Designing global climate and atmospheric chemistry simulations for 1 km and 10 km diameter asteroid impacts using the properties of ejecta from the K-Pg impact
Authors: Toon Owen B. , Bardeen Charles , Garcia Rolando 
Published: May 2016

Abstract. About 66 million years ago an asteroid about 10 km in diameter struck the Yucatan Peninsula creating the Chicxulub crater. The crater has been dated and found to be coincident with the Cretaceous-Paleogene (K-Pg) mass extinction event, one of 6 great mass extinctions in the last 600 million years. This event precipitated one of the largest episodes of rapid climate change in Earth history, yet no modern three-dimensional climate calculations have simulated the event. Similarly, while there is an on-going effort to detect asteroids that might hit Earth and to develop methods to stop them, there have been no modern calculations of the sizes of asteroids whose impacts on land would cause devastating effects on Earth. Here we provide the information needed to initialize such calculations for the K-Pg impactor and for a 1 km diameter impactor. There is considerable controversy about the details of the events that followed the Chicxulub impact. We proceed through the data record in the order of confidence that a climatically important material was present in the atmosphere. The climatic importance is roughly proportional to the optical depth of the material. Several hundred-micron diameter spherules are found globally in an abundance that would have produced an atmospheric layer with an optical depth around 20, yet their large sizes would only allow them to stay airborne for a few days. They were likely important for triggering global wildfires. Soot, probably from global or near-global wildfires, is found globally in an abundance that would have produced an optical depth near 100, which would effectively prevent sunlight from reaching the surface. Nanometer sized iron particles are also present globally. Theory suggests these particles might be remnants of the vaporized asteroid and target that initially remained as vapor rather than condensing on the hundred-micron spherules when they entered the atmosphere. If present in the abundance suggested by theory, their optical depth would have exceeded 1000. Clastics may be present globally, but only the quartz fraction can be quantified since shock features can identify it. However, it is very difficult to determine the total abundance of clastics. We reconcile previous widely disparate estimates and suggest the clastics may have had an optical depth near 100. Sulfur is predicted to originate about equally from the impactor and from the Yucatan surface materials. By mass, sulfur is less than 10 percent of the mass of the spheres and nano-particles. Since the sulfur probably reacted on the surfaces of the soot, nano-particles, clastics and spheres, it is likely a minor component of the climate forcing; however, detailed studies of the conversion of sulfur gases to particles are needed to determine if sulfuric acid aerosols dominated in late stages of the evolution of the atmospheric debris. Numerous gases, including CO2, SO2 (or SO3), H2O, CO2, Cl, Br, and I, were likely injected into the upper atmosphere by the impact or the immediate effects of the impact such as fires across the planet. Their abundance might have increased relative to current ambient values by a significant fraction of current values for CO2, and by factors of 100 to 1000 for the other gases. For the 1 km impactor, nano-particles might have had an optical depth of 1.5 if the impact occurred on land. If the impactor struck a densely forested region, soot from the forest fires might have had an optical depth of 0.1. Only S and I would be expected to be perturbed significantly relative to ambient gas phase values. 1 km asteroids impacting the ocean may inject seawater into the stratosphere as well as halogens that are dissolved in the seawater. For each of the materials mentioned we provide initial abundances and injection altitudes. For particles we suggest initial size distributions and optical constants. We also suggest new observations that could be made to narrow the uncertainties about the particles and gases generated by large impacts.