The environmental effects of impact events differ with respect to time

Local and Global Environmental Effects of Impacts on Earth Elisabetta Pierazzo 1, and Natalia Artemieva 1,2 Dinosaurs became extinct after the impact of a giant asteroid. 1811-5209/12/0008-0055$2.50 DOI: 10.2113/gselements.8.1.55 The environmental effects of impact events differ with respect to time (seconds to decades) and spatial (local to global) scales. Short-term localized damage is produced by thermal radiation, blast-wave propagation in the atmosphere, crater excavation, earthquakes, and tsunami. Global and long-term effects are related to the ejection of dust and climate-active gases (carbon dioxide, sulfur oxides, water vapor, methane) into the atmosphere. At the end of the Cretaceous, the impact of a >10 km diameter asteroid led to a major mass extinction. Modern civilization is vulnerable to even relatively small impacts, which may occur in the near future, that is, tens to hundreds of years. Keywords: impact, shock waves, mass extinction, climate change, comets, asteroids INTRODUCTION Impact events punctuate the terrestrial geologic record (~180 impact craters have been identified so far; Reimold and Jourdan 2012 this issue), but they are not normally associated with environmental catastrophes. Large impacts are rare compared to floods, earthquakes, and other more mundane hazards; indeed, they are so infrequent that they are normally disregarded on the timescale of human evolution. The devastating consequences of a high-velocity impact on the terrestrial ecosystem became apparent in the 1980s, when the revolutionary work of Alvarez et al. (1980) linked the Cretaceous Paleogene (K Pg) mass extinction event 66 million years ago (66 Ma) with the impact of an asteroid larger than 10 km in diameter (see review by Schulte et al. 2010). To date, the K Pg boundary event is the only recognized mass extinction that coincides precisely with a large impact event. Many more impacts of similar size have occurred during Earth s history without substantial influence on life. The principal impact-related cause of environmental catastrophe is still debated, in part because of our incomplete knowledge of the impact process and in part because Earth s ecosystem is extremely complex and its response to the effects of a large impact event is still not well understood. In this review we focus on recent investigations of the environmental effects of impact events. Interested readers will find detailed background material about the environmental effects of impacts in Toon et al. (1997). 1 Planetary Science Institute 1700 E. Fort Lowell Rd, Suite 106, Tucson, AZ, USA 2 Institute for Dynamics of Geospheres, Russian Academy of Science Leninsky pr. 38, bldg.1, Moscow 119334, Russia E-mail: artemeva@psi.edu Author deceased THE IMPACT HAZARD Chapman and Morrison (1994) defined a globally catastrophic impact as one that would disrupt global agricultural production and lead, directly or indirectly, to the death of more than one-fourth of the world s population and the destabilization of modern civilization. They loosely estimated that the threshold for such a global catastrophe is an impact with an object between about 600 m and 5 km in diameter. Such events happen rarely in Earth s history (every 0.1 to 1 million years; Fig. 1). Much smaller objects (down to 50 100 m) collide with the Earth every 100 1000 years and have the potential to cause local devastation. The magnitude of an impact event (defined mainly by the projectile size) is the most important parameter for estimating potential impact effects. Together with the probability that such an impact occurs, magnitude defines the impact risk. The number of bodies of a given size that have impacted Earth over the last 3.9 billion years has been estimated, on the one hand, by combining the observed record of lunar impact craters with the absolute ages of a few impact structures deduced from absolute ages of lunar samples. On the other hand, telescopic observational surveys of small bodies orbiting the Sun, and associated dynamical models, have constrained the current number of possible impactors (with the exclusion of long-period comets) in hazardous, near-earth orbits as a function of size. Such objects are known as near-earth objects (NEOs) and make up most of the impactors hitting the surface of inner Solar System bodies. According to one such study, as of the beginning of 2011, approximately 85% of NEOs larger than 1 km in diameter have now been discovered (see NASA s NEO website, http://neo.jpl.nasa.gov/stats/). Figure 1 shows an estimate of the cumulative population of NEOs versus size compared to observed NEOs (Harris 2008). Given the high confidence that all asteroids larger than 10 km in diameter have been discovered among the current NEO population, the risk of a giant asteroid impact may be excluded for at least the next century. However, well over a hundred objects 1 to 2 km in diameter, and possibly thousands between about 600 m and 1 km in diameter, are probably orbiting in the Earth s neighborhood yet undiscovered. Comets represent only a small fraction (less than 10%) of all potential impactors. Unfortunately, early detection of comets is difficult to achieve. They are generally much fainter than asteroids of comparable size, and they are Elements, Vol. 8, pp. 55 60 55

