Antonio Zamora Podcast
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Penetration Funnels

Penetration funnels are impact craters that form when a projectile strikes a target and transfers its energy without disintegrating. This can only happen when the target material can be displaced by the moving projectile without generating forces that break up the projectile.

Penetration Funnels
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Penetration funnels are impact craters that form when a projectile strikes a target and transfers its energy without disintegrating. This can only happen when the target material can be displaced by the moving projectile without generating forces that break up the projectile.

On February 15, 2013, a meteor exploded over Chelyabinsk, Russia at 9:20 AM local time. The explosion sent a shock wave that shattered windows and wounded approximately 1500 persons. The intense light of the explosion also caused temporary flash blindness in about 200 persons. Many small meteorite pieces were strewn over the snow-covered landscape.

A meteorite piece weighing 654 kilograms, or about 1,442 pounds, penetrated the frozen surface of Lake Chebarkul making a hole eight meters wide. The meteorite piece was recovered seven months later from the bottom of the lake.

In 2015, a publication entitled "Snow compaction during the Chelyabinsk meteorite fall" reported that a few days after the meteorite fall, small fragments with an average size of 3 to 6 centimeters were collected by geologists from the Vernadsky institute. They found 450 meteorite fragments with a total weight of about 4 kilograms distributed over an area 40 kilometers south of the city of Chelyabinsk.

The collected fragments were mostly found in holes in the snow. The funnel walls of the holes showed an irregular topology and consisted of coarse-grained snow. Larger fragments penetrated the 70-centimeter-thick snow layer and reached the frozen ground surface. Smaller fragments made holes 15 to 25 centimeters deep in the shape of funnels of compressed snow that narrowed with depth and had a meteorite at the apex of the conical cavity surrounded by a dense shell of coarse-grained snow.

The image on the left shows a penetration funnel of compressed snow with a meteorite at the apex of the cavity. The soft snow surrounding the penetration funnel was removed to reveal its shape. These penetration funnels were nicknamed "snow carrots" by the researchers and they were sturdy enough that they could be removed for examination. The image on the right shows a snow carrot upside-down with sunglasses for scale.

The scientists developed a computer model for the penetration of a meteorite fragment into the snow layer. The time interval between each of the six frames is 2 milliseconds, starting at 1 millisecond. The red color corresponds to higher density and blue to lower density. The propagating tail beneath the projectile along the axis of symmetry is a numeric artifact. The formation of the funnel-shaped craters in snow and the occurrence of "snow carrots" at their tip is due to the compaction of the porous, fluffy snow by fragments with initial speeds of up to 80 meters per second. The walls of the penetration funnel show an increase in density of up to 18%, and the walls are about 3.4 centimeters thick.

A projectile moving through a viscous medium creates a conical shock wave. The conical cavity created by the passage of a projectile quickly disappears in elastic or fluid targets. The conical cavity made by a bullet in ballistic gel just leaves a tubular trail when the gel rebounds. The conical cavity made by a tennis ball in water disappears in a fraction of a second. However, a viscous target with low elasticity, like modeling clay, retains the conical geometry of the shock wave. This is similar to the meteorite impacts on snow in Chelyabinsk.

The walls of penetration funnels are formed at high pressures and are denser than their surroundings. This accounts for their persistence. A 2018 paper by Davias and Harris describes Carolina Bays as "palimpsest" features that resist erosion. A palimpsest is a manuscript that has been erased and written over, but on which traces of the original writing can still be discerned. This analogy describes well the rims of the Carolina Bays that persist in spite of having been subjected to encroachment by water and wind erosion for thousands of years.

The paper states that "A comprehensive survey of the bays using LiDAR bare-earth digital elevation maps, or DEMs, has documented their planforms as conforming robustly to a handful of ovoid shapes which diverge from an ellipse by subtle, yet distinctive markers. These shapes are cast as six archetypes, each found to be applicable to a specific geographic region."

These are the six archetypes of Carolina Bays referenced by Davias and Harris. They represent subtle variations that can be shortened or stretched to match the eccentricity evident in the LiDAR images by tracing a path along the highest portions of the rims.

Davias and Harris say: "Carolina bay raised rims appear to the naked eye to be gently arcing embankments, yet the LiDAR Digital Elevation Maps show them to be robust landforms tracking precisely to the archetype planform for kilometers. We speculate that these basins possess a palimpsest structural lithology that subdues the gradualistic mechanisms seeking to eradicate them by controlling their own topographic expression through differential weathering and erosion." Davias and Harris go on to suggest that the bays may have been emplaced up to eight hundred thousand years ago.

This image shows two large Carolina Bays. The one on the left has a length of 950 meters, or 3,117 feet. The raised edge of the bay conforms to the oval archetypes described by Davias and Harris. However, the flat portion of the bay, which corresponds to the original conical cavity, has a mathematically elliptical geometry. Zooming out, we see that there are many other Carolina Bays with elliptical geometry and that many of the bays overlap. In general, well preserved Carolina Bays are elliptical. Deviations from the elliptical geometry are usually due to the characteristics of the terrain where the bays are emplaced or by modifications caused by wind and water erosion or soil movement.

This experiment illustrates an oblique impact of an ice projectile on a viscous surface. The impact creates an inclined conical cavity and the overturned flaps produced by the penetration of the projectile become raised rims around the cavity.

Viscous relaxation reduces the depth of the conical cavity and restores the stratigraphy. Hydrostatic pressure increases with depth and horizontal laminar flow is faster at the bottom of the cavity, so the depth of the cavity is reduced from the bottom up in the reverse order in which the cavity was formed by the impact. These pictures show the transformation of inclined conical cavities into shallow elliptical depressions. Viscous relaxation can be speeded up by shaking the experimental container.

This is an example of an oblique impact by an ice projectile viewed along the path of the projectile. The target material is displaced laterally and pushed forward along the path of the projectile. Impact cratering displaces material laterally through horizontal compressive forces and ejects debris ballistically to produce stratigraphically uplifted rims. Viewed from above, the inclined conical cavities, which are conic sections, have an elliptical geometry.

The Glacier Ice Impact Hypothesis published in 2017 proposes that an extraterrestrial impact on the Laurentide Ice Sheet ejected glacier ice boulders, and that oblique secondary impacts of glacier ice liquefied saturated soil and made inclined conical cavities that became the Carolina Bays. The high energy of the impacts that made the bays may be responsible for the extinction of the North American megafauna 12,900 years ago. It is indeed amazing that the sandy rims of the Carolina Bays have withstood erosion for thousands of years and are still the predominant geological features of the Atlantic Seaboard.



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