The idea that the Carolina Bays were produced as a result of an extraterrestrial impact on the Laurentide Ice Sheet by the Great Lakes region has many supporters, but there are diverse proposals for the mechanisms that created the bays including oblique secondary impacts by ice boulders, impacts by slush balls, and deposition of sheets of ejecta slurry with imbedded steam bubbles. This presentation examines the characteristics of high-velocity impacts on an ice sheet and studies the thermodynamics of the ejecta during the ballistic trajectory.
An extraterrestrial impact is characterized by three phases: contact and compression, excavation and modification. The material that gets ejected during the excavation phase experiences launch, ballistic flight, re-entry through the atmosphere and secondary impacts.
The book by Professor Jay Melosh titled "Impact Cratering: A Geologic Process" describes the phases of an extraterrestrial impact in great detail. The contact and compression phase lasts only a few seconds and during this time, the projectile and the target are subjected to great pressures that exceed the yield strength of both the projectile and the target.
The excavation phase is an explosion that occurs when the great pressure of the contact and compression phase is released. A hemispherical cavity is formed by horizontal compressive forces and debris is ejected in ballistic trajectories that form a conical ejecta curtain. An experimental impact on a sand target illustrates the excavation phase of the impact. A curtain of sand is ejected as the impact cavity expands.
Professor Peter Schultz from Brown University has conducted many experiments of high speed impacts using the AMES high-speed gun of the National Aeronautics and Space Administration. The experiments consider impacts on sand targets, like the one we just saw, and also impacts where the sand is overlaid by a sheet of ice. A high speed impact on an ice sheet sends a shockwave that fractures the ice, and the ice pieces are ejected in ballistic trajectories from the impact point.
There are many misconceptions about impacts on an ice sheet. A publication in 2021 described a numerical model that treated the glacier ice layer as a plastic medium. This image illustrates the cells of the ice sheet undergoing plastic deformation. In this model, the volume of the ice cells almost doubles near the impact point.
This is completely wrong. Ice does not behave in this way. The shock wave of an impact fractures the ice sheet without changing the volume of the ice significantly, and ice fragments get ejected in ballistic trajectories from the impact site. The incorrect model leads to the conclusion that the ice just stays at the crater site and melts to form a lake. By contrast, the experimental evidence shows that the ice sheet breaks apart from the shock of the impact and the pieces of ice are ejected far away from the impact zone.
One hypothesis proposes that an extraterrestrial impact in the Michigan peninsula ejected a sheet of slurry several meters thick with imbedded kilometer-size steam bubbles, and that the Carolina Bays formed when the bubbles burst during deposition of the sheet of foam. This hypothetical mechanism does not take into consideration the physical forces of an extraterrestrial impact on an ice sheet and the material properties of ice.
The experiments by Professor Schultz of high-velocity impacts on ice sheets do not show the formation of any slurry, bubbles or foam. The experiments clearly show the fragmentation of the ice sheet and pieces of ice being ejected during the excavation phase of the impact. The confirmed extraterrestrial impacts on the Earth do not have any evidence of slurry sheets with bubbles.
Asteroid and comet impacts on Mars have produced many rampart craters that are accompanied by distinctive fluidized ejecta features. But there is no evidence that the energetic impacts that made these craters produced slurry sheets with kilometer-sized foam bubbles.
A viable hypothesis for the formation of the Carolina Bays has to be guided only by experimental evidence and not by numerical models or hypothetical mechanisms that have not been confirmed by experiments or geological evidence. There is no convincing confirmation that the impact of an asteroid or a comet can make a slurry sheet with kilometer-size steam bubbles capable of creating the Carolina Bays.
The modification phase of an impact starts when the expanding cavity has reached its maximum extent. At this point the force of gravity becomes dominant. For an impact on hard ground, the walls of the crater slide back into the crater, and a pool of molten material with broken rocks called breccia may form at the bottom of the crater.
The modification phase of an impact on an ice sheet is different. The experiments by Professor Schultz show that the ice sheet absorbs much of the energy of the impact and prevents the formation of a typical impact crater. The impact site on a glacier ice sheet would soon be flooded by the surrounding melting ice. Meltwater, rain and wind would erode the terrain, and most of the impact evidence would be washed away in a relatively short geological time. This is probably why the crater for the Younger Dryas cataclysm has not been found, but the likely site is at the point of convergence of the major axes of the Nebraska Rainwater Basins and the Carolina Bays.
A paper by Laydet, et al. tells us that Lake Agassiz started draining eastward at the onset of the Younger Dryas. We don't know for sure, but we can speculate that an extraterrestrial impact on the Laurentide Ice Sheet by the Great Lakes at the onset of the Younger Dryas could have fragmented the ice and opened up the eastern outlet for the drainage of Lake Agassiz. The impact would also have ejected ice boulders that created the Carolina Bays, caused a megafaunal extinction, and dispersed impact microspherules and platinum-rich material that we find today at the stratigraphic Younger Dryas boundary.
An extraterrestrial impact can create a crater in 30 seconds, but that is only part of the story. The material ejected by the impact has to go through different conditions. Some scientists have proposed that any ice boulders ejected by an impact on an ice sheet would have melted by atmospheric friction soon after they were launched.