NO DAMAGE D < 30 m (0.030 km) Airburst in upper atmosphere, no significant ground damage. AIRBURST 30 m < D < 100 m Airburst in lower atmosphere, causes damage similar to nuclear bomb blast above ground. Over ocean, no damage. REGIONAL / TSUNAMI 100 m 1 km Surface impact, on land, makes crater 2 20 km across. In the sea, raises a tsunami that can cause shoreline damage one to a few thousand km distant from the impact point. GLOBAL 1 km 10 km An impact into land or sea may raise enough dust into the stratosphere to cause a global catastrophe, leading to mass starvation, disease, and general disruption of social order. EXTINCTION 10 km and larger possibility of mass extinction, certainly of some species and possibly humans. Estimated cumulative population of near-earth objects Figure 1 (left axis) or impact frequency (right axis) versus projectile size (bottom axis) or impact energy (top axis), compared to the discovered NEOs as of January 2009. The graded color scheme shows the corresponding type of damage expected if NEOs of a given size were to impact the Earth s surface. Figure provided in fond memory of E. Pierazzo by A. W. Harris, Space Science Institute/Large Synoptic Survey Telescope much more difficult to discover before they turn on (i.e. their reflectance increases due to sublimation of volatiles, thus forming the comet s coma and tail) while approaching the inner Solar System. SHORT-TERM EFFECTS Short-term effects, produced over tens of seconds, accompany the atmospheric entry of a cosmic body, its collision with the surface (if any), and crater formation. They are mainly associated with the propagation of shock waves and are extremely intense, but limited to the region around the impact site. A detailed description of the cratering process may be found in Collins et al. (2012 this issue). Airblast The airblast is a shock wave in the atmosphere induced either by the entry process of a cosmic body or by the expansion of an impact plume. The resulting damage from an airblast depends on the peak overpressure (the maximum pressure in excess of the ambient atmospheric pressure, 1.013 bar) and the gas velocity behind the shock, commonly known as the wind speed. Determination of the damage associated with airblasts is routinely based on data from US nuclear explosion tests (Glasstone and Dolan 1977). Any hypervelocity cosmic body hitting Earth produces atmospheric shock waves, but the amount of damage on the surface depends on the size of the impactor. Small stony bodies (diameter <50 m) are efficiently decelerated in the upper atmosphere, where they lose most of their energy through an airburst; the resulting shock waves decay quickly in the atmosphere and reach the surface as a package of harmless acoustic waves. These waves may be used to estimate the meteoroid s parameters. Iron projectiles of similar size could survive the atmospheric passage and create a crater on the surface (e.g. Meteor Crater, Arizona, USA, 1.2 km in diameter). Larger stony bodies (50 300 m) are fully decelerated at lower altitudes, and strong shock waves may reach the surface. A classic example of such an airburst is the Tunguska explosion, which occurred in 1908 over a remote region of Siberia, Russia. The ground damage of this event is represented by a butterfly-shaped region of devastated forest extending over 2000 km 2. Numerical models (e.g. Boslough and Crawford 2008; Artemieva and Shuvalov 2010) indicate that the Tunguska airblast was caused by a 50 100 m diameter object releasing most of its energy at altitudes of 5 15 km. No crater or projectile material has been found at the surface within the damaged zone; the impactor material was carried by the impact plume, dispersed in the upper atmosphere, and then deposited in a large area of the northern hemisphere over a couple of days. The wellknown white nights that occurred all over Europe for two weeks after the event can be explained by a large amount of atmospheric water vapor being lifted by the plume from the troposphere to the mesosphere. Impact location plays an important role in these smallenergy events: if an explosion similar to Tunguska were to occur above a large city (such as Moscow), the blast would completely