Atmospheric cosmic dust fluxes in the mass range, < 102 g are 1-2 orders of magnitude greater than those in the near Earth space

There exists a fairly good consensus on the fluxes of extra-terrestrial particles in the near Earth space, over a very wide mass range (Ceplecha, 1992). We show analytically that ablation and fragmentation of incident objects in the atmosphere can drastically modify the size spectrum of incident extra-terrestrial particles (ETPs) in the mass range, 10^-17 > m < 10² g. We present a plausible model for the fragmentation of extra-terrestrial particles in the atmosphere using the available information principally from two sources: (i) the classic analytical studies of Whipple, and of Dohnanyi, who modeled the observed population of shower and sporadic meteoroids, in terms of their production and destruction, who showed that the observed size distribution can be understood as a result of fragment production in a steady state by mutual collisions, and (ii) data on the observed size distribution and mass wastage of meteorites on their traversal through the atmosphere. We conclude that the population of extra-terrestrial material is radically altered in the atmosphere. There are two important modifications: (i) Secondary particles produced in fragmentation, and in certain size ranges, 10^-17 > m < 10² g, the population is augmented by one to two orders of magnitude, and (ii), vaporized material produced during ablation. A conservative estimate for the latter is about ~50% of the incident flux which ranges between 1.5 x 10¹⁰ -3 x 10¹⁰ g/year on the surface of the earth, depending on the upper limit of the meteoroid size considered. The model calculations of the secondary particles produced in fragmentation are not very sensitive to the choice of the parameters since the secondary fragments produced in ablation arise chiefly from incident fragments of size, 1cm < r₀ < 10² cm (10^-1 g < m < 10⁷ g), the range in which their number-radius spectra is very hard. The results should be treated to be robust within a factor of about 2 - 3. In closing, we would like to mention that we were led to the development of the atmospheric fragmentation model because we found an ice sample from the Greenland Ice Sheet Project 2 to contain appreciable amounts of meteoritic material, the odds for which were estimated to be smaller by more than three orders of magnitude. This led us to think that the fragmentation of large meteorites during their passage through the atmosphere could possibly lead to the production of an appreciable number of small particles, which is eloquently borne out by the results. The reserach was in part funded by ATM-9905299. References Z. Ceplecha, Astron. Astrophys. 263, 361 (1992). J. S. Dohnanyi, Jour. Geophys. Res. 75, 3468 (1970). D. Lal, A. J. T. Jull, G. S. Burr, and D. J. Donahue, Nucl. Instr. Meth. Phys. B172, 623, (2000). F. L. Whipple, Smiththsonian Astophys. Obs. Spec. Rep. 239, 1-46, 1967.