A paper by Shuvalov and Dypvik that modeled extraterrestrial impacts by projectiles of various sizes says that there is no qualitative difference between the 100 m diameter projectile case and the 300 m diameter projectile impact. In both cases, fine ejecta decelerate in the air at a small distance from the launching point and then rise to the stratosphere by air flows induced by the impacts.
In the 1000 meter-scale impact, the mass of ejecta is so large that it moves the atmosphere itself to high altitudes. Thus, the atmosphere cannot decelerate even the fine ejecta and they consequently expand to the rarefied upper atmosphere. In the upper atmosphere, even fine ejecta move more or less ballistically and therefore may travel to high altitudes.
The extraterrestrial object that made the Carolina Bays is estimated to have had a diameter of 2000 to 3000 meters, so it is very likely that the ice boulders that were ejected by the impact on the Laurentide Ice sheet were able to start their ballistic trajectories without significant ablation or melting from atmospheric friction because the ejecta was lifted along with the atmosphere for such a large impact.
Ballistic flight refers to the time that a projectile is in transit from when it is launched to the time that it hits a target. The best way to analyze the trajectories of the ejected ice boulders is by using ballistic equations.
This table shows calculations for different distances and different angles that may be typical for the glacier ice projectiles that made the Carolina Bays. A distance of 1000 kilometers could be traversed by a projectile launched at an angle of 35 degrees with a speed of 3.2 kilometers per second. The projectile would take 6.3 minutes to get to the target, and the maximum height of the trajectory would be 150 kilometers above the surface of the Earth.
A projectile launched at an angle of 45 degrees with a speed of 3.8 kilometers per second can traverse a distance of 1470 kilometers. The projectile would take 9.1 minutes to get to the target, and the maximum height of the trajectory would be 368 kilometers above the surface of the Earth.
From these calculations, we can tell that the secondary impacts of glacier ice ejected from the Laurentide ice sheet started striking the East Coast from 6 to 9 minutes after the extraterrestrial impact in the Michigan area.
The atmosphere of the Earth has a thickness of only 100 kilometers, so all these trajectories were suborbital space flights in the vacuum of space. Without air resistance, the mass of the glacier ice projectiles would not decrease by ablation during their flight above the atmosphere.
In order to understand what happens to the ejected material in space, we need to study this graph showing the behavior of water with regard to pressure and temperature. At a pressure of one atmosphere, water freezes at 0 degrees Celsius and boils at 100 degrees Celsius. At sea level we experience pressure of one atmosphere or about 100 kilopascals, but the density of the atmosphere decreases with elevation above sea level. Denver, Colorado is at an elevation of 1609 meters or 5280 feet above sea level. Water boils at just below 95 degrees Celsius in Denver because of the lower atmospheric pressure.
The triple point of water is the temperature and pressure at which water in its three phases is in equilibrium. At this point, ice, water and water vapor may coexist indefinitely.
Notice that when the pressure is below the triple point, which is 6 thousands of one atmosphere, water can only exist as a solid or a gas regardless of the temperature. Liquid water at a pressure below the triple point is unstable and it boils. The evaporation cools the residual water and it turns into ice leaving only ice and water vapor when the system comes to equilibrium.
The National Weather Service provides a pressure altitude calculator. Six thousands of one atmosphere corresponds to a pressure of 4.56 millimeters of mercury. The calculator shows that at about 90,000 feet or 27.5 kilometers above the surface of the Earth the air pressure is below the triple point of water. This means that any liquid water launched above 27.5 kilometers will quickly turn into ice.
Yes, we can launch slush balls in ballistic trajectories, but as soon as the projectiles go beyond an altitude of 27.5 kilometers above the surface of the Earth, the slush balls become solid ice. This is how we know that the Carolina Bays were not made by slush balls or slurries or foam. It is thermodynamically impossible for liquid water to exist in the vacuum of space, as can be demonstrated in the laboratory.
This experiment puts some water in a beaker, which is then placed in a vacuum chamber. The air is pumped out until the pressure drops below the triple point. The water boils vigorously and the evaporation cools the remaining water. The liquid water then undergoes a phase transition into a solid. When the pressure is restored and the beaker is retrieved, we can see that the remaining water is frozen.
Using our analytical skills we can determine that, in a vacuum, 50 grams of water at 20 degrees Celsius transform into 7.8 grams of water vapor and 42.2 grams of ice. These are not numbers pulled out of the air. They can be calculated using the thermodynamics of phase transitions for water.
This phase transition diagram illustrates the effect of energy on the temperature of water in its solid, liquid, and gaseous phases. The heat for melting or freezing water, also called the Heat of Fusion, is 80 calories per gram at 0°C. It is necessary to remove 80 calories of energy to freeze one gram of water at 0°C. Liquid water requires one calorie per gram to raise or lower the temperature by one degree Celsius. The heat for boiling or condensing water at 100 degrees Celsius, called the Heat of Vaporization, is 539 calories per gram.