destroy the city (Fig. 2) and would also devastate today s global communications system. Earthquakes As the impactor hits the surface, a shock wave propagates through the target, quickly changing the thermodynamic state of material near the impact point in an irreversible process. At the surface, total destruction occurs within the final crater and extends at least one more crater radius farther as a result of the large volume of rock debris ejected from the crater and deposited in a continuous ejecta blanket (Fig. 3). The shock waves propagate along the surface, eventually attenuating into seismic waves, and may cause violent ground shaking up to several crater radii away. The intensity of the resulting seismic wave, which decreases with distance from the impact, depends on the impact energy. Impact experiments suggest that beween 10-3 and 10-5 of the impact energy is transferred to seismic energy (Schultz and Gault 1975). According to the classic Gutenberg-Richter magnitude energy relation, the K Pg impact, which created the Chicxulub crater, ~200 km in Elements 56

diameter, produced a destructive earthquake with a magnitude around 10. Seismic shaking from this giant impact initiated massive submarine landslides that devastated the continental slope many thousands of kilometers from the impact site (Norris et al. 2000). Wünnemann et al. (2010) proposed that these landslides may have caused tsunami (see also next section) and massive releases of methane from excavated methane clathrates, which over time affected the Earth s climate (Day and Maslin 2005). Tsunami Deep oceans cover about two-thirds of the Earth s surface. The exact hazard from impact-related tsunami for coastal communities is still controversial. The main debate centers on whether impact-induced waves can propagate on global scales like a typical tsunami caused by a submarine earthquake or whether such waves decay rapidly due to their different characteristics. Numerical modeling prompted Wünnemann et al. (2010) to conclude that the wave signal is primarily controlled by the ratio between projectile diameter, D, and water depth, W, and that such signals can be roughly classified according to whether impact is in deep or shallow water. In shallow-water impacts (D/W > 0.6, for impactors 1.2 km in diameter and oceans >2 km deep), the collapse of the crater rim produces a wave similar to the solitary waves generated in typical tsunami, which propagate and decay slowly according to shallow-water wave theory. Oceanic impacts are much more likely to be in deep water, where a relatively small body strikes the ocean with D/W < 0.4. In this case, the collapse of the transient crater in the water results in a significantly different and much more complex wave signal characterized by strong nonlinear behavior and rapid decay; thus such an impact would not likely constitute a major hazard for distant coastal communities. The consequences of a tsunami reaching coastal regions cannot be addressed easily in a general context, as they depend on local conditions, such as distance of the coastline from impact, ocean bathymetry, and shore and coastal configuration (Korycansky and Lynett 2007; Wünnemann et al. 2010 and references therein). Although a tsunami constitutes a short-term, mostly localized effect of an oceanic impact, it could have potentially devastating effects in highly populated coastal regions. Thermal Effects and Distal Ejecta Reentry During an impact, a significant fraction of the impactor s kinetic energy is converted into thermal energy that melts and vaporizes both impactor and target. Vaporized/melted material, combined with some fragmented solid material, is then ejected into a very hot (average temperature of 2000 3000 K), rapidly expanding impact plume (Fig. 3). The resulting thermal effect ensuing from the expanding plume depends on the magnitude of the impact event and is limited to areas within sight of the impact plume. Modeling studies indicate that radiation from the Chicxulub plume could have caused wildfires up to 2000 3000 km from the crater (Shuvalov and Artemieva 2002). For very large impacts (projectile diameter >1 km), the expanding impact plume carries material well beyond the Earth s atmosphere. Deposits of these materials are known as plume deposits and have been found around the world at the K Pg boundary (Smit 1999). The reentry of a large amount of ejecta into the upper atmosphere could ignite the surface (Melosh et al. 1990). The fires, in turn, would fill the lower atmosphere with smoke, dust, and pyrotoxins. However, recent, more detailed investigations of this process suggest a smaller effect, for two reasons: (1) as particles settle through the atmosphere and cool, they Butterfly-shaped area of fallen trees caused by the Figure 2 