Impacts from Comets and Asteroids

The evolution of the Earth, and life on Earth, has been punctuated by periodic impacts with extraterrestrial bodies — some leading to mass extinction events. Most famous among these is the 65 Mya Cretaceous-Tertiary (K-T) extinction, during which three quarters of all surface species were eliminated following the impact of a 15 km diameter asteroid travelling at 20 km/s at the Chicxulub site (the cenote ring — sinkholes in the underlying standstone —  is traced on figure 1).

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Figure 1: approximate location of cenote ring (and extrapolation of the semicircle to the marine environment) at the Chicxulub site in the Yucatan penninsula

 

Asteroid densities range between 3000 kg / m^3 (rocky) to 8000 kg / m^3 (metallic), depending on their composition. A back-of-the-envelope calculation provides an estimate of the impactor’s energy:

\text{E}_\text{kinetic} = \frac{1}{2}\text{m}{v}^2

where m ranges between 4\times 10^{16} - 1\times 10^{17} kg and v = 20,000 m / s and the kinetic energy ranges between 8\times 10^{24} - 2\times 10^{25} J [one should note that the impact velocity of an asteroid averages 18 km /s but may range between the Earth’s escape velocity (~10 km/s)  and the solar system’s escape velocity (~ 70 km/s]. Another back-of-the-envelope conversion (one ton of TNT = 4\times 10^9 J) reveals that this impact released an energy equivalent to 2\times 10^9 - 5\times 10^9 megatons of TNT (not a typo).

An interesting thing happens when explosions produce large fireballs. If the fireball is greater than the scale height of the atmosphere (~ 8 km), the fireball will backfire, producing a vacuum straw which draws material from within the fireball directly to the stratosphere and mesosphere. This was certainly the case with the K-T impactor.

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The chemical and climate effects of this event were evaluated by a team from the NCAR and the University of Colorado in 2017. Bardeen and coworkers found catastrophic decadal reductions in surface and sea-surface temperatures following the massive injection of aciniform carbon, black carbon, and charcoal into the mesosphere and stratosphere by the event. They determined that this layer of carbon reduced surface insolation by one-billionfold for several months, and this impact winter didn’t recover to even one percent of the present downwelling flux until two years later. Meanwhile, the carbon layer in the atmosphere produced stratospheric heating between 50 — 100 K, cooking away the ozone layer: enhanced rates of HOx-mediated ozone destruction and reduced rates of ozone production resulted in surface UV flux increases of up to 300% for a period of about six years following.

Such impacts of large, monolithic asteroids or comets are infrequent. The time between expected impacts can be expressed as a power law function of impactor diameter in which D is given in meters and alpha is the fractional composition of ocean (0.7) or land (0.3), depending on the type of impact for which one wishes to compute the recurrence interval. If the impactor is greater than the average depth of the ocean (3.6 km), alpha is unity.

\tau_\text{impact} = \frac{3.71\text{x}10^{-2}\text{D}^{2.377}}{\alpha}

The threat from an asteroid or comet impact is ranked between 0 — 10 on the Torino Scale, in which an increasing metric conveys both the increasing probability of impact and a higher potential destructiveness. Though only known Earth-crossing bodies are ranked on the Torino scale, from the recurrence interval relation, it is evident that the catastrophic Chicxulub event is not a major concern (\tau = 3.13\times 10^8 years); however, smaller events are expected to pose a threat especially to the ozone layer.

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Impacts of smaller bodies with the ocean will vaporize large amounts of ocean water and the trace species within it. Pierazzo et al. (2010) quantify the risk to ozone following such an event, finding that impactors greater than 500 m in diameter (\tau = 97 ky, ~ 60,000 MT TNT) would increases the stratospheric burden of NOx, ClOx, BrOx, and HOx to such an extent that years-long decreases in total column ozone of up to 30% at midlatitudes would occur, resulting in twofold increases in erythemal radiation.

Screenshot_2018-02-05_16-42-24

Similar results are found by Birks et al. (2007), who explore impactors between 150 — 1200 m in diameter. They find that asteroids of 450 m diameter (\tau = 75 ky, ~ 40,000 MT TNT) are likely to cause significant years-long perturbations in total column ozone, and that smaller impactors are likely to only have continental or regional effects, though they do not model the effect of stratospheric soot injections, and these effects may be very significant, depending on the area over which biomass burning occurs.

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This leads me to two very informative articles which were published a few days ago in the Journal of Geology. Wolbach and coworkers produced two comprehensive papers (paper one & paper two) which provide an explanation for the Younger Dryas cooling event (13 kya), which is characterized by a 1300 year disruption in climate, the collapse of the thermohaline circulation, the disappearance of the Clovis culture (note: not the Klobas culture), and the extinction of large animals across the world. The authors provide trace element and isotopic data from ~200 sedimentary and ice core sites around the world indicating that an abrupt and widespread input of carbon to the atmosphere coincided with an enhancement in platinum resultant from comet impact.

They posit that an Earth-crossing comet on the order of 100 km in size fragmented and produced a debris swarm several hundred Earth radii in length, estimating that the recurrence time between an encounter of the Earth and such a debris swarm is on the order of 50 ky. For each encounter, some 10^{13} - 10^{14} g of comet bits will rain down at 30 km/s (comets have an average velocity around 30 km/s at 1 AU) and produce a series of air bursts and land impacts. The authors find from ice core and sedimentary record that up to 10 million square kilometers of the Earth burned following the impact 13 kya (about 5% of the total land surface area of the planet at the time), producing a long-lived impact winter with extreme effect on the climate. Though the direct effects of the comet on the atmosphere dissipated within a decade, feedbacks were perturbed in such a way that the climate system was disrupted for a millennium afterward (causing iceberg calving, meltwater flooding, and subsequent thermohaline compensation).

The authors also briefly discuss the ozone impact of the comet encounter, noting that while soot falls out of the atmosphere after a few months the recovery of ozone lags behind for up to a decade — and that ozone depletion may play a primary role in the extinctions observed during this period.

So, I guess it’s time to add another entry to the stratospheric threat matrix. More on that later, when the work is complete.

 

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