|
|
| Fig. 1: The Chixculub impact in the Yucatan Peninsula. (Source: Wikimedia Commons.) |
From my perspective, a big reason why the Chicxulub impact came to be a popular topic for discussion is that researchers hypothesize it may have caused the mass extinction at the Cretaceous-Paleogene (K-Pg) boundary. While there are many disagreeing researchers and scientists who study the impact ejecta and attempt to determine the exact date of the impact, there are several basic facts that can be certain. About 65.5 million years ago, an asteroid crashed into Chicxulub, Mexico (hence, the name "the Chicxulub Impact"). [1] It left a 180-km-diameter crater in the Yucatan Peninsula, in which the rocks have chemical and isotopic compositions similar to those of tektites found in the K-Pg boundary, which is why the age of the crater points do the K-Pg boundary age. [2] You can see this in Fig. 1. In this essay, I will not discuss the theories that varying scientists posit (on whether the Chicxulub impact actually caused the extinction of dinosaurs and the like). Rather, I will turn to the evidence that they have gathered on energy and climate changes and attempt to summarize it in a fashion that the layman can understand.
When the asteroid hit Chicxulub and created the crater, a large amount of energy was released. In the paper, "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact", the researchers discover that the energy of the Chicxulub impact is simply a function of the mass and velocity of the asteroid. However, in their study, the researchers used 5 main sets of parameters to estimate the energy of the impact: crater size, ejecta volume, melt sheet volume and chemistry, meteoritic content in the global ejecta, and population and size statistics of asteroids and comets in Earth-crossing.
In the method that attempts to determine the mass of the asteroid, the researchers call upon a study conducted by Alvarez and his colleagues. They used both the iridium content and overall mass of the K-Pg boundary, see Fig. 2, to calculate an asteroid mass between 300-3200 gigatons. Later, calculations presented by Vickery and Melosh assume that a 14 km diameter asteroid impacting at 35 km/s is consistent with Alvarez's findings. One of the most difficult aspects of calculating the velocity of the projectile mass was determining whether the projectile mass was an asteroid or a comet. While a comet impact is slightly more probable than an asteroid impact, this probability must be put in the context of rare events given that the Chicxulub impact was indeed rare. From the analysis of the transient crater and the meteoritic material, the results indicate parameters corresponding to an impact energy range of about 6.7 × 1030 to 3.4 × 1031 ergs. After examining the crater size, the meteoritic content for mass, and the impact models for velocity, researchers find indications to believe that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7-3.4 × 1031 ergs of energy. [3]
Let us assume that the object had a mass of M = 3200 gigatons × 1012 kg/gigaton = 3.2 × 1015 kg and that it was traveling at zero velocity until it got sucked in by the earth's gravitational field. In terms of the radius of earth R = 3.67 × 106 m and the acceleration due to gravity at its surface g = 9.8 m sec-2, we have
E = M g R = 3.2 × 1015 kg × 9.8 m sec-2 × 6.37 × 106 m = 1.15 × 1023 joules = 1.15 × 1030 ergs But the energy budget of civilization is 5.0 × 1020 joules/year. So this amount of energy would power civilization for 230 years.
 |
| Fig. 2: Cretaceous-Tertiary boundary, 65 million years ago. (Source: Wikimedia Commons) |
Another important result of the Chicxulub impact was the change in the atmospheric composition. When large asteroids or comets impact planets, large amount of gases are released into the atmosphere. These released gases include H2O, SO2, and CO2, which significantly affect the global environment including the biosphere and as a result, global warming. [4] We must study the shock-induced devolatilization of the impactors and crustal material as they caused the release of these gases.
Carbonate rocks occupy 15-20% by volume of Earth's sedimentary rocks, and given that carbonate rocks are unstable at relatively high temperatures, shock-induced CO2 is released into the environment. In the Chicxulub impact, researchers estimated a 1-2°C increase in surface temperature due to the greenhouse effect of shock-induced CO2. However, CO can also be produced from the devolatilization of carbonate rocks, which can rise the surface temperature higher than the previous estimates observed from CO2 only. From the experiment conducted in "Direct measurements of chemical composition of shock-induced gases from calcite: an intense global warming after the Chicxulub impact due to the indirect greenhouse effect of carbon monoxide", the results showed that twice the amount of CO was released than CO2. After calculating the radiative forcing of tropospheric O3, produced by photochemical reactions promoted by the injection of CO gas into the atmosphere, the researchers find an intense global warming of about 2-5°C occurred in several years after the Chicxulub impact due mainly to the increase in tropospheric O3. [4]
While there are many theories around that discuss whether impact energy and the subsequent global warming actually coincided with the mass extinctions that happened during the K-Pg boundary, several facts are very clear. The impact released large amounts of energy that caused the planet to increase in temperature. The Chicxulub impact was caused by an asteroid or a comet, which was determined by examining the crater size and the meteoritic content. The event happened millions of years ago and caused a permanent change in the Earth's atmospheric composition. But whether that was enough to create the K-Pg boundary is a question still to be determined.
© Jessica Xu. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. R. Hildebrand et al., "Chicxulub Crater: A Possible Cretaceous/Tertiary Boundary Impact Crater on the Yucatán Peninsula, Mexico," Geology 19, 867, (1991).
[2] C. M. Belcher, M. E. Collinson, and A. C. Scott, "Constraints on the Thermal Energy Released From the Chicxulub Impactor: New Evidence From Multi-Method Charcoal Analysis," J. Geol. Soc., London 162, 591 (2005).
[3] K. O. Pope et al., "Energy, Volatile Production, and Climatic Effects of the Chicxulub Cretaceous/Tertiary Impact," J. Geophys. Res. 102, 21645 (1997).
[4] K. Kawaragi et al., "Direct Measurements of Chemical Composition of Shock-Induced Gases From Calcite: an Intense Global Warming After the Chicxulub Impact Due to the Indirect Greenhouse Effect of Carbon Monoxide," Earth Planet. Sci. Lett. 282, 56 (2009).
_boundary_(5090097984).jpg){kind=link}