Tunguska airburst, superimposed onto a map of modern Moscow (population >10 million) partially shield the surface from the infrared (IR) radiation emitted by new particles entering the upper atmosphere (Goldin and Melosh 2009); (2) almost all the K Pg distal ejecta might not have been distributed worldwide ballistically, but might have followed impact-induced atmospheric winds (Artemieva and Morgan 2009). LONG-TERM EFFECTS Long-term effects of impact events are associated with substantial and long-lasting (months to years) changes in Earth s climate system. This occurs whenever large amounts of dust, soot, and climatically active gases are injected into the atmosphere by an impact event, causing a significant perturbation of the atmosphere s chemistry and thermodynamics. The possibility of a release of climatically active gases in an impact event depends on the characteristics of the target. CO 2 and SO x species are released from sedimentary targets (as in the case of the K Pg impact), whereas oceanic impacts inject large amounts of water vapor into the upper atmosphere. Dust and Soot Alvarez et al. (1980) were the first to propose that years of darkness and the resulting elimination of photosynthesis, brought about by the ejection of impact-generated dust into the atmosphere, caused a mass extinction at the K Pg boundary. Toon et al. (1997) distinguished two sources for dust that rises into the upper atmosphere (or beyond) after a large land impact at 25 km s -1 : (1) molten and vaporized rocks, with the total mass equal to ~15 projectile masses; (2) 100 300 projectile masses of solid (pulverized) target materials. In the case of the K Pg impact, dust production has been estimated to correspond to about 5 10 12 tons of initially molten or vaporized droplets, hundreds of micrometers in diameter. The size distribution of pulverized rock is not well constrained; using data from nuclear tests and Elements 57

A B Early stage (8 seconds after impact) of the Chicxulub Figure 3 impact event, modeled as the impact of an asteroid 14 km in diameter hitting the surface at an angle of 45º (from the left) at a velocity of 18 km s -1. (A) Material-density distribution: atmosphere is in blue, sediments in yellow, crystalline basement in reddish brown, projectile in gray. Material density is given by the color shading (more intense color = higher density). The figure shows the growing transient cavity (TC), ejecta curtain (EC), expanding impact plume (IP), and shock waves (SW) in the atmosphere and in the target. (B) Temperature distribution (in kelvins): the plume has a temperature of 1000 3000 K, a range that includes the phase-transition temperatures for many materials; the atmosphere is heated well above 6000 K. laboratory impact studies, O Keefe and Ahrens (1982) determined that only ~0.1% of the total impact ejecta (~0.3 impactor masses) is <1 μm in size. The submicrometer component of the dust would remain in the upper atmosphere for extended periods of time (several months) and ultimately affect the climate. The climatic effect of dust injected into the atmosphere during the first year after the K Pg impact was modeled with an atmospheric general circulation model (GCM) simulation that included the optical effects of a thick dust layer in the lower stratosphere (Covey et al. 1994). The results indicate a strong and patchy cooling on land, with temperature declining by up to ~12 ºC, and a mild cooling (a few degrees) over the oceans, accompanied by a 90% decrease in precipitation for several months. More recently, Luder et al. (2003) integrated a two-dimensional, zoned, average dynamic ocean circulation model with surface radiative fluxes obtained from a 1-D radiation balance model modulated by a thick atmospheric dust layer. They found that the upper 200 m of the oceans could cool by several degrees Celsius in the first year after the impact, while overall deep-sea temperatures might decrease by no more than a few tenths of a degree. According to this model, the dust-related climatic change did not affect the general ocean circulation, which is the main moderator of Earth s climate. These results are confirmed by the marine isotopic record, which shows little evidence for either warming or cooling across the K Pg boundary, suggesting a prompt recovery of the climate within a few decades after the impact. Recently, Harvey et al. (2008) concluded that soot found at several marine K Pg boundary sites is consistent with combustion of fossil organic matter within a 200 km diameter crater. As the authors overestimated the total input from fossil fuel (combustion requires high temperature and, hence, strong shock compression), traditional sources (e.g. ignition of a large fraction of the Cretaceous biomass: Wolbach et al. 1990) have to be