A Strange Angle
A Strange Angle hopes to elucidate on the continuing arguments experienced throughout the scientific community. This is just one of the many issues arising from the scientific debate on the K-Pg mass extinction and the Chicxulub impact crater.
Like a stone skipped across a pond or dropped from a bridge, the trajectory at which an object impacts a surface will dictate the features of its crater. Proposed at the early conception of Chicxulub’s discovery, Alan Hildebrand had asserted with utter conviction the impact occurred at a 45 degree angle. Depending on the mass, speed, diameter, density, angle, and lithology of target rock, the impactor and target played specific roles in the crater’s formation.
From the early discovery of the crater it had been presumed the diameter of the bolide ranged around 10 kilometres. Relative to the composition of the bolide, the diameter at which near identical crater features would be preserved start from around 12 kilometres for a comet to 10 kilometres for an asteroid. Since no living creature survived to record the size of the bolide, observing crater features such as depth, diameter, and ejecta distribution can lead scientists to deduce the diameter of the impacting bolide.
Alan Hildebrand, in his 1995 paper, described the morphology and processes governing the formation of Chicxulub. As craters increase in size, they undergo a gravity driven modification where the floor of the initial transient cavity rebounds upwards, and the crater margins collapse inwards, to form broad, shallow, complex craters. As the size increases further, this central peak is replaced by a peak ring, typically an irregular ring of hills and massifs, that lack prominent asymmetrical boundary scarps.
With the continued investigation into the morphology of Chicxulub, it could be said with certainty, the crater measured 180 kilometres in diameter and formed a multi-ring basin (Hildebrand, 1995). Defining the craters structure is distinctive asymmetries revealing an elongate central gravity high (trending northwest) encircled by a horseshoe-shaped gravity low (Hildebrand, 1991; Sharpton, 1993; Schultz, D’Hondt, 1996). Continuing to interpret the evidence collected from the gravity mapping surveys, from the distribution of the ejecta blanket and general morphology of the crater, Alan Hildebrand concluded the bolide impacted at around a 45 degree angle. Any irregular anomalies encountered in Chicxulub’s structure could be explained from the depth at which the transient cavity reached after excavation. From implementing scaling laws, Alan was able to formulate the transient cavity reached a depth of around 35 kilometres, with the maximum excavation depth reaching 12 kilometres (Morgan et al., 1997). Since the depth of the transient cavity reached such extensive depths, subsequent faulting and slumping would have quickly filled the gaping hole, creating the multi-ring basin we observe today.
A limiting factor continuing to hamper investigative procedures was the limited accessibility to samples. With approximately half of Chicxulub crater being submerged by water, gaining access to new samples for analysis cost astronomical amounts, amounts inaccessible to the team. Despite this hindrance, the team would forgo their limitations and piece together tangible conclusions.
Peter Schultz, a Ph.D in astronomy, became Associate Professor in the Department of Geological Studies at Brown University in 1984. Having completed various projects with NASA, Peter Schultz was promoted to the Science Coordinator at the NASA Ames Vertical Gun Range, which conducted experiments on the ballistic trajectories of crater forming processes. In 1996, Peter Schultz, with the help of Steven D’Hondt from the University of Rhode Island, published a paper in the journal of Geology claiming the Chicxulub impact occurred at an oblique angle of 20-30 degrees instead of the previously though 45. Rooted to the proposed claims was the asymmetrical geophysical signatures first discovered by Alan Hildebrand. Since Chicxulub’s structure exhibited distinctive asymmetrical features, Peter Schultz postulated from the ballistic trajectory experiments done in lab, an oblique impact best explained the features created. From Hildebrand (1991) and Sharpton’s (1993) prior papers, the gravity maps revealed an elongate central gravity high encircled by a horse-shoe shaped gravity low. Peter Schultz, continuing to instrument in lab models to interpret the crater, speculated that the bolide impacted from the Southeast sending a vast vapour cloud of rock hurling to the Northwest. To provide support for his claims he turned to observing the distribution of ejecta material across North America. In his 1996 paper, Peter Schultz stated there was a presence of less-shocked crystalline basement material downrange from the impact, greater preservation of meteoric spinels due to decreased pressure from the oblique impact angle, larger overall shocked quartz grains, and thicker two component layers in North America.
