depth of heating between
the two surfaces. It is nearby to assume that due to a higher degree of heating the leading surface offered a
slightly higher coefficient of extension than the trailing surface and thus could
be stretched further before the metal gave in.
Evidence for atmospheric ablation subsequent to the break up is present
in the shape of regmaglypts extended over the edges and onto the shear
surfaces. One of the expansion cracks from the fringe continues on the flat
surface where it shows extensive carving and widening into a channel like
groove. The smoothness and uniformity with which the 3 mm crack has been
extended to a depth and width of 12 x 12 mm as well as the absence of other
exaggeration features on the specimen suggest
a forming through atmospherical ablation rather than through terrestrial
erosion.
The Maltahöhe mass not only lends proof to the hypothesis of one or
several violent atmospheric fragmentations of the Gibeon meteoroid but also
delivers evidence for a continuing ablation of the falling bodies subsequent
to this violent airburst. Judged by regular ablation rates of other
iron meteorites (~ 0.2 cm/sec, Lovering et al. 1960) and by the depth
to which the expansion crack was hollowed out, it may be presumed that
the break up occurred early during the descent, allowing considerable
ablation through the continuing supersonic plasma flux.
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