added to reproduce the total amount of soot at the K Pg boundary. Soot is a strong absorber of short-wave radiation; even minor amounts of soot injected into the upper atmosphere during impact could prevent solar radiation from reaching Earth s surface and significantly enhance the climatic effects associated with dust injection. Climate-Active Gases Carbon Dioxide CO 2 is a strong greenhouse gas; in the atmosphere it is transparent to visible solar radiation while absorbing IR radiation. An increase of CO 2 in the atmosphere increases the trapping of IR radiation emitted by the Earth, causing a net increase of temperature at the Earth s surface. The K Pg impactor hit a target containing significant sedimentary carbonate, which would have been dissociated to produce CO 2 during the impact. Estimates of the amount of CO 2 released during the K Pg impact vary by an order of magnitude due to (1) uncertainties in the value of shock pressure needed for decarbonation (this value depends on target porosity); (2) the relative proportions of dolomite and limestone, the main carbonate rocks at the impact site; (3) the rate of recombination of CO 2 with highly reactive residual oxides; and (4) the impact scenario. Even assuming the largest estimate of CO 2 released into the atmosphere, around 6000 Gt (1 Gt = 10 12 kg), the K Pg impact would have increased the end-cretaceous atmospheric inventory by at most ~50%, causing a perturbation of the solar energy flux of between 1.2 and 3.4 W m -2, which is comparable to the estimated perturbation by greenhouse gases due to industrialization (Pierazzo et al. 2003). A potentially much larger contribution to atmospheric CO 2, hypothesized to originate from global wildfires, now appears unlikely in view of new assessments of the lower intensity of the IR heat pulse from reentering ejecta (Goldin and Melosh 2009). Sulfur Oxides The production of sulfate aerosols from the release of SO 2 and water vapor into the stratosphere is well documented for volcanic eruptions. Micrometer-sized sulfate aerosols scatter visible solar radiation and can be strong absorbers of IR radiation, causing a net cooling of the Earth s surface. During the K Pg impact, dissociation of evaporitic layers in the sediments produced orders of magnitude more sulfur oxides than giant volcanic eruptions, although the exact proportion of the various oxides (SO 2 and SO 3 ) is still unclear. Sulfur dioxide must be oxidized to SO 3 prior to forming sulfate aerosols. The potential effects of impactrelated sulfate production in the stratosphere have been Elements 58

investigated using 1-D atmospheric models combined with simple coagulation models. Assuming 200 Gt of SO 2 and the availability of water vapor, Pope et al. (1997) found a >50% reduction in the amount of solar radiation transmitted to the atmosphere for up to 10 years after the impact, which would have caused continental surface temperatures to approach the freezing point for several years. Using a similar approach but incorporating a mixture of SO 2 and SO 3 and a different estimate of oxidation time for SO 2, Pierazzo et al. (2003) obtained a slightly shorter duration for the sulfate effect, with a 50% reduction in solar radiation transmission for 4 to 5 years after the impact. The sulfate aerosol effect has a much longer duration than the effect of dust injection, mainly because of the long residence time of SO x and water vapor in the upper atmosphere, which results in the continuous formation of new sulfate aerosols over a period of years. Water Vapor The consequence of injecting a large amount of water into the upper atmosphere is an important, yet poorly constrained, global effect from oceanic impacts. Pierazzo (2005) estimated that a K Pg-sized impact into a deep ocean would inject into the upper atmosphere an amount of water equivalent to more than three times the amount of water vapor that an unperturbed middle atmosphere can hold at saturation (0.2 g cm -2, or ~1000 Gt). In the atmosphere, water molecules supply the free radicals OH and HO 2 to the HO x catalytic cycle that destroys ozone. Furthermore, dissolved salts from seawater (whose average salinity is 35 ) would deposit about 65 Gt of Cl and 3 Gt of S in the upper atmosphere, contributing to ozone destruction and the formation of sulfate aerosols. The atmospheric injection of a large amount of seawater would, GLOSSARY Acoustic wave a weak elastic wave propagating through a medium with constant speed (speed of sound) Catalytic cycle a multistep chemical reaction that is accelerated by the involvement of a catalyst (a reagent that is not consumed by the reaction, but changes the rate of the reaction) Ejecta curtain debris thrown from an impact crater. Although each ejected fragment follows a ballistic (parabolic) trajectory, the times and velocities of ejection are organized so that most of the debris lies on the surface of an expanding inverted cone. Impact plume the fastest and hottest material ejected from an impact crater; a mixture of partially vaporized projectile and target materials that expands nonballistically Meteoroid a solid object moving in interplanetary space, considerably smaller in size than an asteroid and considerably larger than an atom or molecule (International Meteor Organization). This term is often used to designate any cosmic body impacting the Earth without specification of its size or type. Pyrotoxin a toxic agent produced by combustion (e.g. carbon monoxide, carbon dioxide, hydrogen cyanide, nitrogen dioxide) Shock wave a compression wave propagating through a medium (solid, liquid, gas, or plasma) faster than sound. Shock waves are characterized by an abrupt, nearly discontinuous change in pressure, density, and energy. Zonally averaged, monthly mean fractional changes of Figure 4 the atmospheric ozone column following the impact of a 1 km diameter asteroid in the central Pacific Ocean. Values are calculated with respect to the unperturbed case, assuming a January 1, 1995 impact. Ozone depletion below -0.5 (green and cyan colors) exceeds the recorded historic minima of the ozone hole above the South Pole in the mid-1990s. Reprinted from Pierazzo et al.(2010), with permission from Elsevier therefore, cause a large perturbation of the atmosphere s chemistry and its radiative balance (because water is a strong absorber of infrared radiation). Recently, Pierazzo et al. (2010) concluded that even smaller impacts would cause a major perturbation of the normally dry upper atmosphere. Combining numerical impact simulations with a whole atmosphere GCM, including interactive chemistry, they found that impact in the ocean of a 1 km diameter asteroid could produce a multiyear ozone depletion comparable to the mid-1990s ozone hole (Fig. 4). The upper atmosphere s ozone protects the biosphere against harmful ultraviolet-b radiation (UV-B, 280 315 nm), and Pierazzo et al. (2010) found that such an ozone depletion would result in an increase in UV-B to a level far in excess of that currently experienced anywhere on the Earth s surface. Important biologic repercussions associated with increased surface UV-B irradiance include increased incidence of erythema (skin-reddening) and cortical cataracts; decreased plant height, shoot mass, and foliage area; and damage to molecular DNA. CONCLUDING REMARKS Impact events have punctuated Earth s geologic past. They brought local if not global devastation, affecting the equilibrium of the terrestrial ecosystem by perturbing the climate and affecting the evolution of life. Although only the largest (and rarest) impact events can result in mass extinction of life, the more frequent impacts of midsize objects may still have dramatic consequences. Little is yet known about the potential consequences for the environment and biosphere of impacts below the threshold for causing mass extinction, but nevertheless large enough to cause (near-)global catastrophe. Human civilization is fragile and might be affected by a much smaller impact than the one that ended the dinosaurs reign. Our natural resources can barely support the growing world population, and we must rely on the development of new technologies and alternative/advanced energy sources. Impacts of midsize asteroids could perturb the environment and climate enough to cause partial crop failure worldwide for a short period of time. Significant damage to sensitive infrastructure, such as dams, nuclear power plants, and high-risk chemical facilities, would reduce energy production and further affect the environmental pollution. Elements 59

Overall, a calamity resulting from an impact is a low-probability event, which raises difficult questions about how humanity should best prepare for, or mitigate against, such a disaster. What is clear is that future research should focus on quantifying the potential threat of small-to-mediumsize impacts, including their risk to civilization as we know it. Impact risk mitigation is well summarized in a recent National Research Council report (NRC 2010). Finally, interested readers can estimate impact-related short-term effects online by going to the site www.purdue.edu/ ImpactEarth (see also Collins et al. 2005), which provides general first-order-accuracy estimates. REFERENCES Alvarez LW, Alvarez W, Asaro F, Michel HV (1980) Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108 Artemieva N, Morgan J (2009) Modeling the formation of the K Pg boundary layer. Icarus 201: 768-780 Artemieva N, Shuvalov V (2010) Tunguska Explosion - Final Remarks. 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