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Ice Boulder Re-entry

The Glacier Ice Impact Hypothesis published in 2017 proposes that the Carolina Bays were made by secondary impacts of ice boulders ejected by an extraterrestrial impact on the Laurentide Ice Sheet. One of the objections against the hypothesis is the claim that the ice boulders would not have been able to withstand the aerodynamic forces of re-entry and that they would have disintegrated in the atmosphere just like a shooting star.

The invention of LiDAR in the early 1960s discovered a great number of bays by emphasizing the small differences in elevation. It is estimated that there are more than 500,000 Carolina Bays. In some areas, the bays are so dense that all the ground is completely covered by them.

LiDAR image of the Carolina Bays
LiDAR image of Carolina Bays 20 miles (32 km) southwest of Fayetteville, NC

The mathematically precise elliptical geometry of the Carolina Bays can be confirmed by fitting them with ellipses using the least squares method. Since ellipses are conic sections, it is likely that the bays originated as inclined conical cavities or penetration funnels. The orientations of the Carolina Bays and the Nebraska Rainwater Basins converge by the Great Lakes. The width-to-length ratios of the bays allow us to calculate the launch angles, and ballistic equations let us calculate the launch speeds required to get the ice boulders to their destinations.

A 2013 publication by Colgan and Arenson contains information on the fundamentals of open-pit glacier ice excavation from a variety of sources. The authors say that while ice is technically a non-Newtonian fluid over long timescales, it may be regarded as a low-density and low-strength rock, analogous to coal, for the practical purpose of ice excavation over short timescales.

The publication includes graphs of the compressive strength and the tensile strength of ice at various temperatures. The graph by Colgan and Arenson shows that the compressive strength of ice increases with decreasing ice temperature, from a minimum of about 3 megapascals at zero degrees Celsius to a maximum of about 40 megapascals at minus fifty degrees Celsius.

Max Q is the greatest force that is experienced by a body traveling through the atmosphere. Max q, or maximum dynamic pressure, is the point when an aerospace vehicle's atmospheric flight reaches the maximum difference between the fluid dynamics total pressure and the ambient static pressure.

The dynamic pressure Q is defined as one-half the air density, represented by the Greek letter rho, multiplied by the square of the projectile's velocity. The air density can be obtained from a public domain aeronautical table that lists the attributes of the atmosphere at different altitudes. Notice that the air density decreases with altitude and it is most dense at sea level. This table displays the air density from an altitude of 20 kilometers above the surface of the Earth to sea level in 5 kilometer intervals. These values allow us to estimate the dynamic pressure at various altitudes.

Calculating Max q for speeds of 3.5 and 4.0 kilometers per second, we can see that the aerodynamic forces are smaller than the compressive strength of the glacier ice, which has a high range of 15 megapascals. This means that the ice boulders could have survived re-entry through the atmosphere without breaking up. Some weak ice boulders with a compressive strength of 3 megapascals could have fragmented at an altitude of 5 kilometers above the surface during re-entry, but at a speed of 3.5 kilometers per second, the pieces would have hit the ground 1.4 seconds after the fragmentation. Prof. Melosh has written that a strike by a crushed but closely grouped collection of fragments would produce a single crater although it could have an anomalous morphology. The fact that the ice impacts produced inclined conical cavities that transformed into elliptical basins instead of fragmenting upon contacting the ground also provides evidence of the large compressive strength of the glacier ice boulders.


Topics about the Carolina Bays



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