Atmos. Chem. Phys., 16, 13185–13212, 2016 www.atmos-chem-phys.net/16/13185/2016/ doi:10.5194/acp-16-13185-2016 © Author(s) 2016. CC Attribution 3.0 License. Designing global climate and atmospheric chemistry simulations for 1 and 10 km diameter asteroid impacts using the properties of ejecta from the K-Pg impact Owen B. Toon1, Charles Bardeen2, and Rolando Garcia2 1Department of Atmospheric and Oceanic Science, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA 2National Center for Atmospheric Research, Boulder, CO, USA Correspondence to: Owen B. Toon ([email protected]) Received: 15 April 2016 – Published in Atmos. Chem. Phys. Discuss.: 17 May 2016 Revised: 28 August 2016 – Accepted: 29 September 2016 – Published: 27 October 2016 Abstract. About 66 million years ago, an asteroid about surface. Nanometer-sized iron particles are also present glob- 10 km in diameter struck the Yucatan Peninsula creating the ally. Theory suggests these particles might be remnants of the Chicxulub crater. The crater has been dated and found to be vaporized asteroid and target that initially remained as va- coincident with the Cretaceous–Paleogene (K-Pg) mass ex- por rather than condensing on the hundred-micron spherules tinction event, one of six great mass extinctions in the last when they entered the atmosphere. If present in the great- 600 million years. This event precipitated one of the largest est abundance allowed by theory, their optical depth would episodes of rapid climate change in Earth’s history, yet no have exceeded 1000. Clastics may be present globally, but modern three-dimensional climate calculations have simu- only the quartz fraction can be quantified since shock fea- lated the event. Similarly, while there is an ongoing effort to tures can identify it. However, it is very difficult to determine detect asteroids that might hit Earth and to develop methods the total abundance of clastics. We reconcile previous widely to stop them, there have been no modern calculations of the disparate estimates and suggest the clastics may have had an sizes of asteroids whose impacts on land would cause dev- optical depth near 100. Sulfur is predicted to originate about astating effects on Earth. Here, we provide the information equally from the impactor and from the Yucatan surface ma- needed to initialize such calculations for the K-Pg impactor terials. By mass, sulfur is less than 10 % of the observed mass and for a 1 km diameter impactor. of the spheres and estimated mass of nanoparticles. Since the There is considerable controversy about the details of sulfur probably reacted on the surfaces of the soot, nanopar- the events that followed the Chicxulub impact. We proceed ticles, clastics, and spheres, it is likely a minor component through the data record in the order of confidence that a cli- of the climate forcing; however, detailed studies of the con- matically important material was present in the atmosphere. version of sulfur gases to particles are needed to determine The climatic importance is roughly proportional to the opti- if sulfuric acid aerosols dominated in late stages of the evo- cal depth of the material. Spherules with diameters of sev- lution of the atmospheric debris. Numerous gases, including eral hundred microns are found globally in an abundance CO2, SO2 (or SO3/,H2O, CO2, Cl, Br, and I, were likely that would have produced an atmospheric layer with an op- injected into the upper atmosphere by the impact or the im- tical depth around 20, yet their large sizes would only allow mediate effects of the impact such as fires across the planet. them to stay airborne for a few days. They were likely im- Their abundance might have increased relative to current am- portant for triggering global wildfires. Soot, probably from bient values by a significant fraction for CO2, and by factors global or near-global wildfires, is found globally in an abun- of 100 to 1000 for the other gases. dance that would have produced an optical depth near 100, For the 1 km impactor, nanoparticles might have had an which would effectively prevent sunlight from reaching the optical depth of 1.5 if the impact occurred on land. If the Published by Copernicus Publications on behalf of the European Geosciences Union. 