Zodiacal dust and deep-sea sediments.
The recent detection of radioactive Al(26) in marine sediments has led to the conclusion that it is brought into the earth's atmosphere by micrometeorites which have been exposed, in interplanetary space, to solar high-energy protons. The Al(26) method is not precise enough to yield directly a reliable value for the mass accretion rate to the earth to better than about 3 orders of magnitude, but is sufficiently accurate to allow a crucial decision between two widely differing of interplanetary dust which have been proposed to explain observations of the zodiacal light. The two models lead to Al(26) concentrations which would differ by about 5 orders of magnitude. Thus, the presence of Al(26) is consistent with the zodiacal dust model with particles of some tens of microns rather then with submicron particles. From this model a mass accretion to the earth then be calculated which is set at 1250 (upper limit, 2500; lower limit, 250) tons per day, or 2.8 x 10(-15) g/cm(2) sec, or 4.5 x 10(11) g over the earth per This value does not depend on the flux of the solar high-energy particles, which may be uncertain by an order of magnitude or more. The presence of Al(26) supports the idea that an important fraction of the dust is stony in composition material density, and thus eliminates some more exotic dust models, as such one consisting entirely of carbon grains. We may also conclude that the accreted dust particles have been in the solar system and exposed to protons from solar high-energy particles for a time interval which is greater than a significant of the Al(26) half-life (0.74 x 10(6) years).
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