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The Accretion of Meteoric Matter and Cosmic Dust

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THE ACCRETION OF METEORIC MATTER AND COSMIC DUST The interplanetary space beyond the Earth's atmosphere is by no means as empty as it was believed to be as recently as the beginning of the 20th century, when space data were still very meager. In actual fact, this space is saturated with rapidly moving high-energy elementary particles, known as cosmic rays, which come from the Sun and from distant stars and stellar systems (galaxies). Moreover, there are also a great number of larger material formations moving through the space outside the atmosphere, such as dust grains and splinters and lumps of rocky material and iron-nickel compounds. Although the density of the meteoric matter is very low in comparison with that of the closely packed matter on planets and on the Sun, still it is now clear that interplanetary space is far from empty.

During its annual journey around the Sun, the Earth sweeps up the meteoric particles that it encounters along the way. This may be compared with what happens when an automobile moving along the road runs into a swarm of gnats and the bodies of the insects, crushed by collision with the vehicle, are deposited on the windshield. The accumula tion (accretion) of meteoric matter takes place on the surface of the planet. When they strike the Earth's atmosphere at velocities of 12 to 70 km/sec (which is much higher than the speeds of artificial satellites launched from powerful rockets), the larger meteoric particles heat up, become melted, and then vaporize. As a result of this disintegration of meteoric bodies at heights of 70 to 120 km, a fine meteoric dust is produced in the atmosphere. This dust is composed of spheroidal particles with diameters ranging from several microns to a millimeter /1/. The dust settles in the Earth's atmosphere for about a month's time. In addi tion, the Earth's atmosphere is invaded by large numbers of tiny meteoric particles which are unaffected by interaction with the air. Because of their small mass, these particles are slowed down in the very tenuous upper layers of the atmosphere and thus are not heated up in the relatively dense meteoric zone (70-120 km) the way the more massive meteoric bodies are. This is the so-called cosmic dust.

The meteoric and cosmic dust settling onto the Earth's surface increases the mass of the planet and contributes to the material composition of the Earth's crust. At first sight, the part played by meteoric dust

might seem to be negligible. However, if we take into account that the accretion of meteoric matter has been going on for at least 4 or 5 billion years, and also that the cloud of meteoric matter around the Sun during the initial stage of dust accumulation by the Earth and the other planets must have been incomparably denser, then of course the picture is different. The accumulation of the particles of a protoplanetary meteoric cloud forms the basis of Shmidt's well-known cosmogonic hypothesis /2/. It may not be possible to accept all the aspects of this hypothesis, but Shmidt's proposal that meteoric accretion played a larger part in the past seems to be quite plausible.

New factual data on the rates of accumulation of meteoric matter by the Earth have been obtained via direct studies of this matter in interplanetary space by means of artificial satellites and space rockets. Previously, comparatively large meteoric particles (a millimeter or some tenths of a millimeter in size) were studied by visual means and by radar. The total mass of meteoric matter falling onto the Earth was then estimated at a few (2 to 5) tons per day /3/. However, the picture became quite different after new direct counts of meteoric particles in space were made using instruments mounted on satellites and rockets. Soviet and American investigators have used very different techniques for recording direct encounters between the surface of a space vehicle and the meteoric grains in interplanetary space. The pits produced on plates of soft metal or polished steel by collisions with dust grains have been counted; impacts of meteoric particles have been recorded using microphones with piezoelectric crystals or photomultipliers; finally, other ingenious and diverse observation methods have been used. The counts of the number of collisions were then telemetered back to Earth or else studied upon recovery of the satellite or rocket. In this way, we have been able to obtain information on grains of meteoric matter 4 or 5 microns in diameter with masses as low as one billionth of a gram gram]. Such measurements are feasible both because of the extremely sensitive instruments used and because of the enormous energies of collisions with meteoric particles, the average collision velocity being about 40 km/sec.

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