THE BRIGHT NIGHTS STRADDLING THE TUNGUSKA CATASTROPHE: CAUSED BY COMETARY DUST IN THE MESOSPHERE, OR BY NATURAL GAS IN THE THERMOSPHERE?
Wolfgang Kundt Bonn University, Germany
The bright nights preceding and following the 1908 Tunguska catastrophe in Europe and Western Asia, 29 June till 2 July, above northern latitudes of 42 degrees are unique in recorded history except for the 1883 Krakatoa volcanic eruption.
Based on the cometary infall hypothesis, Bronshten (1991) explains them via thin cloud layers at heights between 50 and 70 km, formed from settling cometary dust. He argues that such infalling dust gets softly braked, from speeds of about 30 km/s down to 30 m/s, if entering the atmosphere almost horizontally and at a narrow, mass-dependent impact parameter which corresponds to infall heights between 100 and 180 km. For a given mass, this soft-impact height is sensitively fixed to within some km. Sunlight at grazing incidence must scatter twice in succession on such clouds in order to illuminate the western Eurasian continent down to 42 degrees northern latitude.
In order to make his explanation work - i.e. to reduce the daylight illumination by only a factor of 10-5.7 +- 0.5 - Bronshten is forced to push several of his parameters to the limit. From the more than three 'inferred' meteoritic infall trajectories, he has to choose the easternmost, at a rather low inclination angle. For a sufficiently high infalling dust density, he requires a bright comet, like Halley's, brighter than could have gone unnoticed weeks before the assumed impact, whereby an overidealized geometry helps with another factor of ten. For strong scattering, the grains should be moderately heavy, some 10-9) g, whereas for illuminating distant enough latitudes, the clouds should hang high, i.e. their grains should be light, some 10-13 g. Why have the noctilucent clouds - at heights of 85 km - never given rise to similar bright nights, whereas Krakatoa has done so?! And how can the tails of a comet extend over four successive days, with one of them travelling ahead of its nucleus ?! (For an outgoing comet, there are no trailing tails).
Instead, I interpret the bright nights of Krakatoa and Tunguska as due to natural gas - mainly methane - which can rise supersonically to the thermobase, some 200 km high, where it is reheated both by solar irradiation and by slow burning (with the surrounding atomic oxygen), and can rise again subsonically,to <= 600 km. Convected water vapour (from partial burning) will freeze out as snow flakes, thus directly scattering the sunlight at thermospheric heights. Angular-momentum conservation of the rising gas can explain its westward drift from the venting site.