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Possible Climatic Perturbations Produced by Impacting Asteroids and Comets

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D. A. Kring, H. J. Melosh & D. M. Hunten: Possible Climatic Perturbations
Produced by Impacting Asteroids and Comets, Meteoritics 30(5), 530
(September 1995)

Abstract (refs removed):
Studies of the Chicxulub impact event suggest that large volumes of
evaporites and carbonates in the target may have been vaporized, enhancing
concentrations of S-rich aerosols and CO2 in the atmosphere, which may have,
in turn, been partly responsible for the mass extinction that occurred at
the Cretaceous-Tertiary boundary. We note that in this and other impact
events the projectile is also vaporized and propose that it may be an
additional source of climatically-active elements in the atmosphere. In
particular, we suggest asteroids and comets that are vaporized during
hypervelocity impact events can inject large masses of S into the
stratosphere where it can then affect the radiation budget of the Earth,
alter the chemistry of the ozone layer, and eventually be converted to
sulfuric acid rain. We also suggest that these vaporized objects can inject
large masses of Cl and Br into the stratosphere where they, in addition to
previously described NOx, can destroy ozone and, in the case of Cl, also
acidify rainwater. To quantify these types of perturbations, we calculated
the masses of S, Cl, and Br that can be injected into the stratosphere
following the impact of several classes of asteroids and model comets. The
calculations indicate, for example, that relatively small carbonaceous
asteroids, 0.3 km in diameter, contain 5 times more S than the entire modern
stratosphere, or approximately the same as that produced by the 1982
eruption of El Chich[ó]n. Using current cratering rates, we infer that these
types of perturbations occur at an average rate of 1 per 10,000 years.
Larger impact events, capable of injecting 10^15 g of S into the
stratosphere and possibly causing mean surface temperatures to decrease by 2
degrees C for 3 years or longer, occur at an average rate of 1 per 1 million
years. These results indicate that significant S can be added to the
stratosphere even in those cases where the target (like that at Chicxulub)
does not contain vast evaporite deposits. Specific calculations of 17 past
impact events on Earth indicate that stratospheric S was enhanced up to 10^5
times the current value; it seems likely that some of the larger events,
like the 35 +/- 5 Ma Popigai impact event and the 73 +/- 3 Ma Kara and
Ust-Kara impact events, produced enough S-rich aerosols to perturb the
climate. In addition to S, asteroids with 0.1 to 10 km diameters inject 3 x
10^8 to 9 x 10^14 g of Cl and 2 x 10^5 to 5 x 10^12 g of Br into the
stratosphere, not including any Cl and Br produced from target lithologies
(like seawater), nor any Cl and Br produced from biomass burning that may
have occurred after the impact. These calculations indicate that impacting
asteroids with diameters of a few tenths of a kilometer or larger can inject
enough Cl and Br into the stratosphere to destroy a significant portion of
the stratospheric ozone. Unfortunately, it is difficult to determine the
exact magnitude and duration of these climatic perturbations because they
depend in large part on the still uncertain chemistry and microphysics that
occur in the vapor plume as it rises and cools, and in the stratosphere once
the ejecta is deposited there. Nonetheless, it seems clear that asteroid and
comet impacts are an important source of climatically-active material and
that relatively small impact events (and, thus, those that occur frequently)
can potentially affect the global environment.