A consequence of the oblique angle as further told by Peter Schultz are the environmental repercussions created from the vapour cloud of melt rock launched towards North America. Due to the force of the impacting object at the 20-30 degree angle, the vast majority of excavation material would have been launched down range from the incoming trajectory. Being hot enough to instantaneously combust plant material, the produced vapour cloud destroyed everything along its path except some extremely lucky aqueously submerged organisms (Schultz, D’Hondt, 1996).
Stored in the sediment, preserved in time, was the pollen from regrowth after the impact had occurred and the climate had returned to a semi-hospitable state. Retrieved from the pollen samples were astonishingly large amount of fern spores. Fern is a naturally advantageous species that can quickly colonize areas reduced in species competition and deficient in mineral nutrients. This perceived ‘Fern Spike,’ indicated to Peter Schultz that North America had experienced the most environmentally devastating consequences after the impact, resulting in a much larger extinction rate then anywhere else on the globe (Schultz, D’Hondt, 1996). A ‘Fern Spike’ was also discovered off the coast of Japan, one small, seemingly unrelated discrepancy amongst the pollen analyzed. This however didn’t dissuade Schultz. The almost complete biota extinction of North America proved to Peter Schultz that the oblique impact hypotheses was true.
Many controversies and arguments had arisen from the discovery of Chicxulub crater, publicized from the media, the dinosaur extinction garnered massive attention from the general public. The issue with this publicity was the potential for pseudoscience to become intertwined with the legitimate work being conducted by scientists like Peter Schultz and Alan Hildebrand. The gold rush for evidence brought about by the discovery of Chicxulub crater and the allure of dinosaurs had casted a lot of doubt on the validity of opposing arguments, diminishing the explanatory power for some scientific theories.
Neutron Activation Analysis
The process of Neutron Activation Analysis requires a small sample of material to become sealed in a polyethylene or silica fused tube, suspended in the core of a nuclear reactor, and bombarded in a sea of neutrons (Muecke, 1980). Reacting only with a small number of atoms, the neutrons produce radioisotopes of the parent isotope present. Relative to the abundance of elements comprising the sample, each of the elements occurring possess a unique likelihood of receiving the neutron. Characterized by its neutron capture cross section, elements with larger cross section value will more readily form radioisotopes.
Hoping to better illustrate this, if a soil sample containing both magnesium and sodium are prepared for analysis, and the magnesium present in the soil has a much greater ratio then that of the sodium. The element that can more readily accept the incoming neutron will show up to a greater extent then the element that does not. Henceforth, if the Sodium better receives the neutron, it will have a larger ratio of radioisotopes in the sample then the magnesium (Hildebrand, Personal Communication, 2016).
From the new daughter isotopes formed, each possesses a characteristic energy and unique half-life related to its stable form. Quickly preceding the formation of the daughter isotope, the sample is transported to the lab to undergo radio assay, the detection of decaying nuclei. Through the process of half-life measurements or gamma ray spectrometry the radioisotopes are identified. The identity of the daughter isotope will reveal that of the parent isotope during analysis. To obtain the desired quantitative data, the quantity of parent isotope is observed by the decays of related daughter isotopes present. For a successful analysis to occur, the daughter isotope must have a large enough half-life as to not decay before detection, and also a short enough half-life so that the detection can occur in a relatively small amount of time (Muecke, 1980).
When Neutron Activation Analysis occurs a relatively stable form of the radioisotope needs to be chosen to ensure the above criteria is met. Iridium, with a large thermal neutron absorption cross section, has 2 naturally occurring isotopes and 34 lab constructed radioisotopes. To perform the detection tests, the activation of iridium 191 begins. Attaching itself to the atom, the newly collected neutron transforms the stable iridium 191 into the unstable iridium 192, which then later decays to the parent isotope. Possessing a 73.83 day half-life period, ¹?²I provided both the appropriate qualitative data and half-life period. Comparatively, iridium’s remaining radioisotopes have half-lives ranging from 24 hours to a few seconds, making the data collection almost impossible. Via this technique the iridium anomaly was discovered.