Atmospheric cosmic dust fluxes in the size range 10_-4 to 10 centimeters

We show that the long-time-averaged "effective" flux of extraterrestrial particles (ETPs) on the Earth over a wide range of particle sizes is significantly different from that of particles incident on the Earth's atmosphere, as a result of the ablation and fragmentation of larger incident objects in the atmosphere. We have analytically deduced the "modified" size spectrum of the ETPs using the equilibrium fragmentation model developed by Dohnanyi for meteoroid population in the interplanetary space and direct observations on fragmentation of a large number of meteorites in the atmosphere. Our model shows that most of the secondary particles are produced in the fragmentation of objects of radii from 1 cm to 1 m, the size range where the differential number spectrum is relatively much flatter, compared to the spectrum on either end. A significant fraction of impacting mass, about 50%, is volatilized during ablation and would be globally deposited, as will particles of size smaller than a few microns, in contrast to the remaining mass of particles of greater than 10-20 μm size, which we expect would be deposited close to the point of impact of the object. Although the long-time-averaged particle fluxes of smaller particles are greatly enhanced, these particles would not be sampled efficiently in stratospheric collections, since they are produced in fragmentation of larger size objects whose impact rates are small. Thus, stratospheric collections would be biased toward collection of the primary incident ETPs, whereas terrestrial accumulations representing large (area × time) products, as in the case of ocean sediments, would efficiently sample the fragmented particles.