An analysis of the physical, chemical, optical, and historical impacts of the 1908 Tunguska meteor fall

Abstract

A detailed analysis of the physical nature and photochemical after effects of the explosive cometary meteor Tunguska is presented. The physical manifestations of the event (the acoustic and seismic waves, forest damage, and so on) are shown to be consistent with the entry of a 5-million-ton object into the Earth's atmosphere at 40 km sec −1. The meteor apparently had a very low effective density (<0.01 g/cm 3) due either to its intrinsic porous structure, to shattering in orbit far from the Earth, or to breakup upon initial impact with the Earth's upper atmosphere. Aerodynamic calculations are used to demonstrate that the shock waves emanating from the falling meteor could have generated up to 30 million tons of nitric oxide (NO) in the stratosphere and mesosphere. The photochemical consequences of such an immense Tunguska-related NO injection are investigated with the aid of a fully interactive one-dimensional chemical-kinetics model of atmospheric trace constituents. The first year after the Tunguska fall (from mid-1908 to mid-1909) a 35–45% hemispherical ozone depletion is predicted with the model; declining but still substantial ozone depletions are calculated in subsequent years. Atmospheric transmission data collected by a research team of the Smithsonian Astrophysical Observatory (APO) at Mount Wilson, California, from 1908 to 1911 are analyzed for ozone absorption in the Chappuis bands. Statistical analysis of the APO data reveals an ozone variation of 30 ± 15% over this period, supporting the theoretical predictions. The optical anomalies which followed the Tunguska event are reviewed for evidence of NO x O x chemiluminescent emissions, NO 2 solar absorption, and meteoric dust turbidity. The chemical afterglows are shown to be intense enough to account for some of the unusual night-time light displays seen after the fall, but not widespread enough to explain the “light nights” and glowing skies reported throughout Eurasia. These phenomena appear to be related to the dust and water vapor deposited by the meteor at the cold summer mesopause, resulting in the formation of dense noctilucent clouds. Only circumstantial optical evidence for a large Tunguska NO 2 enhancement is found, which can not be used to calibrate independently the NO injection by the meteor. The suggestion of a dust veil created by the Tunguska explosion is revealed by the APO transmission data. We deduce that nearly 1 million tons of pulverized dust may have been deposited in the mesosphere and stratosphere by the Tunguska fall, which agrees with previous estimates of the meteor mass influx. Possible climate changes triggered by the Tunguska event are investigated. The most important climate anomaly identified in the post-Tunguska era is a 0.3°K cooling of the Northern Hemisphere which lasted for almost a decade. Several large volcanic eruptions occurred during this period which also played a role in the temperature change. However, radiation transport calculations are reported which suggest that Tunguska contributed to the cooling trend. The lessons of Tunguska for other important geophysical problems, such as ozone/weather coupling and the ancient extinction of the dinosaurs, are also explored. It is concluded that more rigorous investigations of the physics and chemistry of the Tunguska event are warranted.