The beaker is filled to the 50 milliliter mark with water. Water has a density of one gram per milliliter, so this represents an initial weight of 50 grams of water in the beaker. The experiment is conducted at room temperature, which is about 20 degrees Celsius. Let X equal the number of grams of water that evaporates removing 539 calories per gram, and then 50-X is the number of grams of water that turns into ice. In order to freeze the water, its temperature has to be lowered from 20 degrees to 0 degrees, which requires one calorie per gram for each change of one degree. After the temperature of the water reaches 0 degrees, 80 calories per gram need to be removed for the phase transition from liquid water into solid ice at 0 degrees.
We set up an equation showing that the calories to freeze 50-X grams of water at 20 degrees Celsius are equal to the calories removed by the evaporation of X grams of water. Solving the equation, we find that 7.8 grams of water need to evaporate to reduce the temperature of the remaining water to zero degrees and freeze it.
This experiment proves that there can be no slush balls in the vacuum of space. For every 100 grams of water in the vacuum of space, the evaporation of 13 to 25 grams is enough to turn the remaining water into ice, depending on the water's initial temperature. Above 27.5 kilometers from the Earth's surface water can only exist as a solid or a gas because the atmospheric pressure is below the triple point of water.
When the glacier ice boulders re-entered the atmosphere they were subjected to ablation from aerodynamic friction. The atmosphere is only 100 kilometers thick, but the ice boulders came at oblique angles through about 174 kilometers of atmosphere, which at 3 km/sec requires 58 seconds, and at 4 km/sec the transit time is 44 seconds. The ballistic sedimentation of the ejecta curtain was a saturation bombardment where many ice chunks followed others closely. Slipstreaming reduced the air resistance and ablation for projectiles traveling close to each other. The water vapor created by atmospheric friction created vapor trails that quickly turned into black clouds.
Many people believe that ice boulders would have completely evaporated during re-entry. I set up an experiment where I heated a chunk of ice with a propane torch for one minute to simulate re-entry through the atmosphere. The initial weight of the ice was 388 grams and it lost only 38 grams after the experiment. This is a loss of less than 10 percent of the initial weight. Some of the ice boulders that made the Carolina Bays measured about 180 meters in diameter which is about the size of Yankee Stadium. Such enormous chunks of ice would not be easily vaporized.
During the excavation stage of an impact, material is ejected in the form of an expanding conical ejecta curtain. The innermost ejecta are launched first and travel fastest. Ejecta originating further from the center are launched later and move more slowly, falling nearer the rim. Large heavier material falls closer to the impact point and small lighter material falls further away.
The ballistic sedimentation of the ice boulders that made the Carolina Bays was complete by about 10 minutes after the impact. Plants and animals within 1500 kilometers of the epicenter were destroyed by the ice impacts. The extinction was localized from the Rocky Mountains to the Atlantic Coast, but it was aggravated by the destruction of habitat and the onset of a global winter that made life hard for any survivors outside the kill zone. The time of emplacement of the Carolina Bays varies widely using radiocarbon and Optically Stimulated Luminescence dating methods. The time at which the megafauna became extinct may be the most reliable way of dating the event.
We can estimate the size of the ice projectiles based on the diameters of the Carolina Bays using a calculator developed by Professor Jay Melosh and Ross Beyer. The calculator assumes circular craters, so an adjustment needs to be made based on the area of the ellipse. A Carolina Bay with a diameter of 1 kilometer would have been made by an ice projectile with a diameter of 180 meters. The impact at a speed of 3 kilometers per second would have a kinetic energy equivalent to 3.03 megatons of TNT. As a rough estimate, the diameter of the ice projectile is one fifth the length of the impact cavity.
Well preserved Carolina Bays have a mathematically elliptical geometry that can be confirmed by plotting points along the perimeter of the bay and fitting them with an ellipse using the least squares method. Ellipses are conic sections. This implies that the bays originated as inclined conical cavities, also called a penetration funnels.
This is an experimental impact of an ice projectile on a viscous target consisting of equal parts of sand and clay. Oblique impacts produce inclined conical cavities that look elliptical when viewed from above. Looking at the impact in slow motion reveals many details about how the cavity is formed.
This physical experimental model makes it possible to investigate the mechanism by which raised rims and overlapping bays are formed, and also how viscous relaxation reduces the depth of the cavity and restores the stratigraphy.
Using the distance of a bay from the Michigan peninsula and the angle of impact obtained from the width-to-length ratio, we can calculate the speed and height of the ballistic trajectories. The size of the ice projectile and its speed can be used to calculate the kinetic energy of the impact. The saturation bombardment by such energetic impacts had a devastating ecological effect.
The application of the laws of physics to the analysis of the Carolina Bays provides a mathematical foundation that can be used to estimate the size of the extraterrestrial object that impacted the Laurentide Ice Sheet using the law of conservation of energy. The kinetic energy required to create the Carolina Bays corresponds to the impact of a stony asteroid with a diameter of 3 kilometers or to the impact of a faster icy comet with a diameter of 2 kilometers. The study of the Carolina Bays is crucial for understanding the history of North America at the end of the Ice Age.