13186 O. B. Toon et al.: Designing climate and chemistry simulations for asteroid impacts impactor struck a densely forested region, soot from the for- that the environmental effects of impacts scale with the im- est fires might have had an optical depth of 0.1. Only S and pactor energy, or cube of the diameter, not diameter (or crater I would be expected to be perturbed significantly relative to size). The Popigai object likely had about 12 % of the energy ambient gas-phase values. One kilometer asteroids impacting of the Chicxulub object. Surprisingly, except for collisions the ocean may inject seawater into the stratosphere as well as in the ocean (Pierazzo et al., 2010), climate models have not halogens that are dissolved in the seawater. been used to determine the destruction that might be caused For each of the materials mentioned, we provide initial by objects near 1 km in diameter, a suggested lower limit to abundances and injection altitudes. For particles, we suggest the size of an impactor that might do significant worldwide initial size distributions and optical constants. We also sug- damage (e.g., Toon et al., 1997). gest new observations that could be made to narrow the un- Here, we describe the parameters that are needed to ini- certainties about the particles and gases generated by large tialize three-dimensional climate and atmospheric chemistry impacts. models for the Chicxulub impact and for a 1 km diameter as- teroid impact. Nearly every aspect of the K-Pg impact event is uncertain and controversial. We will address some of these uncertainties and controversies and make recommendations 1 Introduction and definitions for the initial conditions that seem most appropriate for a cli- mate model, based upon the geological evidence. We will About 66 million years ago, an asteroid around 10 km in di- also suggest the properties of the initial impact debris from a ameter hit the Earth near the present-day Yucatan village of 1 km diameter asteroid. Chicxulub and created an immense crater whose age coin- There are numerous observed and predicted components cides with the Cretaceous–Paleogene (K-Pg) global mass ex- of the Chicxulub impact debris. The distal debris layer, de- tinction (Alvarez et al., 1980; Schulte et al., 2010; Renne et fined to be the debris that is more than 4000 km removed al., 2013). There is an enormous literature concerning this from the impact site, is thought to contain material that re- event and its aftermath. Surprisingly, however, there are very mained in the atmosphere long enough to be globally dis- few papers about the changes in climate and atmospheric tributed. This distal layer, sometimes called the fireball layer chemistry caused by the debris from the impact while it was or the magic layer, is typically only a few millimeters thick in the atmosphere, and no studies based on modern three- (Smit, 1999). As discussed below, the layer includes 200 µm dimensional climate models. Nevertheless, this event was al- sized spherules, 50 µm sized shocked quartz grains, 0.1 µm most certainly one of the largest and most dramatic short- sized soot, and 20 nm sized iron-rich material. term perturbations to climate and atmospheric chemistry in We discuss each of the components of the distal layer Earth’s history. in detail below. In brief, we find the following: the large There is substantial evidence for many other impacts in spherules are not likely to be of importance to the climate Earth’s history as large or larger than that at Chicxulub, because they would have been removed from the atmosphere mostly in the Precambrian (e.g., Johnson and Melosh, 2012a; in only a few days. However, they may have initiated global Glass and Simonson, 2012). There is also a growing effort to wildfires. The shocked quartz grains, one of the definitive find asteroids smaller than the one that hit Chicxulub, but pieces of evidence for an impact origin as opposed to vol- whose impact might have significant global effects, and to canic origin of the debris layer, are likely only a small frac- develop techniques to stop any that could hit the Earth. For tion of the clastic debris. It is difficult to identify the rest of example, as of 17 November 2015, NASA’s Near Earth Ob- the minerals produced by crushing because there is material ject Program identifies 13 392 objects whose orbits pass near in the layer that might have been produced long after the im- Earth. Among these objects, 878 have a diameter of about pact by erosion and chemical alteration of the large spheres 1 km or larger, and 1640 have been identified as “potentially or from the ambient environment. One major controversy hazardous asteroids”, which are asteroids that pass the Earth surrounding the clastic material is the fraction that is sub- within about 5 % of Earth’s distance from the sun, and are micron sized. Particles larger than a micron will not remain larger than about 150 m diameter. in the atmosphere very long and, therefore, are less likely to There is evidence for such smaller impacts in recent ge- affect climate. Unfortunately, the submicrometer portion of ologic history from craters, osmium variations in sea cores the clastics in the distal layer, which might linger in the at- (Paquay et al., 2008), and spherule layers (Johnson and mosphere for a year or more, has not been directly measured.