Comment on “On the influx of small comets into the Earth's upper atmosphere II. Interpretation”

An influx of small water snow comets at a rate of ~20/minute has been suggested by Frank et al. (1986b) as the cause of large transient decreases of atmospheric dayglow intensities at ultraviolet wavelengths observed by Frank et al. (1986a). The comet motion was assumed to be direct (prograde) and generally confined to the ecliptic plane. The comets were taken to be in Earth-crossing orbits with perihelia and aphelia ranging from the orbit of Venus to beyond Pluto's orbit. This model is untenable for several reasons: (1) The comets would evaporate within a few periods unless they were thoroughly manfled, and no residue of the required large quantity of mantle material has been found. (2) Iridium abundance measurements in deep sea sediments limit the dust fraction of the suggested small comets to less than 10-4; in contrast, known comets seem to be nearly half dust. (3) Frank et al. (1986b) assumed the small comets to be fully